1

Chapter - 1

Introduction

1.1 Variable Speed Drive:

Rotational industrial loads require operation at any one of a wide

range of operating speeds. Such loads are generally termed as variable

speed drives or adjustable speed drives. The variable speed drive systems

are also an integral part of automation. They help to optimize the process

to reduce the investment costs, energy consumption, and energy cost.

The system efficiency can be increased by the introduction of variable

speed drive operation in place of constant speed operation [1, 2].

There are three basic types of variable speed drive systems: electrical

drives, hydraulic drives and finally mechanical drives. In this thesis, only

electrical drives are focused. Drives employing electric motors are known

as electrical drives. Block diagram of an electric variable speed drive

system is shown in Fig 1.1. It consists of three basic components: the

electric motor, the power electronic converter and the control system.

The electric motor is connected directly or indirectly (through gears) to

the load. The power electronic converter controls the power flow from

power supply to the motor by appropriate control of power

2

semiconductor switches. The recent advances in power semiconductor

devices and converter topologies, electric variable speed drives are

witness a revolution in a wide variety of applications such as machine

tools and robotics drives, fans, pumps, compressors, paper mill, steel

industries, automation, traction applications, ship propulsion and

cement mills.

Fig.1.1. Block diagram of an electric variable speed drive system

1.2 Classification of Variable Speed Drives:

According to the type of electric motor, the electric variable speed

drives can be classified into two categories.

1. DC motor drives

2. AC motor drives

Power Supply

Power Electronic

Converter Gear System

Drive Control Unit

Load

Supervisory Control System

Electric

Motor

3

1.2.1 DC Motor Drives:

Traditionally, separately excited DC machines are the obvious choice

for applications in variable speed drives, where good dynamic response

and steady state performance are required. The control of a separately

excited dc motor is very straightforward, because of the commutator

within the motor. The commutator and brush allow the developed torque

of the motor to be proportional to the armature current if the field

current is held constant. The dc machines also have the excellent

dynamic performance over a wide range of operating conditions due to

inherent decoupling between field flux and armature current.

Applications are used in steel industries, robotic drives, printers,

machine tools, textile and paper industries, etc. On the other hand, dc

machines are inherently bulky, require frequent maintenance, have low

torque-to-weight ratio, in addition to having commutation problems.

Moreover, the mechanical commutator limits the maximum applicable

voltage to about 1500V and the maximum power capacity to a few

hundred kilowatts. The commutator also limits the maximum armature

current and its rate of change.

1.2.2 AC Motor Drives:

AC motors exhibit highly coupled, nonlinear and multi variable

structures as opposed to much simpler decoupled structures of

separately excited DC motors. The AC motors have a number of

advantages: light weight, inexpensive and have low maintenance

4

compared with DC motors. They require control of frequency, voltage and

current for variable speed applications. However, the advantages of AC

drives outweigh the disadvantages. AC drives replace the DC drives in

many domestic and industrial applications. The AC motor drives can be

classified into two categories.

1. Induction motor drives

2. Synchronous motor drives

1.2.2.1 Induction Motor Drives:

The three phase induction motors drives can be classified into two

types, namely

• Squirrel-cage induction motor drives

• Slip-ring induction motor drives

Both the motors are electrically equivalent as long as attention is

confined to the fundamental sine-waves of voltage, current, flux, etc

except the former has rotor-winding terminals permanently shorted

inside the motor. In case of slip-ring induction motor, the terminals of

the rotor three-phase winding are externally available to the user.

1.2.2.1.1 Squirrel-cage Induction Motor Drives:

The Nikola Tesla exhibited a crude type of three-phase induction

motor at the Frankfort exhibition of 1891. An improved construction,

with a distributed stator winding and a cage rotor, was built by Dolivo

Dobrowolsky in conjunction with the Maschinenfabrik Oerlikon and

described in 1893. This motor is the most widely used motor in the

5

industry. Traditionally, it has been used in constant and variable speed

drive applications that do not cater for fast dynamic processes. Because

of recent development of several new control technologies, such as vector

control, sensorless control and direct torque control (DTC), the situation

is changing rapidly. Squirrel-cage induction motors are much cheaper

and more rugged than the dc motor. They require little maintenance.

They can be designed as totally enclosed motors to operate in dirty and

explosive environments. All these features make them attractive for use

in industrial drives. The some of speed control methods are listed below,

which are widely used.

• Scalar control

• Vector control or Field Oriented Control (FOC)

• Sensorless control

• Direct Torque Control (DTC)

1.2.2.1.2 Slip-ring Induction Motor Drives:

The slip-ring induction motors with three rotor slip rings have been

used in adjustable speed drives for many years. In early slip-ring

induction motor drives, adjustable speed is achieved by dissipating the

energy in external resistances, connected to the slip-ring terminals of the

rotor. Modern slip-ring induction motor drives use an inverter to recover

the power from the rotor circuit, feeding it back to the supply system. So,

the speed control methods employed for slip-ring induction motor drives

are

6

• Rotor resistance control

• Slip power recovery schemes (Static Kramer drive & Static

Scherbius drive)

Generally, slip-ring induction motors are used for high power

applications where a small speed range is required.

1.2.2.2 Synchronous Motor Drives:

The speed of synchronous motors with constant rotor excitation is

determined by the stator supply frequency and the number of poles. So,

a variable frequency static inverter can extend its operation as a variable

speed drive. The main applications are gearless rolling mills, mine hoists,

traction, etc.

