Module 1DC machines- Principle of operation of dc generator, constructional details, emf equation, types of generators. Principle of operation of dc motors. Electrical and mechanical characteristics of dc series, shunt and compound motors, applications. AC motors- Principle of operation, rotating magnetic field, single phase and three phase induction motors.

DC MACHINES – INTRODUCTIONA dc machine can be considered as a 2 – port network where energy conversion from electrical to mechanical domain (or vice versa) takes place. If energy is flowing in direction as shown in fig 1 (from electrical to mechanical domain), i.e., we give an electrical input to the dc machine and get a mechanical output (in the form of rotation of shaft, etc), then the machine is a dc motor. If energy flow is in opposite direction, (from mechanical domain to electrical domain), i.e., we give a mechanical input in the form of rotating the shaft or so, and we get an electrical output, we can call that dc machine as a dc generator.

On the electrical domain, the variables are E (potential) and i (current). Mechanical domain variables are T (Torque) and ω (angular velocity). CONSTRUCTIONAL FEATURES [Explain with neat sketches the main parts of a dc machine. Describe their functions. June 2009, KU] Constructionally, dc generator and dc motor are same.A dc machine can be considered to have 4 main parts. 1. Field system2. Armature3. Commutator4. Brushes

1. FIELD SYSTEM____________________________________________________________________________________________Q. What is its need?A. The field system creates a uniform magnetic field within which the armature rotates.____________________________________________________________________________________________Q. What is the need of producing a magnetic field?A. Consider the case of a simple water pump. It pumps water from a low elevated area (e.g. well) to a tank situated in an elevated area.

The pump does not produce water. It just causes a mechanical pressure which forces the existing water into the elevated tank against its weight. In the same way, the generator does not produce electricity, but creates potential difference which causes the electric current to flow from low potential terminal to high potential terminal in the machine. The generator operates on the principle of production of dynamically induced emf. i.e., whenever flux is cut by the conductor, dynamically induced emf is produced in it according to the laws of electromagnetic induction, which will cause a flow of current if the circuit is closed.____________________________________________________________________________________________Q. What is needed to create this dynamically induced emf?A. Here we come back to our discussion; we need a magnetic field (field system), conductor and motion of conductor with respect to the field to create dynamically induced emf. ____________________________________________________________________________________________Electromagnets are preferred in comparison with permanent magnets on account of their greater magnetic effects and field strength regulation which can be controlled by regulating the magnetizing current. The field system consists of four parts.1) Yoke or Frame2) Pole Cores3) Pole shoes4) Magnetizing coilsYoke provides mechanical supports for poles and acts as a protecting cover for whole machine. It acts as the frame of the machine and carries magnetic flux produced by the poles. In small generators yokes are made of cast iron. But for large machines, cast steel/rolled steel is employed owing to their weight-less nature.

Pole Cores are made of cast steel and usually of circular cross section. They are used to carry the coils of insulated wires which carry the field current (or exciting current). [Q. What is the need of exciting current?A. It’s simply to excite the field magnet which is usually electromagnetic in nature. It behaves as a magnet when it is excited (when current is passed through a coil round it). Of course, we need the magnet to produce the field.] Pole shoe serves two purposes. They spread out flux in air gap and also being of larger cross-section; reduce the reluctance of magnetic path. They also support the exciting/field coils. They are always laminated to avoid heating and eddy current losses caused by fluctuations in flux distribution on the pole face due to movement of armature slots and teeth.Magnetizing/Field coils/windings consists of copper wire/strip, are former-wound for correct dimension. Then former is removed and wound-coil is put into place over core. When current (exciting or field current) is passed through these coils, they electromagnetise the poles which produce the necessary flux that is cut by revolving armature conductors.[Explain the functions of field winding in a DC machine. June 2006, KU]2. Armature Core:- In dc machine, armature is rotating. It houses the armature conductors/coils and causes them to rotate and hence cut the magnetic flux of field magnets. In addition to this most important function is to provide a low reluctance path to flux through armature. This is done by making the air gap between pole pieces and armature as small as possible. Larger the air gap, greater magnetizing/exciting current is needed. Armature core is cylindrical/drum shaped and is built of usually circular sheet steel discs/laminations and keyed to shaft.

The slots are either die-cut or punched on outer periphery of disc and keyway is located on inner diameter. In small machines, armature stampings are directly keyed to shaft. Usually these laminations are perforated for air ducts which permit axial flow of air through armature for cooling purposes.Armature winding is an arrangement of conductors to develop desired emfs by relative motion in a heteropolar magnetic field. In winding, conductor or group of conductors are distributed in different ways in slots all over the periphery of the armature. The conductors maybe connected in series and parallel combinations depending upon the current and voltage rating of the machine.[Explain the functions of armature winding in a DC machine. June 2006, KU]Functions of armature:- 1. Permits rotation for mechanical – generator action.2. Since it houses the conductors, emf is induced in them.3. Provides low reluctance path for magnetic flux.3. Commutator:-It facilitates collection of current from armature conductors. It rectifies and converts ac current induced in armature conductors into unidirectional current in the external load circuits. It is of cylindrical structure and is built up of wedge-shaped segments of high conductivity hard-drawn or drop forged copper. These segments are insulated from each other by thin layers of mica.4. Brushes and bearings:- Brushes collect current from commutator. Usually they are made of carbon or graphite and are rectangular shaped blocks. Brushes are housed in brush holders mounted on a spindle and brushes can slide in a box open at both ends. Ball bearings are frequently used. Heavy duty machines prefer roller bearings.

Copper brushes are used for machines designed for large currents at low voltages. The shaft is used to transfer the mechanical power from or to the machine. Principle of DC generatorThe dc generator operates on the principle of production of dynamically induced emf. i.e., whenever flux is cut by the conductor (armature conductor, carrying current), dynamically induced emf is produced in it according to the laws of electromagnetic induction, which will cause a flow of current if the circuit is closed.As the coil rotates in a magnetic field formed by the poles, a voltage is induced in the coil. When a coil – side moves from one pole to next pole, the polarity of induced emf changes – but polarity at brushes don’t change as the commutator also rotates with the coil. Thus a unidirectional voltage is available across the brushes. Armature winding As we discussed in the beginning, we need a magnetic field (field system), conductor and motion of conductor with respect to the field to create dynamically induced emf. In an armature winding, conductor or group of conductors are distributed in different ways in slots in the periphery of armature. Depending on the current and voltage rating of the machine, the armature windings may be connected in series and parallel. Below are given certain terms and their descriptions. It will be meaningful to go through it before dealing with the core topic of armature windings.Conductor: - The length of wire that lies in the magnetic field is called conductor. Hereafter we use the symbol Z to represent the number of conductors in an armature winding.

Turn: - A conductor induces a certain amount of emf in a magnetic field. In cases where we need to induce a larger amount of emf, we have to connect conductors in series. When the two conductors lying in a magnetic field are connected in series, so that resultant emf induced in them is doubled (compared to the emf induced in one conductor), it is known as a turn.Coil: - When one or more turns are connected in series and the two ends of it are connected to adjacent commutator segments (in lap winding) it is termed as a coil. Coil Side: - Each coil has two sides called coil sides which are embedded in two different slots nearly a pole pitch apart.Pole Pitch [Dec 2008, KU]:- It is defined as the number of conductors per pole. If there are 48 conductors for 4 poles, then pole pitch will be 48/4 = 12 conductors per pole.Front Pitch [Dec 2008, KU]:- It is defined as distance in terms of number of armature conductors between the second conductor of one coil and the first conductor of next coil which are connected to the same commutator segment. It is denoted byY f . Back Pitch [Dec 2008, KU]:- It is defined as distance in terms of number of armature conductors between the last and first conductors of the coil. It is denoted by Y b .

Resultant Pitch: - It is defined as distance in terms of number of armature conductors between start of one coil and start of next coil to which it is connected. Commutator pitch: - It is distance measured in terms of commutator segments between the segments to which the two ends of a coil are connected.

Coil span or coil pitch [Dec 2008, KU]:- Same as Back Pitch.

Above fig gives the cross-sectional view of a modern d.c. machine showing the main parts. Armature windings, along with the commutators, form the heart of the dc machine. The emf is induced here and its effective use enhances the output of machine. There are mainly two types of winding – Lap and Wave windingDC generator - e.m.f equation [Nov/Dec 2007, Nov/Dec 2006, KU] Let Φ = flux/pole in WeberZ = total number of armature conductorsP = No of polesA = No of parallel paths in armatureE = induced emf in any parallel path in armatureGenerated emf Eg = emf generated in any 1 of the parallel paths i.e., EAverage emf generated/conductor by Faraday’s laws of electromagnetic induction = dΦdt volt

Now flux cut/conductor in one revolution dΦ = ΦP WbNo. of revolutions/second = N/60So time for 1 revolution dt = 60/N secondEmf generated per conductor = dΦdt = ΦPN60 VoltSo generated emf, Eg = ΦPZN60 A VA=2 for simplex wave winding = P for simplex lap winding______________________________________________________________________________________________Q. A 4 pole generator with wave wound armature has 51 slots each having 24 conductors. The flux/pole is 0.01 Wb. At what speed must the armature rotate to give an induced emf of 220V. What will be the voltage developed if the winding is lap connected and armature rotates at the same speed?[NOV/DEC 2006, KU]A. Flux/pole,Φ=0.01Wb No of conductors = No of slots × No of conductors per slot Z = 51× 24 = 1224 conductorsNo of poles, P = 4; As it’s a wave winding, A =2E=ΦZNP

60 A

N=60 AEΦZP

=539 .21 rpm

For the same voltage to be developed if the winding is lap connected, take A = P=4 E=ΦZNP

60 A

E=0.01×1224×539.21×460×4

=109 .99V

______________________________________________________________________________________________Q. A 4-pole dc shunt generator has 24 slots with 10 conductors per slot and is rotated at 1000rpm. The flux per pole is 0.03Wb. Calculate the generated emf if the armature is lap-wound. [June 2009, KU]A. No of poles, P =4For a lap winding, we know that A = P = 4Flux per pole, Φ = 0.03WbN = 1000rpmNo of conductors, Z = No of slots × No of conductors per slot = 24× 10 = 240

Eg=ΦZNP60 A

Volts=0.03×240×1000×460×4

=120V

______________________________________________________________________________________________EXCITATION OF DC MACHINE AND CLASSIFICATION BASED ON EXCITATION (June 2006, KU) In dc machine, the field coils or field winding is excited by the current in order to produce the magnetic flux. The production of the useful magnetic flux with the application of electric current to the field windings is generally known as excitation. Generally two types of excitation are possible in a dc machine, they are(i) Separately – excited dc machine(ii) Self – excited dc machine1) SEPARATELY – EXCITED DC MACHINE

In this method of excitation, the field winding is energized or excited by a separate dc source. With the application of separate dc source, the field coils are energized and produce magnetic flux. This type of excitation is known as separate excitation. 2) SELF – EXCITED DC MACHINEIn this method of excitation, the field coils or windings are excited by the machine itself. So, the excitation of filed winding by machine itself is to produce magnetic flux is known as self – excitation. The dc machines are distinguished according to the connection of field and armature winding. The main types of dc machines classified on excitation are :-1) Separately excited generator2) Series generator3) Shunt generator4) Compound generator

1) Separately excited dc generators: - As we have discussed earlier that in separate excitation, the field winding is energized by a separate dc source which produces magnetic flux. The connection diagram for separately excited dc generators is shown below.

In the diagram,

I f=field currentV t=terminal voltage (armature )

2) SERIES GENERATORIn series generator, principal of operation is based on self – excitation method. The circuit for series generators is shown below.

I a=armature current

I se=series field current

R se=series field winding

V t=Terminal voltageIn series generators, the field windings are in series with the armature windings. The operation of series generator is based on the armature current which flows through the series field winding and then magnetic flux is produced. Since the armature current is large, series field winding consist a few turns of wire of large cross sectional area. 3) SHUNT GENERATORSThe shunt generator is also self – excited dc machine. In this type of machine, field winding is connected in parallel to the armature winding. The connection diagram is as shown.

