Ikbal Khan 4 Sare File of 8 Semdoc

7/29/2019 Ikbal Khan 4 Sare File of 8 Semdoc

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DC DRIVES

A

Seminar Report

Submitted

in

partial fulfillment

for the award of Degree of

Bachelor of Technology

in Department of Electrical Engineering

Submitted to: Guide by: Submitted by:

Mr. Vinesh Agarwal Miss. Shruti jain Ikbalkhan

Head of Department Sr. lecturer 09EIMEE023

(Electrical Engineering) (Electrical Engineering)

Department of Electrical Engineering

INSTITUTE OF TECHNOLOGY AND MANAGEMENT, BHILWARA


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INSTITUTE OF TECHNOLOGY & MANAGEMENT, BHILWARA

ELECTRICAL ENGINEERING DEPARTMENT

CERTIFICATE

This is to certify that the Seminor Report titled DC DRIVES was prepared by IKBAL KHAN of Institute of

Technology & Management, Bhilwara in partial fulfillment of the requirement as a subject under the

Rajasthan Technical University during the IVth Year VIIIth Semester.

Miss. Shruti jain Miss. Shruti jain Mr. Vinesh Agarwal

(Signature of Guide) (Seminar Coordinator EE Deptt) (Head EE Deptt.)


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ACKNOWLEDGEMENT

I wish to express my deep sense of gratitude towards my guide Miss Shruti jain for his guidance

and encouraging support which were invaluable for the completion of this work. My sincere

thanks are to Mr. Vinesh agarwal & N. K. Mathur (Principal, ITM Bhilwara) for his valuable

inputs in completion of the thesis work.

I take immense pleasure in thanking all the faculty members and my friends for their valuable

assistance in the seminar work. Finally, yet importantly, I would like to express my heartfelt

thanks to my beloved parents, for their help, and support in all the circumstances and kept my

moral always high.

.

Ikbal khan


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CONTENTS

TITLE PAGE NO

Certificate

Acknowledgement

Contents i

List of Figures iii

Abstract 1

Chapter 1 Introduction 2

1.1 DC Motor Drive 2

1.2 Advantages of DC Motor 2

1.3 Disadvantages 2

1.4 Type of DC Motor 3

1.4.1 Separately Excited DC Motor. 3

1.4.2 Series Wound or Series Motor. 3

1.4.3 Shunt Wound or Shunt Motor. 3

1.4.4 Compound Motor. 4

Chapter 2 Starting & breaking 5

2.1 Starting 5

2.2 Breaking 7

2.2.1 Type of Electrical Braking 7


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2.2.1.1 Plugging 8

2.2.1.2 Dynamic or Rheostatic Breaking 10

2.2.1.3 Regenerative Breaking 12

Chapter 3 Speed control 15

3.1. Speed Control Of dc Drive 15

3.1.1 Armature Voltage Control 15

3.1.1.1Multiple Voltage Control 15

3.1.1.2 Ward-Leonard System 15

3.1.2 Field Flux Control 16

3.1.3 Armature Resistance Control 18

Chapter 4 Controlled rectifier dc drive 21

4.1 Controlled Rectifier fed DC Drives 21

4.1.1 Single-Phase Fully-Controlled Rectifier 21

4.1.2 Single-Phase Half-Controlled Rectifier 24

4.1.3 Three-Phase Fully Controlled Rectifier 26

Chapter 5 Chopper controlled dc drive & dual converter 30

5.1 Chopper Controlled DC Drive 30

5.2 Dual Converter 31

Chapter 6 Conclusion v

Chapter 7 Referance vi


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LIST OF FIGURE

FIGURE NO. DESCRIPTION OF FIGURE PAGE NO.

Chapter 1

1.1 Series wound or series motor 3

1.2 Shunt wound or shunt motor 4

1.3 Compound motor 4

Chapter 2

2.1 Starting of a dc shunt motor 6

2.2 Plugging operation in separately excited dc motor 8

2.3 Plugging operation in dc series motor 8

2.4 Plugging speed-torque curve 10

2.5 Dynamic breaking operation in separately excited dc motor 11

2.6 Dynamic breaking in a dc series excited motor 11

2.7 Dynamic breaking speed-torque curve 12

2.8 Regenerative breaking speed-torque char 13

Chapter 3

3.1 Armature voltage control 16

3.2 Field Flux Control 17

3.3 Field flux (rated) 18

3.4 Armature Resistance Control 18


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3.5 Speed-torque curve of motor 19