In this thesis, main attention is given to squirrel-cage induction motor

drives only.

1.3 Control Strategies for Squirrel-cage Induction Motor:

Induction motors are known as workhorses of industry. These are

most widely used motors due to their lower cost, rugged construction

and high power to volume/weight ratio. When operated directly from the

ac line voltage, induction motor operates nearly at constant speed.

However by means of power electronic converters, it is possible to change

the speed of an induction motor. Even though the induction motors are

desirable, their speed control is not as straight forward as that of a dc

motor. Therefore, it was natural for the researchers to think of ways,

which would take the induction motor control closer to that of a dc

7

motor. The various speed control methods, which are used to control the

speed of induction motors discussed in this section.

1.3.1 Volts/Hz

fv Control of Induction Motor:

The volts/Hz control of induction motor is by far the most popular

method of speed control because of its simplicity, and these types of

motors are widely used in industry [2, 3]. In this control, for adjustable

speed applications, supply frequency is varied. However, voltage is

required to be proportional to frequency so that the flux remains

constant, neglecting the stator resistance drop. Hence, in this method,

fv is held constant. In steady state operation, the machine air gap flux

is approximately related tof

v . As the frequency nearly approaches zero,

near zero speed, the magnitude of the stator voltage also tends to zero

and this low voltage is absorbed by the stator resistance. Therefore, at

low speed by injecting the boost voltage, the stator resistance drop is

compensated, so that rated air gap flux and hence the full load torque is

available up to zero speed. At steady state operation, if load torque is

increased, the slip increases within stability limit and a balance will be

maintained between the developed torque and the load torque.

Problems with Volts/Hz Control:

• If the supply voltage to the inverter fluctuates, the air gap flux will

vary.

8

• Also, increase in stator resistance with temperature results in

variation in air gap flux. Hence, in constant f

v control scheme

the air gap flux may drift as a result, torque sensitivity with slip

frequency or stator current will vary. If correct f

v ratio is not

maintained, the flux may be weak or may saturate.

• Torque pulsations are present at low speeds owing to presence of

fifth, seventh and eleventh and higher harmonics.

• Because of the presence of low frequency harmonics, the motor

losses are increased at all speeds causing the derating of the

motor.

These drawbacks can be overcome with the help of vector control

technique where an induction motor is controlled on the same principles

as a separately excited dc motor in which torque component and the flux

component are decoupled.

1.3.2 Vector Control of Induction Motor:

In 1971, F. Blaschke proposed a scheme, which aims at the control of

induction motor like a separately excited dc motor, called Field Oriented

Control (FOC) or vector control. As in the dc machines, torque control in

ac machines is achieved by controlling the motor currents. However, in

contrast to a dc machine, current phasor has to be controlled. This is the

reason for the terminology ‘vector control’. In the vector control, the

induction motor is analyzed in a synchronously rotating reference frame

9

where the sinusoidal variables appear as dc quantities. The torque and

the flux components are identified and controlled independently to

achieve a good dynamic response. Vector controlled techniques

incorporating fast microprocessors and DSPs have made possible the

application of induction motors for high performance applications where

traditionally only dc drives were applied. However, it should be noted

that:

• In dc machines, the armature current and main flux distribution

are fixed in space and where the torque can be established by

independently controlling the excitation flux and armature current.

• Where as in ac machine, it is much more difficult to realize this

principle because these quantities are coupled and are stationary

with respect to the stator and rotor. They also depend on modulus,

frequency and phase angles of stator current.

There are essentially two general methods of vector control [2].

They are:

1. Direct vector control or Feedback method, developed by F.

Blaschke

2. Indirect vector control or Feedforward method, developed by

K. Hasse.

These two methods are differently essential by how the unit vector is

generated for the control.

10

1.3.2.1 Direct Vector Control of Induction Motor:

The direct vector control depends on the generation of unit vector

signals from the stator or rotor flux signals. The air-gap flux signals can

be measured directly or estimated from the stator voltage or current

signals. In these systems, rotor speed is not required for obtaining rotor

field angle information. Here, the actual motor currents are converted to

synchronously rotating frame currents using park transformation. The

resulting dc quantities are compared with the reference d-axis and q-axis

components. The outputs of the controllers are used to generate the

pulsewidth modulated signals for switching the devices in the inverter

bridge feeding the motor. The main disadvantages of this method are:

• Direct method of rotor flux estimation depends on the machine

parameter; the rotor resistance variation, especially, becomes

dominant due to the temperature variation and skin effect.

• The direct method of vector control can be applied typically above

10% of the base speed because of difficulty in accurate flux signal

synthesis at low speeds.

Hence, due to these disadvantages, normally indirect method of vector

control is preferred.

1.3.2.2 Indirect Vector Control of Induction Motor:

In this method, the unit vector signal that transforms the

synchronously rotating stator voltages into stationary frame signals has

been generated from the speed signal and slip signal. The drive can easily

11

be operated from zero speed to constant power field-weakening region. It

is the most popular vector control method in industry. The main

disadvantages of this method are:

• The machine parameter variation affects the slip gain, and

correspondingly, both static and dynamic performances of the

drive are affected.

• The on-line tuning for parameter variation is more difficult.

