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32 Drives Types and Speciﬁcations 855

Harmonic filter

& Pony motor MG

24 pulse converter

AC in: 14-25.7Hz

DC link reactor

Static excitation

6.6kV, 60Hz

FIGURE 32.11 Schematic diagram of shaft generator.

the power rating of VSD correctly to meet performance

requirements and required life expectancy.

New areas of VSD applications are emerging as the power

electronics advances and become more reliable at afford-

able prices. Packaged drives up to several hundred kWs are

becoming a commodity product, and end-users do not need

to involve a third party during speciﬁcation, installation,

and commissioning. Integrated motors are likely to increase

their popularity, possibly with new types of power converters,

e.g. matrix.

Further Reading

1. Dury, W., “Electrical Variable Speed Drives Mature Consumable

or Radical Infant,” Power Engineering Journal, April 1999, Vol.13,

No.2, IEE, London.

2. Guide to Harmonics with AC drives. ABB Technical Guide No.6.

3. Bose, B.K., “Power Electronics and Variable Frequency Drives:

Technology and Applications,” Piscataway, NJ: IEEE Press 1997.

4. Siemens, Complete Guide Series to Variable Speed Drives, “Knowl-

edge of Motor Duty – Key to Proper Planning of Drives,” 3.6.

5. Hobbs, P.J., “Electrical Variable Speed Drives Saves Energy,” Confer-

ence on Development in Variable Speed Drives for Fluid Machinery,

ImechE 1981, C101/81.

6. Richmond, A.W., Apprentice Engineers’ Handbook No. 2, “Servos

and Steppers,” Drives and Controls Publication, Kamtech Publishing

Ltd, Croydon, Surrey.

7. Shakweh, Y., “Aspects of Limited Motion Actuators and Sub-kW

Unipolar Drives,” PhD thesis, August 1989, London University.

8. Shakweh, Y. and Lewis, E., “Assessment of MV converter stack

topologies,” Power Electronics Specialist Conference 99.

9. Jouanne, et al., “Application Issues for PWM Adjustable Speed

AC Motor Drives,” IEEE Industry Application Magazine, Septem-

ber/October 1998, pp. 10–18.

10. Manz, L., “Motor Insulation System Quality for IGBT Drives,” IEEE

Industry Application Magazine, January/February 1997, pp. 51–55.

11. Chen, S. and Lipo, T.A., “Circulating type Motor Bearing Cur-

rent in Inverter Drives,” IEEE Industry Application Magazine,

January/February 1998, pp. 32–38.

12. Hargis, C., “Electro-Magnetic Compatibility – a Basic Guide for

Power Engineers,” Control Techniques Technical Publications,

Powys, UK.

13. Shakweh, Y. and Auﬂeger, P., “Multi-Megawatts, Medium Volt-

age, PWM Voltage Source Sine-Wave Converter For Industrial

Drive Applications,” Power Electronics & Variable Speed Drives

Conference (PEVD’98), UK, London, 21–23 September 98.

14. Shakweh, Y., Lewis, E.A., and Gent, A., “High-power drives

for mining applications,” Minmech 98, South Africa, September

1998.

15. Crane, A. and McCoy, T.J., “EMC design for a 19 MW PWM motor

drive.” 1999 IEEE Industry Applications Society Annual Meeting’99,

Vol.3, pp. 1590–1995.

16. Stromquist, R. and Gustafson, S., “SUBSIS – World’s First Separation

and Injection System,” ABB Review, 6/1998.

17. Clegg, B., et al., “The Application of Drives and Generator Technol-

ogy to a Modern Container Ship,” IEE, PEVD98, London, UK.

18. Elliott, N.J., “Novel Application of a Linear Synchronous Motor

Drive,” IEE Colloquium on “Update on New Power Electronics

Techniques,” IEE, London, May 23 1997.

19. Shakweh, Y., “Power Devices for MV PWM VSI Converters”, Power

Engineering Journal, IEE, U.K., December 1999.

20. Richmond, A.W., ‘A Practical Engineer’s handbook, Industrial Elec-

tric Drives’, Drives & Controls Publications, Kamtech Publishing Ltd,

Croydon, Surrey, UK.