3.6 Torque & power limitation in combined armature voltage and field control

20

Chapter 4

4.1 Single-phase fully-controlled rectifier 22

4.2 Discontinuous conduction waveform 23

4.3 Continuous conduction mode 24

4.4 Single-Phase Half-Controlled Rectifier 25

4.5 Discontinuous mode waveform 25

4.6 Continuous conduction waveform 26

4.7 Three-Phase Fully Controlled Rectifier 27

4.8 Three-Phase Fully Controlled Rectifier waveform 28

4.9 Three-Phase Half-Controlled Rectifier 29

Chapter 5

5.1 Chopper Control 30

5.2 Waveform 31

5.3 Voltage Current Diagram 32

5.4 Single-Phase Dual Converter 32


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ABSTRACT

DC SRC drives and motors remain common in industries such as metals, cranes, mining and

printing. The current trend is to replace DC systems with new AC drives and motors to reduce

maintenance overhead. This can, however, often be a significant task that requires the machinery

to be taken out of service for an extended period while mechanical and electrical rework

is completed. Utilizing existing DC motors and upgrading the DC drives is often the

most attractive option. DC motors are usually well built and capable of offering many more years

of service to help lower project costs and minimize disruption and risks.

Control Techniques DC drives are now based on our industry-leading AC drive technology to

deliver enhanced motor performance, reliability and system integration options Where speeds

may be selected from several different pre-set ranges, usually the drive is said to be adjustable

speed. If the output speed can be changed without steps over a range, the drive is usually referred

to as variable speed. Adjustable and variable speed drives may be purely mechanical

(termed variators), electromechanical, hydraulic, or electronic. DC motors have been used in

industrial applications for years. Coupled with a DC drive, DC motors provide very precise

control. DC motors can be used with conveyors, elevators, extruders, marine applications,

material handling, paper, plastics, rubber, steel, and textile applications to name a few.


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Chapter 1

INTRODUCTION

1.1 DC Motor Drive

DC drives are DC motor speed control systems. Since the speed of a DC motor is directly

proportional to armature voltage and inversely proportional to motor flux (which is a function of

field current), either armature voltage or field current can be used to control speed. Several types

of DC motors are described in the electric motor article. The electric motor article also describes

electronic speed controls used with various types of DC motors.

A DC motor is an electric motor that runs on direct current (DC) electricity. DC motors were used

to run machinery, often eliminating the need for a local steam engine or internal combustion

engine. DC motors can operate directly from rechargeable batteries, providing the motive power

for the first electric vehicles. Today DC motors are still found in applications as small as toys and

disk. Electric drives for motor is used to draw electrical energy from the mains and supply the

electrical energy to the motor at whatever voltage, current and frequency necessary to achieve the

desired mechanical output. The drive is relatively simple and cheap (compared to induction motor

drives. But DC motor itself is more expensive Due to the numerous disadvantages of DC motor, it

is getting less popular. DC drives are windily used in applications requiring adjustable speed,

good speed regulation and frequent starting, breaking and reversing. Some important applications

are rolling mill, paper mills, mine winders, hoists etc.

1.2Advantages of DC Motor1. Ease of control.

2. Deliver high starting torque.

3. Near-linear performance.

4. Wide speed range

5. Very precise speed control

1.3Disadvantages


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1. High maintenance.

2. Large and expensive (compared to induction motor.)

4. Low efficiency.

5. High initial cost

1.4 Type of DC Motor

1. Separately excited dc motor.

2. Series wound or series motor.

3. Shunt wound or shunt motor.

4. Compound motor.

1.4.1. Separately Excited DC Motor

In a separately excited motor field winding is energized from a separate voltage source in order to

produce flux in the machine. So long the machine operates in unsaturated condition the flux

produced will be proportional to the field current. In order to implement shunt connection, the

field winding is connected in parallel with the armature.

1.4.2. Series Wound or Series Motor

The series-wound motor has the field connected in series with the armature. Although the series

wound motor offers high starting torque, it has poor speed regulation. Series-wound motors are

generally used on low speed, very heavy loads.