The control methods discussed so far require a speed sensor for

closed operation. The speed sensor has several disadvantages from

standpoint of drive cost, reliability and noise immunity. The torque is

controlled indirectly.

1.3.3 Sensorless Vector Control of Induction Motor:

Sensorless vector control of an induction motor drive essentially

means vector control without any speed sensor [2, 4]. Here the

terminology ‘sensorless’ refers to only the speed and shaft sensors. It is

possible to estimate the speed signal from machine terminal voltages and

currents. The speed estimation methods can generally be classified as

follows:

• Slip calculation

• Direct synthesis from state equations

• Model referencing adaptive system (MRAS)

• Speed adaptive flux observer

• Extended kalman filter (EKF)

12

• Slot harmonics

Many of the sensorless techniques depend on the machine

parameters, temperature, saturation levels, etc.

1.3.4 Direct Torque Control of Induction Motor:

In the mid 1980s the Direct Torque Control (DTC) principle was

developed by Takahashi and Noguchi for low and medium power

applications and Direct Self Control (DSC) principle was established by

Depenbrock for high power applications. As the name suggests, DTC or

DSC regulates the motor torque and flux directly. In the DTC approach,

the reference torque and reference flux are compared to the estimated

motor torque and the estimated stator flux respectively, both employing

hysteresis controllers. The torque and flux hysteresis controller output

logic signals are evaluated in an optimal switching logic table to generate

the inverter switching device gate signals. The generation of inverter

switching state is made to restrict the stator flux and electromagnetic

torque errors within the respective flux and torque hysteresis bands and

to obtain the fastest torque response and highest efficiency at every

instant. The DTC scheme is found to be very promising and valuable as

compared to FOC. But, DTC has few drawbacks such as more steady

state ripple in flux, torque and current and variable switching frequency

due to hysteresis bands.

13

1.4 Literature Review and State of the Art Assessment:

1.4.1 Introduction:

The one and a half century of progress in the electric machines field,

about three quarters of a century of progress in the power electronics

field, and about half a century of progress in the micro-

electronics/macro-electronics and control fields are inherited in the state

of the art pulsewidth modulated voltage source inverter (PWM-VSI)

drives. Since they involve various disciplines of engineering and there

has always been a strong demand for them in the market, PWM-VSI

drives have continuously drawn the attention of many researchers all

around the world. Among the various PWM-VSI drives, the induction

motor drives with cage type machines have found wide range of

applications and have become the workhorse of industry due to their

simplicity and ruggedness. These motors can be fed from current source

inverters (CSI) or voltage source inverters (VSI), and used as variable

speed drives. Recent advances in semiconductor technology have led to

new generations of fast-acting power semiconductor switches like GTOs,

MOSFETs, IGBTs, and more recently IGCTs. The performance and

characteristics of these switches strongly favor the VSI topology over the

CSI one. This has been a major reason for VSI fed induction motor drives

becoming more popular than CSI fed induction motor drives. Pulsewidth

Modulation (PWM) strategies are required for switching the devices in a

14

VSI appropriately to generate variable voltage, variable frequency, 3-

phase AC required for the variable speed induction motor drive.

Following a brief review of the various control techniques for induction

motor drives and state of the art DTC of induction motor drives will be

described and the fundamental contributions to the area will be

discussed in detail.

1.4.2 Control Techniques for Induction Motor Drives:

The various speed control techniques for three-phase squirrel cage

induction motors are

• Constant Volts per Hertz Control

• Field Oriented Control (FOC) or Vector Control

• Sensorless Vector control

• Direct Torque Control (DTC)

1.4.2.1 Constant Volts per Hertz Control:

The block diagram of volts per hertz control of induction motor is

given in Fig. 1.2. In this method, the inverter output voltage is varied

proportionally to the reference frequency such that constant stator flux is

maintained. In an induction motor drive, this operating mode results in

shunt speed-torque characteristics (linear portion of the torque-speed

curve), yielding low slip frequency and therefore high energy efficiency

and good speed regulation. Therefore, the method gained wide

acceptance in many industrial and residential induction motor drive

15

speed regulation applications as given by B.K. Bose [2], R. Krishnan [3],

D.A. Bradley et al [6] and B. Mokrytzki [7-8].

Fig. 1.2 Open-loop volts per hertz speed control of induction motor

The performance of the fv control is not satisfactory, because the

rate of change of voltage and frequency has to be low. A sudden

acceleration or deceleration of the voltage and frequency can cause a

transient change in the current, which can result in drastic problems.

Moreover, they exhibit limited speed response, poor load torque

disturbance characteristic, and inferior low speed characteristics. Some

efforts were made to improve fv control performance, but none of these

improvements could yield a fv torque controlled drive systems and this

made DC motors a prominent choice for variable speed applications. In

IM

Diode Bridge Rectifier

L

C

Sine Triangle PWM G

+ +

Vo

V*

ω*

AC supply

16

printing press applications, packaging applications, servo applications

with very high resolution position control etc, where precise control is

mandatory, the performance of fv drives is not satisfactory. In such

type of applications, traditionally DC motor drives have been employed

with a shaft encoder. Typical application areas of fv drives are pumps,

ventilation systems, etc. which have passive torque-speed characteristics

and no precise speed regulation requirement.

