8/12/2019 Unit - 4 ED

1/67

Unit -4

Control of DC Drives

Introduction to Electric Drives

Mayank Goyal

8/12/2019 Unit - 4 ED

2/67

Introduction Industries are increasingly demanding process automation.

The variable speed drives are essential controlling elementsin automation systems.

With the advent of power electronics, today variable electricdrive systems are smaller in size, efficient, reliable and meetsall stringent demands of the various industries of modernera.

DC motors have been extensively used in variable speeddrives and position control due to their versatile controlcharacteristics.

DC motors can provide high starting torques, large range ofspeed control and their speed control methods are simplerand less expensive than those of ac motors.

Mayank Goyal

8/12/2019 Unit - 4 ED

3/67

Mayank Goyal

Dc motors have inherent disadvantages in that they need

regular maintenance, they are tailor-made hence not readily

available for replacements and are bulky in size. Added to

this, due to commutator sparking, they are simply not

suitable in hazardous areas like chemical and petrochemical

plants or mines.

Separately excited dc motors controlled by thyristorconverters are the most widely used motor drive systems in

industry. The thyristor converter provides variable armature

voltage for the drive motor.

8/12/2019 Unit - 4 ED

4/67

Basic Machine Equations

Mayank Goyal

The dc machine consists of a stationary field winding and a

rotating armature winding as shown in figure below.

8/12/2019 Unit - 4 ED

5/67

Steady state equivalent circuit of

armature

Mayank Goyal

8/12/2019 Unit - 4 ED

6/67

Commonly Used DC Motors

Mayank Goyal

8/12/2019 Unit - 4 ED

7/67

Speed Control

Mayank Goyal

Speed of DC motors can be controlled by any of the

following methods: Armature voltage control

Field flux control

Armature resistance control

8/12/2019 Unit - 4 ED

8/67

Speed torque curves

Mayank Goyal

8/12/2019 Unit - 4 ED

9/67

Speed Torque Curves (Contd)

Mayank Goyal

8/12/2019 Unit - 4 ED

10/67

Speed Torque Curves (Contd)

Mayank Goyal

8/12/2019 Unit - 4 ED

11/67

Mayank Goyal

Armature voltage control is preferred because of high

efficiency, good transient response and good speed

regulation. But it can provide speed control only below base(rated) speed because the armature voltage cannot be allowed

to exceed rated value.

For speed control above base speed, field flux control is

employed. In a normally designed motor, the maximumspeed can be allowed up to twice rated speed and in specially

designed machines it can be six times rated speed.

8/12/2019 Unit - 4 ED

12/67

Mayank Goyal

8/12/2019 Unit - 4 ED

13/67

Mayank Goyal

In separately excited motor, flux is controlled by varying

voltage across field winding and in a series motor it is

controlled either by varying number of turns in the fieldwinding or connecting a diverter resistance across the field

winding.

In armature resistance control, speed is varied by wasting

power in external resistors that are connected in series withthe armature. Since it is an inefficient method of speed

control, it was used in intermittent load applications where

the duration of low speed operation forms only a small

proportion of total running time, for example in traction. Ithas, however, been replaced by armature voltage control in

all applications.

8/12/2019 Unit - 4 ED

14/67

Braking

Mayank Goyal

Electric drive operates in all the four quadrants.

Mechanical braking has a number of disadvantages: frequent

maintenance and replacement of brake shoes, lower life,

braking power is always wasted as heat.

These disadvantages are overcome by the use of electrical

braking in which the motor is made to work as a generator

converting mechanical energy to electrical energy and

producing torque in a direction so as to oppose the motion.

8/12/2019 Unit - 4 ED

15/67

Electric Braking

Mayank Goyal

In electric braking, the motor is made to work as a generator

converting mechanical energy to electrical energy andproducing torque in a direction so as to oppose the motion.

It is of three types:

Regenerative Braking

Dynamic or rheostatic braking

Plugging or reverse voltage braking

8/12/2019 Unit - 4 ED

16/67

Regenerative Braking

Mayank Goyal

In regenerative braking, generated energy is supplied back to thesource. For this to happen, following condition should be satisfied:

Eb> E and negative Ia

For a source of fixed voltage of rated value, regenerative braking ispossible only for speeds higher than rated and with a variablevoltage source it is also possible below rated speeds.