Fig 1.1 Series Wound or Series Motor


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1.4.3. Shunt Wound or Shunt Motor

Shunt-wound motors have the field controlled separately from the armature winding. With

constant armature voltage and constant field excitation, the shunt-wound motor offers relatively

flat speed-torque characteristics. The shunt-wound motor offers simplified control for reversing,especially for regenerative drives.

Fig 1.2. Shunt Wound or Shunt Motor

1.4.4. Compound Motor

The compound-wound DC motor utilizes a field winding in series with the armature in addition to

the shunt field, to obtain a compromise in performance between a series and a shunt wound type

motor. The compound-wound motor offers a combination of good starting torque and speed

stability.

Fig 1.3 Compound Motor


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Chapter 2

STARTING & BREAKING

2.1 Starting

For a machine to start, the torque developed by the motor at zero speed must exceed that

demanded by load. Maximum current that a dc motor can safely carry during starting is limited by

the maximum current can be commutated without sparking. For normally designed machines.

Twice the rate current can allowed to flow and for specially designed machines it can be 3.5

times. The induced emf at starting point is zero as the = 0.The armature current with rated

applied voltage is given by V/ , where is armature circuit resistance.

Normally the armature resistance of a dc machine is such as to course 1 to 5 present drop at full

load current. Hence, the starting current tends to rise to several times the full load current. A

separately excited DC motor started by an armature rheostat is shown in the figure. The field

current is kept at rated value of 1.6 Amp in this case. The rated armature current is 10 A.

Normally the armature current

Ia = (Vdc-Eb) / (Ra+Rext) (2.1)

Where

Eb is the back emf and Vdc is the applied armature voltage. When initially the motor is started,

the back emf is zero because the speed is zero and hence the armature current,

Ia = Vdc/ (Ra +Rext) .(2.2)

Where

Ra is the armature resistance.

Torque developed by the motor

Te = KIa (2.3)


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Where = Back emf constant of the motor

= main flux of the motor

Ia = Armature current

The back emf developed by the motor at any speed of rad/sec is

Eb = K (2.4)

The power output is the product of Eb and Ia.Initially the current will be very large if no external

resistance is included due to the back emf being zero. So, the motor starter generally consists of a

large resistance in series with the armature circuit which is cut down slowly as the motor picks up

speed. This is being emulated in this experiment with the help of a series rheostat next.

Fig. 2.1 Starting of a DC Shunt Motor


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2.2 Breaking

In any drive, break is very important to stop the motor. It should work for a reasonably short

period of time. Breaking in drive works as a generator developing a negative torque, which

opposes the motion is called breaking. Whenever an electric drive is disconnected from the suppythe speed of motor is gradually decreases and becomes zero. Breaking torque can be applied

either by mechanical brakes or elctro-dynamically. Elctrodynamically breaking can be applied by

a separate eddy current brahe or driving motor can be made to work as a brake, while stopping, be

employed for any one of the following purposes. Sometimes it is desirable to stop a d.c. motor

quickly. This may be necessary in case of emergency or to save time if the motor is being used

for frequently repeated operations. The motor and its load may be brought to rest by us in either

(1) mechanical (friction) braking or (2) electric braking. In mechanical breaking, the motor is

stopped due to the friction between the moving parts of the motor and the brake shoe i.e. kinetic

energy of the motor is dissipated as heat. Mechanical braking has several disadvantages including

non-smooth stop and greater stopping time

1. Reducing the time taken to stop.

2. Stop exactly at specified point, e.g., in lifts.

3. Feeding beck to the soppy.

During the electric braking motor current tends to exceed the safe limit. Appropriate changes are

made to ensure that the current is restricted within safe limit. When electric braking may persist

for long periods, maximum current is usually restricted to the rated value.

2.2.1 Type of Electrical Braking

There are three type braking are as follow.

1. Plugging2. Dynamic or rheostatic braking

3. Regenerative breaking


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2.2.1.1 Plugging

Plugging or reverse current braking In this method of braking connection of motor are changed in

such way that motor develops torque in opposite direction for braking condition to the

movement of the rotor. Thus the motor speed will gradually decrease till zero speed is reached. Inthis method, connections to the armature are reversed so that motor tends to rotate in the opposite

direction, thus providing the necessary braking effect. When the motor comes to rest, the supply

must be cut off otherwise the motor will start rotating in the opposite direction.

