SPECIAL ELECTRICAL MACHINES

UNIT-I

1. What are the types of rotor available in synchronous reluctance motor? Salient rotor Radially laminated rotor Axially laminated rotor

2. What are the applications of synchronous reluctance motor? Fiber-spinning mills Industrial process equipment Metering pumps Wrapping and folding machines

3. Compare synchronous reluctance motor and induction motorSynchronous reluctance motor Induction motor

1.

2.

3.

4.

Better efficiency

Highcost

Low power factor.

Used for low and medium power application.

Efficiency is low compared withsynchronous reluctance motor.

Lowcost

High power factor.

Used for high power application.

4. Classify the different types of synchronous reluctance motor. Cageless Line-start Radial Axial

5. What is meant by reluctance torque in synchronous reluctance motor?The torque exerted by the reluctance motor because of the tendency of the salient poles to align themselves in the minimum reluctance position. This torque is called reluctance torque.

6. Draw the voltage and torque characteristics of synchronous reluctance motor.

7. Write the torque expression for Synchronous reluctance motor.

8. Express and explain the voltage and torque equation of a synchronous motor.

V= E- j Isd Xsd-j Isq Xqs

9. What are the merits of 3-phase brushless permanent magnet synchronous motor?a. No field winding, therefore no field cu loss.b. The length of the motor is less as there is no mechanical commtator.c. Size of the motor becomes less.d. Better ventilation as armature is accommodated in the stator.e. Motor can be designed for higher voltages subjected to the constraints posed by the

power semiconductor devices.f. It is possible to have very high speed.g. Motor can be operated in hazardous atmospheric condition.h. Efficiency is better.i. It is self starting motor, speed can be controlled.

Regenerative braking is possible.10. What is meant by SYNREL motors?A reluctance motor that utilizes an ac rotating field, which allows for the possibility of extremely smooth torque and good operation to low speeds.

11. Give the difference between synchronous reluctance motor and switched reluctance motor.

Synchronous reluctance motor switched reluctance motor.

1. In order to have self starting capability and bidirectional control,the rotor of a SRM has lesser poles than the stator

The motor has the same number of poles on stator and rotor.

2. The stator of SRM has salient poles with concentrated coils like a d.c motor

The stator of SRM is cylindrical type with distributed winding.

3. The stator of SRM is cylindrical The stator has a smooth bore except fro

type with distributed winding.In SRM ,both stator and rotor have salient poles.

slotting.

4. Excitation is a sequence of current pulses applied to each phase.

Excitation is a set of polyphase balanced sine wave currents.

12. Give the operating principle of radial flux motor.The rotor of radially laminated or flux barrier type synchronous reluctance motor comprises a rotational shaft .the rotor core which is formed as a plurality of steel plates are laminated to one another, the steel plate having a shaft hole for inserting the rotational shaft, a plurality of flux barrier groups spaced from one another in a circumferential direction and having a plurality of flux barriers spaced from one another in a radial direction.

13. Write the various design parameters of a synchronous reluctance motor. Power factor Copper loss and core loss Cost Efficiency

14. Define reluctance torque Reluctance torque or alignment torque is experienced by a ferromagnetic object placed in an external magnetic field, which causes the object to line up with the external magnetic field. An external magnetic field induces an internal magnetic field in the object and because of this torque is produced.

15. What is vernier motor? A vernier motor is an unexcited reluctance type synchronous motor which has the feature of high torque at low speed. This feature is based on the principle vernier or magnetic gearing effect such that a small displacement of the rotor produces large displacement of the axes of maximum and minimum permeance.

UNIT-II

1. Draw the equivalent circuit of winding in stepper motor.

2. What are the applications of stepper motor?disk drives and tape drives.Electric watches.Dot matrix and line printers.X-Y recordersGraph plottersMachine toolsRoboticsSpace vehicles.IC fabrication.

3. Name the various modes of excitation in stepper motor. single phase on or full step operation. Two phase on mode. Half step operation (Annexure 1 – phase on and 2 – phase on mode) Micro stepping operation.