8/12/2019 Unit - 4 ED

17/67

Regenerative Braking (Contd)

Mayank Goyal

In series motor as speed increases, armature current and

therefore flux decreases. Consequently, condition of

regenerative braking cannot be achieved. Thus, regenerativebraking is not possible.

Regenerative braking should only be used when there are

enough loads to absorb the regenerated power.

8/12/2019 Unit - 4 ED

18/67

Dynamic Braking

Mayank Goyal

In dynamic braking, motor armature is disconnected from the source andconnected across a resistance R

B

.

The generated energy is dissipated in RB andRA.

8/12/2019 Unit - 4 ED

19/67

Plugging

Mayank Goyal

For Plugging, the supply voltage of a separately excited motor isreversed so that it assists the back emf in forcing armature current inreverse direction. A resistance RB is also connected in series witharmature to limit the current.

For plugging of series motor armature alone is reversed.

8/12/2019 Unit - 4 ED

20/67

Schemes for D.C. Motor Speed Control

Mayank Goyal

The most common form of variable speed d.c. drive is

based on the control of armature voltage. The speed of a d.c.motor has to be controlled from an a.c. or d.c. source.

The two basic schemes of d.c. motor speed control are:

Controlled rectifier fed-drive

Armature voltage control using d.c. chopper.

8/12/2019 Unit - 4 ED

21/67

Mayank Goyal

8/12/2019 Unit - 4 ED

22/67

Single Phase Separately Excited Drives

Mayank Goyal

In phase controlled d.c. drives, an a.c. to d.c. phase

controlled converter is used to control the d.c. drive motor. Controlled rectifier for d.c. drives are widely used in

applications requiring a wide range of speed control and/or

frequent starting, braking and reversing.

Some applications are in rolling mills, paper mills, printingpresses, machine tools, mine winders, etc.

8/12/2019 Unit - 4 ED

23/67

Mayank Goyal

The basic circuit arrangement for a single phase separatelyexcited d.c. motor is shown below.

8/12/2019 Unit - 4 ED

24/67

Single Phase Half Wave converter Drives

Mayank Goyal

In this circuit the motor current is always discontinuous, resulting inpoor motor performance.

This type of converter is employed only for motors below 400 W.

8/12/2019 Unit - 4 ED

25/67

Single Phase Semiconverter Drives

Mayank Goyal

Semi Converters are one quadrant converters, that is, they have onepolarity voltage and current at d.c. terminals.

In drive system using semiconverters, regeneration or reverse power flow frommotor to a.c. supply is not possible

8/12/2019 Unit - 4 ED

26/67

(i) Continuous armature current

Mayank Goyal

8/12/2019 Unit - 4 ED

27/67

Mayank Goyal

8/12/2019 Unit - 4 ED

28/67

(ii) Discontinuous Armature Current

Mayank Goyal

The armature current becomes discontinuous for large values of firing angle,high speed and low values of torque.

If the armature current is discontinuous, the no-load speeds will be higher and

the speed regulation will be poor in region of discontinuous armature current.

8/12/2019 Unit - 4 ED

29/67

Single Phase Full Converter Drives

Mayank Goyal

A full converter is a two quadrant converter in which the voltage polarity of theoutput can reverse, but the current remains unidirectional because of the unidirectionalthyristors.

It is limited to applications up to 20 HP.

8/12/2019 Unit - 4 ED

30/67

(i) Continuous Armature Current

Mayank Goyal

8/12/2019 Unit - 4 ED

31/67

Mayank Goyal

For firing angle greater than 90o, Eais negative. If the motor back emf Ebis reversed, it

will behave as dc generator and will feed power back to the a.c. supply. This is known as

the inversion operation of the converter and this mode of operation is used in the

regenerative braking of the motor.

8/12/2019 Unit - 4 ED

32/67

Mayank Goyal

8/12/2019 Unit - 4 ED

33/67

(ii) Discontinuous Armature Current

Mayank Goyal

8/12/2019 Unit - 4 ED

34/67

Effect of Armature Circuit Inductance

Mayank Goyal

Addition of inductance in the motor armature circuit hassignificant effects on the performance of the converter motor

system.

It reduces the region of discontinuous motor current.

If a large inductor is used, the motor will operate essentially atconstant speed for a particular firing angle over a wide range of

torque. This significantly improves the speed regulation of the

drive.