The shunt field is made of large number of turns of fine wire as it receives the full output voltage (since it is connected in parallel) and small field winding current. R sh=Resistance of shunt field winding

I sh=shunt field current

V t=Terminal voltage

I a=Armature current

4) COMPOUND GENERATORThis is also a self – excited dc machine in which, the machine itself excites the field winding. In compound generation, both the series and parallel field windings are connected across the armature. Thus, the machine having both series and field winding is known as compound generators. There are two types of connections possible in compound generators. They are:-a) Short shunt compound generatorsb) Long shunt compound generatorsThe connection diagram is as shown:-

In short shunt, the shunt field is connected in parallel with the armature. Whereas, in long shunt compound generator, both series and shunt fields are parallel with the armature. I a=armature current

I sh=shunt field current

I se=series field current

R sh=shunt field winding

R se=series field winding

V t=terminal voltage CHARACTERISTICS OF DIFFERENT GENERATORSAT NO LOADAt no load, the generated emf is

EG∝ΦN

¿ EG=KΦNK = constant of proportionalityΦ = magnetic fluxN = Speed of rotation in RPM

EG depends directly in the flux which is produced by the ampere turns of the field coils when speed is constant. Since the number of turns is constant, the flux depends only on the field current. When I f=0 (when field circuit is open circuited), a small voltage Eg appears. This is due to the effect of residual magnetism. When the field current increases, the generated emf also increases linearly upto the knee of the magnetization curves. After the knee point is achieved, increase in I f cause’s magnetic field to saturate. So we can conclude that, a large increase in field current is required to obtain desired increase in voltage above the knee point or saturation region. The magnetization curve for a separately excited dc machine is drawn below.

In the above curve, the curve does not start from the origin, but from a little above on the y – axis due to residual magnetism. ________________________________________________________________________________________________Voltage build up process (Nov/Dec 2006, KU) Before loading a shunt generator, it is allowed to build up its voltage. Usually, there is always present some residual magnetism in its poles, hence a small emf is produced initially. This emf circulates a small current in the field circuit which increases the pole flux (provided field circuit is properly connected to armature, otherwise

this current may wipe off the residual magnetism). When flux is increased, generated emf is increased which further increases the flux and so on. This is known as voltage build up process.____________________________________________________________________________________________APPLICATION OF LOADa) SEPARATELY EXCITED DC GENERATORGenerally the terminal voltage decreases when the armature winding resistance increases and the expression is given as

V t=EG−I aRa

V t=TerminalVoltage

I a=Armature current (load current )

Another factor for decrease in terminal voltage is armature reaction (discussed later on). Due to this, the field flux decreases and gets distorted, which ultimately results in decrease of terminal voltage. b) SERIES GENERATOR (WITH LOAD)

Above figure shows circuit diagram for series generator with load. In series generator (remember that the armature and field winding are connected in series here, so current flows through both and voltage drop is created in both the windings), when the load is connected, there is a voltage drop in the field and armature winding. The voltage drop in series generator with application of load is given by expression.

V t=EG−I a (R A+RS )V t=terminal voltage

I a=Armature current

Ra=Armature resistance

RS=series field resistanceThe load characteristics of dc series generator can be shown as below.

c) DC COMPOUND GENERATORIn dc compound generator, the shunt field and series field is connected in parallel with the armature. When the load is connected across the armature, the voltage drop appears in series, shunt and armature winding( I A RA ). This voltage drop appears due to weakened flux and distortion of flux path from the original. The load characteristic of dc compound generator is shown below.

_______________________________________________________________________________Q. A short shunt compound generator supplies a load current of 100A at 250 volts. Generator has following winding resistances. Shunt field = 130 ohms. Armature = 0.1 ohms and series field = 0.1 ohms. Find the emf generated if the brush drop is 1V per brush. [June 2007, KU]Series drop¿ I aR se=¿ 100(0.1) = 10VI sh=

250+10130

=260130

=2 A I a=I L+ I sh=102 AEg=V +seriesdrop+brushdrop+ I aRa ¿250V +10V +2V+120 (0.1V )=274V_______________________________________________________________________________Q. Explain the terms critical resistance and critical speed as applied to dc shunt generator. [Dec 2008 KU]____________________________________________________________________________________The critical resistance is the slope of critical resistance line.

ie . ,RC=∆ E∆ I f

=DECD

=tan θ

Similar to the critical resistance there is a concept of critical speedNC. We know thatE∝N . As speed decreases, the induced emf decreases and we get O.C.C. below the O.C.C. at normal speed. If we go on reducing the speed, at a particular speed we will get O.C.C. just tangential to normal field resistance line.Thus the speed at which the machine just excites for the given critical field resistance is called the critical speed of a shunt generator denoted as NC .

DETERMINING CRITICAL SPEED

It is known that as speed changes, the OCC also changes, similarly for different shunt field resistances, the corresponding lines are also different.As discussed earlier, the speed for which the given field resistance acts as critical resistance is called the critical speed, denoted asNC. Thus if the line is drawn representing given R sh then OCC drawn for such a speed to which this line is tangential to the initial portion represents the critical speed NC. _______________________________________________________________________________________________Q. Conditions in which a self excited d.c generator fails to build up voltage [June 2009, KU]A. (i) Residual magnetism may not be there in the poles.(ii) Direction of rotation may be wrong(iii) Field resistance may be more than critical resistance(iv) There may be disconnection in field winding.(v) Brush contact may be poor.

(vi) The field coils may be connected with the armature to oppose the EMF due to residual magnetism_______________________________________________________________________________________________APPLICATIONS OF DC GENERATORS1. SEPARATELY EXCITED GENERATORS:(i) The separately excited generators are usually more expensive than self – excited generators as they require a separate source of supply. Consequently they are generally used where self – excited generators are relatively unsatisfactory. These are used in Warn Leonard speed control systems as self excitation would be unsuitable at lower voltages. (ii) These generators are also used where quick & requisite response to control is important (as separate excitation gives quicker and more precise response to changes in resistance of field circuit).2. SHUNT GENERATORS(i) These generators are used as exciters for supplying current required to excite fields of AC generators. ii) Used to charge batteriesiii) In lathes, fans, pumps disc and band saw drive requiring moderate torques.SERIES GENERATORSi) Series arc lightingii) Series incandescent lightingiii) As series booster for increasing voltage across feeder carrying current furnished by some other sourcesiv) Supplying field current for regenerative braking of DC locomotives.COMPOUND GENERATORS

i) Used to supply power to railway circuits, motors of electrified steam rail – roads, industrial motors, elevator motors.ii) As arc welding generatorsiii) Rolling mills and other loads requiring large momentary toques.LOSSES IN DC MACHINE (Nov/Dec 2007, KU)The losses in a d.c. machine (generator or motor) may be divided into three classes viz (i) copper losses (ii) iron or core losses and (iii) mechanical losses. All these losses appear as heat and thus raise the temperature of the machine. They also lower the efficiency of the machine.

1. Copper lossesThese losses occur due to currents in the various windings of the machine

Note. There is also brush contact loss due to brush contact resistance (i.e., resistance between the surface of brush and surface of commutator). This loss is generally included in armature copper loss.2. Iron or Core lossesThese losses occur in the armature of a d.c. machine and are due to the rotation of armature in the magnetic field of the poles. They are of two types viz., (i) hysteresis loss (ii) eddy current loss.

(i) Hysteresis loss

Hysteresis loss occurs in the armature of the d.c. machine since any given part of the armature is subjected to magnetic field reversals as it passes under successive poles. Fig. shows an armature rotating in two-pole machine. Consider a small piece ab of the armature. When the piece ab is under N-pole, the magnetic lines pass from a to b. Half a revolution later, the same piece of iron is under S-pole and magnetic lines pass from b to a so that magnetism in the iron is reversed. In order to reverse continuously the molecular magnets in the armature core, some amount of power has to be spent which is called hysteresis loss. It is given by Steinmetz formula. This formula is

In order to reduce this loss in a d.c. machine, armature core is made of such materials which have a low value of Steinmetz hysteresis co-efficient e.g., silicon steel.(ii) Eddy current lossIn addition to the voltages induced in the armature conductors, there are also voltages induced in the armature core. These voltages produce circulating currents in the armature core as shown in Fig1. These are called eddy currents and power loss due to their flow is called eddy current loss. The eddy current loss appears as heat which raises the temperature of the machine and lowers its efficiency. Core resistance can be greatly increased by constructing the core of thin, roundIf a continuous solid iron core is used, the

resistance to eddy current path will be small due to large cross-sectional area of the core. Consequently, the magnitude of eddy current and hence eddy current loss will be large. The magnitude of eddy current can be reduced by making core resistance as high as practical. The iron sheets called laminations [See Fig. 2]. The laminations are insulated from each other with a coating of varnish. The insulating coating has a high resistance, so very little current flows from one lamination to the other. Also,because each lamination is very thin, the resistance to current flowing through the width of a lamination is also quite large. Thus laminating a core increases the core resistance which decreases the eddy current and hence the eddy current loss.

Eddy current loss ,Pe=K eBmax2 f 2t 2V Watts

K e=constant dependingon the electrical resistance of core and system of unit usedBmax=Maximumflux density∈Wb /m

2

f = frequency of magnetic reversal in Hzt = Thickness of lamination in mV = volume of core in m3 It may be noted that eddy current loss depends upon the square of lamination thickness. For this reason, lamination thickness should be kept as small as possible.3. Mechanical lossesThese losses are due to friction and windage.(i) friction loss e.g., bearing friction, brush friction etc.(ii) windage loss i.e., air friction of rotating armature.

These losses depend upon the speed of the machine. But for a given speed, they are practically constant.Note. Iron losses and mechanical losses together are called stray losses.Constant and Variable LossesThe losses in a d.c. generator (or d.c. motor) may be sub-divided into(i)constant losses (ii) variable losses.(i) Constant lossesThose losses in a d.c. generator which remain constant at all loads are known asconstant losses. The constant losses in a d.c. generator are:(a) iron losses(b) mechanical losses(c) shunt field losses(ii) Variable lossesThose losses in a d.c. generator which vary with load are called variable losses.The variable losses in a d.c. generator are:a) Copper loss in armature winding (I a2 Ra )b) Copper loss in series field winding (I se2 R se)

Total losses = Constant losses + Variable lossesNote. Field Cu loss is constant for shunt and compound generators.DC MOTORSDC motors are seldom used in ordinary applications because all electric supply companies furnish alternating current However, for special applications such as in steel mills, mines and electric trains, it is advantageous to convert alternating current into direct current in order to use dc motors. The reason is that speed/torque characteristics of dc motors are much more superior to that of ac motors. Therefore, it is not surprising to note that for industrial drives, dc motors are as popular as 3- phase induction motors.DC Motor Principle:

A machine that converts dc power into mechanical power is known as a dc motor. Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. The direction of this force is given by Fleming’s left hand rule and magnitude is given by; F =BIl newtons Basically, there is no constructional difference between a dc motor and a dc generator. The same dc machine can be run as a generator or motor. Working of DC Motor: Consider a part of a multipolar dc motor as shown in Fig. (1). When the terminals of the motor are connected to an external source of dc supply:(i) The field magnets are excited developing alternate N and S poles;(ii) The armature conductors carry ^currents. All conductors under Npole carry currents in one direction while all the conductors under S-pole carry currents in the opposite direction. Suppose the conductors under N-pole carry currents into the plane of the paper and those under S-pole carry currents out of the plane of the paper as shown in Fig.(1). Since each armature conductor is carrying current and is placed in the magnetic field, mechanical force acts on it. Referring to Fig.(1) and applying Fleming’s left hand rule, it is clear that force on each conductor is tending to rotate the armature in anticlockwise direction. All these forces add together to produce a driving torque which sets the armature rotating. When the conductor moves from one side of a brush to the other, the current in that conductor is reversed and at the same time it comes under the influence of next pole which is of opposite polarity. Consequently, the direction of force on the conductor remains the same.