4. Distinguish the half step and full step operation of a stepper motorHALF STEPIt is the alternate one phase on and two phase on mode operation. Here the rotor rotates through half of the full step angle.FULL STEPFull step operation or single phase on mode is the one in which at a time only one phase winding is energized, due to which on stator winding is energized and causes the rotor to rotate to some angle.

5. Define load angle.Load angle (or Torque angle): For a synchronous generator, the magnetic field rotates at synchronous speed and the rotating magnetic field is created in the stator. These two fields are not fully aligned. The stator field lags the rotating field. This lagging expressed in angle is called load angle. 

6. What is the need if suppressor circuits in stepper motor?Suppressors are circuits that are used to ensure fast decay ofcurrent through the winding when the transistor is turned OFF.

7. Define the micro stepping mode of stepper motor.

Micro stepping means, the step angle of the VR stepper motor is very small. It is also called mini - stepping. It can be achieved by two phases simultaneously as in 2-phase on mode but with the two currents deliberately made unequal.

8. Name the various driver circuits used in stepper motor. L/R driver circuits Chopper drive circuits Wave drive (one phase on) Full-step drive (two phases on) Half-stepping Microstepping

9. Write the principle of operation of stepping motors.A stepper motor is a brushless DC motor whose rotor rotates in a discrete angular displacements when its stator winding are energized in a programmed manner.

10. Give the difference between single and multi-stack stepping motors.Single –stack

It has one magnetically isolated section. Micro stepping cannot be achieved. High speed operation and less resolution.

Multi stack It has two or more magnetically isolated section. Micro stepping can be achieved. Low speed operation and high resolution.

11. Mention the applications of micro stepping VR stepper motor.Micro stepping is mainly used where very fine resolution is required. The applications are printing and photo type setting. AVR stepper motor with micro stepping provides very smooth low - speed operation and high resolution.12. Write the features of stepper motor which are responsible for its wide spread use?

o The no. of pulses per time unit determine the motor speed.o If more than one motor is driven from the same source then they will maintain

perfect synchronization.o Stepper motor can rotate in both directions.o It can sustain a holding torque at zero speed.o Stepper motor can be interfaced with digital circuit directly.o It is used for lo power position control applications.o For controlling motor feedback is not required but an encoder or position sensor is

used to control motor accurately. This reduces feedback control complexity.o Stepper motor has a lower output and efficiency as compared to other motors.

13. What is the function of drive circuit in stepping motor?The output from the logic sequence generator signals are low level signals which are too weak to energize stepper motor windings. To increase the voltage, current and power levels of the logic sequence output by using power semiconductor switching circuit. This circuit is called power drive circuit.

14. Define step angle in stepping motor.Step angle is defined as the angle through which the stepper motor shaft rotates for each command pulse. It is denoted as β.

15. Calculate the stepping angle for a 3 phase,24 pole permanent magnet stepper motor.M=3, Nr=24β=360/(3*24)β =5֗

16. Define torque constant of a stepper motor.The torque constant of stepper motor is defined as the initial slope of the torque current curve of the stepper motor.

17. Draw the block diagram of the drive systems of a stepping motor

18. What is meant by hybrid stepper motors?A hybrid stepper motor combines the features of both PM and VR stepping motors.19. Give the classification of stepper motor.

Variable reluctance stepper motor Permanent magnet stepper motor Hybrid stepper motor

20. Define the term slewing.It is the range of stepping rates at which the stepper motor can run in the slov mode, with losing synchronism, at a given load torque.

The slewing range spans stepping rates

UNIT-III

1. Define energy ratio

This energy ratio cannot be called as efficiency. As the stored energy R is not wasted as a loss but it is feedback to the source through feed back diodes..

2. Draw the torque-speed characteristics of SRM

3. Illustrate the different modes of operation of switched reluctance motora. Low speed operation mode.b. High speed operation mode.

4. Give the advantages of sensor less operation of switched reluctance motor.Low cost, Reliable and it avoids additional cost size.

5. What is the need of a rotor positioning sensor in Switched reluctance motor?It is necessary to use a rotor position sensor for commutation and speed feedback. The turning ON and OFF operation of the various devices of power semiconductor switching circuit are influenced by signals obtained from rotor position sensor

6. Write any four applications of SRM? 1. Domestic applications like vacuum cleaner2. General purpose industrial drives3.fans4. automobile applications.

