DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

Name of the course : POWER SEMICONDUCTOR DRIVES

Name of the Dept. : ELECTRICAL AND ELECTRONICS ENGINEERING

Name of the Faculty : M. KRISHNA, Assistant Professor

Class : IV Year B. Tech. EEE –I Semester

Academic year : 2019-20

UNIT –I

Control of DC motors by single phase and three phase converter

Introduction of Electrical Drives

Electrical drives

Systems employed for motion control are called drives. And many employ any of the

prime movers such as diesel or petrol engines, as or steam turbines, hydraulic motors and

electric motors for supplying mechanical energy for motion control. Drives employing

electrical motors are known as electrical drives

Block Diagram of Electric Drive

The block diagram of an electric drive is shown below, and the load in the diagram

signifies different kinds of equipment which can be built with an electric motor such as

washing machine, pumps, fans, etc. The electric drive can be built with source, power

modulator, motor, load, sensing unit, control unit, an input command.

Electric Drive Block Diagram

Power Source

The power source in the above block diagram offers the necessary energy for the system.

And both the converter and the motor interfaces by the power source to provide

changeable voltage, frequency and current to the motor.

Power Modulator

This modulator can be used to control the o/p power of the supply. The power

controlling of the motor can be done in such a way that the electrical motor sends out the

speed-torque feature which is necessary with the load. During the temporary operations,

the extreme current will be drawn from the power source.

The drawn current from the power source may excess it otherwise can cause a voltage

drop. Therefore the power modulator limits the motor current as well as the source.

The power modulator can change the energy based on the motor requirement. For

instance, if the basis is direct current & an induction motor can be used after that power

modulator changes the direct current into alternating current. And it also chooses the

motor’s mode of operation like braking otherwise motoring.

Load

The mechanical load can be decided by the environment of the industrial process & the

power source can be decided by an available source at the place. However, we can choose

the other electric components namely electric motor, controller, & converter. Control Unit

The control unit is mainly used to control the power modulator, and this modulator can

operate at power levels as well as small voltage. And it also works the power modulator

as preferred. This unit produces the rules for the safety of the motor as well as power

modulator. The i/p control signal regulates the drive’s working point from i/p toward

the control unit.

Sensing Unit

The sensing unit in the block diagram is used to sense the particular drive factor such as

speed, motor current. This unit is mainly used for the operation of closed loop otherwise

protection.

Motor

The electric motor intended for the specific application can be chosen by believing various

features such as price, reaching the level of power & performance necessary by the load

throughout the stable state as well as active operations.

Main factors which decide the choice of electrical drives

Nature of electric supply

Whether AC or DC supply is to be used for supply ·

Nature of the drive

Whether the particular motor is going to drive individual machine or a group of

machines

Capital and running cost ·

Maintenance requirement ·

Space ad weight restrictions ·

Environment and location ·

Nature of load

Whether the load requires light or heavy starting torque

Whether load torque increases with speed remain constant

Whether the load has heavy inertia which may require longer straight time ·

Electrical characteristics of motor

Starting characteristics,

Running characteristics,

speed control and Braking characteristics

Size, rating and duty cycle of motors

Whether the motor is going to the operator for a short time or whether it has to run

continuously intermittently or on a variable load cycle

Mechanical considerations

Type of enclosures, type of bearings, transmission of drive and Noise level.

Due to practical difficulties, it may not possible to satisfy all the above considerations.

In such circumstances, it is the experience and knowledge background which plays a

vital role in the selection of the suitable drive. The following points must be given

utmost important for the selection of motor.

The factors are:

Nature of the mechanical load driven

Matching of the speed torque characteristics of the motor with that of the load

Starting conditions of the load

Classification of Electrical Drives

Usually, these are classified into three types such as group drive, individual drive, and

multi-motor drive. Additionally, these drives are further categorized based on the

different parameters which are discussed below.

Electrical Drives are classified into two types based on supply namely AC drives &

DC drives.

Electrical Drives are classified into two types based on running speed namely Constant

speed drives & changeable speed drives.

Electrical Drives are classified into two types based on a number of motors namely

Single motor drives & multi-motor drives.