Significance of Back EMF: The presence of back emf makes the dc motor a self-regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load. When the motor is running on no load, small torque is required to overcome the friction and windage losses. Therefore, the armature current Ia is small and the back emf is nearly equal to the applied voltage.If the motor is loaded, the first effect is to cause the armature to slow down and hence the back emf E falls. The decreased back emf allows a larger current to flow through the armature and larger current means increased driving torque. Thus, the driving torque increases as the motor slows down. The motor will stop slowingdown when the armature current is just sufficient to produce the increased torque required by the load.If the load on the motor is decreased, the driving torque is momentarily in excess of the requirement so that armature is accelerated. As the armature speed increases, the back emf E also increases and causes the armature current Ia to decrease. The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque required by the load. It follows; therefore, that back emf in a dc motor regulates the flow of armature current i.e., it automatically changes the armature current to meet the load requirement.Voltage and power Equations of DC Motor:

Let in a dc motor,

V = applied voltageE = back e.m.f.Ra = armature resistanceIa = armature currentV=E+IaRa. By multiplying this equation by Ia, we getV=E I a+ I a

2RaThis is known as power equation of the dc motor.VIa = electric power supplied to armature (armature input)EIa = power developed by armature (armature output)I a2Ra= electric power wasted in armature (armature Cu loss)Thus out of the armature input, a small portion (about 5%) is wasted as a I a2Ra and the remaining portion EIa is converted into mechanical power within the armature

Types of DC Motors: Like generators, there are three types of dc motorscharacterized by the connections of field winding in relation to thearmature:(i) Shunt-wound motor(ii) Series-wound motor(iii) Compound-wound motorArmature Torque of DC Motor: Consider a pulley of radius r meter acted upon by a circumferential force of F Newton which causes it to rotate at N r.p.m.

Then torque T = F .r Newton-meter (N.m)Work done by this force in one revolution =Force.distance=F.2π r JoulePower developed = F .2π r N Joule/second or Watt (N in r.p.s unit)= (F.r).2πN Watt∴2π N = Angular velocity ω in radian/second∴Power developed = T.ω watt or P = T ω WattMoreover, if N is in r.p.m., thenω= 2 π N/60 rad/s∴P=2 πN

60. T

Losses in a DC Motor: The losses occurring in a dc motor are the sameas in a dc generator. These are:(i) copper losses and Iron losses or magnetic losses(ii) Mechanical lossesAs in a generator, these losses cause (a) an increase of machine temperature and (b) Reduction in the efficiency of the dc motor.The following points may be noted:(i) Apart from armature Cu loss, field Cu loss and brush contact loss, Cu losses also occur in interpoles (commutating poles) and compensating windings. Since these windings carry armature current (Ia), Loss in interpole winding = I a2 x Resistance of interpole winding

Loss in compensating winding =I a2 x Resistance of compensating winding(ii) Since dc machines (generators or motors) are generally operated at constant flux density and constant speed, the iron losses are nearly constant.(iii) The mechanical losses (i.e. friction and windage) vary as the cube of the speed of rotation of the dc machine (generator or motor). Since dc machines are generally operated at constant speed, mechanical losses are considered to be constant.Efficiency of a DC Motor: Like a dc generator, the efficiency of a dc motor is the ratio of output power to the input power i.e.

Efficiency ,η=outputinput

×100= outputoutput+losses

×100

As for a generator, the efficiency of a dc motor will be maximum when: Variable losses = Constant lossesTherefore, the efficiency curve of a dc motor is similar in shape to that of a dc generator.A - B = Copper lossesB - C = Iron and friction lossesOverall efficiency, ηc = C/AElectrical efficiency, ηe = B/AMechanical efficiency, ηm = C/BDC Motor Characteristics: There are three main types of dc motors:Shunt motors, series motors and compound motors. The performance of a dc motor can be judged from its characteristic curves known as motor characteristics; following are the three important characteristics of a dc motor:

(i) Torque and Armature current characteristic (Ta/Ia): It is known as electrical characteristic of the motor.(ii) Speed and armature current characteristic (N/Ia): It is very important characteristic as it is often the deciding factor in the selection of the motor for a particular application.(iii) Speed and torque characteristic (N/Ta): It is also known as mechanical characteristic.Characteristics of Series Motors1. Ta/Ia Characteristic:- We know that Ta∝ ΦIa. In this case, as field windings also carry the armature current, Φ ∝ Ia up to the point of magnetic saturation. Hence, before saturation,Ta∝ Φ Ia and Ta∝ I a2At light loads, Ia and hence Φ is small. But as Ia increases, Ta increases as the square of the current. Hence, Ta/Ia curve is a parabola. After saturation, Φ is almost independent of Ia hence Ta ∝ Ia only. So the characteristic becomes a straight line. The shaft torque Tsh is less than armature torque due to stray losses. It is shown dotted in the figure. So we conclude that (prior to magnetic saturation) on heavy loads, a series motor exerts a torque proportional to the square of armature current.Hence, in cases where huge starting torque is required for accelerating heavy masses quickly as in hoists and electric trains etc., series motors are used.

2. N/Ia Characteristics:- Variations of speed can be deduced from the formula:N ∝ E

ΦChange in E, for various load currents is small and hence may be neglected for the time being. With increased Ia, Φ also increases. Hence, speed varies inversely as armature current. When load is heavy, Ia is large. Hence, speed is low (this decreases E and allows more armature current to flow). But when load current and hence Ia falls to a small value, speed becomes dangerously high. Hence, a series motor should never be started without some mechanical (not belt-driven) load on it otherwise it may develop excessive speed and get damaged due to heavy centrifugal forces so produced. It should be noted that series motor is a variable speed motor.3. N/Ta or mechanical characteristic:- It is found from above that when speed is high, torque is low and vice-versa.Characteristics of Shunt Motors1. Ta/Ia Characteristic:- Assuming Φ to be practically constant (though at heavy loads, φ decreases somewhat due to increased armature reaction) we find that Ta ∝ Ia.Hence, the electrical characteristic, is practically a straight line through the origin. Shaft torque is shown dotted. Since a heavy starting load will need a heavy starting current, shunt motor should never be started on (heavy) load.2. N/Ia Characteristic:

If Φ is assumed constant, then N ∝ E. As E is also practically constant, speed is, for most purposes, constant.

Both E and Φ decrease with increasing load. However, E decreases slightly more than φ so that on the whole, there is some decrease in speed. The drop varies from 5 to 15% of full-load speed, being dependent on saturation, armature reaction and brush position. Hence, the actual speed curve is slightly drooping as shown by the dotted line in the figure. But, for all practical purposes, shunt motor is taken as a constant-speed motor. Because there is no appreciable change in the speed of a shunt motor from no-load to full load, it may be connected to loads which are totally and suddenly thrown off without any fear of excessive speed resulting. Due to the constancy of their speed, shunt motors are suitable for driving shafting, machine tools, lathes, wood-working machines and for all other purposes where an approximately constant speed is required.3. N/Ta Characteristic can be deduced from (1) and (2) above.Compound Motors: These motors have both series and shunt windings. If series excitation helps the shunt excitation i.e. series flux is in the same direction; then the motor is said to be cumulatively compounded. If on the other hand, series field opposes the shunt field, then the motor is said to be differentially compounded. The characteristics of such motors lie in between those of shunt and series motors.

(a) Cumulative-compound Motors: Such machines are used where series characteristics are required and where, in addition, the load is likely to be removed totally such as in some types of coal cutting machines or for driving heavy machine tools which have to take sudden cuts quite often. Due to shunt windings, speed will not become excessively high but due to series windings, it will be able to take heavy loads. In conjunction with fly-wheel (functioning as load equalizer), it is employed where there are sudden temporary loads as in rolling mills. The fly-wheel supplies its stored kinetic energy when motor slows down due to sudden heavy load. And when due to the removal of load motor speeds up, it gathers up its kinetic energy. Compound-wound motors have greatest application with loads that require high starting torques or pulsating loads (because such motors smooth out the energy demand required of a pulsating load). They are used to drive electric shovels, metal-stamping machines, reciprocating pumps, hoists and compressors etc. (b) Differential-compound Motors: Since series field opposes the shunt field, the flux is decreased as load is applied to the motor. This results in the motor speed remaining almost constant or even increasing with increase in load (because, N ∝Eb/Φ. Due to this reason, there is a decrease in the rate at which the motor torque increases with load. Such motors are not in common use. But because they can be designed to give an accurately constant speed under all conditions, they find limited application for experimental and research work. One of the biggest drawback of such a motor is that due to weakening of flux with increases in load, there is a tendency towards speed instability and motor running away unless designed properly.

AC MotorPrinciples of OperationThe principle of operation for all AC motors relies on the interaction of a revolving magnetic field created in the stator by AC current, with an opposing magnetic field either induced on the rotor or provided by a separate DC current source. The resulting interaction produces unstable torque, which can be coupled to desired loads throughout the facility in a convenient manner. Prior to the discussion of specific types of AC motors, some common terms and principles must be introduced.

Figure            Induction Motor : - Circuitry diagramSlip It is virtually impossible for the rotor of an AC induction motor to turn at the same speed as that of the rotating magnetic field. If the speed of the rotor were the same as that of the stator, no relative motion between them would exist, and there would be no induced EMF in the rotor. (Recall from earlier modules that relative motion between a conductor and a magnetic field is needed to induce a current.) Without this induced EMF, there would be no interaction of fields to produce motion. The rotor must, therefore, rotate at some speed less than that of the stator if relative motion is to exist between the two.The percentage difference between the speed of the rotor and the speed of the rotating magnetic field is called slip. The smaller the percentage, the closer the rotor speed is to the rotating magnetic field speed. Percent slip can be found by using Equation (1).

Slip=NS−NR

N S

×100%¿

whereNS= synchronous speed (rpm) NR= rotor speed (rpm)The speed of the rotating magnetic field or synchronous speed of a motor can be found by using Equation (2).NS=

120 fP

whereNS→speed of rotating field (rpm )

f →frequency of rotor current (Hz )

P→total number of polesQ.      A two pole, 60 Hz AC induction motor has a full load speed of 3554 rpm. What is the percent slip at full load?Synchronous speed , NS=

120 fP

=120 (60 )2

=3600 rpm

Slip , s=N S−N R

NS

×100%=3600−35543600

×100%=1.3%

The torque of an AC induction motor is dependent upon the strength of the interacting rotor and stator fields and the phase relationship between them. Torque can be calculated by using Equation (3).T=KΦ IRcosθRwhere

T=torque

K=constant

Φ=stator magnetic flux

IR=rotor current ( A )

cosθR=power factor of rotor

During normal operation, K , Φ , cosθR  are, for all intents and purposes, constant, so that torque is directly proportional to the rotor current. Rotor current increases in almost direct proportion to slip. The change in torque with respect to slip (Figure 4) shows that, as slip increases from zero to -10%, the torque increases linearly. As the load and slip are increased beyond full-load torque, the torque will reach a maximum value at about 25% slip. The maximum value of torque is called the breakdown torque of the motor. If load is increased beyond this point, the motor will stall and come to a rapid stop. The typical induction motor breakdown torque varies from 200 to 300% of full load torque.  Starting torque is the value of torque at 100% slip and is normally 150 to 200% of full-load torque. As the rotor accelerates, torque will increase to breakdown torque and then decrease to the value required to carry the load on the motor at a constant speed, usually between 0-10%.

Figure 4            Torque vs SlipSingle Phase Induction MotorThe single phase induction motor in its simple form is structurally the same as a poly – phase induction motor having a squirrel cage rotor, the single phase induction motor has single winding on the stator which produces stationary mmf in space which is alternating in time.