7. State the reluctance principle.A reluctance motor is an electric machine in which the torque is produced by the tendency of its movable part to move to a position where the inductance of the excited winding is maximised.

8. List the characteristics of switched reluctance motor.The switched reluctance motor is a doubly-salient, singly-excited motor. This means that it has salient poles on both the rotor and the stator but only one member (usually the stator) carries windings. The rotor has no windings, magnets, or cage windings, but is built up from a stack of salient-pole laminations. Low inertia and simple manufacturing, Losses appear only on the stator and easy to cool, No magnets and so permissible rotor temperature is higher than in PM motors, Torque is independent of the polarity of phase current. Reduction in no of semiconductor devices in controller Open circuit voltage and

Short circuit current are zero or very small under faulty condition, Immune from shoot through failure, High starting torque, extremely high speeds possible

9. Write the principle of operation of Switched reluctance motor.The switched reluctance motor is a double salient, singly-excited motor. This means that it has salient pole on both the rotor and the stator, but only one member crries windings. The rotor has no windings, magnets (or) cage winding. It works on vai1é reluctance principle.

10. Write the principle of operation of a variable reluctance stepper motor.It is usually fed by a square wave of phase current without rotor position feed back.It designed to maintain step integrity rather than to achieve efficient power conversion.

11. List out the advantages and disadvantages of the converter circuit with two power semiconductor devices and two diodes per phase.

Merits:a. Control of each phase is completely independent of the other phase.b. The converter is able to freewheel during the copping period at low speeds, which helps

to reduce the switching frequency and thus switching losses of the converter.c. The energy from the off going phase is feedback to the source which results in useful

utilization of energy.Demerits:Higher number of switches required in each phase which makes the converter expensive and also used for low voltage applications.12. What is the significance of closed loop control in switched reluctance motor?

Switched reluctance motor is always operated with closed loop control. Normally we have to use a rotor position sensor for commutation and speed feedback. Here the phase windings are energized by using power semiconductor circuit. He turning on and off operation of the various semiconductor devices are influenced by signals obtained from rotor position sensor. It is the main significance of closed loop control in SR motor.

13. List out the advantages of switched reluctance motor Construction is simple and robust. Rotor carries no windings, no slip rings, no brushes, less maintenance. There is no permanent magnet. Ventilating system is simpler as losses takes place mostly in the stator. Power semi conductor switching circuitry is simpler No shoot through fault likely to happen power short circuits, Developed torque doesn’t depends upon the polarity of current in the phase Winding. The operation of the machine can be easily change from motoring mode to generating

mode by varying the region of conduction. It is possible to get very high speed. Depending upon the requirement T- characteristics can be achieved. It is the self starting machine. Energy stored in the phase winding is fed back to the supply through the feedback diodes

during off period.

14. Write the torque equation of switched reluctance motor?

15. Write the voltage equations of a Switched Reluctance motor?V=iR+L(∂i/∂t)+eiR- the ohmic dropL(∂i/∂t)-emf due to incremental inductancee-self induced emf

16. List the methods of rotor position sensing in Switched Reluctance motor.Optical encoder, resolver, Speed sensors and Hall Effect sensor.

17. Why SR machines popular in adjustable speed drives?Rotor is simple and it tends to have a low inertia, The stator is simple to wind, In most applications the bulk of the losses appear on the stator, which is relatively easy to cool, Due to the absence of magnet the maximum permissible rotor temperature may be higher than in PM motors, Under fault conditions the open circuit voltage and short circuit current are zero or varying small, Extreme by high speeds are possible.

UNIT-IV

1. What is electronic commutator? It is equipment used in PM brushless DC motor to transfer the current to the armature.In this, power semiconductor devices are used as switched devices.usually six switching devices are employed in a normal electronic commutator.hence ,the armature should have three tappings which can be connected either in star or in delta.