Electrical Drives are classified into two types based on control parameter namely

stable torque drives & stable power drives.

Compare AC and DC drives

Advantages of Electrical Drives

The advantages of electrical drives include the following.

These dries are obtainable with an extensive range of speed, power & torque.

Not like other main movers, the requirement of refuel otherwise heat up the motor is

not necessary.

They do not contaminate the atmosphere.

Previously, the motors like synchronous as well as induction were used within stable

speed drives. Changeable speed drives utilize a dc motor.

They have flexible manage characteristics due to the utilization of electric braking.

At present, the AC motor is used within variable speed drives because of

semiconductor converters development.

Disadvantages of Electrical Drive

The disadvantages of electrical drives include the following.

This drive cannot be used where the power supply is not accessible.

The power breakdown totally stops the entire system.

The primary price of the system is expensive.

The dynamic response of this drive is poor.

The drive output power which is obtained is low.

By using this drive noise pollution can occur.

Applications of Electrical Drives

The applications of electrical drives include the following.

The main application of this drive is electric traction which means transportation of

materials from one location to another location. The different types of electric tractions

mainly include electric trains, buses, trolleys, trams, and solar-powered vehicles

inbuilt with battery.

Electrical drives are extensively used in the huge number of domestic as well as

industrial applications which includes motors, transportation systems, factories,

textile mills, pumps, fans, robots, etc.

These are used as main movers for petrol or diesel engines, turbines like gas otherwise

steam, motors like hydraulic & electric.

1

UNIT-II

DC DRIVES

2.2 Conventional methods of speed control

N = Eb / φ

1. By varying the resistance in the armature circuit (Rheostatic control) 2. By varying the flux (flux control) 3. By varying the applied voltage (voltage control)

Solid state speed control of DC motor

The DC motor speed can be controlled through power semiconductor switches.Here,the power semiconductor switches are SCR (thyristor),MOSFET,IGBT,This type of speed control is called ward-Leonard drive.

2.3 Types of DC drives

1. Phase controlled rectifier fed DC drives a. According to the input supply

i. Single phase rectifier fed DC drives ii. Three phase rectifier fed DC drives

b. According to the quadrant operation

i. One quadrant operation ii. Two quadrant operation iii. Four quadrant operation

2. Chopper fed DC drives i. One quadrant chopper drives ii. Two quadrant chopper drives iii. Four quadrant chopper drives

2.4 Single phase controlled rectifier fed DC Drives

Fig (2.4) Single phase controlled rectifier fed DC Drives

Here AC supply is fed to the phase controlled rectifier circuit.AC supply may be single

phase or three phase.Phase controlled rectifier converts fixed AC voltage into variable

DC voltage .

Here the circuit consists of SCR’s.By varying the SCR firing angle the output voltage

can be controlled. This variable output voltage is fed to the DC motor.By varying the

motor input voltage,the motor speed can be controlled.

AC

SOURCE

PHASE

CONTROLLED

RECTIFIER

DC

MOTOR

LOAD

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2.5 Single phase controlled rectifier fed separately excited DC motor Drives

Figure shows block diagram of single phase controlled rectifier fed separately excited DC motor. The armature voltage is controlled by means of a half wave controlled or half controlled or full convener. l¢ AC supply is fed to the single phase controlled rectifier. This controlled rectifier converts fixed AC voltage into variable DC voltage. By varying the firing angle of this converter, we can get variable DC voltage. The field winding is fed from the AC supply through a diode bridge rectifier.

Fig (2.5) Single phase controlled rectifier fed separately excited DC motor Drives

The armature circuit of the DC motor is represented by its back emf Eb, amature

resistance Ra and armature inductance La as shown in figure .

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2.6 Single Phase Half wave controlled Rectifier fed DC Drives (one quadrant

converter)

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Fig (2.6) Single Phase Half wave controlled Rectifier fed DC Drives (one quadrant

Converter)

Figure shows single phase half wave controlled rectifier drive. Assume armature current Ia is constant. Here, the motor is separately excited DC motor. Motor is operated from single phase half wave controlled rectifier. Motor field winding is fed through separate DC source. During the positive half cycle SCR T is forward biased. At ωt = , SCR T is triggered and comes to the on state, Then the positive voltage is fed to the motor.