Suppose the rotor is at rest and 1 – phase supply is given to stator winding. The current flowing in the stator winding gives rise to an mmf whose axis is along the winding and it is pulsating in nature. It varies from positive maximum to zero to negative maximum and this pulsating mmf induces current in the short – circuited rotor of motor which results in mmf. The induced current is due to transformer action and direction of current is such that mmf developed opposes the stator mmf. Since the torque developed is proportional to sine of angle between the two mmf, and since the angle is zero, net torque will be zero and hence rotor remains stationary.

Consider, the pulsating field being resolved into two revolving fields of constant magnitude and rotating in opposite directions as in fig. Each field has a magnitude equal to half the maximum length of original pulsating phasor.

These component waves rotate in opposite direction at synchronous speed. The component waves f and b are shown in fig. In case of 3 – phase induction motor, there is only 1 forward magnetic field and hence torque is developed and so motor is self – starting. But, in single phase induction motor, each of these component mmf waves produce induction motor action but the corresponding torques are in opposite direction. When the rotor is at rest, forward and backward field produce equal and opposite torques and hence no net torque is developed on the motor. So, the motor remains stationary. If forward and backward air gap fields remain equal when rotor is revolving, each of component fields would produce a torque – speed characteristic similar to that of polyphase induction motor with negligible leakage impedance.The resultant torque – speed characteristic is algebraic sum of two component curves shows that if motor is started by auxillary means, it will produce torque in whatever direction it is started.

When single phase supply is connected to stator, and rotor is given a push along the forward rotating field, the relative speed between the rotor and forward rotating field goes on decreasing and hence magnitude of induced currents also decreases and hence mmf due to induced current in the rotor decreases and its opposing effect due to forward rotating field decreases. Thus forward rotating field becomes stronger as rotor speeds up. For backward rotating field,

relative speed between rotor and backward field increases, as rotor rotates and so rotor emf increases. Mathematically ,

T∝ I2

I=Im sinωt

T=K Im2 sin2ωt=K Im

2 (1−cosωt )2

Starting methods of single phase induction motorsPermanent-split capacitor motorOne way to solve the single phase problem is to build a 2-phase motor, deriving 2-phase power from single phase. This requires a motor with two windings spaced apart 90o electrical, fed with two phases of current displaced 90o in time. This is called a permanent-split capacitor motor in Figure below

This type of motor suffers increased current magnitude and backward time shift as the motor comes up to speed, with torque pulsations at full speed. The solution is to keep the capacitor (impedance) small to minimize losses. The losses are less than for a shaded pole motor. This motor configuration works well up to 1/4 horsepower (200watt), though, usually applied to smaller motors. The direction of the motor is easily reversed by switching the capacitor in series with the other winding. This type of motor can be adapted for use as a servo motor, described elsewhere is this chapter.Capacitor-start induction motorIn Figure below a larger capacitor may be used to start a single phase induction motor via the auxiliary winding if it is switched out by

a centrifugal switch once the motor is up to speed. Moreover, the auxiliary winding may be many more turns of heavier wire than used in a resistance split-phase motor to mitigate excessive temperature rise. The result is that more starting torque is available for heavy loads like air conditioning compressors. This motor configuration works so well that it is available in multi-horsepower (multi-kilowatt) sizes.

Capacitor-run motor induction motorA variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large capacitor for high starting torque, but leave a smaller value capacitor in place after starting to improve running characteristics while not drawing excessive current. The additional complexity of the capacitor-run motor is justified for larger size motors.

Resistance split-phase motor induction motorIf an auxiliary winding of much fewer turns of smaller wire is placed at 90o electrical to the main winding, it can start a single phase induction motor. (Figure below) With lower inductance and higher resistance, the current will experience less phase shift than the main winding. About 30o of phase difference may be obtained. This coil produces a moderate starting torque, which is disconnected by a

centrifugal switch at 3/4 of synchronous speed. This simple (no capacitor) arrangement serves well for motors up to 1/3 horsepower (250 watts) driving easily started loads.

This motor has more starting torque than a shaded pole motor (next section), but not as much as a two phase motor built from the same parts. The current density in the auxiliary winding is so high during starting that the consequent rapid temperature rise precludes frequent restarting or slow starting loads.Applications

The split phase induction motors are used for fans, blowers, centrifugal pumps and office equipments. Typical ratings vary from 1/20 to HP; in this range, they are the lowest cost motors available.½ The capacitor start motors are used for compressors, pumps, refrigeration and air – conditioning equipments and other hard to start – loads.The capacitor start capacitor run motors are manufactured in a number of sizes from 1/8 to HP and are used in compressors, ¾conveyors, pumps and other high torque loads. The permanent split capacitor motors are manufactured in the range of 1/20 HP to HP ¾

and are used for direct connected fans, blowers, centrifugal pumps and loads requiring low starting torque. The shaded pole motors are used in toys, hair driers, desk fans, etc.Three phase induction motor: Working Principle When three phase supply is given to the three phase stator winding of the induction motor, a rotating magnetic field is developed around the stator which rotates at synchronous speed. This rotating magnetic field passes through the air gap and cuts the rotor conductors which were stationary. Due to the relative speed between the stationary rotor conductors and the rotating magnetic field, an emf is induced in the rotor conductors. As the rotor conductors are short circuited, current starts flowing through it. And as these current carrying rotor conductors are placed in the magnetic field produced by the stator, they experiences a mechanical force i.e. torque which moves the rotor in the same direction as that of the rotating magnetic field.  The induction motor can't run at the synchronous speed because at synchronous speed the induction motor cannot develop any torque to move the rotor from its stationary position.

The electrical section of the three-phase induction motor consists of the fixed stator or frame, a three-phase winding supplied from the three-phase mains and a turning rotor. There is no electrical connection between the stator and the rotor. The currents in the rotor are induced via the air gap from the stator side. Stator and rotor are made of highly magnetizable core sheet providing low eddy current and hysteresis losses.Rotating FieldBefore discussing how a rotating magnetic field will cause a motor rotor to turn, we must first find out how a rotating magnetic field is produced. Figure 1 illustrates a three-phase stator to which a three-phase AC current is supplied.

The windings are connected in wye. The two windings in each phase are wound in the same direction. At any instant in time, the magnetic field generated by one particular phase will depend on the current through that phase. If the current through that phase is zero, the resulting magnetic field is zero. If the current is at a maximum value, the resulting field is at a maximum value. Since the currents in the three windings are 120° out of phase, the magnetic fields produced will also be 120° out of phase. The three magnetic fields will combine to produce one field, which will act upon the rotor. In an AC induction motor, a magnetic field is induced in the rotor opposite in polarity of the magnetic field in the stator. Therefore, as the magnetic field rotates in the stator, the rotor also rotates to maintain its alignment with the stator's magnetic field. The remainder of this chapter's discussion deals with AC induction motors.

Figure 1 :- Three-Phase StatorFrom one instant to the next, the magnetic fields of each phase combine to produce a magnetic field whose position shifts through a certain angle. At the end of one cycle of alternating current, the magnetic field will have shifted through 360°, or one revolution (Figure 2). Since the rotor has an opposing magnetic field induced upon it, it will also rotate through one revolution.For purpose of explanation, rotation of the magnetic field is developed in Figure 2 by "stopping" the field at six selected positions, or instances. These instances are marked off at 60° intervals on the sine waves representing the current flowing in the three phases, A, B, and C. For the following discussion, when the current flow in a phase is positive, the magnetic field will develop a north pole at the poles labeled A, B, and C. When the current flow in a phase is negative, the magnetic field will develop a north pole at the poles labeled A', B', and C'.

Figure 2            Rotating Magnetic FieldAt point T1, the current in phase C is at its maximum positive value. At the same instance, the currents in phases A and B are at half of the maximum negative value. The resulting magnetic field is established vertically downward, with the maximum field strength developed across the C phase, between pole C (north) and pole C' (south). This magnetic field is aided by the weaker fields developed across phases A and B, with poles A' and B' being north poles and poles A and B being south poles.

At Point T2, the current sine waves have rotated through 60 electrical degrees. At this point, the current in phase A has increased to its maximum negative value. The current in phase B has reversed direction and is at half of the maximum positive value. Likewise, the current in phase C has decreased to half of the maximum positive value. The resulting magnetic field is established downward to the left, with the maximum field strength developed across the A phase, between poles A' (north) and A (south). This magnetic field is aided by the weaker fields developed across phases B and C, with poles B and C being north poles and poles B' and C' being south poles. Thus, it can be seen that the magnetic field within the stator of the motor has physically rotated 60°.At Point T3, the current sine waves have again rotated 60 electrical degrees from the previous point for a total rotation of 120 electrical degrees. At this point, the current in phase B has increased to its maximum positive value. The current in phase A has decreased to half of its maximum negative value, while the current in phase C has reversed direction and is at half of its maximum negative value also. The resulting magnetic field is established upward to the left, with the maximum field strength developed across phase B, between poles B (north) and B' (south). This magnetic field is aided by the weaker fields developed across phases A and C, with poles A' and C' being north poles and poles A and C being south poles. Thus, it can be seen that the magnetic field on the stator has rotated another 60° for a total rotation of 120°.At Point T4, the current sine waves have rotated 180 electrical degrees from Point T1 so that the relationship of the phase currents is identical to Point T1 except that the polarity has reversed. Since phase C is again at a maximum value, the resulting magnetic field developed across phase C will be of maximum field strength. However, with current flow reversed in phase C the magnetic field is established vertically upward between poles C' (north) and C (south). As can be seen, the magnetic field has now physically rotated a total of 180° from the start.At Point T5, phase A is at its maximum positive value, which establishes a magnetic field upward to the right. Again, the magnetic field has physically rotated 60° from the previous point for a total rotation of 240°.

At Point T6, phase B is at its maximum negative value, which will establish a magnetic field downward to the right. The magnetic field has again rotated 60° from Point T5 for a total rotation of 300°.Finally, at Point T7, the current is returned to the same polarity and values as that of Point T1. Therefore, the magnetic field established at this instance will be identical to that established at Point T1. From this discussion it can be seen that for one complete revolution of the electrical sine wave (360°), the magnetic field developed in the stator of a motor has also rotated one complete revolution (360°). Thus, you can see that by applying three-phase AC to three windings symmetrically spaced around a stator, a rotating magnetic field is generated.When alternating current is applied to the stator windings of an AC induction motor, a rotating magnetic field is developed. The rotating magnetic field cuts the bars of the rotor and induces a current in them due to generator action. The direction of this current flow can be found using the left-hand rule for generators. This induced current will produce a magnetic field, opposite in polarity of the stator field, around the conductors of the rotor, which will try to line up with the magnetic field of the stator. Since the stator field is rotating continuously, the rotor cannot line up with, or lock onto, the stator field and, therefore, must follow behind it.

Module 2Power devices- power BJT, power MOSFET and IGBT - steady state and switching characteristics. Drive requirements. Design of simple drive circuits for power BJT, power MOSFET and IGBT. Principle of DC motor control. Principle of PWM switching control. Two quadrant, four quadrant converter circuit. Controlled rectifiers. Principle of phase controlled converter operation. Single phase half wave and full wave controlled rectifiers with R, RL and battery loads.

Power BJTAs the name Power BJT suggests, these are high power versions of conventional small signal junction transistors with individual current ratings of several hundred amperes and voltage ratings of several hundred volts.Transistors are current controlled devices i.e. the operation of the switch is specified by the current input at its control terminal. There is a minimum threshold current to ensure the proper ON state specified by the parameter,

h fe=ICIBWhen a transistor is used as a controlled switch, the control current input is provided at the base terminal. The control circuit is connected between the base and emitter. The power terminals of the switch are the collector and the emitter.The figure given below shows an NPN Bipolar junction power transistor.

The output characteristic is a plot of the current IC through the switch versus the voltage V CE across it for a fixed value of the current.