21. Direct Torque Control, ABB Technical Guide No.1.

Motor Drives

M. F. Rahman

School of Electrical Engineering and

Telecommunications, The

University of New South Wales,

Sydney, New South Wales 2052,

Australia

D. Patterson

Northern Territory Centre for Energy

Research, Faculty of Technology,

Northern Territory University,

Darwin, Northern Territory 0909,

Australia

A. Cheok

Department of Electrical and

Computer Engineering, National

University of Singapore, 10 Kent

Ridge Crescent, Singapore

R. Betz

Department of Electrical and

Computer Engineering,

University of Newcastle,

Callaghan, New South Wales,

Australia

33.1 Introduction......................................................................................... 858

33.2 DC Motor Drives .................................................................................. 859

33.2.1 Introduction • 33.2.2 DC Motor Representation and Characteristics • 33.2.3 Converters

for DC Drives • 33.2.4 Drive System Integration • 33.2.5 Converter-DC Drive System

Considerations

33.3 Induction Motor Drives.......................................................................... 865

33.3.1 Introduction • 33.3.2 Steady-state Representation • 33.3.3 Characteristics and Methods

of Control • 33.3.4 Vector Controls

33.4 Synchronous Motor Drives ..................................................................... 877

33.4.1 Introduction • 33.4.2 Steady-state Equivalent-circuit Representation of the Motor

• 33.4.3 Performance with Voltage-source Drive • 33.4.4 Characteristics under Current-source

Inverter (CSI) Drive • 33.4.5 Brushless DC Operation of the CSI-driven Motor

• 33.4.6 Operating Modes • 33.4.7 Vector Controls

33.5 Permanent-magnet AC Synchronous Motor Drives ..................................... 885

33.5.1 Introduction • 33.5.2 The Surface-magnet Synchronous Motor

• 33.5.3 The PM Sinewave Motor

33.6 Permanent-magnet Brushless DC Motor Drives .......................................... 891

33.6.1 Machine Background • 33.6.2 Electronic Commutation • 33.6.3 Current/Torque

Control • 33.6.4 The Signal Processing for Producing Switch Drive Signals from Hall Effect

Sensors and Current Sensors • 33.6.5 Summary

33.7 Servo Drives ......................................................................................... 902

33.7.1 Introduction • 33.7.2 Servo Drive Performance Criteria • 33.7.3 Servo Motors, Shaft

Sensors, and Coupling • 33.7.4 The Inner Current/Torque Loop • 33.7.5 Sensors for

Servo Drives • 33.7.6 Servo Control-loop Design Issues

33.8 Stepper Motor Drives............................................................................. 907

33.8.1 Introduction • 33.8.2 Motor Types and Characteristics • 33.8.3 Mechanism of Torque

Production • 33.8.4 Single- and Multi step Responses • 33.8.5 Drive Circuits

• 33.8.6 Micro Stepping • 33.8.7 Open-loop Acceleration–Deceleration Proﬁles

33.9 Switched-reluctance Motor Drives ............................................................ 915

33.9.1 Introduction • 33.9.2 Advantages and Disadvantages of Switched-reluctance Motors

• 33.9.3 Switched-reluctance Motor Variable-speed Drive Applications • 33.9.4 SR Motor and

Drive Design Options • 33.9.5 Operating Theory of the Switched-reluctance Motor: Linear

Model • 33.9.6 Operating Theory of the SR Motor (II): Magnetic Saturation and Nonlinear

Model • 33.9.7 Control Parameters of the SR Motor • 33.9.8 Position Sensing

33.10 Synchronous Reluctance Motor Drives ...................................................... 926

33.10.1 Introduction • 33.10.2 Basic Principles • 33.10.3 Machine Structure

• 33.10.4 Basic Mathematical Modeling • 33.10.5 Control Strategies and Important Parameters

• 33.10.6 Practical Considerations • 33.10.7 A Syncrel Drive System • 33.10.8 Conclusion

References ............................................................................................ 932

Further Reading .................................................................................... 932

857

858 M. F. Rahman et al.

33.1 Introduction

The widespread proliferation of power electronics and ancil-

lary control circuits into motor control systems in the past two

or three decades have led to a situation where motor drives,

which process about two-thirds of the world’s electrical power

into mechanical power, are on the threshold of processing

all of this power via power electronics. The days of driving

motors directly from the ﬁxed ac or dc mains via mechanical

adjustments are almost over.