Plugging of a d.c. shunt motor. Note that armature connections are reversed while the connections

of the field winding are kept the same. As a result the current in the armature reverses. During the

normal running of the motor the back e.m.f. Eb opposes the applied voltage V. However, when

armature connections are reversed, back e.m.f. EB and V act in the same direction around the circuit.

Fig. 2.2 Plugging Operation in Separately Excited DC Motor

Fig. 2.3 Plugging Operation in DC Series Motor


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Electrical breaking torque,

I= + / ..(2.5)

= + / (2.6)

Back emf is proportional to motor speed and flux, i.e.

...(2.7)

..(2.8)

Substituting value in equation

= [ + N (2.9)

..(2.10)

In the case of series motor, proportional to the current and the value of the torque can only be

determined from the magnetization curve.


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Fig. 2.4 Plugging Speed-Torque Curve

This is the simplest type of breaking. This method can be applied to direct current motor or

alternating current induction and synchronous motors. The main disadvantage is heavy in-rush

current oat the time of breaking.

2.2.1.2 Dynamic or Rheostatic Breaking

In this method, the armature of the running motor is disconnected from the supply and is

connected across a variable resistance R. However, the field winding is left connected to the

supply. The armature, while slowing down, rotates in a strong magnetic field and, therefore,

operates as a generator, sending a large current through resistance R. This causes the energy

possessed by the rotating armature to be dissipated quickly as heat in the resistance. As a result,

the motor is brought to standstill quickly.

Dynamic braking of a shunt motor. The braking torque can be controlled by varying the

resistance R. If the value of R is decreased as the motor speed decreases, the braking torque may

be maintained at a high value. Ata low value of speed, the braking torque becomes small and the

final stopping of the motor is due to friction. This type of braking is used extensively in

connection with the control of elevators and hoists and in other applications in which motors must

be started, stopped and reversed frequently.


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Fig. 2.5 Dynamic Breaking Operation in Separately Excited Motor

Fig. 2.6 Dynamic Breaking in Series Excited Motor

Electrical breaking torque

(2.11)

..(2.12)


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..(2.13)

When fast breaking is desired, consist of a few sections. This permits breaking even where

supply fails.

Fig. 2.7 Dynamic Breaking Speed-Torque Curve

The second method of breaking is advantageous compared to the first one as, if the direction of

rotation of the machine armature reverses, the machine will fail excite in the first case and

therefore will not produce any breaking effect. The machine will build up in series and being

short-circuited on them, will provide emergency breaking. Rheostat breaking cannot be employed

with 3-phase induction motors.

2.2.1.3 Regenerative Breaking

In the regenerative braking, the motor is run as a generator. As a result, the kinetic energy of the

motor is converted into electrical energy and returned to the supply. Shows two methods of


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regenerative braking for a shunt motor. In one method, field winding is disconnected from the

supply and field current is increased by exciting it from another source. As a result, induced e.m.f.

E exceeds the supply voltage V and the machine feeds energy into the supply. Thus braking

torque is provided up to the speed at which induced e.m.f. and supply voltage are equal. As the

machine slows down, it is not possible to maintain induced e.m.f. at a higher value than the

supply voltage. Therefore, this method is possible only for a limited range of speed.(b) In a

second method, the field excitation does not change but the load causes the motor to run above

the normal speed (e.g., descending load on a crane).As a result, the induced e.m.f. E becomes

greater than the supply voltage V. The direction of armature current I, therefore, reverses but the

direction of shunt field current If remains unaltered. Hence the torque is reversed and the speed

falls until E becomes less than V.

If the emf generated by motor is greater than the supply voltage, the power will be fed back into

the supply. The shunt motor depends upon its exciting current and speed. If the field is

disconnected from the supply and the field current increase by exciting it form another source, the

induce emf will exceed to supply voltage and the motor will fed energy into the supply.

Fig. 2.8 Regenerative Breaking Speed-Torque Char


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Advantage

1. A part of energy is returned to the supply system, so that energy consumption for the run is

considerably reduced.

2. A higher value of breaking retardation is obtain, so that the vehicle can be brought to restquickly and run time can be considerably reduced.

3. Higher speed are possible while going down the gradients.

Disadvantage

2. The additional equipment is required for control of regeneration and for protection of

equipment and machines.

3. Owing to the recaptured energy the operation of the substations becomes complicated and

difficult.


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Chapter 3

SPEED CONTROL

3.1Speed Control Of dc Drive

Speed can be controlled by following methods.