1.4.2.2 Vector Control:

In the volts per hertz control, the voltage and frequency are the basic

control variables of the induction motor. In a voltage fed drive, both the

torque and air gap flux are functions of voltage and frequency. This

coupling effect is responsible for the sluggish response of the induction

motor and moreover, the system is easily prone to the instability.

However, the continuous progress in induction motor control theory,

power electronics, and digital signal processors yielded the modern

vector controlled induction motor drives [9-15] which can match the

performance and reliability characteristics of dc drives and cost less. The

invention of vector control, which is also known as decoupling,

orthogonal, transvector or field oriented control (FOC) in the beginning of

1970s, and the demonstration that an induction motor can be controlled

like a separately excited dc motor, brought a renaissance in the high-

performance speed control of induction motor drives. Modern vector

controlled squirrel cage induction motor drives meet the demanding

17

performance criteria of most high performance speed control

applications. In the vector control method, an ac machine is controlled

like a separately excited dc machine. This analogy is explained by B.K.

Bose [2] as in Fig. 1.3.

Fig. 1.3 (a) Separately excited dc motor (b) vector controlled induction motor.

In a dc machine, neglecting the armature demagnetization effect and

field saturation, the torque is given by

fate IIkT '= (1.1)

Where aI is the armature or torque component of current and fI is the

field or flux component of current. In a dc machine, the control variables

aI and fI can be considered as orthogonal or decoupled “vectors”. In

normal operation, the field current is set to maintain the rated field flux

aI fI

(a)

IM

Vector

Control

Inverter

*qsi

*dsi

(b)

18

and torque is changed by changing the armature current. Since the

current fI or the corresponding field flux is decoupled from the

armature current, the torque sensitivity remains maximum in both

transient and steady state operations. This mode of control can be

extended to an induction motor also if the machine operation is

considered in a synchronously rotating reference frame where the

sinusoidal variables appear as dc quantities. In Fig. 1.3 the induction

motor with inverter and vector control is shown with two control inputs,

*dsi and *

qsi . The currents *dsi and *

qsi are the direct-axis component and

quadrature-axis component, respectively, of the stator current, where

both are in a synchronously rotating reference frame. In vector control

*dsi is analogous to the field current fI and *

qsi is analogous to the

armature current aI of a dc machine. Therefore, the torque can be

expressed as

*** 'ˆ dsqstqsmte iikikT == ψ (1.2)

Thus the similarity between the production of the electromagnetic

torque in a compensated dc machine and in symmetrical, smooth air gap

induction machine has been established. However, it should be noted

that:

• In dc machines, the armature current and main flux distribution

are fixed in space and where the torque can be established by

independently controlling the excitation flux and armature current.

19

• Where as in an induction machine, it is much more difficult to

realize this principle because these quantities are coupled and are

stationary with respect to the stator and rotor. They also depend

on modulus, frequency and phase angles of stator current.

The search for simple control schemes, similar to those used for dc

machines has led to the development of “vector controlled schemes”.

There are essentially two general methods of vector control. They are:

• Direct vector control or Direct Field Oriented Control (DFOC)

• Indirect vector control or Indirect Field Oriented Control (IFOC)

These methods are differentiated on how the unit vector signals are

generated from stator, rotor or air-gap flux signals. The DFOC method

was presented by F. Blaschke [9] and it employs flux sensors. The IFOC

method was presented by K. Hasse [10] and it employs a shaft encoder to

close the speed loop.

In FOC, the magnetizing flux and torque producing components of the

stator currents are properly and independently distributed both during

steady state and dynamic conditions. As explained by D.W. Novotny and

T.A. Lipo [11] and Rik W. De Doncker and D.W. Novotny [12], by

regulating each component independently with a high performance

current controller, the drive torque can be controlled in the same precise

manner as the DC machine. Since installing flux sensors in the stator or

the air gap of a machine is difficult, and the operation is not reliable, the

DFOC method is practically rarely employed in its original form.

20

Employing flux observers, the DFOC method provides high performance

torque control, in particular in the high speed region where the stator

resistance voltage drop is small compared to the stator EMF and the

stator flux observer is highly accurate. The stator flux oriented DFOC

method is attractive for traction, spindle tool etc, applications which

require operation in a wide field weakening region. However, near zero

speed the stator flux observer estimator error becomes substantial due to

the dominance of the stator resistive voltage component over the nearly

zero EMF and the DFOC method looses performance. In a large number

of applications requiring high performance in the low speed operating

region the rotor flux oriented IFOC method is utilized.

With accurate parameter adaptation, the IFOC based induction

machine drives can provide servo performance in a wide speed region.

Since the torque regulation quality of an FOC induction motor drive is

mainly dependent on the current controller accuracy and bandwidth,

high performance motion control requires high performance current

regulators. The hysteresis type current controllers which have superior

dynamic performance have not gained acceptance in motor drives due to

the difficulty in controlling their switching frequency and significant

waveform distortion. Employing high switching frequency IGBT devices

and high performance digital signal processors or microprocessors, high

performance current controlled drives provide high torque/speed

bandwidth, hence high motion quality. High performance FOC drives

21

have been successfully employed in industrial and servo drive

applications which are summarized by T. Kume and T. Iwakane [13]. The

evolution of FOC drives from concept to industrial products and

successful applications has been summarized by W. Leonard in [14-15]

in detail.