The marriage of power electronics with motors has meant

that processes can now be driven much more efﬁciently with a

much greater degree of ﬂexibility than previously possible. Of

course, certain processes are more favorable to certain types

of motors, because of the more favorable match between their

characteristics. Historically, this situation was brought about

by the demands of the industry. Increasingly, however, power

electronic devices and control hardware are becoming able to

easily tailor the rigid characteristics of the motor (when driven

from a ﬁxed dc or ac supply source) to the requirements of the

load. Development of novel forms of machines and control

techniques therefore has not abated, as recent trends would

indicate.

It should be expected that just as power electronics equip-

ment has tremendous variety, depending on the power level

of the application, motors also come in many different types,

depending on the requirements of application and power level.

Often the choice of a motor and its power electronic drive

circuit for application are forced by these realities, and the

application engineer therefore needs to have a good under-

standing of the application, the available motor types, and

the suitable power electronic converter and its control tech-

niques. Table 33.1 gives a rough guide of combinations of

suitable motors and power electronic converters for a few

typical applications.

TABLE 33.1 Typical motor, converter, and application guides

Motor Type of converter Type of control Applications

Brushed dc motor Thyristor ac–dc converter Phase control, with inner current

loop

Process rolling mills, winders, locomotives, large cranes,

extruders, and elevators

GTO/IGBT/MOSFET

chopper

Pulse-width modulator(PWM)

control with inner current loop

Drives for transportation, machine tools, and ofﬁce

equipment

Induction motor (cage) Back-back thyristor Phase control Pumps, and compressors

IGBT/GTO inverter/

cycloconverter

PWM V-f control General-purpose industrial drive such as for cranes, pumps,

fans, elevators, material transport and handling,

extruders, and subway trains

IGBT/GTO Vector control High-performance ac drives in transportation, motion

control, and automation

Induction motor (slip-ring) Thyristor ac–dc converter Phase control with dc-link

current loop

Large pumps, fans, and cement kilns

Synchronous motor (excited) Thyristor ac–dc converter DC-link current loop Large pumps, fans, blowers, compressors, and rolling mills

Synchronous motor (PM) IGBT/MOSFET inverter PWM current control High-performance ac servo drives for ofﬁce equipment,

machine tools, and motion control

For many years, the brushed dc motor has been the natural

choice for applications requiring high dynamic performance.

Drives of up to several hundred kilowatts have used this type

of motor. In contrast, the induction motor was considered

for low-performance, adjustable-speed applications at low and

medium power levels. At very high power levels, the slip-ring

induction motor or the synchronous motor drive were the

natural choices. These boundaries are increasingly becoming

blurred, especially at the lower power levels.

Another factor for motor drives was the consideration for

servo performance. The ever-increasing demand for greater

productivity or throughput and higher quality of most of the

industrial products that we use in our everyday lives means

that all aspects of dynamic response and accuracy of motor

drives have to be increased. Issues of energy efﬁciency and har-

monic proliferation into the supply grid are also increasingly

affecting the choices for motor-drive circuitry.

A typical motor-drive system is expected to have some of

the system blocks indicated in Fig. 33.1. The load may be a

conveyor system, a traction system, the rolls of a mill drive,

the cutting tool of a numerically controlled machine tool,

the compressor of an air conditioner, a ship propulsion sys-

tem, a control valve for a boiler, a robotic arm, and so on.

The power electronic converter block may use diodes, metal-

oxide semiconductor ﬁeld effect transistors (MOSFETS), gate

turn-off thyristors (GTOs), insulated gate bipolar transistors

(IGBTs), or thyristors. The controllers may consist of sev-

eral control loops, for regulating voltage, current, torque, ﬂux,

speed, position, tension, or other desirable conditions of the

load. Each of these may have their limiting features purposely

placed in order to protect the motor, the converter, or the

load. The input commands and the limiting values to these

controllers would normally come from the supervisory con-

trol systems that produce the required references for a drive.