2. What are the merits of the BLDC motor drives? Advantages

1. There is no field winding so that field copper loss is neglected.2. Length of the motor is very small as there is no mechanical commutator, so that size becomes very small.3. Better ventilation because of armature accommodated in the stator.4, Regenerative braking is possible.5. Speed can be easily controllable.6. Motor can be designed for higher voltages subjected to the constraint caused bythe power semi conductor switching circuit.7. It is possible to have very high speeds.

3. Write the difference between electronic and mechanical commutator.

4. Classify the types of BLDC motor. BLPM square wave motor BLPM sine wave motor5. How the demagnetization occurs in PMBLDC motor?

During the normal operation of motor, when the torque and back emf are constant, if the field flux level becomes low, then demagnetization occurs.

6. What is meant by Permeance Co-efficient?The line drawn from the origin through the operating point is called load line and absolute value of its slope normalized to µ0 is called permeance coefficient. Permeance coefficient= µrec((1+PrlRg)/(PmoRg))where µrec=relative recoil permeability,Rg=air gap reluctance,Pmo=internal leakage permeance,Prl=normalized rotor leakage permeance.

7. List some permanent magnet materials. Alnico Rare earth magnet NdFeB magnet Sm2 Co17 magnet Ceramic magnet.

8. Write the principle of operation of PMBLDC motor.When a D.C supply is switched on to the motor the armature winding draws a current. The current distributes with the stator armature winding depends upon rotor position and the devices turned on. As per faradays law of electromagnetic induction a emf is dynamically induced in the armature conductors. This back emf as per lenz law opposes the cause. As a result developed torque reduces. Finally the rotor attain the steady speed.

9. What is meant by demagnetisation in PMBLDC motor.In the absence of externally applies ampere turn, the magnets operating point is at the intersection of demagnetization curve and the load line.

10. . Draw the magnetic equivalent circuit of PMBLDC motor

11. Compare permanent magnet brushless D.C motor with permanent magnet synchronous motor.Brushless DC motor 1. No Brushes. Maintenance problems (RFI, sparking, ignition and fire accidents) eliminated. 2. More cross sectional available for armature windings. Conduction of heat through the frame is improved. 3. Increase in electric loading is possible, providing a greater specific torque. Higher efficiency. 4. Space saving, higher speed possible, with reduced inertia. 5. Maximum speed limited by retention of magnet against centrifugal force.6. Shaft position sensor is a must

7. Complex electronics for controller. P.M. Commutator motor1. Commutator based DC machines need carbon brushes, so sparking and wear and tear is un avoidable. 2. Armature winding is inside and the magnet is on the stator outside. 3. Efficiency less. Commutator restricts speed. 5. Magnet is on the stator. No problem. 6. Not mandatory. 7. Simple

12. What is commutation?Because of hetropolar magnetic field in the air gap of DC machine the emf induced in the armature conductors is alternating in nature. This emf available across brushes as unidirectional emf because of commutator and brushes arrangement. It provides less spark, easy to control, less maintenance, more efficient, small in size (compact).

13. Compare and contrast conventional DC motors and PMBLDC motors.Features Conventional DC motor PMBL DC motorMechanical structure

Maintenance

Winding connection

Commutation method

Detecting method

Reversing method

Field magnets on the stator

Maintenance is high

Ring connectionThe simplest: Delta connection

Mechanical contact between brushes and commutator

Automatically detected by brushes

By a reverse of terminal voltage.

Field magnets on the rotor

Low maintenance

The highest grade: Dell or star- connected three phase connection.Normal: star-connected three phase winding with grounded neutral point or four-phase connection. The simplest: Two-phase connection.

Electronic switching usingpower semi conductor devicesie. transistors, MOSFETS.

Rotor position can be detected by using sensor i.e., Hall sensor; optical encoder.Rearranging logic sequencer

14. .How are the directions of rotations reversed in case of PLBLDC motor?The direction of rotation can be reversed by reversing the logic sequences in PMBLDC motor15. What are the advantages of PMBLDC motor?

Advantages1. Better ventilation because of armature accommodated in the stator.2. Regenerative braking is possible.3. Speed can be easily controllable.4. Motor can be designed for higher voltages subjected to the constraint caused by5. the power semi conductor switching circuit.