1.4.2.3 Sensorless Vector Control:

Sensorless vector control induction motor drive essentially means

vector control without any shaft encoder or speed sensor. An incremental

shaft mounted speed encoder; usually an optical type is required for

closed loop speed or position control in vector controlled drives. A speed

encoder is undesirable in a drive because it adds cost and poses

reliability problems, besides the need for a shaft extension and mounting

arrangement. To reduce total hardware complexity, cost and to increase

mechanical robustness, it is desirable to eliminate speed and position

sensors in vector-controlled drives. Drives operating in hostile

environments or in high speed drives speed sensors cannot be mounted.

To replace the sensor, the information of the rotor speed is extracted

from measured stator voltages and currents at the motor terminals.

Continuing research has concentrated on the elimination of the speed

sensor at the machine shaft without deteriorating the dynamic

performance of drive control system. Speed estimation is an issue of

particular interest with induction motor drives where the mechanical

speed of the rotor is generally different from the speed of the revolving

22

magnetic field. The advantage of speed sensorless induction motor drives

are reduced hardware complexity, lower cost, reduced size of the drive

machine, elimination of the sensor cable, better noise immunity,

increased reliability and less maintenance requirements.

The pioneering work in the shaft encoderless motor speed control area

was reported by R. Jötten and G. Maeder in 1983 [16]. They employed

the induction motor fundamental model to estimate the slip frequency

and the back emf of the machine and provided a closed loop controller to

regulate the slip such that superior dynamic performance could be

obtained in a wide speed region, including the field weakening region.

Although a large variety of shaft encoderless control methods have been

reported from that time to the present date, only a few found practical

applications, which are given by B.K. Bose [2], Peter Vas [4], Tsugutoshi

Othani et al [17], C. Ilas et al [18] and J. Holtz [19-20].

1.4.2.4 State of the Art DTC of Induction Motor Drives:

In addition to vector control systems, instantaneous torque control

yielding fast torque response can also be obtained by employing Direct

Torque Control (DTC) [21, 23] or Direct Self Control (DSC) [22, 24]. As

the name suggests, the DTC method regulates the motor torque and flux

directly. In the mid 1980s the DTC principle was developed and

discussed by Isao Takahashi and T.Noguchi [21] for low and medium

power applications and DSC principle was established by M. Depenbrock

[22] for high power applications. In this thesis, the attention is mainly

23

focused on the DTC scheme. In the DTC approach, the reference torque

is compared to the estimated motor torque and the reference stator flux

is compared to the estimated stator flux, both employing hysteresis

controllers. The torque and flux hysteresis controller output logic signals

are evaluated in an optimal switching logic table to generate the inverter

switching device gate signals. The generation of inverter switching state

is made to restrict the stator flux linkage and electromagnetic torque

errors within the respective flux and torque hysteresis bands and to

obtain the fastest torque response and the highest efficiency at every

instant. The DTC scheme is found to be very promising and valuable as

compared to FOC. Moreover, using the DTC it is possible to obtain a good

dynamic control of the torque without any speed sensors or position

sensors on the machine shaft. Thus, DTC can be considered as

sensorless type control techniques.

In DTC, the stator flux can be calculated from the motor terminal

voltages and stator resistance. Variations in the stator resistance result

in significant errors in the stator flux, especially at low speeds. This

problem can be overcome by using the slip relation from indirect rotor

flux field orientation to locate the position of the rotor flux. The rotor flux

position is then used to locate the position of the stator flux. Also, the

motor speed and rotor resistance are used to calculate the position of the

stator flux at low speeds. With this, the advantages of DTC scheme are

maintained over the entire speed range as explained by Thomas G.

24

Habetler et al [25]. Moreover, the robust start and improved operation in

the zero speed region can be achieved easily by introducing the

additional carrier signal to the torque comparator input as given by

Kazmierkowski and Kasprowicz [26].

Thus, unlike FOC, DTC operates with closed torque and flux loops but

without current controllers. In spite of its simplicity, DTC allows a good

torque control in steady state and transient operating conditions to be

obtained. Moreover, DTC has simple and robust control structure and is

not sensitive to rotor parameters. A review of recently used DTC

algorithms for VSI fed induction motor drives has been presented and

discussed by Giuseppe S. Buja and Marian P. Kazmierkowski [27]. A

detailed comparison between FOC and DTC, emphasizing advantages

and disadvantages are provided by Domenico Casadei et al [28] and

concluded that DTC might be preferred for high dynamic applications.

Hence, the DTC scheme was introduced in commercial products by Asea

Brown Boveri (ABB) and therefore created wide interest. This is very

significant industrial contribution and it has been stated by ABB that

DTC is the latest ac motor control method and it can be considered to be

next generation motor control technologies. Therefore, DTC has gaining

more industrial applications such as high performance applications,

electric vehicle applications, etc as explained by Peter Vas [4], James N.

Nash [29], Pekka Tiitinen and Surandra [30] and Jawad Fiaz et al [32].

25

Though DTC has high dynamic performance, it has few drawbacks

that can be summarized as high current, torque and flux ripple, variable

switching frequency due to hysteresis controllers and high noise level at

low speeds, etc. The effect of torque and flux hysteresis band amplitudes

on the performance of induction motor drive has been studied by D.