This supervisory control system is normally more concerned

33 Motor Drives 859

Power Electronic

Converter

Motor

Transmission

Load

Drive Controllers

Drive References

Supervisory Control System

AC supply

FIGURE 33.1 Block diagram of a typical motor-drive system.

with the overall operation of the process rather than the

drive.

Consequently, a vast array of choices and technologies

exist for a motor-drive application. Against this background,

this chapter gives a brief description of the dominant forms

of motor drives in current usage. The interested reader is

expected to consult the further reading material listed at the

end of this chapter for more detailed coverage.

33.2 DC Motor Drives

33.2.1 Introduction

Direct-current motors are extensively used in variable-speed

drives and position-control systems where good dynamic

response and steady-state performance are required. Examples

are in robotic drives, printers, machine tools, process rolling

mills, paper and textile industries, and many others. Con-

trol of a dc motor, especially of the separately excited type,

is very straightforward, mainly because of the incorporation

of the commutator within the motor. The commutator-brush

allows the motor-developed torque to be proportional to the

armature current if the ﬁeld current is held constant. Classical

control theories are then easily applied to the design of the

torque and other motion loops of a drive system.

The mechanical commutator limits the maximum applica-

ble voltage to about 1500 V and the maximum power capacity

to a few hundred kilowatts. Series or parallel combinations of

more than one motor are used, when dc motors are applied

in applications that handle larger loads. The maximum arma-

ture current and its rate of change are also limited by the

commutator.

33.2.2 DC Motor Representation and

Characteristics

The dc motor has two separate sources of ﬂuxes that interact to

develop torque. These are the ﬁeld and the armature circuits.

Because of the commutator action, the developed torque is

given by

T = Ki

(33.1)

where i

and i

are the ﬁeld and the armature currents, respec-

tively, and K is a constant relating motor dimensions and

parameters of the magnetic circuits.

The dynamic and the steady-state responses of the motor

and load are given by

Dynamic Steady-state

= R

+eV

= R

+E (33.2)

= R

(33.3)

e = Ki

ω E = KI

ω (33.4)

T = J

dω

+Dω + T

T = J ω + T

(33.5)

where J , D, and T

are the moment of inertia, damping fac-

tor, and load torque, respectively, referred to the motor, and

the subscripts a and f refer to the armature and ﬁeld circuits,

respectively. R, L, I, and E refer to resistance, inductance, cur-

rent, and back-emf of the motor in associated circuits referred

by the subscripts.

860 M. F. Rahman et al.

(a)

(b)

FIGURE 33.2 Representation of the separate and series excited motor

circuits: (a) separately excited dc motor circuit and (b) series excited dc

motor circuit.

Small servo-type dc motors normally have (PM)

permanent-magnet excitation for the ﬁeld, whereas larger size

motors tend to have separate ﬁeld-supply, V

, for excitation.

The separately excited dc motors represented in Fig. 33.2a,

have ﬁxed ﬁeld excitation, and these motors are very easy to

control via the armature current that is supplied from a power

electronic converter. Thyristor ac–dc converters with phase

angle control are popular for the larger size motors, whereas

the duty-cycle controlled pulse-width modulated switching

dc–dc converters are popular for servo motor drives. The

series-excited dc motor has its ﬁeld circuit in series with the

armature circuit as shown in Fig. 33.2b. Such a connection

gives high torque at low speed and low torque at high speed,

a pseudo constant-power-like characteristic that may match

traction-type loads well.

Torque–speed characteristics of the separately and series

excited dc motors are indicated in Figs. 33.3a and b, respec-

tively. The speed of the separately excited dc motor drops with

load, the net drop being about 5–10% of the base speed at

full load. The voltage drop across the armature resistance and

the armature reaction are responsible for this. Operation of

the motor above the base speed at which the armature volt-

age reaches for the rated ﬁeld excitation is by means of ﬁeld

weakening, whereby the ﬁeld current is reduced in order to

increase speed beyond the base speed. The armature voltage

is now maintained at the rated value actively, by overriding

the ﬁeld control if required. Note that the range of ﬁeld con-

trol is limited because of the magnetic nonlinearity of the ﬁeld

circuit and the problem of good commutation at weak ﬁeld.