6. It is possible to have very high speeds.16. Give the expression for the emf and torque of a PMBLDC motor.

eph = 2 Bg rlm Tph volts.Where Bg = flux density in the air gap r - radius of the airgap l - length of the armature m - angular velocity in mech. rad/sec Tph - number of turns per phase.Tph = 4 Bg rl Tph Iwhere Bg = flux density in the air gap r - radius of the airgap l - length of the armature I - current flow through the motor Tph - number of turns per phase.

17. What are the various kinds of permanent magnets? There are basically three different types of permanent magnets which are used in small DC motors Alnico magnets, Ferrite or ceramic magnet, and Rare - earth magnet (samarium – cobalt magnet ). 18. What is meant by multiphase brushless motors? A multi-phase brushless motor including a stator having a plurality of drive coils each corresponding to a specific phase and a rotor having a plurality of field magnet poles of successively alternating polarity. The stator further has a plurality of Hall generators for detecting the positions of the rotor and a speed sensor for detecting the rotational speed of the rotor. 19.Give the uses of sensors in motors. It is used to identify the position of the rotor and it is also used to excite the coils in proper manner. 20.List some applications of BLPM DC motor.

Fans, Pump drive, Traction and Hydraulic power steering, precision high speed spindle drivers for hard disk drivers etc.,.

21. Why brushless permanent magnet motor is called as electronically commutated motor?

The switching instants of the individual transistor switches, Q1 – Q6 with respect to the trapezoidal emf wave is synchronized with the rotor. So switching the stator phases synchronously with the emf wave make the stator and rotor mmfs rotate in synchronism. Thus, the inverter acts like an electronic commutator that receives switching logical pulses from the rotor position sensor. This is why a BLDC drive is also commonly known as an electronically commutated motor (ECM). 22. What is meant by slotless motor?

In slotless motor, the stator teeth are removed and resulting space is partially filled with addition of copper.

23.What are the types of sensors used with PMBLDC motors? Hall effect sensors are most commonly used for speed, position sensing with PMBLDC motors. Optical Disc based sensors are also used. Presently rotor position sensors are avoided by using alternative methods called as Sensorless control methods, which uses terminal emf measurement, third harmonic voltage measurement, flux estimation and neuro – fuzzy techniques etc.

UNIT-V

1. What are the types of materials used in permanent magnet motor? AlnicoRare earth magnetNdFeB magnetSm2 Co17 magnetCeramic magnet.

2. What is self-control in PMSM?Self control ensures that for all operating points the armature and rotor fields move exactly at the same speed.As the rotor speed changes the armature supply frequency is also changes proportionally so that the armature field always moves (rotates) at the same speed as the motor. The armature and rotor field move in synchronism for all operating points. Here accurate tracking of speed by frequency is realised with the help of rotor position sensor.3. State the power controllers for PM synchronous machines

PWM inverter using power MOSFETS with microprocessor control. PWM inverter using BJT‟s with microprocessor control (up to 100 KW).

4. Classify the different types of PMSM.a) General classification: There are two types of PMSM : 1) Surface mounted rotor:

further classified as a) Projected type b) Insert type 2)interior( or buried) rotorb) Classification based on rotor configuration: 1)Peripheral 2) Interior 3)Claw-pole 4)Transverse

5. Differentiate square wave and sine wave motor.

Features BLPM Square wave motor BLPM Sine wave motorFlux densitydistribution

Flux per pole (Փ) Փ = BgTlWb

T = 2𝜋𝑟/ 2𝑝

Փ = BavTl = ( 2 𝐵/̂ 𝜋 ) πl Wb

RMS value of the line

current to the motor

Irms= √ 2 /3 Id Irms= I = 𝐼𝑚 /√2

6. Define the term load angle.The phase angle introduced between the induced emf phasor, E and terminal voltage phasor , V during the load condition of an Alternator is called load angle. In PMSM the load angle is the angle between stator field and rotor field when the machine is rotated at synchronous speed. It is represented as δ.