Casadei et al [33] and Jun-Koo Kong et al [34-35]. The amplitude of the

flux hysteresis band mainly affects the motor current distortion in terms

of low order harmonics. Small flux hysteresis bands lead to sinusoidal

current waveforms, while small torque hysteresis bands allow smoothed

torque to be generated. On the other hand, small hysteresis bands

usually determine high switching frequency thereby increasing the

switching losses. Moreover, the switching frequency of the torque

controller has a peak value at medium speed due to the effect of back

emf, while that of the flux controller is proportional to operating speed

[35]. The analytical determination of the relationships between the

applied voltage vector and the corresponding torque and flux variations

is given by D. Casadei et al [36], from which, it has been observed that

the effects produced by a voltage vector are strongly dependent on both

rotor speed and voltage vector direction relative to the rotor flux. The

maximum torque variation is obtained by applying a voltage vector along

the direction perpendicular to the rotor flux vector. Thus, the presence of

torque and flux hysteresis bands in DTC causes ripples in stator current,

stator flux and torque that results in more harmonics in the line current.

26

To increase the dynamic performance of DTC and to decrease the

ripple in torque, various switching control strategies had been proposed

in the literature. The effect of the applied voltage on the torque response

is strongly dependent on rotor angular speed. To tackle the problem of

stator flux drooping at low speeds, to reduce the harmonic contents in

the stator current and to reduce the switching frequency, the method of

“variable switching sectors” for DTC has been proposed by CG Mei et al

[37]. To reduce the ripple in torque further, a series of switching control

strategies have been presented by E. Galvan et al [38] and G. Escobar et

al [39]. In conventional DTC (CDTC), which was proposed by Takahashi,

the selected voltage vector is not always the best one since only the

sector is considered where the flux linkage space vector lies without

considering its accurate location. As the CDTC has a fewer number of

selectable voltage vectors, it causes higher ripples in the flux and torque.

To overcome this problem, a unified flux control (UFC) method for DTC

has been developed by Joon Hyoung Ryu et al [40]. In UFC, a voltage

space vector is calculated for a deadbeat action and a minimum-distance

vector selection scheme replaces the switching vector look-up table to

minimize the flux and torque ripples. Moreover, torque ripple can be

reduced, by applying a suitable voltage vector from the switching table

for the time interval needed by the torque to reach the upper or lower

limit of the band, where the time interval is calculated from a suitable

modeling of the torque dynamics as explained by Vanja Ambrožič et al

27

[41]. This method is also known as band-constrained technique in which,

depending on the inverter voltage vector and the operating conditions,

the time interval may extend over several sampling periods. Therefore,

the inverter switching frequency settles automatically to the minimum

value.

Now a days, the intelligent controllers like fuzzy, neuro and neuro-

fuzzy controllers play a major role in industrial applications. To improve

the dynamic performance of torque and flux, stator resistance

estimation, stator flux estimation, tuning procedure, etc, intelligent

control algorithms given by Sayeed A. Mir et al [42], I.G. Bird and H.

Zelaya De La Parra [43], Fatiha Zidani and Rachid [44], Luis Romeral et

al [45] and Pawel Z. Grabowski et al [46] can be implemented to the DTC

algorithm. As there are no sector borders, there is no current and torque

distortion caused by the sector changes. During the low speed region

also, the performance can be improved by using fuzzy logic or neuro-

fuzzy controllers.

A substantial reduction in torque, flux and current ripples could be

obtained using the discrete space vector modulation (DSVM) algorithm

developed by D. Casadei et al [47] and Xin Wei et al [48]. DSVM uses

prefixed time intervals within a cycle period that results more number of

voltage vectors with respect to those used in conventional DTC. The

increased number of voltage vectors allows the definition of more

accurate switching tables in which the selection of voltage vectors is

28

made according to the rotor speed, the flux error and torque error. In

DSVM, one sampling time period is divided into ‘m’ equal time intervals.

One of the VSI voltage vectors is applied in each time interval. The

number of voltage vectors, which can be generated is directly related to

‘m’. However, a good compromise between the errors compensation and

the complexity of the switching tables is achieved by choosing m = 3 [47-

48]. Using DSVM algorithm with three equal time intervals, 36

synthesized non-zero voltage vectors are obtained. If the stator flux

vector is assumed to be in first sector, then 19 voltage vectors can be

used. Then, different voltage vectors are chosen for different speed

ranges. A fuzzy logic controller can be designed to select synthesized

voltage vectors in DSVM based DTC [48]. Thus, DSVM allows the

performance of DTC scheme in terms of flux and torque ripple and

current distortions to be improved without increasing the complexity of

the power circuit and the inverter switching frequency.

To overcome the problem of variable switching frequency, and torque

ripple, few controllers have been proposed in the literature [49-50]. This

can be done in two ways. In first method, the optimal switching instant is

calculated at each switching cycle to satisfy the ripple minimum

condition based on the instantaneous torque slope equations. In second

method the conventional three-level torque hysteresis comparator is

replaced by a new controller, which consists of two triangular waveform

generators, two comparators and a PI controller. To operate the DTC

29

algorithm with constant switching frequency and to reduce the torque

ripple few methods have been proposed in [51].