(a)

(b)

, Nm

−T, Nm

−T

−V

, rad/sec

−w

, rad/sec

−w

increases

−V

increases

FIGURE 33.3 Torque–speed characteristics of the (a) separately and

(b) series excited motors.

Usually, the top speed is limited to about three times the base

speed. Note also that ﬁeld weakening results in reduced torque

production per ampere of armature current. Depending on

the type of load, the armature current, and speed change as

dictated by Eqs. (33.1)–(33.5).

For the separately excited dc motor, assuming that the ﬁeld

excitation is held constant, the transfer characteristic between

the shaft speed and the applied voltage to the armature can

be expressed as indicated in the block diagram of Fig. 33.4.

If we ignore the load torque T

, the transfer characteristic is

1 +sT

1/R

s +D

(s)

T(s)

(s)

−

(s)

FIGURE 33.4 Transfer characteristic block diagram of a separately

excited motor.

33 Motor Drives 861

given by

(

)(

Js +D

)

(33.6)

the characteristic roots of which are given by



s +



s +



= 0 (33.7)

If we compare this with a standard second-order system, the

undamped natural frequency and damping factor are given by







(33.8)

and

σ = ξω





(33.9)

where, T

= mechanical time constant =

in which

= Ki

= K

in SI units, and T

= electrical time

constant =

The speed response of the motor around an operating speed

to the application of load torque on the shaft is given by

ω

T

1 +sT

(

1 +sT

)(

Js +D

)

(33.10)

33.2.3 Converters for DC Drives

Depending on the application requirements, the power con-

verter for a dc motor may be chosen from a number of

topologies. For example, a half-controlled thyristor converter

or a single-quadrant pulse-width modulated (PWM) switch-

ing converter may be adequate for a drive that does not

require controlled deceleration with regenerative braking. On

the other hand, a full four-quadrant thyristor or transistor con-

verter for the armature circuit and a two-quadrant converter

for the ﬁeld circuit may be required for a high-performance

drive with a wide speed range.

The frequency at which the power converter is switched, e.g.

100 Hz for a single-phase thyristor bridge converter sup-

plied from a 50 Hz ac source (or 300 Hz for a three-phase

thyristor bridge converter), 20 kHz for a PWM MOSFET

H-bridge converter, and so on, has a profound effect on the

dynamics achievable with a motor drive. Low-power switching

devices tend to have faster switching capability than high-

power devices. This is convenient for low-power motors since

these are normally required to be operated with high dynamic

response and accuracy.

33.2.3.1 Thyristor Converter Drive

Consider the dc drive of Fig. 33.5 for which the armature

supply voltage v

to the motor is given by

max

cos α (33.11)

where V

max

is the peak value of the line-line ac supply voltage

to the converter and α is the ﬁring angle. The dc output volt-

age v

is controllable via the ﬁring angle α, which in turn is

controlled by the control voltage e

as the input to the ﬁring

control circuit (FCC). The FCC is synchronized with the mains

ac supply and drives individual thyristors in the ac–dc con-

verter according to the desired ﬁring angle. Depending on the

load and the speed of operation, the conduction of the current

may become discontinuous as indicated in Fig. 33.6a. When

this happens, the converter output voltage does not change

with the control voltage as proportionately as with the con-

tinuous conduction. The motor speed now drops much more

with the load as indicated by Fig. 33.6b. The consequent loss of

gain of the converter may have to be avoided or compensated

if good control over speed is desired.

Power Supply

max

sin wt

FIGURE 33.5 A two-quadrant single-phase thyristor bridge converter

drive.

The output voltage of the simple, two-pulse ac–dc converter

of Fig. 33.5 is rich in ripples of frequency nf, where n is an

even integer starting with two and f is the frequency of the ac

supply. Such low-frequency ripple may derate the motor con-

siderably. Converters with higher pulse number, such as the

6- or 12-pulse converters deliver much smoother output volt-

age and may be desirable for more demanding applications.