B ki O i f R ifi C ll d

8/12/2019 Unit - 4 ED

35/67

Braking Operation of Rectifier Controlled

Separately Excited Motor

Mayank Goyal

8/12/2019 Unit - 4 ED

36/67

Mayank Goyal

The reversal of the motor emf with respect to the rectifier

terminal can be done by any one of the following changes:

An active load coupled to the motor shaft may drive it in the

reverse direction. In this case no changes are required in thearmature connection with respect to the rectifier terminals.

The field current may be reversed with the motor running in

the forward direction. In this case also, no changes are required

in the armature connection. The motor armature connection may be reversed with respect

to the rectifier output terminals, with the motor running in the

forward direction.

8/12/2019 Unit - 4 ED

37/67

Single Phase Series D.C. motor Drives

Mayank Goyal

Series motors are particularly used for applications thatrequire high starting torque such as cranes, hoists, elevators,

vehicles, etc.

Si gl Ph S i t D i

8/12/2019 Unit - 4 ED

38/67

Single Phase Semiconverter Drives

Mayank Goyal

8/12/2019 Unit - 4 ED

39/67

Mayank Goyal

8/12/2019 Unit - 4 ED

40/67

Mayank Goyal

8/12/2019 Unit - 4 ED

41/67

Single Phase Full Converter Drives

Mayank Goyal

8/12/2019 Unit - 4 ED

42/67

Mayank Goyal

8/12/2019 Unit - 4 ED

43/67

Power Factor Improvement

Mayank Goyal

Phase controlled rectifiers are widely used because these

converters are simple, less expensive, reliable, and do notrequire any commutation circuit. However, the supply power

factor in phase controlled converters is low when the

output voltage is less than the maximum, i.e. when the firing

angle is large. The displacement angle between the supplyvoltage and current increases as the firing angle increases and

the converter draws more lagging reactive power, thereby

decreasing the power factor.

The power factor operation is a major concern in variablespeed drives and in high power applications.

P F I (C d )

8/12/2019 Unit - 4 ED

44/67

Power Factor Improvement (Contd)

Mayank Goyal

The various techniques to improve power factor in phase

controlled converters are:

Phase Angle Control (PAC)

Semiconverter operation of full converters

Asymmetrical Firing

Extinction angle control (EAC) Symmetrical angle control (SAC)

Pulse Width Modulation (PWM)

8/12/2019 Unit - 4 ED

45/67

Phase Angle Control

Mayank Goyal

Phase angle control is achieved by varying the firing angle.

In both semiconverter and full converter systems, powerfactor and the displacement factor decreases as the output

voltage decreases (or as the firing angle increases).

The semiconverter provides some improvement in power

factor, displacement factor and lower order harmonics. In this section, mathematical analysis done in the topic

Performance measures for two pulse converters has to be

repeated.

8/12/2019 Unit - 4 ED

46/67

Semiconverter operation of full converters

Mayank Goyal

A full converter system is commonly used where regeneration isrequired. However, a semiconverter is used where regeneration isnot required.

It is possible with some complexity in the control logic circuit tooperate a full converter as semiconverter in both rectifying modeand inverting mode in order to exploit the better performancecharacteristics of semiconverter.

8/12/2019 Unit - 4 ED

47/67

Mayank Goyal

8/12/2019 Unit - 4 ED

48/67

Mayank Goyal

This control scheme, although it improves power factor in

both single phase and three phase converters, is not

recommended for three phase converters because of three

major disadvantages:

Even harmonic currents are present in the supply line current.

Third harmonic ripple is present in the output.

There is a danger of commutation failure.

A t i l Fi i g

8/12/2019 Unit - 4 ED

49/67

Asymmetrical Firing

Mayank Goyal

The improvement in power factor can also be achieved by atechnique known as asymmetrical firing.

In this scheme thyristors are triggered at different angles,whereas in the symmetrical firing scheme thyristors aretriggered at same firing angle.

8/12/2019 Unit - 4 ED

50/67

8/12/2019 Unit - 4 ED

51/67

Mayank Goyal

Asymmetrical firing produces several disadvantages: It generates dc and even harmonic currents in the supply line

current.

If sufficient inductance is not present in the motor armature

circuit, asymmetrical firing makes the motor current verypeaky and discontinuous.

The disadvantages outweigh the advantages of minor

improvement in power factor and therefore, asymmetrical

firing is only of theoretical interest.