It is necessary to ensure a saturated ON state, by providing adequate base drive current, for the safe and satisfactory operation of the switch.Therefore, the minimum base current to ensure the saturated ON state is given by IB=

IChFE

;hFE istransistor parameter

Power transistors switch on and off much faster than thyristors. They may switch on in less than 1μs and turnoff in less than 2μs.Therefore power transistors can be used in applications where frequency is as high as 100kHz.These devices are very delicate. They fail under certain high voltage and high current conditions. They should be operated within its specified limits, known as safe operating area (S0A).The output characteristics of CE configuration is as shown below

The characteristics depict the relation between collector current(IC) and collector to emitter voltage (VCE) for different values of base current. These characteristics are of npn transistor. These characteristics have three regions i.e., active region, saturation and cut off region.The input characteristics depict the relationship between base current IB and emitter to base voltage for different values of collector to emitter voltage VCE.

The input characteristics is shown in the figure below

The V-I characteristics (output characteristics of power npn transistor is shown in figure below

As in the case of lower power BJT, this characteristics depict the relationship between collector current(IC) and collector to emitter voltage(VCE) for different values of base current IB.It is seen that these characteristics have some special features very different from those for lower power BJT. These features are as follows.1. For substantial values of collector current, there is maximum value of collector emitter voltage which the device can sustain; it is denoted

as BVsus in above figure. If IB= 0 the maximum voltage which can be sustained by the device increases to BVCEO . (The voltage (VCE) when the base is open circuited).The voltage BVCBO is the breakdown voltage when the emitter is open circuited.2. The primary breakdown is due to the avalanche breakdown of C-B junction. In this region the current and the power dissipation can be very high. Therefore this region should be avoided.3. In the region marked second breakdown, the C-E voltage decreases substantially and the collector current is high. This region is due to thermal runaway. A cumulative process occurs in this region and the device gets destroyed. In this breakdown, power dissipation is not uniformly spread over the entire volume of the transistor but is rather restricted to highly localized areas. Therefore the chances of the device getting destroyed are high.4. A quasi saturation (between saturation and active region)region exists. This region is due to the lightly doped drift collector region.

Q. The transistor in the circuit has the following data:V CE (sat )=1.5V ;hFE=50 ;V BE ( sat )=1.8V

a) Determine the minimum value of V ¿ necessary to ensure a satisfactory ON state.b) Determine total ON state power dissipation in switch and its break up into collector dissipation and base dissipation.

a) The ON state current is given by[ (V−V CE (sat ) )

R ]=100−1.55=19.7 A

Theminimum IB is given by 19.7hFE

=19.750

=0.394 A

V ¿=0.394 (10 )+1.8=5.74V

b) The collector power dissipationP1=ICV CE ( sat )=19. .7 (1.5 )=29.55W

The base power dissipationP2=IBV BE (sat )=1.8 (0.394 )=0.71WThetotal internal power dissipation is given by P1+P2=30.26WConstructional Features of a Power BJT • A power BJT has vertically oriented alternating layers of n type and p type semiconductor materials as shown in fig.

The vertical structure is preferred for power transistors because it maximizes the cross sectional area through which the on state current flows. Thus, on state resistance and power lass is minimized. • In order to maintain a large current gain “β” (and hence reduce base drive current) the emitter doping density is made several orders of magnitude higher than the base region. The thickness of the base region is also made as small as possible. • In order to block large voltage during “OFF” state a lightly doped “collector drift region” is introduced between the moderately doped base region and the heavily doped collector region. Practical Power transistors have their emitters and bases interleaved as narrow fingers. This is necessary to prevent “current crowding” and consequent “second break down”.

Switching characteristics of a Power Transistor In a power electronic circuit the power transistor is usually employed as a switch i.e. it operates in either “cut off” (switch OFF) or saturation (switch ON) regions. However, the operating characteristic of a power transistor differs significantly from an ideal controlled switch in the following respects.

• It can conduct only finite amount of current in one direction when “ON” • It can block only a finite voltage in one direction. • It has a voltage drop during “ON” condition • It carries a small leakage current during OFF condition • Switching operation is not instantaneous • It requires non zero control power for switching Turn On characteristics of a Power Transistor

Before t = 0, the transistor was in the “OFF” state. To utilize the increased break down voltage (VCBO) the base-emitter junction of a Power Transistor is usually reverse biased during OFF state. So, only negligible leakage current flows through the transistor. Power loss due to this leakage current is thus limited. The entire load current flows through the diode and VCE is clamped approximately to VCC .

To turn the transistor ON at t = 0, the base biasing voltage VBB changes to a suitable positive value. Charge redistribution starts at the base-emitter junction which is similar to charging of a capacitor. The rising base current that flows during this period is analogic to capacitor charging current. Finally at t = td the BE junction is forward biased. The junction voltage and the base current settles down to their steady state values. During this period, called the “Turn ON

delay time” no appreciable collector current flows. The values of iO and VCE remains essentially at their OFF state levels. At the end of the delay time (td ON) the minority carrier density at the base region quickly approaches its steady state distribution and the collector current starts rising while the diode current (id) starts falling. At t = tdON + tri the collector current becomes equal to the load current (and id becomes zero) IL. At this point D starts blocking reverse voltage and VCE becomes unclamped. tri is called the current rise time of the transistor. At the end of the current rise time the diode D regains reverse blocking capacity. The collector voltage VCE which has so far been clamped to VCC because of the conducting diode “D” starts falling towards its saturation voltage VCE (sat). The initial fall of VCE is rapid. During this period the switching trajectory traverses through the active region of the output characteristics of the transistor. At the end of this rapid fall (tfv1) the transistor enters “quasi saturation region”. The fall of VCE in the quasi saturation region is considerably slower. At the end of this slow fall (tfv2) the transistor enters “hard saturation” region and the collector voltage settles down to the saturation voltage level VCE (sat) corresponding to the load current IL. Turn ON process ends here. The total turn on time is thus, TSW (ON) = td (ON) + tri + tfv1 + tfv2. Turn Off Characteristics of a Power Transistor During Turn OFF a power transistor makes transition from saturation to cut off region of operation. Just as in the case of Turn ON, substantial redistribution of minority charge carriers is involved in the Turn OFF process. The following waveform shows the turn – off

characteristics.

The “Turn OFF” process starts with the base drive voltage going negative to a value -VBB. The base-emitter voltage however does not change from its forward bias value of VBE(sat) immediately, (this is due to the excess, minority carriers stored in the base region). A

negative base current starts removing this excess carrier at a rate determined by the negative base drive voltage and the base drive resistance. After the storage time of the transistor, the remaining stored charge in the base becomes insufficient to support the transistor in the hard saturation region. At this point the transistor enters quasi saturation region and the collector voltage starts rising with a small slope. After a further time interval “trv1” the transistor completes traversing through the quasi saturation region and enters the active region. The stored charge in the base region at this point is insufficient to support the full negative base current. VBE starts falling forward –VBB and the negative base current starts reducing. In the active region, VCE increases rapidly towards VCC and at the end of the time interval “trv2” exceeds it to turn on D. VCE remains clamped at VCC, thereafter by the conducting diode D. At the end of trv2 the stored base charge can no longer support the full load current through the collector and the collector current starts falling. At the end of the current fall time tfi the collector current becomes zero and the load current freewheels through the diode D. Turn OFF process of the transistor ends at this point. The total Turn OFF time is given by Ts (OFF) = ts + trv1 + trv2 + tfi.Power MOSFETA Power MOSFET is a specific type of metal oxide semiconductor field-effect transistor (MOSFET) designed to handle significant power levels. Compared to the other power semiconductor devices (IGBT, Thyristor...), its main advantages are high commutation speed and good efficiency at low voltages. It shares with the IGBT an isolated gate that makes it easy to drive.

Power MOSFETs are voltage controlled majority carrier devices having three terminals, source (S), gate (G) and drain (D) for low power high frequency switching applications. As there is no charge – storage mechanism, MOSFET operates at much higher speed than BJT. To maintain MOSFET in the conducting state, continuous application of gate – to – source voltage is required.

Under steady – state conditions, gate draws very small current of order of nano – amperes. But, during turn – ON and turn – OFF it could be higher due to charging and discharging of gate capacitance.

Structure of Power MOSFETPower MOSFETs are of 2 types : (i) depletion MOSFETs (D – MOSFETs) and (ii) enhancement MOSFETs (E – MOSFETs)The following figure shows an n – channel D – MOSFET which is formed on a p – type silicon substrate, with two heavily doped n+ silicon layers for low – resistance connections.

The gate is insulated from channel by a thin layer of silicon dioxide. The substrate is connected to source. Voltage V GS is applied across gate and source terminals. In an n – channel MOSFET, if V GS is made negative, then electrons in n – channel will get repelled and a depletion region will be created below oxide layer near the gate. This results in narrowing the effective channel and a higher resistance results between the drain and source. In this case, device works like an open switch with very small current flowing between source and drain. If V GS=−V P , pinch−off voltage ,current between source and drain will be almost zero If V GS is made positive, then the channel width

increases resulting in reduction in RDS. Then the current between the source and drain increases. This is the ON – state of the device.Steady – State model of Power MOSFET

Fig shows a MOSFET working as a switch. Under steady – state conditions, current input to gate is small. Therefore, as seen from input side, we can visualize an open circuit. As seen across the drain and source, we can visualize a voltage – controlled current source as in fig.

Power IGBTINSULATED GATE BIPOLAR TRANSISTORS (IGBTs)Like the power MOSFET

It is a voltage controlled switch, Its switching control requirements are practically the same as for a power MOSFET. The switching speeds of IGBTs are higher than those of BJTsLike the power BJT Its ON state voltage drop is typically lower than that of a power MOSFET The IGBT has no integral reverse diode. The IGBT has no significant reverse voltaic blocking capability. The maximum reverse voltage is typically well below 10 V.

WORKING & STRUCTURE OF IGBT

Fig (a) shows the structure of power IGBT and fig (b) shows its symbol.The working and structure are similar to that of Power MOSFET. The main difference is that, in IGBT there is an additional p+¿¿ layer over n−¿drain layer ¿of power MOSFET structure. This p+¿¿ region constitutes the collector of IGBT. Emitter in IGBT is identical to the source in power MOSFET. The switching control voltage for IGBT is applied across gate an emitter just as done in power MOSFET. How IGBT works?Its operation is quite similar to that of power MOSFET. The difference is that, due to the injection of holes from top p+¿ zone intonzone , ¿ resistance offered by n region to current flowing through device in ON state is reduced. This effect is called conductivity modulation of n region.

Fig above, shows the current flow paths in IGBT when positive gate – to – emitter control voltage (above threshold value) is applied. When this voltage is applied, it creates an n – channel as in fig (a). This channel connects the n+¿ ¿emitter zone of IGBT to middle n – region. A pnp transistor is formed out of the top p+¿¿ zone, middle n region and lower p region (labeled in fig(b)). The top p+¿¿ region or the collector functions as emitter of this pnp transistor. The middle n – region constitutes the base of the pnp transistor. The circuit model on this theory is shown in fig (b). STEADY STATE CHARACTERISTICS

Fig a shows the steady state – output characteristics of an IGBT near the saturation region showing BJT like characteristics. Modern IGBTs use trench – gate technology to reduce ON – state drop further inorder to reduce their ON – state power loss. The device doesn’t show any second breakdown characteristics like a BJT. It has a square SOA which is limited thermally like a MOSFET due to its positive temperature co – efficient. So, an IGBT converter can be designed with or without a snubber. Fig b shows steady – state transfer characteristics which is plot of ICv/s V ¿ .SWITCHING CHARACTERISTICS

Switching chara of IGBT for a typically highly inductive load is shown in fig above. This is similar to that of MOSFET. Turn – ON delay time t d is time required for gate – emitter capacitance to charge to threshold level V T to bring the device into conduction. During this time V CE drops from steady – state value V CE to 0.9V CE and the collector current iC rises from zero to 0.1IC where IC is the steady – state value. Rise time t r is the time required for iC to rise from 0.1IC to steady state

value IC. During this time, vCE drops from 0.9V CE to 0.1V CE. Total time required for turning ON is tON=t d+t r .At turn – OFF, t S is the tiime required for v¿ to fall from steady – state value V ¿¿V T because of discharging of gate – emitter capacitance . During this time interval, iC falls from iC¿0.9 iC . Time required for iC to fall from 0.9IC to 0.2IC is t f 1 and to fall from 0.2IC to 0.1IC, it is t f 2. So, total turn OFF time is given by tOFF=t S+t f 1+t f 2.