In recent years, to overcome the problem of ripples and varying

switching frequency a voltage modulation algorithm, which is known as

Space Vector Pulsewidth Modulation (SVPWM) has been used in the

literature [52-68]. The recently reported SVPWM algorithm [58-67] has

become very popular over the last decade. In this method, the reference

is provided as a voltage space vector, which is sampled once in every

subcycle and an average voltage vector equal to the sampled reference

voltage vector is generated by time-averaging of the different voltage

vectors produced by the inverter. The SVPWM is a superior PWM

technique for three phase inverter drives compared to the traditional

regularly sampled triangular comparison technique. Space vector

approach has the advantages of lower current harmonics and a possible

higher modulation index compared with the three phase sinusoidal

modulation method and ease of digital implementation.

A novel scheme was reported by Thomas G. Habetler et al [52] that

calculate the inverter switching pattern directly in order to control the

torque and flux in a dead beat fashion over a constant switching period.

This is accomplished by calculating the voltage space vector required to

control the torque and flux on a cycle-by-cycle basis using the calculated

flux and torque errors sampled from the previous cycle and estimated

value of the back EMF in the machine.

30

To get constant switching frequency and to increase the inverter

switching frequency for the same sampling frequency, the symmetrical

regular sampled SVM technique was used by Yen-Shin Lai and Jian-Ho

Chen [53] for inverter control of the DTC based drive. A new SFVC based

DTC was reported by D. Casadei et al [54] along with a simple closed loop

flux estimator to improve the drive performance in the very low speed

region, including zero speed. Further, a simplified DTC algorithm based

on SVPWM was reported by Lixin Tang et al [55-56], in which instead of

the switching table and hysteresis controllers, a PI controller and

reference flux vector calculator (RFVC) were used to determine reference

stator flux linkage vector. The RFVC generates the reference flux vector

according to the error in the torque, which is based on the current

estimated flux linkage vector. Moreover, a special SVM pattern has been

used to reduce the switching frequency of the inverter. Further, closed

loop digital control for both flux and torque was implemented by Cristian

Lascu et al [57] in a SVPWM based DTC to improve the transient

performance and steady state ripple and to preserve the robustness. A

sensorless hybrid DTC drive based on SVPWM for high volume low cost

applications was reported by Cristian Lascu and Andrzej M.

Trzynadlowski [57]. In this hybrid method, under the transient operating

conditions, the drive is controlled by using the classical bang-bang DTC

and in the steady state, using linear torque and flux controllers, the

control system generates a reference voltage vector for the inverter.

31

Now a days, the attention is paid to determine the switching losses

of the inverter. The dependency of the switching losses of a bridge leg of a

PWM converter system with a pulse rate was explored by Johann W.

Kolar et al [64]. The modern PWM methods can be separated into two

groups. In the continuous PWM (CPWM) methods, the modulation waves

are always within the triangular peak boundaries and within every

carrier cycle triangle and modulation waves intersect, and, therefore, on

and off switching occur. In the discontinuous PWM (DPWM) methods, the

modulation wave of a phase has at least one segment which is clamped

to the positive or negative dc rail for at most a total of 120o, therefore,

within such intervals the corresponding inverter leg discontinues

modulation. Since no modulation implies no switching losses, the

switching loss characteristics of CPWM and DPWM methods are

different. Hence, in recent years the attention is paid on the DPWM

methods. Several DPWM methods have been reported in the literature

[64-68]. A few carrier based DPWM methods are reported by Ahmet

Hava et al [65-67]. Among these, Depenbrock’s DPWM1 and Ogasawara’s

DPWM methods have gained recognition due to their low harmonic

distortion at high voltage utilization and the controllability of the

switching losses. Simple and powerful analytical and graphical carrier

based PWM tools have been presented in [67]. Also, expressions for

harmonic distortion and switching loss factor (SLF) are given. Moreover,

the performance characteristics of various PWM methods have been

32

compared. The switching loss and waveform quality indicate SVPWM at

low modulation and DPWM methods at the high modulation range have

superior performance. Based on this comparison, a high-performance

generalized DPWM (GDPWM) method, with superior high modulation

operating range performance characteristics was presented in [66]. The

GDPWM algorithm is suitable for most high performance PWM-VSI drive

applications. Also, an algorithm combining GDPWM and conventional

SVPWM to maximize the drive performance in the whole modulation

range is developed. Also, the relationship between zero sequence signal

and space vectors and the relationship between the distribution of zero

vectors and different carrier based PWM modulators are systematically

investigated without dependence on the load. Then the expressions for

the continuous modulating signals for the switching devices of the VSI

required to generate unbalanced three phase voltages are given by

Olorunfemi Ojo [68]. Then, for the generation of balanced and

unbalanced phase voltages, a GDPWM scheme, in which the modulation

signals of the switching devices are inherently discontinuous, is reported

by Olorunfemi Ojo [68]. In this method, by varying a parameter various

discontinuous modulating signals can be generated.