At ωt = П, freewheellng diode comes to the forward biased state and SCR comes to the off state, because of reverse voltage. Dur.ing the negative half cycle, SCR T is in off state, and freewheeling diode conducts upto 2П+

toП-T on

П to 2П + – FD on

During the period, П to 2П+ current is positive but output voltage is zero because

of closed path (FD - motor -FD).

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Fig (2.6.1) Single Phase Half wave controlled Rectifier fed DC Drives, Wave form

Torque of the separately excited motor is given by

T φIa

Φ - constant

T Ia

T = KmIa

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2.7 Single phase fully controlled rectifier fed DC drives

The drive circuit is shown in the fig .motor is shown by its equivalent circuit.Filed

supply is not shown.The ac input voltage is defined by

Vs = Vm sin ωt

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Fig (2.7) Single phase fully controlled rectifier fed DC drives

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Fig (2.7.1) Single phase fully controlled rectifier fed DC drives, Discontinuous

Conduction waveforms.

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Fig (2.7.2 ) Single phase fully controlled rectifier fed DC drives, Continuous

Conduction waveforms.

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Fig (2.7.3) Single phase fully controlled rectifier fed DC drives, continuous Conduction

waveforms.(Rectification Mode)

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Fig (2.7.4) Single phase fully controlled rectifier fed DC drives, Continuous

Conduction waveforms.(Inversion Mode)

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The motor terminal voltage and current waveforms for the dominant discontinuous

and continuous conduction modes are shown in figure .T1, T2 are gated at ωt = , these SCRS will get turned on only if Vm sin > E. Thyristors T1 and T2 are given gate signals from to П and thyristors T3 and T4 are given gate signals from (П+ ) to 2П.

When armature current does not flow continuously the motor is said to operate in discontinuous conduction When current flows continuously the conduction is said to be continuous.

ln discontinuous conduction modes, the current starts flowing with the turn-on thyristors T1 and T2 at ωt = . Motor gets connected to the source and its terminal voltage equals Vs, At some angle , known as extinction angle load current decays to zero Here > П.As T1 T2 are reverse biased after ωt= П , this pair commutated at ωt= ,

when ia= 0 From to П + , no SCR- conducts ,the motor terminal voltage jumps from Vm sin to E as shown in figure.

At ωt=П+ , as pair T3 T4 is triggered, load current starts to build up again as before and load voltage Va follows Vs, waveform as shown. At П+ , ia falls zero, Va, changes from Vm sin(П+ ) to E as no SCR conducts.

In continuous conduction mode during the positive half cycle thyristors T1, T2 are forward biased At ωt = ,T1,T2 are turned on.As a result ,supply voltage Vm sin immediately appears across thynstors T3 T4 as a reverse bias, these are turned off by natural commutation At ωt= П+ forward biased SCRs T3, T4 are triggered causing turn off of T1 and T2 Figure and figure shows rectification and inversion mode voltage and current waveforms.

Steady state Analysis of Discontinuous Conduction.

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Average output voltage

Average output Current

Speed Equation (ωm)

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Steady State Analysis of continuous conduction

Average output voltage

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Speed ωm

RMS value of output current

RMS value of source current

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Average Value of thyristor current

RMS value of Thyristor current

Assume no loss in the converter

Input power = output power

VsIscosφ = VaIa

Cosφ = 2√2 cos

П

Speed –torque characteristics

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2.8 Three Phase controlled Rectifier fed DC drives

For large power dc drives, three phase controlled rectifiers are used, three phase

controlled rectifier circuits give more number of voltage pulses per cycle of supply

frequency .this makes motor current continuous and filter requirement also less.

The number of voltage pulses per cycle depends on the number of thyristors and their

connections for three phase controlled rectifiers.

Semi converters and full converters are most commonly used in practice.Dual

converters are used in reversible drives having power ratings of several mega watts in

steel industry and heavy applications.

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2.8.1 Three phase fully controlled rectifier fed separately excited DC motor drive.

Three phase-full converters are used industrial applications upto 1500 kW drives. It

is a two quadrant convener i.e., the average output voltage is either positive or

negative `but average output current is always positive.