A high-performance dc drive for a rolling mill drive may

consist of such converter circuits connected for bidirectional

operation of the drive, as indicated in Fig. 33.7. The interfacing

of the FCC to other motion-control loops, such as speed and

position controllers, for the desired motion is also indicated.

Two fully controlled bridge ac–dc converter circuits are used

back-to-back from the same ac supply. One is for forward and

the other is for reverse driving of the motor. Since each is a

two-quadrant converter, either may be used for regenerative

braking of the motor. For this mode of operation, the braking

converter, which operates in the inversion mode, sinks the

862 M. F. Rahman et al.

(a) (b)

Discontinuous

Conduction

<90°

>90°

FIGURE 33.6 Converter output voltage and motor torque–speed characteristic with discontinuous conduction.

DCM

−−−

ref

AC MAINS

ref

FIGURE 33.7 Bidirectional speed and position control system with a back-to-back (dual) thyristor converter.

motor current aided by the back-emf of the motor. The energy

of the overhauling motor now returns to the ac source.

It may be noted that the braking converter may be used

to maintain the braking current at the maximum allowable

level right down to zero speed. A complete acceleration–

deceleration cycle of such a drive is indicated in Fig. 33.8.

During braking, the ﬁring angle is maintained at an appro-

priate value at all times so that the controlled and predictable

deceleration takes place at all times.

The innermost control loop indicated in Fig. 33.7 is for

torque, which translates to an armature current loop for a dc

drive. Speed and position control loops are usually designed

as hierarchical control loops. Operation of each loop is sufﬁ-

ciently decoupled from each other so that each stage can be

designed in isolation and operated with its special limiting

features.

33.2.3.2 PWM Switching Converter Drive

Pulse-width modulated switching converters have traditionally

been referred to as choppers in many traction, and forklift-type

drives. These are essentially PWM dc–dc converters operating

from rectiﬁed dc or battery mains. These converters can also

operate in one, two, or four quadrants, offering a few choices to

meet application requirements. Servo drive systems normally

use the full four-quadrant converter of Fig. 33.9 which allows

the bidirectional drive and regenerative braking capabilities.

For forward driving, the transistors T

, and diode D

are used as a buck converter which supplies a variable voltage,

, to the armature given by

= δV

(33.12)

where V

is the dc supply voltage to the converter, and δ is

the duty cycle of the transistor T

33 Motor Drives 863

DCM DCM DCM

−

−−−−

Forward

Running

Forward

Braking

Reverse

Acceleration

Reverse

Running

Reverse

Braking

Forward

Acceleration

Forward

Running

FWD FWD

FWD

FWD FWD

REV

REV REV

REV

REV REV

FIGURE 33.8 Converter conduction and operating duty of a bidirectional drive.

DCM

+ +

−

ref

AC MAINS

ref

−V

FIGURE 33.9 Bidirectional speed and position control system with a PWM transistor bridge drive.

864 M. F. Rahman et al.

The duty cycle δ is deﬁned as the duration of the ON-time

of the modulating (switching transistor) as a fraction of the

switching period. The switching frequency is normally dictated

by the application and the type of switching devices selected

for the application.

During regenerative braking in the forward direction, tran-

sistors T

and diode D

are used as a boost converter that

regulates the braking current through the motor by auto-

matically adjusting the duty cycle of T

. The energy of the

overhauling motor now returns to the dc supply through

the diode D

, aided by the motor back-emf and the dc sup-

ply. Again, note that the braking converter, comprising T

and D

, may be used to maintain the regenerative braking

current at the maximum allowable level right down to zero

speed.

Figure 33.10 shows a typical acceleration–deceleration cycle

of such a drive under the action of the control loops indicated

in Fig. 33.9.

Four-quadrant PWM converter drives such as that in

Fig. 33.10 are widely used for motion- control equipment

in the automation industry. Because of the signiﬁcant devel-

opment of power switching devices, switching frequencies of

10–20 kHz are easily attainable. At such frequencies, virtually

no derating of the motor is necessary.