So far, a number of PWM techniques have been discussed for VSI fed

induction motor drives. The techniques for the generation of PWM

waveforms can be broadly divided into:

• Offline PWM generation techniques

33

• Online PWM generation techniques

Offline PWM techniques are those where the switching instants of the

inverter are stored in the form of lookup tables, which are previously

calculated and used. The online PWM techniques are more common

where the fundamental cycle is divided into many subcycles in each of

which the volt-second balance is maintained. The online PWM

techniques can be further subdivided into two categories on the basis of

approach, namely the triangle comparison (TC) approach and the space

vector (SV) approach. In the TC approach, three-phase modulating waves

are compared against a common triangular carrier to determine the

switching instants of the three phases. The most common and popular

modulating waves are sinusoidal waves. Any triplen frequency

component can be added as zero sequence components to the 3-phase

sinusoidal waves. The choice of these triplen frequency components is a

degree of freedom in this approach. In the SV approach, the voltage

reference is provided in terms of a revolving space vector. The magnitude

and the frequency of the fundamental component are specified by the

magnitude and frequency respectively of the reference vector. The

reference vector is sampled once in every subcycle. The inverter is

maintained in different states for appropriate durations such that an

average voltage vector equal to the sampled reference vector is generated

over the given subcycle. The inverter states used are the two zero voltage

vectors, and the two active voltage vectors, whose voltage vectors are the

34

closest to the commanded voltage vector. The division of the zero voltage

vector duration between the two zero states is a degree of freedom in the

space vector approach. This division of zero vector time in a subcycle is

equivalent to adding a common-mode component to the 3-phase average

pole voltages. The same PWM waveform can be generated based on both

the approaches as explained by G. Narayanan and V. T. Ranganathan

[63].

The ripple in torque can also be decreased by using the multilevel

inverters. An increase in the number of levels improves the torque quality

reducing the ripple amplitude. Therefore, by using the multilevel

inverters, the torque performance of direct torque control of induction

motor in high power and medium power applications can be improved as

explained by Kyo-Beum Lee et al [69], A. Damiano et al [70], Zhuohui

Tan et al [71] and José Rodríguez [72]. But, in the multi level concept,

though the torque performance is improved, the cost and complexity will

be increased.

Though the look-up table based 3-level inverter fed DTC drives give

good performance when compared with the look-up table based 2-level

inverter fed DTC drives, it gives varying switching frequency operation of

the inverter and gives more harmonic distortion. To obtain constant

switching frequency operation and to achieve superior waveform quality,

various PWM algorithms have been proposed in the literature. Nabae,

et.al. proposed a PWM algorithm for neutral point clamped (NPC) 3-level

35

inverter in [73]. Nowadays, the multilevel inverter fed drives are

becoming popular in many industrial and electrical vehicles applications

especially for medium and high power applications [74-77]. A detailed

survey on the multilevel inverters and various topologies of the multilevel

inverters are discussed in [77]. The waveform quality can be increased by

increasing the number of levels. But, as the number of level increases,

the complexity involved in the PWM algorithm and power circuit also

increases. To decrease the complexity involved in the PWM algorithms for

a multilevel inverter, several simplified PWM algorithms have been

proposed in the literature. A simplified SVPWM algorithm has been

proposed for a three-level inverter by suing the concept of SVPWM

algorithm for a two-level inverter in [78-79]. In this algorithm, the

switching times can be calculated similar to a two-level inverter.

However, this algorithm requires angle and sector calculations, which

increases the complexity of the PWM algorithm as the number of levels

increases.

To decrease the complexity of the SVPWM algorithm, it is

necessary to avoid the angle and sector calculations. A simplified

approach for SVPWM algorithm is proposed in [80], which uses

instantaneous phase voltages only for the calculation of gating times of

the inverter. The same approach is extended to the various

discontinuous PWM algorithms along with the SVPWM algorithm in [81].

However, these approaches are proposed for two-level inverters only. The

36

same approach is extended to a n-level inverter in [82]. This algorithm

also uses instantaneous phase voltages only. By using the concept of

effective time, the algorithm is extended for multilevel inverters under

both linear and over modulation regions.

Nowadays, many researchers have been focused their interest on

open-end winding induction motor drives in medium power applications.

The open-end winding induction motor drives offer many advantages

when compared with the normal drives. The open-end winding induction

motor drives fed by two inverters on either ends. By using the two 2-level

inverters on both sides of the winding, the phase voltages can be

obtained similar to the three-level inverter. To control these two 2-level

inverters, various PWM approaches are presented in the literature [83-

92].

Among the various PWM algorithms, decoupled and nearest sub-

hexagonal centre PWM (NSHCPWM) algorithm are popular approaches

for open-end winding induction motor drives. In both the approaches,

the two inverters will be operated with 180 degrees phase shift. Though

the implementation of decoupled PWM algorithm is simple, it gives more

harmonic distortion in line currents and voltages [91]. Hence, nowadays

the research interests have been focused on NSHCPWM algorithm. In

[83], a look-up table based NSHCPWM algorithm has been presented.

However, this approach will generate large common mode voltage

variations. To overcome the drawbacks of NSHCPWM algorithm, which is

37

presented in [83], various approaches have been proposed in the

literature [86-92].

Thus, though the DTC offers good dynamic performance, it has few

drawbacks such as steady state ripple in torque, flux and current,

varying switching frequency and sensitive to load torque disturbances.

Hence alternatives must be explored to reduce the steady state ripple in

torque, flux and current and memory size and to get constant switching

frequency. Mainly, this research is focused on the various PWM

algorithms to overcome the problems of steady state ripple, switching

frequency variations and memory size. Moreover, various simplified PWM

algorithms have been presented for multilevel inverters and open-end

winding induction motor drives.

1.5 Summary:

Recently, the DTC is gaining popularity in the high-performance

applications due to its numerous advantages. Moreover, the PWM

algorithms also attracting many researchers nowadays due to the

advantages of PWM algorithms. A detailed literature survey and the state

of art of DTC drives and various PWM algorithms have been presented in

this chapter.