1. Armature voltage control

2. Field flux control

3. Armature resistance control

3.1.1 Armature Voltage Control

In this method, the voltage source supplying the field current is different from that which supplies

the armature. This method avoids the disadvantages of poor speed regulation and low efficiency as

in armature control method. However, it is quite expensive. Therefore, this method of speed control

is employed for large size motors where efficiency is of great importance.

3.1.1.1 Multiple Voltage Control

In this method, the shunt field of the motor is connected permanently across a-fixed voltage source.

The armature can be connected across several different voltages through suitable switchgear. In this

way, voltage applied across the armature can be changed. The speed will be approximately

proportional to the voltage applied across the armature. Intermediate speeds can be obtained by

means of a shunt field regulator.

3.1.1.2 Ward-Leonard System

In this method, the adjustable voltage for the armature is obtained from an adjustable-voltage

generator while the field circuit is supplied from a separate source. This is illustrated in The armature

of the shunt motor M (whose speed is to be controlled) is connected directly to a d.c. generator G

driven by a constant-speed a.c. motor A. The field of the shunt motor is supplied from a constant-

voltage exciter E. The field of the generator G is also supplied from the exciter E. The voltage of the

generator G can be varied by means of its field regulator. By reversing the field current of generator


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G by controller FC, the voltage applied to the motor may be reversed. Sometimes, a field regulator is

included in the field circuit of shunt motor M for additional speed adjustment. With this method, the

motor may be operated at any speed up to its maximum speed.

Advantages

1. The speed of the motor can be adjusted through a wide range without resistance losses which

results in high efficiency

2. The motor can be brought to a standstill quickly, simply by rapidly reducing the voltage of

generator G. When the generator voltage is reduced below the back e.m.f. of the motor, this back

e.m.f. sends current through the generator armature, establishing dynamic braking. While this takes

place, the generator G operates as a motor driving motor A which returns power to the line.

3. This method is used for the speed control of large motors when a d.c. supply is not available. The

disadvantage of the method is that a special motor-generator set is required for each motor and the

losses in this set are high if the motor is operating under light loads for long periods.

Fig. 3.1 Armature Voltage Control

3.1.2 Field Flux Control

In this method, the flux produced by the series motor is varied and hence the speed. The variation

of flux can be achieved in the following ways:

(1) Field diverters. In this method, a variable resistance (called field diverter) is connected in

parallel with series field winding as shown in Fig. Its effect is to shunt some portion of the line


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current from the series field winding, thus weakening the field and increasing the speed (Q N

1/f). The lowest speed obtainable is that corresponding to zero current in the diverter (i.e., diverter

is open). Obviously, the lowest speed obtainable is the normal speed of the motor. Consequently,

this method can only provide speeds above the normal speed. The series field diverter method is

often employed in traction work.

(2) Armature diverter. In order to obtain speeds below the normal speed, a variable resistance

(called armature diverter) is connected in parallel with the armature as shown in. The diverter

shunts some of the line current, thus reducing the armature current. Now for a given load, if Ia is

decreased, the flux f must increase (Q T f Ia). Since N 1/f, the motor speed is decreased. By

adjusting the armature diverter, any speed lower than the normal speed can be obtained.

Fig. 3.2 Field Flux Control

(3) Tapped field control. In this method, the flux is reduced (and hence speed is increased) by

decreasing the number of turns of the series field winding The switch S can short circuit any part

of the field winding, thus decreasing the flux and raising the speed. With full turns of the field

winding, the motor runs at normal speed and as the field turns are cut out, speeds higher than

normal speed are achieved.


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Fig. 3.3 Field Flux (rated)

3.1.3 Armature Resistance Control

In this method, a variable resistance is directly connected in series with the supply to the complete

motor. This reduces the voltage available across the armature and hence the speed falls. By

changing the value of variable resistance, any speed below the normal speed can be obtained.

This is the most common method employed to control the speed of d.c. series motors. Although

this method has poor speed regulation, this has no significance for series motors because they are

used in varying speed applications. The loss of power in the series resistance for many

applications of series motors is not too serious since in these.

Fig. 3.4 Armature Resistance Control


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Applications

The control is utilized for a large portion of the time for reducing the speed under light-load

conditions and is only used intermittently when the motor is carrying full-load.