E i i A l C l

8/12/2019 Unit - 4 ED

52/67

Extinction Angle Control

Mayank Goyal

8/12/2019 Unit - 4 ED

53/67

Mayank Goyal

In a semiconverter system, performance of the converter

under extinction angle control is similar to that of phase

angle control, with the exception that displacement factor isleading in extinction angle control, whereas it is lagging in

phase angle control.

Symmetrical Angle Control

8/12/2019 Unit - 4 ED

54/67

Symmetrical Angle Control

Mayank Goyal

8/12/2019 Unit - 4 ED

55/67

Mayank Goyal

In this control scheme, the supply current pulse is placed

symmetrically with respect to the supply voltage peak and

therefore, the fundamental current is in phase with thesupply voltage. This makes the displacement factor unity and

improves the power factor.

P l Width M d l ti (PWM) C t l

8/12/2019 Unit - 4 ED

56/67

Pulse Width Modulation (PWM) Control

Mayank Goyal

8/12/2019 Unit - 4 ED

57/67

Mayank Goyal

In phase angle control, extinction angle control and symmetricalangle control schemes, the supply current consists of one pulseper half cycle and the lowest order harmonic is the third. It isvery difficult to filter out the lowest order harmonic current,especially third.

The lowest order harmonics can be eliminated and/or reduced ifthe supply current has more than one pulse per half cycle.

In a sinusoidal PWM control, the displacement factor is unity andthe power factor is improved.

The lowest order harmonic is the fifth for four pulses per halfcycle and the seventh for six pulses per half cycle. Therefore,lower order harmonics that are difficult to filter out are eliminatedor reduced by selecting the appropriate number of pulses per halfcycle.

Th Ph S t l E it d D i

8/12/2019 Unit - 4 ED

58/67

Three Phase Separately Excited Drives

Mayank Goyal

Three phase drives are used for high power applications, up

to megawatts power level. The three phase controlledrectifiers provide power to these large horse power drives.

Three phase drive is better as compared to single phase

drive because the output ripple is small, the ripple frequency

is large and filtering requirement is less. Moreover, in a three phase drive, the armature current is

mostly continuous and therefore, the motor performance is

better as compared to that of single phase drives.

Three phase half wave converter drives

8/12/2019 Unit - 4 ED

59/67

Three phase half wave converter drives

Mayank Goyal

This drive is impractical for most industrial applications because the supply

currents would contain harmonics.

=3 3

2 cos

Where Em is the peak phase voltage of a star connected 3 phase a.c. supply

Three Phase Semiconverter Drives

8/12/2019 Unit - 4 ED

60/67

Three Phase Semiconverter Drives

Mayank Goyal

It is a one-quadrant drive and is limited to applications in the range of 15 150 HP.

The field converter will be single phase or three phase.

8/12/2019 Unit - 4 ED

61/67

Mayank Goyal

8/12/2019 Unit - 4 ED

62/67

8/12/2019 Unit - 4 ED

63/67

Mayank Goyal

8/12/2019 Unit - 4 ED

64/67

Mayank Goyal

D C Chopper Drives

8/12/2019 Unit - 4 ED

65/67

D.C. Chopper Drives

Mayank Goyal

Control of a d.c. motors speed by a chopper is required

where the supply is d.c. (or from a battery) or an a.c. voltage

that has already been rectified to a d.c. voltage.

The most important applications of choppers are in the speed

control of d.c. motors used in industrial or traction drives.

Choppers are used for the control of d.c. motors because of anumber of advantages, such as high efficiency, flexibility in

control, light weight, small size, quick response and

regeneration down to very low speeds.

Chopper controlled d.c. drives have also applications inservos in battery operated vehicles such as forklift trucks,

trolleys, and so on.

D C Chopper Drives (Contd )

8/12/2019 Unit - 4 ED

66/67

D.C. Chopper Drives (Contd)

Mayank Goyal

Because of the flexible control characteristics, separately

excited dc motors are used in servo applications.

Earlier, only dc series motors were used for traction. But

now separately excited dc motors are also employed in

traction system.

The high starting torque was the main reason for using dcseries motor but it has certain limitations:

The field current control is not static.

Regenerative braking is difficult.

Because of the limitations of series motors, separately excitedmotors are now preferred even for traction applications.

Chopper fed D C Series Motor

8/12/2019 Unit - 4 ED

67/67

Chopper fed D.C. Series Motor