In order to satisfy the requirements of a drive application,

simpler versions of the drive circuits indicated in Figs. 33.7

and 33.9 may be used. For instance, for a unidirectional drive,

half of the converter circuits indicated earlier may be used.

DCM DCM DCM DCM DCM DCM

Forward

Running

Forward

Braking

Reverse

Acceleration

Reverse

Running

Reverse

Braking

Forward

Acceleration

Forward

Running

−−

−−−

−

FIGURE 33.10 Converter conduction and operating duty with a PWM bridge transistor drive.

Further simpliﬁcation of the drive circuit is possible, if

regenerative braking is not required.

33.2.4 Drive System Integration

A complete drive system has a torque controller (armature cur-

rent controller for a dc drive) as its inner most loop, followed

by a speed controller as indicated in Fig. 33.11.

The inner current loop is often regulated with a propor-

tional plus integral (PI) type controller of high gain. The rest

of the inner loop consists of the converter, the motor arma-

ture, which is essentially an R–L circuit with the armature

back-emf as disturbance, and the current sensor. The current

sensor is typically an isolated circuit, such as a Hall sensor or

a direct current transformer (DCT). A well-designed torque

(armature current) loop behaves essentially as a ﬁrst-order

lag system. Together with the mechanical inertia load, this

loop can be indicated as the middle Bode plot of Fig. 33.12,

in which 1/T

represents the current-controlled system band-

width. (Note that the damping factor D and the load torque

indicated in Fig. 33.11 have been neglected in this descrip-

tion for simplicity.) The current loop is normally designed by

analyzing the block diagram of Fig. 33.11, the converter and

the PI controller for the current loop using Bode analysis or

other control-system design tools.

The next step is usually the design of the speed controller.

The 0-db intercept of 1/Js

(

1 +T

)

is normally too low. Again,

if a PI controller is selected for the speed loop, its Bode plot

33 Motor Drives 865

+sL

Js + D

ref

CONVERTER

ref

−−

FIGURE 33.11 Structure of a closed-loop speed-control system with a dc drive.

log ω

Current

Controller

Bandwidth

Speed

Controller

Bandwidth

K(1+T

s(1+T

)

Js(1+T

1.0

10.0

100.0

FIGURE 33.12 Typical current and speed control-loop designs for the

system of Fig. 33.11.

is superimposed on the current-controlled system as indicated

in Fig. 33.12 to obtain the desired speed-control bandwidth.

33.2.5 Converter-DC Drive System

Considerations

Several operational factors need to be considered in applying

a dc drive. Some of the important ones are as follows:

(1) The armature current may be rich in harmonics.

This is particularly true for thyristor converters. The

feedback of these current ripples into the FCC may

cause overloading of individual switches and trip-

ping. Adequate ﬁltering is necessary to avoid such

problems.

(2) Since the converter is switched at regular intervals

while the current controller operates continuously,

current overshoot may occur because of the delay in

the FCC. The current controller gains must be limited

to limit this overshoot.

(3) The switching frequency of the converter should be

selected according to the desired motor-current rip-

ple, supply input current harmonics, and the dynamic

performance of the drive.

(4) Ripple in the speed-sensor output limits the perfor-

mance of the speed controller. Analog tachogenerator

output is particularly noisy and deﬁnes the upper

limit of the speed-control bandwidth. Digital speed

sensors, such as encoders and resolvers, alleviate this

limit signiﬁcantly.

33.3 Induction Motor Drives

33.3.1 Introduction

The ac induction motor is by far the most widely used motor

in the industry. Traditionally, it has been used in constant

and variable-speed drive applications that do not cater for fast

dynamic processes. Because of the recent development of sev-

eral new control technologies, such as vector and direct torque

controls, this situation is changing rapidly. The underlying rea-

son for this is the fact that the cage induction motor is much

cheaper and more rugged than its competitor, the dc motor, in

such applications. This section starts with the induction motor

drives that are based on the steady-state equivalent circuit of

the motor, followed by vector-controlled drives that are based

on its dynamic model.

33.3.2 Steady-state Representation

The traditional methods of variable-speed drives are based on

the equivalent circuit representation of the motor shown in

Fig. 33.13.