(a) Separately excited (b) Series

Figure 3.5 Speed-Torque Curve of Motor

in the method speed is varied by wasting power is external resistor that are connected in series

with the armature. Since is an inefficient method of speed control, it was used in intermittent load

applications where the duration of the low speed operation from only a small proportion of the

total running time.


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Fig. 3.6 Torque & Power Limitation in Combined Armature Voltage and Field Control


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Chapter 4

CONTROLLED RECTIFIER FED DC DRIVES

4.1 Controlled Rectifier fed DC Drives

Controlled rectifiers provide variable dc voltage from an ac source of fixed voltage controlled

rectifier fed dc drives are also known as Static Ward-Leonard drives.

These are four types

1. Single-phase fully-controlled rectifier

2. Single-phase half-controlled rectifier

3. Three-phase fully-controlled rectifier

4. Three-phase half-controlled rectifier

4.1.1 Single-Phase Fully-Controlled Rectifier

Single phase fully controlled bridge converter. It is one of the most popular converter circuits and

is widely used in the speed control of separately excited dc machines. Indeed, the RLE load

shown in this figure may represent the electrical equivalent circuit of a separately excited dc

motor. The single phase fully controlled bridge converter is obtained by replacing all the diode of

the corresponding uncontrolled converter by thyristors. Thyristors T1 and T2 are fired together

while T3

and T4

are fired 180 after T1

and T2. From the circuit diagram of Fig 4.1 It is clear that

for any load current to flow at least one thyristor from the top group (T1, T

3) and one thyristor

from the bottom group (T2, T

4) must conduct. It can also be argued that neither T

1T

3nor T

2T

4can

conduct simultaneously. For example whenever T3

and T4

are in the forward blocking state and a

gate pulse is applied to them, they turn ON and at the same time a negative voltage is applied

across T1 and T2 commutating them immediately. Similar argument holds for T1 and T2.

For the same reason T1T

4or T

2T

3can not conduct simultaneously. Therefore, the only possible

conduction modes when the current i0

can flow are T1T

2and T

3T

4. Of course it is possible that at a


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given moment none of the thyristors conduct. This situation will typically occur when the load

current becomes zero in between the firings of T1T

2and T

3T

4. Once the load current becomes

zero all thyristors remain off. In this mode the load current remains zero. Consequently the

converter is said to be operating in the discontinuous conduction mode. Fig 4.2 shows the voltage

across different devices and the dc output voltage during each of these conduction modes. It is to

be noted that whenever T1

and T2

conducts, the voltage across T3

and T4becomesv

i. Therefore

T3

and T4

can be fired only when viis negative i.e, over the negative half cycle of the input supply

voltage. Similarly T1

and T2

can be fired only over the positive half cycle of the input supply. The

voltage across the devices when none of the thrusters conduct depends on the off state impedance

of each device. The values listed in Fig 4.2 assume identical devices. Under normal operating condition

of the converter the load current may or may not remain zero over some interval of the input

voltage cycle. If i0

is always greater than zero then the converter is said to be operating in the

continuous conduction mode. In this mode of operation of the converter T1T

2and T

3T

4conducts

for alternate half cycle of the input supply.

Fig. 4.1 Single-Phase Fully-Controlled Rectifier

During the +ive half cycle, thyristors and are operated from while duringive half

cycle.

1. Discontinuous conduction mode.

2. Continuous mode

1. Discontinuous Conduction Mode


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we have assumed that the converter operates in continuous conduction mode without paying

attention to the load condition required for it. The voltage across the R and L component of the

load is negative in the region - t + . Therefore i0

continues to decrease till a new pair

of thyristor is fired at t = + . Now if the value of R, L and E are such that i0becomes zero

before t = + the conduction becomes discontinuous. Obviously then, at the boundary

between continuous and discontinuous conduction the minimum value of i0

which occurs at t =

and t = + will be zero. We obtain the condition for continuous conduction as.

Fig. 4.2 Discontinuous Conduction Waveform

The drive operation is

+ + E = (4.1)

For duty interval when motor connect to the source

= 0 for ..(4.2)

.(4.3)

.(4.4)

1. Continuous Conduction Mode

The continuous conduction mode of operation i0 never becomes zero, therefore, either T1T2 or

T3T

4conducts. The waveforms of different variables in the steady state. The firing angle of the

converter is . The angle is given by


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1Esin=2V (4.5)

It is assumed that at t = 0-

T3T

4was conducting. As T

1T

2are fired at t = they turn on

commutating T3T

4immediately. T

3T

4are again fired at t = + . Till this point T

1T

2conducts.