7. Write the advantages and disadvantages of PM synchronous motor?Advantages: Runs at constant speed, No field winding, no field copper loss, better efficiency, High power density, Lower rotor inertia, Robust construction of rotor, No sliding contact hence requires less maintenance .Disadvantages: Loss of flexibilities of field flux control, Demagnetization effect and High cost.

8. Define synchronous reactance in permanent magnet synchronous motor.The synchronous reactance is the fictitious reactance employed to account for the voltage effects in the armature circuit produced by the actual armature leakage reactance and the change in the airgap flux caused by the armature reaction. For an ideal two pole sine distributed three phase winding with Ns turns in series per phase neglecting the leakage inductance of the slots and end-turns, the expression for synchronous reactance is given by µ0 Xs= 3𝜋𝜇0𝑁𝑠 2 𝑙𝑟1𝜔 /8𝑝2𝑔′′

9. Give the applications of permanent magnet synchronous motor. a. Used as a direct drive traction motor.

b. Used as high speed and high power drives for compressors, blowers, conveyors, fans, pumps, steel rolling mills, main line traction, ship propulsion and air craft facilities.

10. What are the assumptions made in the derivation of emf equation for PMSM? a. Flux density distribution in the air gap is sinusoidal. b. Rotor rotates with uniform angular velocity. c. Armature winding consists of full pitched, concentrated similarly located coils of equal number of turns.

11. Briefly explain the vector control of PM synchronous motor.BLPM SNW motor is usually employs for variable speed applications. For this we keep V/f constant and vary V and f to get the desired speed and torque. From the theory of BLPM SNW motor it is known that as the speed is varied from a very low value up to the corner frequency, the desired operating point of current is such that Id =0 and I is along q-axis. 2)When the frequency is more than the corner frequency it is not possible to make Id=0, due to voltage constraints. In such a case a better operating point for current is obtained with minimum I value after satisfying the voltage constraints.Controlling BLPM SNW motor taking into considerations the above mentioned aspects is known as vector control of BLPM SNW motor.

12. Mention the various assumptions in deriving the EMF equation of permanent magnet synchronous motor.

Flux density distribution in the air gap is sinusoidal, Rotor rotates with an uniform angular velocity of ωm ( rad/sec), Armature winding consists of full pitched, concentrated similarly located coils of equal number of turns.13. Write torque and EMF equation of PM synchronous motor

T=3EIsin β/ωm N-m ωm is the angular velocity,T is the torque produced, β is the torque angle or power angle, E is the induced emf .Eph = 4.44 f Фm Kw Tph volts, This is the rms value of induced emf per phase, where f = Frequency in Hertz, Фm = flux per pole, Kw = Winding factor, Tph = Turns per phase

14. Write the significance of power controllers of permanent magnet synchronous motorsThe 180º magnet arcs were assumed to produce a rectangular distribution of flux density in the airgap. The phase windings are assumed to be star connected. The 120º pole arc motor is less efficient than 180º pole arc motor, the phase current waveforms of delta connected converters is shown above. In this machine , the effect of fringing flux, slotting and commutation overlap combine to produce torque ripple.

15. Define synchronous reactance in PMSM Synchronous reactance is the sum of armature leakage reactance and ficititious reactance

Xs = Xl + Xa

16. Draw the output phasor diagram of PMSM.

17. Draw the torque / speed characteristics of PM synchronous motor.

18. Brief up the advantages of load commutation in permanent magnet synchronous motors.Commutation of thyristors by induced voltages of load is known as “load commutation”. Here, frequency of operation is higher and it does not require commutation circuits.

19. Write down the expressions for the self and synchronous reactance of permanent magnetsynchronous motor.

The sum of the magnetizing and leakage reactance define synchronous reactance. XS X M Xl , Xs = Synchronous reactance, XM = Magnetizing reactance, Xl = Leakage reactance

20. What is the magnitude of stator current in PMSM to achieve demagnetization? The magnitude of stator current in PMSM to achieve partial demagnetization of the magnets is given byI=Eq/Xs amps

Which is many times greater than the normal continuous rating of the motor windings or the converter. 22. What is meant by vector control.

PMSM are employed for variable speed applications. The process of controlling voltage and frequency to get the desired speed and torque is known as vector control of PMSM.