Principle of PWM switchingPWM Modulation

PWM, or Pulse Width Modulation is a powerful way of controlling analog circuits and systems, using the digital outputs of microprocessors. Defining the term, we can say that PWM is the way we control a digital signal simulating an analog one, by means of altering its state and frequency of this.The PWM signal

This is how a PWM signal would look like:

 

The square wave

The PWM is actually a square wave modulated. This modulation infects on the frequency (clock cycle) and the duty cycle of the signal. Both of those parameters will be explained in details later but keep in mind that a PWM signal is characterized from the duty clock and the duty cycle. The amplitude of the signal remains stable during time (except of course from the rising and falling ramps). The clock cycle is measured in Hz and the duty cycle is measured in hundred percent (%).Clock cycle and Duty cycle parameters

These are the basic parameters that characterize a PWM signal. The first parameter, the clock cycle, is easy to understand. It is the frequency of the signal measured in Hz.The other parameter has to do with the switching time of the signal. Take a look in the following three signals:

All three signals shown above are square wave oscillations modulated as per their oscillation width, so called "duty cycle". They have the same frequency (t1), but they differ on the width of the positive state (t2). The duty cycle is the percentage of the positive state compared to the period of the signal. So:Period (T) = 1Frequency (F)

A 10% duty cycle means that the positive stated remains positive for 10% of the period of the signal.

Example:Suppose that the above signals have a frequency of 1000Hz. This means that their period is 1/1000 =>

T = 0.001 Sec => T = 1mSec The first signal has 10% duty cycle. This means that during one full period, it remains positive for 10% of the total period:

t2 = 10 x T100And this comes to t2 = 0.1mSec. In the first example, the positive state will remain for 0.1 mSec.With the same way we can calculate the t2 (positive state) of the other two signals:Signal 2, 40%:

t2 = 40 x 1mSec / 100 = 0.4 mSec Signal 3, 90%:

t2 = 90 x 1mSec / 100 = 0.9 mSec Usage of the PWMVoltage and power control

One of the most popular usages of PWM is the control of voltage delivered to loads. Those loads could be for example an LED which

would utilize a LED dimmer, or a motor that could be a simple DC motor and would be converted into a controlled speed DC motor, as used for example in the modern PC motherboard fans. But how can PWM control the voltage?The idea is simple. A PWM signal with 100% duty cycle would deliver 100% of the voltage. It would be like a DC power supply. But by altering the duty cycle, the result is to reduce the area of the power delivered to the load as shown below:

 The total power delivered to the connected load each time, is the area under the positive state of the PWM (the drawn area shown on the above drawing). It is clearly seen that by altering the duty cycle, we can alter the power delivered by the supply. And because the wave form is a square wave, the power supplied each time is calculated by:

PDELIVERED = PSUPPLIED x Duty – Cycle  Suppose now that the frequency is high, and at the output of the PWM generator a capacitor is connected like the following schematic:

Following, you can see the resulting output voltage, when the above circuits operates with duty cycle 10% (left waveforms) and with 90% (right waveforms)

The above example is the principal of the operation for the switching power supplies.

Two quadrant Operation of Phase – Controlled Converter An operation with particular value of dc voltage and dc current can be located on the graph using respective axes for these two quantities. The horizontal axis for +ve dc current and vertical axis for +ve dc voltage are used. On basis of reference polarity for voltage and reference direction of current, all operating points in rectification mode will be located in first quadrant – quadrant for positive voltage and +ve current as shown below.

For inversion mode, all operating points will be in the 4th quadrant – the quadrant for +ve current, but negative voltage. The control circuit for a phase – controlled converter can be designed to operate in both modes, and to facilitate changeover from 1 mode to other. Such a converter is known as two – quadrant converter. An example of 2 – quadrant phase – controlled converter is in the area of bulk power HVDC transmission. It is more economical to use dc at high voltage to interconnect 2 ac power systems and transmit power from 1 system to other. Fig shows block diagram of main features of HVDC link between 2 ac power networks, labeled British and French power grid. The 3 terminals labeled R1 ,Y 1∧B1 constitute the 3 – phase bus of ac network of British power grid. Similarly R2 , Y 2∧B2 constitute the 3 – phase bus of ac network of French power grid. The inductance L outside the converter block absorbs the ripple. The block labeled “converter at British power grid” and “converter at French power grid” consist of transformers and SCR bridges. When the power flow is to be from the British power grid network to the French power grid network, the converter on British side will be working in rectification mode, i.e., in quadrant 1 and converter on French side will be working in inversion mode, i.e., in quadrant 4.

The operating modes of the two converter stations will be reversed when power flow from the network in France to network in Britain is reversed.

Four quadrant Operation of Phase – Controlled ConverterA single converter needs the addition of either a change over contact to reverse the armature connections, or a means of reversing the field current in order to change the relationship between i) converter voltage and ii) direction of rotation of motor. It is the connection of two fully controlled converters back – to – back across the load circuit. Such as system is known as dual converter. Both voltage and current of either polarity are obtained with a dual converter. In full converters, the direction of current cannot be reversed because of unidirectional property of the thyristor, but polarity of the output voltage can be reversed. Thus the full converter can be operated in first quadrant if firing angle < 90ᵒ(both Edc1 , Idc 1 positive ). If firing angle > 90 , it can be operated in 4ᵒ th quadrant. Edc 1 is positive and I dc1 is negative. Therefore in first quadrant, power flows from ac source to dc source and in 4th quadrant, power flows from dc source to ac source.

DC motor control principle

The basic principle of a DC motor is the production of a torque as a result of the flux interaction between a “field” produced on the STATOR (either produced by a permanent magnet, or a field winding) and the current circulating in the “armature” windings on the ROTOR. In order to produce a torque of constant sign, the armature winding loops are connected to a set of “brushes” which commutates the current appropriately in each loop according to their geometric position. The commutator is a MECHANICAL RECTIFIER. Note that reversal of either the field current or the armature current results in a torque in the opposite direction. However, reversal of both fields does not change the torque direction, hence it can be used as a “universal motor” with DC or AC feed if both windings are in series.

Basic Equations of a DC MachineUnder steady state conditions (assumes all time varying quantities have a constant average value).

V f=I f R f ( field winding )

E=KVω I f (counter emf∨Back EMF )

V a=Ra I a+E=Ra I a+KVω I f (armature voltage )

T e=K t I f I a=A+Bω+T load (electrical torque )

Pd=T eω (developed power )

Speed Controlω=

V a−Ra I aKV I fThe different speed control methods.1) V a (VoltageControl )2) I f (Field control )3) I a (with I f ¿ )→Demand Torque

For speeds, less than the base/rated speed, the armature current and field currents are maintained at fixed values (hence constant torque operation), and the armature voltage controls the speed. For speeds, higher than the base speed, the armature voltage is maintained at rated value, and field current is varied to control the speed.

Drive requirements1) Limits of Speed Range : The range over which the speed control is necessary for the load, similarly how hard is it to control the speed and the speed regulator also affects the choice of motor.2) The efficiency: The motor efficiency varies as load varies so the efficiency consideration under variable speed operation affects the choice of the motor.3) Braking: The braking requirements from the load point of view. Easy and effective braking are requirements of a good drive.4) Starting requirements: The starting torque necessary for the load, the corresponding starting current drawn by the motor also affects the selection of drive.5) Power factor: It is well known that running of motors with low power factor values is not at all economical. While the pf varies with the load conditions in same motors. Hence type of load and running pf of motor are essential considerations while selecting a drive.

DRIVE CIRCUITS FOR MOSFETLow power digital circuits (TTL or CMOS) can be easily used to directly drive the gate of a power MOSFET. A CMOS – based driver circuit and an open collector TTL – based driver circuit are shown in fig.

There are pulse transformer – based driver circuits where the pulse transformer provides the isolation needed to drive different MOSFETs and different voltage levels. The size of the transformer significantly reduces when the operating frequency is high.

DRIVE CIRCUITS FOR IGBTBasically, an IGBT is a MOSFET – driven power BJT. Thus, the driver circuits used for MOSFET are applicable for IGBT, too. It includes fabrictated – driver circuits as well as custom – built driver circuits.

DRIVE CIRCUITS FOR BJTA properly designed base – drive circuit improves reliability by minimizing switching time and switching losses and allows device to

operate in efficient modes. Ideal base current and voltage waveforms are shown in fig below.

Initially a high current pulse should superimpose over the normal turn – on base current supplied from a current source. It brings the BJT rapidly into conduction. Thus it reduces turn – on switching time and switching losses. Instead of simply stopping the input drive base current, the turn – off switching/ transition time can be reduced by applying a negative base current. Normally, the base drive circuits are placed as near as possible to the device. So, additional base and emitter terminals are provided for base – drive circuits. It reduces the length and inductance of connecting wires, minimizes effect of noise/stray signals and clamps the oscillation in the base – drive signal. Since a high current and high power source is required for driving base of a power BJT, it is difficult when the emitter is not at ground potential. When BJTs are connected in bridge configuration, or load is connected between emitter and ground, then emitter has a floating potential. Its potential changes from zero to supply voltage and from supply voltage to zero, when BJT is switched off and

switched ON respectively. Following figure shows a simple base drive circuit for power BJT.

In fig a, a pnp transistor amplifies input – drive signal and IC ultimately drives main power BJT. In fig b, a transistor totem – pole arrangement is shown which reduces power dissipation in the base – drive circuit. Although this circuit operates even with unipolar dc power supply, negative –

voltage rail speeds up the turn – off transition (turn – off time reduces). In fig c, a p – channel MOSFET – based – driver circuit is shown. BJTs are drived by both unipolar as well as bipolar dc supply voltage. The emitter – to – base breakdown voltage rating specifies the maximum negative – rail voltage. Normally – 8 to – 6 V is used. 2 QUADRANT

4 quadrant

PRINCIPLE OF PHASE CONTROLLED RECTIFIERSThe principle of phase control operation involves a control of on – off switching which connects an ac supply to a load for controlled fraction of each cycle. In phase control, power semi – conductor devices like thyristors are turned on by governing the phase angle of

ac wave signal. The thyristor is turned on by applying a short pulse to its gate and turned off easily by commutation.

SINGLE PHASE HALF WAVE WITH R LOAD

Conduction angle=π-αPerformance of Single-phase, half-wave controlled rectifiers with pure resistive loadFor the positive half cycle of input voltage, the thyristor T1 is forward biased and when the thyristor is fired at ωt = α, it conducts and the input voltage appears across the load. When the input voltage goes negative at ωt = π, the thyristor is reversed biased and it is turned off. The delay angle α, is defined as the time the input voltage starts to go positive to the time the thyristor is fired.

SINGLE PHASE FULL WAVE CONVERTER WITH R LOAD

Fig a shows the circuit for a single phase full wave converter with purely resistive load. Thyristors T 1∧T2 are fired simultaneously at angle α in positive half cycle and T 3∧T 4 are fired at angle π + α during negative cycle. Since load is purely resistive, currents goes to 0 at π, 2π. . Voltage and current waveform are shown in fig b. Current flow is discontinuous. The average value of output voltage is simply twice the output voltage of half wave rectifier with resistance load.Averageload voltage=V 0=

V m

π(1+cos α )

Averageload current=V 0

R

Q. Explain what will happen if a free-wheeling diode is connected across the load in single phase half wave controlled rectifier.The free wheeling diode will remain off till ωt = π since the positive load voltage across the load will reverse bias the diode. However, beyond this point as the load voltage tends to become

negative the free wheeling diode comes into conduction. The load voltage is clamped to zero there after. As a result i) Average load voltage increases ii) RMS load voltage reduces and hence the load voltage form factor reduces. iii) Conduction angle of load current increases as does its average value. The load current ripple factor reduces.