The period of conduction of different thyristors are pictorially depicted in the second waveform

(also called the conduction diagram)

Fig. 4.3 Continuous Conduction Mode

The average value output is given by

(4.6)

(4.7)

4.1.2 Single-Phase Half-Controlled Rectifier

The working and principle same as single phase fully-controlled rectifier in discontinuous

connection mode, when .


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Fig. 4.4 Single-Phase Half-Controlled Rectifier

it can be stated that for any load current to flow one device from the top group (T1

or T3) and one

device from the bottom group must conduct. However, T1

T3

or D2

D4

cannot conduct

simultaneously. On the other hand T1 D4 and T3 D2 conducts simultaneously whenever T1 or T3 are

on and the output voltage tends to go negative. Therefore, there are four operating modes of this

converter when current flows through the load. Of course it is always possible that none of the

four devices conduct. The load current during such periods will be zero. The operating modes of

this converter and the voltage across different devices during these operating modes.

Fig. 4.5 Discontinuous Mode Waveform

.(4.8)


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Fig. 4.6 Continuous Conduction Waveform

.(4.9)

4.1.3 Three-Phase Fully Controlled Rectifier

The three-phase bridge rectifier circuit has three-legs, each phase connected to one of the three

phase voltages. Alternatively, it can be seen that the bridge circuit has two halves, the positive

half consisting of the SCRs S1, S3 and S5 and the negative half consisting of the SCRs S2, S4 and

S6. At any time, one SCR from each half conducts when there is current flow. If the phase

sequence of the source be RYB, the SCRs are triggered in the sequence S 1, S2 , S3 , S4, S5 , S6 and

S1 and so on.

the R-phase voltage is the highest of the three-phase voltages when q is in the range from 30o

to

150o. It can also be seen that Y-phase voltage is the highest of the three-phase voltages when q is

in the range from 150o

to 270oand that B-phase voltage is the highest of the three-phase voltages

when q is in the range from 270o

to 390oor

30

oin the next cycle. We also find that R-phase

voltage is the lowest of the three-phase voltages when q is in the range from 210o

to 330o. It can

also be seen that Y-phase voltage is the lowest of the three-phase voltages when q is in the range

from 330o

to 450oor

90

oin the next cycle, and that B-phase voltage is the lowest when q is in the

range from 90o

to 210o. If diodes are used, diode D1 in place of S1 would conduct from 30

oto

150o, diode D3

would conduct from 150

oto 270

oand diode D5 from 270

oto 390

oor

30

oin the


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34

next cycle. In the same way, diode D4would conduct from 210

oto 330

o, diode D6

from 330

oto

450o

or90

oin the next cycle, and diode D2

would conduct from 90

oto 210

o. The positive rail of

output voltage of the bridge is connected to the topmost segments of the envelope of three-phase

voltages and the negative rail of the output voltage to the lowest segments of the envelope.

Fig. 4.7 Three-Phase Fully Controlled Rectifier

If SCRs are used, their conduction can be delayed by choosing the desired firing angle. When the

SCRs are fired at 0o

firing angle, the output of the bridge rectifier would be the same as that of the

circuit with diodes. For instance, it is seen that D1 starts conducting only after q = 30o. In fact, it

can start conducting only after q = 30o, since it is reverse-biased before q = 30

o. The bias across

D1 becomes zero when q = 30

o

and diode D1 starts getting forward-biased only after q =30

o

.When vR(q) = E*Sin (q), diode D1 is reverse-biased before q = 30

oand it is forward-biased

when q > 30o. When firing angle to SCRs is zero degree, S1 is triggered when q = 30

o. This

means that if a synchronizing signal is needed for triggering S 1, that signal voltage would lag

vR(q) by 30o

and if the firing angle is a, SCR S1 is triggered when q = a + 30o. Given that the

conduction is continuous, the following table presents the SCR pair in conduction at any instant.


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35

Fig. 4.8Three-Phase Fully Controlled Rectifier waveform

(4.10)


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36

4.1.4 Three-Phase Half-Controlled Rectifier

Fig. 4.9 Three-Phase Half-Controlled Rectifier

(4.11)

.(4.12)


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37

Chapter 5

CHOPPER CONTROLLED DC DRIVE & DUAL CONVERTER

5.1 Chopper Controlled DC Drive

A transistor chopper controlled separately excited motor .transistor is operated periodically

with period T and remain on duration

During on-period of transistor.