SINGLE PHASE HALF WAVE WITH RL LOADThyristor is turned on by gating signal at ωt = α. The load voltage V 0 at that time becomes equal to source voltage V S as in diagram. But, due to load inductance, current rises from zero gradually at time of triggering. After some time I 0 attains its maximum value and then begins to decrease. Though load voltage is sinusoidal, load current is non – sinusoidal. At ωt = π, I 0 is not zero though V 0 is zero due to the load inductance L. At some angle β, I 0 reduces to 0 and SCR is turned off and so, it is reverse biased by supply voltage. During period between ωt = π and ωt= β, magnetic energy stored in inductor is delivered back to supply. After ωt= β, V 0=0∧I 0=0. At ωt=2π + α, when SCR is triggered again, V 0 is applied to load causing load current flow. γ=β−αγ is the conduction angle and β is called extinction angle.

From graph it is seen that load current is zero at ωt = α, firing angle and ωt = β, the extinction angle. Instantaneous value of load current is given byI 0=

VM

Z [sin (ωt−Φ )−sin (α−Φ ) exp(−RωL (ωt−α ))]; for α<ωt<β

where Z=√R2+ (ωL )2∧Φ=tan−1(ωLR )Average value of output voltage is obtained fromV 0=

12π

∫α

β

V m sinωt dωt

V 0=V m

2π(cos α−cos β )

Average voltage drop across inductor = 0.I 0=

V 0

R=V m

2 πR(cos α−cos β )

With Free – wheeling diode

With highly inductive loads, thyristor tends to conduct continuously bringing in the need for triggering. This means that the trigger circuit has lost control over the thyristor since it is conducting. This situation is overcome if thyristor is forcibly turned off at instants when ωt = π, 2π, 3π, . . . This is achieved by connecting a proper diode of proper rating across load as shown in fig. At ωt = π, source voltage V S is zero and just after that instant, it tends to reverse, forward biasing the free – wheeling diode. Thus, the diode begins to conduct at ωt = π and permits the application of a reverse voltage across the conducting thyristor. Simultaneously, current carried by thyristor is transferred to diode thus, reducing SCR current to zero. This ensures turn – off of thyristor. The diode connected across the load is referred to as free – wheeling diode (FD). The load current decays during free – wheeling period, but it is assumed that the current does not become zero at 2π + α, the next instant of triggering. The voltage across load is the supply voltage when thyristor is conducting and it is almost zero when free – wheeling diode is ON. This means that load voltage does not reverse at any instant of time during the operation. Average value of load voltage=V 0=

V m

2π(1+cosα )

Two modes of operation are specified in this circuit.

One mode, is the conduction mode when the thyristor is ON and power flow is from supply to load. There is another mode, called freewheeling mode, in which energy stored in inductor is delivered to load resistance through the circulating free – wheeling diode current. This in contrast to the stored energy in the inductor returning to the source as observed in single – phase half wave circuit feeding simple RL load. Therefore, it may be concluded that the power delivered to the RL load with a free – wheeling diode is more for a given firing angle. Advantages of free – wheeling diode1) The load current waveform is improved2) The input pf is increased as the energy stored in inductor is delivered to the load instead of going back to supply.3) The gate is permitted to have control over thyristor

SINGLE PHASE FULL WAVE WITH RL LOADFig shows the circuit.

The output waveforms depends on value of inductance L and firing angle α. If inductance value is large, output current will be continuous. SCRs T 1 and T 2 will continue to conduct till SCRs T 3 and T 4 are fired. Since polarity of input voltage is already reversed, firing of reverse biasesT 1 and T 2, and turns them off. The load current shifts from pair T 1∧T2 ¿ pair T3∧T 4. If value of inductance is very large, then output current will be constant and ripple free. Thyristor T 1∧T2conduct from α to π + α and thyristors T 3and T 4 from π + α to 2π + α and so on.The waveforms are as shown below.

The dc output voltage V 0 is given byV 0=

22π

∫α

α+π

V msinωt dωt=2V m

πcos α

The circuit & waveforms on addition with Free wheeling diode is as shown below.

V 0with free−wheelin gdiode=V m

πcosα

SINGLE PHASE HALF WAVE WITH RLE LOAD

Figure shows half wave controlled rectifier with RLE load.

The emf E may be battery source or back emf of dc motor. Presence of a voltage source in the load tends to reverse bias the non – conducting SCR during period ωt = 0 to ωt = αC. Here αC is the critical angle which is the minimum angle of delay for thyristor turn – on.αC=sin

−1 EV mE is battery voltage and V m is maximum value of supply voltage. Load current becomes pulsating during presence of emf E. The load voltage is equal to supply voltage when thyristor is in conducting state and it is equal to E when thyristor is OFF. Following figure shows trigger pulse, load voltage, load current and thyristor voltage waveforms.

SINGLE PHASE FULL WAVE WITH RLE LOADThe following figure shows the circuit.

Voltage E corresponds to the battery emf of the load circuit. Thyristor pair T 1 , T 2 is simultaneously triggered at α while the pair T 3 , T 4 is gated together after π radians in each cycle. Load circuit is assumed to be inductive so as to make the load current I 0 continuous. When T 1 , T 2 pair is ON, output voltage is same as the supply voltage V ab. When T 3 , T 4 pair is ON, output voltage is the supply voltage V ba=−V ab. Following figure shows the waveforms.

Average value of output voltage = V 0.V 0=

2V m

πcos α

Module 3 Basic configurations of switched mode inverter-principle of PWM switching schemes for square wave and sine wave output. Single phase inverters-half Bridge, full bridge and push pull inverter, voltage

source inverter. Block diagram of UPS Induction motor drives Speed control by varying stator frequency and voltage. Principle of vector control. Comparison of vector control and scalar control. Voltage source inverter driven induction motor, application of PWM for induction motor drive.

PULSE WIDTH MODULATIONA popular drive configuration consists of a VSI operating on the PWM concept inorder to economize on power semiconductor switching stages. The operating principle consists of chopping the basic inverter square wave output voltage in fig below inorder to control fundamental frequency voltages.

In simplest form, a saw – tooth wave is used to modulate the chops as in fig. The saw – tooth has a frequency which is a multiple of three times the sine wave frequency, allowing symmetrical three – phase voltages to be generated from a three – phase sine wave set and one saw – tooth waveform. This method controls line – to – line voltage from zero to full voltage by increasing magnitude of saw tooth or sine – wave signal, with little notice to harmonics being generated. The most important areas of voltage in spectrum, apart from fundamental occur at carrier or saw – tooth frequency and its two sidebands, and to a significant extent at each multiple of these frequencies in the spectrum. When carrier frequency is at six times the fundamental frequencies, the triple harmonics cancel in the system; however, phase waveforms of fig shown don’t have half – wave symmetry, hence even harmonics are present.

Single phase half bridge INVERTERS

The inverter provides a.c. load voltage from a d.c. voltage source. The semiconductor switches can be BJTs, thyristors, Mosfets, IGBTs etc. The choice of power switch will depend on rating requirements and ease with which the device can be turned on and off. A single-phase inverter will contain two or four power switches arranged in half-bridge or full-bridge topologies. Half-bridges have the maximum a.c. voltage limited to half the value of the full d.c. source voltage and may need a centre tapped source. Full-bridges have the full d.c. source voltage as the maximum a.c. voltage. Where the d.c. source voltage is low, e.g. 12V or 24V, the voltage drop across the conducting power switches is significant and should be taken into account both in calculation and in selection of the switch. The a.c. load voltage of the inverter is essentially a square wave, but pulse- width-modulation methods can be used to reduce the harmonics and produce a quasi-sine wave. If higher a.c. voltages than the d.c. source voltage are required, then the inverter will require a step-up transformer. The output frequency of the inverter is controlled by the rate at which the switches are turned on and off, in other words by the pulse repetition frequency of the base, or gate, driver circuit. Thyristors would only be used in very high power inverters, since on the source side there is no voltage zero, and a forced commutation circuit would be required to turn the thyristor off. The main objective of static power converters is to produce an ac output waveform from a dc power supply. These are the types of waveforms required in adjustable speed drives (ASDs), uninterruptible power supplies (UPS), static var compensators, active filters, flexible ac transmission systems (FACTS), and voltage compensators, which are only a few applications. For sinusoidal ac outputs, the magnitude, frequency, and phase should be controllable. According to the type of ac output waveform, these topologies can be considered as voltage source inverters (VSIs), where the

independently controlled ac output is a voltage waveform. These structures are the most widely used because they naturally behave as voltage sources as required by many industrial applications, such as adjustable speed drives (ASDs), which are the most popular application of inverters. Similarly, these topologies can be found as current source inverters (CSIs), where the independently controlled ac output is a current waveform. These structures are still widely used in medium-voltage industrial applications, where high-quality voltage waveforms are required.

Single-Phase Voltage SourceInverters

Single-phase voltage source inverters (VSIs) can be found as half-bridge and full-bridge topologies. Although the power range they cover is the low one, they are widely used in power supplies, single-phase UPSs, and currently to form elaborate high-power static power topologies. Half-Bridge VSIFigure shows the power topology of a half-bridge VSI,where two large capacitors are required to provide a neutralpoint N, such that each capacitor maintains a constant voltage V i/2.

Because the current harmonics injected by the operation of the inverter are low-order harmonics, a set of large capacitors (C+ and C-) is required. It is clear that both switches S+ and S- cannot be on simultaneously because a short circuit across the dc link voltage source V i would be produced. There are two defined switch states (states 1 and 2) and one undefined (state 3) switch state as shown in Table below.

In order to avoid the short circuit across the dc bus and the undefined ac output voltage condition, the modulating technique should always ensure that at any instant either the top or the bottom switch of the inverter leg is on.

Figure shows the ideal waveforms associated with the half-bridge inverter shown in Fig. . The states for the switches S+ and S- are defined by the modulating technique, which in this case is a carrier-based PWM.

Full-Bridge VSIFigure shows the power topology of a full-bridge VSI.

This inverter is similar to the half-bridge inverter; however, a second leg provides the neutral point to the load. As expected, both switches S1+ and S1- (or S2+ and S2-) cannot be on simultaneously because a short circuit across the dc link voltage source V i would be produced. There are four defined switch states (states 1, 2, 3, and 4) and one undefined (state 5) switch states as shown in Table .

The undefined condition should be avoided so as to be always capable of defining the ac output voltage. In order to avoid the short circuit across the dc bus and the undefined ac output voltage condition, the modulating technique should ensure that either the top or the bottom switch of each leg is on at any instant. It can be observed that the ac output voltage can take values up to the dc link value V i ,which is twice that obtained with half-bridge VSI topologies.Several modulating techniques have been developed that are applicable to full-bridge VSIs. Among them are the PWM (bipolar and unipolar) techniques.

Ideal waveforms for the unipolar SPWM

PUSH – PULL INVERTER

Fig shows a push – pull inverter circuit. It requires a transformer with a centre – tapped primary. Initially we assume that output current of i0 flows continuously in the circuit. When the switch T 1 is ON and T 2 is OFF, T 1 will conduct for a positive value of i0 and D1 will conduct for a negative value of i0. So, irrespective of direction of i0 , v0=V d /n , where n is the transform turns ratio between the primary half and secondary windings, as in fig above. Similarly when T 2 is ON and T 1 is OFF, v0=−V d /n. A push – pull inverter can be operated in a PWM or a square – wave mode and waveforms are identical to those for half – bridge and full – bridge inverters. The output voltage for figure equals,

V̇ 01=ma .V d

n(ma≤1 )¿(1)

¿ ,V d

n< V̇ 01<

4π

V d

n(ma>1 )¿(2)

In a push – pull inverter, peak switch voltage and current ratings are V T=2V d ; IT=i0. peak∈¿

______________________________________________________________________________________________What is advantage of push – pull circuit?The main advantage is that no more than one switch in series conducts at any instant of time. This is important, if the dc input to converter is from a low – voltage source like battery, where voltage drops across more than 1 switch in series will result in significant energy efficiency reduction. Also, the control drives for the two switches have a common ground. It is, however difficult to avoid dc saturation of transformer in push pull inverter.

______________________________________________________________________________________________The output current, which is secondary current of transformer, is slowly varying current at fundamental output frequency. During switching interval, it can be assumed as a constant. When switching occurs, current shifts from one half to other half of the primary winding. This requires very good magnetic coupling between those two half – windings in order to reduce the energy associated with leakage inductance of the two primary windings. This energy will be dissipated in the switches or in snubber circuits that are used to protect the switches. This is a general phenomenon with all inverters with isolation where current in one of windings is forced to go to zero with every switching. In pulse – width – modulated push – pull inverter, for producing sinusoidal output, transformer must be designed for fundamental output frequency. The number of turns will therefore be high compared to a transformer designed to operate at the switching frequency in a switch – mode dc power supply. This will result in a high transformer leakage inductance, which is proportional to square

of number of turns, provided all other dimensions are kept constant. This makes it difficult to operate a sine – wave – modulated PWM push – pull inverter at switching frequencies higher than approximately 1kHz. UPSBasic Concept

The UPS is designed so that there is one source of power, used under normal conditions, known as primary power source (ac mains) and secondary source (battery) that comes into action if primary source is disrupted. It changes from primary source to secondary source when it detects that the primary source has failed. It automatically switches back from the secondary power source to primary when it is detected that primary source has returned to normal.

The power available from mains is ac. But all batteries provide dc. So, in an UPS, there should be a circuit to convert ac to dc for battery charging called converter. Similarly, there is a device converting dc from battery to ac as required by load. This is called an inverter. These are the main components of UPS.

Types of UPS1) ON Line (inverter preferred) UPS2) OFF Line (Line preferred) UPS3) Line Interactive UPSOn Line/True UPS

In this type of UPS, there are two power sources and a transfer switch that selects between them. This type of UPS uses battery as its primary power source and ac mains power as its secondary power source. Under normal operation, the UPS is running out off the battery while line power runs the battery charger. Rectifier converts ac mains to dc and inverter converts dc to ac and is given to load. Thus there are two conversions in this type of UPS and hence its called double conversion ON line UPS. An inverter is always working in normal conditions and it is hence called inverter preferred. In fig above, the dark line shows the normal operation.

If the power goes out, the inverter and load continues to work on the battery. Only the battery charger fails in such a case. This path is shown in fig above. The time required by UPS to transfer on battery is called transfer time which is important characteristics of UPS. But in ON line UPS, there is no transfer time and UPS instantly switches over to the battery when mains fail. The load keeps running without any kind of interruption. Only battery starts run down as there is no line power to charge it.The main advantage to the on-line UPS is its ability to provide an electrical firewall between the incoming utility power and sensitive electronic equipment. While the standby and line-interactive UPS merely filter the input utility power, the double-conversion UPS provides a layer of insulation from power quality problems. It allows control of output voltage and frequency regardless of input voltage and frequency.

Offline/Standby UPS (SPS)The offline / standby UPS (SPS) offers only the most basic features, providing surge protection and battery backup. The protected equipment is normally connected directly to incoming utility power. When the incoming voltage falls below a predetermined level the SPS turns on its internal DC-AC inverter circuitry, which is

powered from an internal storage battery. The SPS then mechanically switches the connected equipment on to its DC-AC inverter output. The switchover time can be as long as 25 milliseconds depending on the amount of time it takes the standby UPS to detect the lost utility voltage. The UPS will be designed to power certain equipment, such as a personal computer, without any objectionable dip or brownout to that device.

Line Interactive UPS

The line-interactive UPS is similar in operation to a standby UPS, but with the addition of a multi-tap variable-voltage autotransformer. This is a special type of transformer that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of the transformer.This type of UPS is able to tolerate continuous under-voltage brownouts and overvoltage surges without consuming the limited reserve battery power. It instead compensates by automatically selecting different power taps on the autotransformer. Depending on the design, changing the autotransformer tap can cause a very brief output power disruption, which may cause UPSs equipped with a power-loss alarm to "chirp" for a moment.This has become popular even in the cheapest UPSs because it takes advantage of components already included. The main 50/60 Hz transformer used to convert between line voltage and battery voltage needs to provide two slightly different turns ratios: one to convert the battery output voltage (typically a multiple of 12 V) to line voltage, and a second one to convert the line voltage to a slightly higher battery charging voltage (such as a multiple of 14 V). Further, it is easier to do the switching on the line-voltage side of the transformer because of the lower currents on that side.To gain the buck/boost feature, all that is required is two separate switches so that the AC input can be connected to one of the two primary taps, while the load is connected to the other, thus using the main transformer's primary windings as an autotransformer. The battery can still be charged while "bucking" an overvoltage, but while "boosting" an under-voltage, the transformer output is too low to charge the batteries.Autotransformers can be engineered to cover a wide range of varying input voltages, but this requires more taps and increases complexity, and expense of the UPS. It is common for the autotransformer to cover a range only from about 90 V to 140 V for 120 V power, and then switch to battery if the voltage goes much higher or lower than that range.

In low-voltage conditions the UPS will use more current than normal so it may need a higher current circuit than a normal device. For example to power a 1000-watt device at 120 volts, the UPS will draw 8.32 amperes. If a brownout occurs and the voltage drops to 100 volts, the UPS will draw 10 amperes to compensate. This also works in reverse, so that in an overvoltage condition, the UPS will need less current.

Induction motor drives Speed control by varying stator frequencySynchronous speed , NS=

120 fPThe expression for the synchronous speed indicates that by changing the stator frequency also, speed can be changed. This can be achieved by using power electronic circuits - inverters. Depending on the type of control scheme of the inverter, the ac generated may be variable-frequency- fixed -amplitude or variable-frequency - variable-amplitude type. Power electronic control achieves smooth variation of voltage and frequency of the ac output. This when fed to the machine is capable of running at a controlled speed. However, consider the equation for the induced emf in the induction machine.

V=4.44NΦm fwhere N is the number of the turns per phase, Φm is the peak flux in the air gap and f is the frequency. Inorder to reduce the speed, frequency has to be reduced. If the frequency is reduced while the voltage is kept constant, thereby requiring the amplitude of induced emf to remain the same, flux has to increase. This is not advisable since the machine likely to enter deep saturation. If this is to be avoided, then flux level must be maintained constant which implies that voltage must be reduced along with frequency. The ratio is held constant in order to maintain the flux level for maximum torque capability. Actually, it is the voltage across the magnetizing branch of the exact equivalent circuit that must be maintained constant, for it is that which determines the induced emf. Under conditions where the stator voltage drop is negligible compared the applied voltage, eqn. V=4.44 f ΦmNis valid.In this mode of operation, the voltage across the magnetizing inductance in the ’exact’ equivalent circuit reduces in amplitude with reduction in frequency and so does the inductive reactance. This implies that the current through the inductance and the flux in the

machine remains constant. The speed torque characteristics at any frequency may be estimated as before. There is one curve for every excitation frequency considered corresponding to every value of synchronous speed. The curves are shown below. It may be seen that the maximum torque remains constant.

Stator Voltage Control for controlling speedStator Voltage Control of an Induction Motor is used generally for three purposes – (a) to control the speed of the motor (b) to control the starting and braking behaviour of the motor (c) to maintain optimum efficiency in the motor when the motor load varies over a large range. It is simple in hardware and reliable compared to the more complex Variable Frequency Drives as far as speed control application is concerned. However, it turns out to be a somewhat dissipative method of speed control and results in lowered efficiency and rotor over heating. Assume that we have a fixed frequency variable magnitude pure sine wave source available. Torque at any particular slip is proportional to (Voltage)2 in an Induction Motor. Fig. 1 below shows

the Torque-Speed curves of an Induction Motor at various voltages assuming sinusoidal voltage. Also included is the torque-speed curve of a typical fan load. Note that the torque-speed curves of the Induction Motor clearly indicates that it is a motor designed for a large running slip. Otherwise the curves would have been steeper than this around full load rated speed. It can be clearly seen that the speed of the fan can be varied more or less uniformly in the range of 80% to 30% of synchronous speed of the motor by varying the voltage between 100% to 30%.

Had the torque-speed curves of the Induction Motor been steep around synchronous speed (as it is in the case of a well designed Squirrel cage Motor with a running slip in the range of 2% to 5%) the possible speed range in the case of a fan load with voltage control would have been lower than this. The motor will pull out at around 55% voltage or so. Moreover, even the available speed variation will be highly non-uniform with the voltage variation. Hence it is necessary that an Induction Motor intended for speed control applications using stator voltage control has to be designed with a higher running slip and hence lower full load efficiency. (Because higher full-load slip implies higher rotor resistance). Such motors are usually designed with a full load slip value of 12%. Obviously their rotor must be of special design in order to withstand the higher rotor losses developed in the rotor.

The range of speed control available by voltage control is a strong function of nature of load torque variation with load. With a constant torque load, the available speed range is more limited than in the case of a fan load and the motor pulls out at voltage levels closer to 100%. Thus fan loads which have low starting torque demand, pump loads with little or no static head component in the system curve, blower loads with small starting torque demand etc. are the loads suitable for speed control by voltage variation.Principle of vector control. Comparison of vector control and scalar controlVector Control (field oriented control) offers more precise control of ac motors compared to scalar control. So, they are used in high performance drives where oscillations in air gap flux linkages are intolerable. eg., robotic actuators, servos, etc. In scalar control, there is an inbuilt coupling effect because both torque and flux are functions of voltage or current and frequency. This results in sluggish response and is prone to instability due to 5th order harmonics. Vector control decouples these effects. Scalar Control of ac drives produces good steady state performance but poor dynamic response. This makes it clear in the deviation of air gap flux linkages from their set values. This variation occurs in both magnitude and phase.

Principle of Vector ControlThe induction motor is a singly fed machine, receiving electrical input from the stator side only. The input current into the stator is ultimately responsible for creating both the flux in machine and current, which together produce the torque. The objective in vector control of induction motor is to separate out the stator current component responsible for creating flux and the one responsible for

creating the torque, and control the two independently, in a decoupled manner, as in a separately excited DC motor. There are different ways of implementing vector control strategy. A typcial choice is the reference frame fixed to stator flux linkage spac vector and oriented in direction of this space vector as reference direction. Alternatively, it could be a reference frame fixed to rotor flux linkage space vector and oriented in its direction.

Voltage source inverter driven induction motorThe variable frequency ac supply can be given to induction motor for its speed control using voltage source inverter. The VSI has stiff dc voltage at its input terminals. Due to low internal impedance, terminal voltage of VSI fairly remains constant with variations in load. If there is short circuit across its terminals, then it causes current to rise very fast. This is due to low time constant of its internal impedance. This fault must be cleared by fast acting fuse links.There are many VSI circuits which can be used to obtain variable frequency, variable voltage supply. The speed of induction motor has to be controlled from ac or dc source. Different configurations are:-

Voltage source inverter is a type of dc link converter which is a two stage conversion device. Firstly three phase supply is rectified using a rectifier on the supply side. This rectified dc is converted to ac of desired frequency with help of inverter on load side. The smoothening is achieved with help of inductance in dc link.The instantaneous voltage at machine terminals is always directly proportional to dc link voltage while current is function of load admittance.

Application of PWM for induction motor drive.Figure shows a speed controller based on a PWM inverter. In this case, the time delay introduced by link capacitor can be avoided. While the voltage and frequency can now be changed almost instantly, acceleration and deceleration limits area gain used to prevent changing the slip too rapidly, which could result in over current condition.