(5.1)

At t = is turned-off.

..(5.2)

..(5.3)

Fig. 5.1 Chopper Control

.(5.4)


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38

Fig. 5.2 Waveform

5.2 Dual Converter

Dual convertor means it converts ac to dc and again it converts dc to ac this phenomenon of

converting two types of signal are in the same circuit is called as dual convertor. The fully

controlled converter can produce a reversible direct output voltage with output current in one

direction, and in terms of a conventional voltage/ current diagram(Fig.1:) is said to be capable of

operation in two quadrants, the first and fourth. Such a range of operation is useful for certain

purposes, examples being the control of a dc torque motor, i.e. a motor used to provide

unidirectional torque with reversible rotation (Fig.2), and a DC transmission link between two AC

systems in which power can be transmitted in either direction according to the polarity of the

voltage with current flows always in one direction. Equally a converter may be used under

steady- stage conditions in the first quadrant only but transiently in the second quadrant in order

to extract energy from the load quickly and thereby improve the response of the system to

changing command signals.

If four-quadrant operation of a DC motor is required, i.e. reversible rotation and reversible torque,

a single converter needs the addition of either a change - over contractor to reverse the armature

connections or a means of reversing the field current in order to change the relationship between

the converter voltage and the direction of rotation of the motor. Both of these are practicable in

suitable phase and three-phase dual-converter using bridge type converter circuits. The output

terminals of each converter having the same potential are connected together through a reactor.


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39

The four possible quadrants of converter operation thus resulting can be translated into four

(steady-state) combinations of motor-torque and rotation.

Fig. 5.3 Voltage Current Diagram

Fig. 5.4 Single-Phase Dual Converter

The basic principle of operation of dual converter can be explained with reference to the

simplified equivalent diagram of the DC circuit. In this simplified representation, assumption is

made that the dual converters are ideal and they produce pure DC output terminals. Each two-quadrant converter is assumed to be a controllable direct voltage source, connected in series with

a diode. Diode D1 and D2 represent the unidirectional current flow characteristics of the

converters. The current in load circuit can, however, flow in either direction.


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40

Chapter 6

CONCLUSION

Once again, everything worked as it was suppose to. You can control a dc motor using

theLMD18245 H-Bridge. Be aware there are many, many different types of motor controller ic's

out there. It would be wise to look at some of them and even try using them to see the

different. Along side looking at different motor controllers, it would also be a good idea to look at

different types of motors. Some would be: Stepper Motors & Servo Motors. We will have

tutorials from controlling these motors coming in the future.

This increase in speed will then proportionately increase the CEMF. The speed and CEMF

willcontinue to increase until the armature current and torque are reduced to values just large

enoughto supply the load at a new constant speed.
http://www.pyroelectro.com/parts/LMD18245http://www.pyroelectro.com/parts/LMD18245http://www.pyroelectro.com/parts/LMD18245http://www.pyroelectro.com/parts/LMD18245


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Chapter 7

REFERANCE

[1] http://www.eurodcdrive.org/info/encyclopedia/n/dcdrive.htm[2] F. Clarke "The Cost and Benefit of Energy Technology in the Global Context", Proceedings,

IEA International Conference on Technology Responses to Global Environmental

Challenges, 1991 .

[3] Microscopic calculations of dc drives.[4] R. A. Bradley , E. C. Watts and E. R. Williams Report to the Congress of the United States,

Limiting Net Greenhouse Gas Emissions in the United States, vol. I, 1991.

[5] R. C. Lind Discounting for Time and Risk in Energy Policy, pp.444 -449 1982 :Resourcesfor the Future.

[6] J. A. Edmonds , H. M. Pitcher , D. Barns , R. Baron and M. A. Wise Modeling FutureGreenhouse Gas Emissions: The Second Generation Model Description, 1991 :Pacific

Northwest Laboratory.

[7] R. W. Bussard, "System Technical and Economic Features of QED-Engine-Driven Space.
http://www.eurodcdrive.org/info/encyclopedia/n/dchttp://www.eurodcdrive.org/info/encyclopedia/n/dchttp://www.eurodcdrive.org/info/encyclopedia/n/dc

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