Fig (2.8.1) Three phase fully controlled rectifier fed separately excited DC motor drive.

(Circuit Diagram).

Fig(2.8.2) Quadrant Diagram

The circuit consists of six thyristors. Here, there are two groups of thyristors, one is positive group and another one is negative group. Here, thyristors Tl, T3, T5 forms a

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positive group, whereas thyristors T4, T6, T2 forms a negative group. The positive group thyristors are tumed on when the supply voltages are positive and negative group thyristors are tumed on when the supply voltages are negative. The operation of this convener is easily understand by using line voltages instead of phase voltages,

For or = 60°, T1 is turned on at П/3+ 60 =120°, T2 at ωt = l80°, T3 at ωt = 240° and so on. When T1 is turned on at ωt = l20°, T5 is turned off. T6 is already conducting. As T1 and T6 are connected to R and Y respectively, load voltage must be very as shown in fig.

When T2 is turned on, T6 is commutated. As T1 and T2 are now eonducting, the load

voltage is vrb, figure. In this way, load voltage waveform can be drawn with thyristors

in sequence.

For = 120°, T1 is triggered at ωt = l80°, T2 is triggered at ωt = 240° and so on, The output voltage waveform is shown in figure. From this waveform, the average output

voltage is negative. This means that dc source is delivering power to ac source.

This operation is called line commutated inverter operation. For is 0 to 90°, this converter operates rectification mode (power flows from source to load) and 90° to 180

converter operates an inversion mode (power flows from load to source). It can work in

the inverter mode only if the load has a direct emf E. It is a regenerative braking mode.

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Fig (2.8.3) Three phase fully controlled rectifier (Motoring mode)

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Fig (2.8.4) Three phase fully controlled rectifier (Regenerating Breaking mode)

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Fig (2.8.5) Speed – Torque curve

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Control strategies

The average output voltage can be controlled through or α by opening and closing of the semiconductor switch periodically.

(i) Time ratio control method (TRC)

1. Fixed frequency

2. Variable frequency

(ii) Current limit control (CLC)

Time ratio control- pulse width control

The ratio of on-time to chopper period is controlled

CONSTANT FREQUENCY TRC

The chopping period T is kept fixed and the on period of the switch is varied to control the

duty cycle ratio.

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VARIABLE FREQUENCY TRC

The duty ratio is varied by keeping ton constant and varying T, or by varying both T and ton

In this control, low output voltages are obtained at very low chopper frequencies. This will affect

the motor performance.

CURRENT LIMIT CONTROL(Point by point control)

The duty ratio is controlled by controlling the load current between certain specified

maximum and minimum values. When the load current reaches maximum value, the switch

disconnects the load from the source and reconnects ie when the current reaches a specified

minimum value.

Two types of control provided for chopper control

1. Power control or motoring control

2. Regenerative braking control

2.9 CHOPPER DRIVES

Fixed

DC Variable DC

Fig (2.9) Basic block diagram of chopper

Fixed DC voltage is fed to the Dc chopper circuit.DC chopper converts fixed DC

into Variable DC voltage.This variable DC voltage is fed to the motor.By varying the DC

voltage ,the motor speed can be controlled.

2.10 Advantages of DC chopper control

1. High eficiency

2. Flexibility in control

3. Light weight

4. Small size

5. Quick response

6. Regeneration down to very low speeds.

DC

Chopper

DC

Motor

Load

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2.11 Applications of DC chopper Drives

1. Battery operated vehicle

2. Traction motor control in electric traction

3. Trolly cars

4. Hoists

5. Electric braking

2.12 Types of DC chopper drives

1. First quadrant chopper or type A chopper

2. Second quadrant or type B chopper

3. Two quadrant type A chopper or type C chopper

4. Two quadrant type B chopper or type D chopper

5. Four quadrant chopper or type E chopper

2.12.1 First Quadrant or Type-A or Motoring Chopper

In the past, series motor was used in traction, because it has high starting torque. It

has number of limitations. The field of the series motor cannot be controlled easily by

static means

If field control is not employed, the series motor must be designed with its base speed

equal to the higher desired speed of the drive. The higher base speeds are obtained

using fewer turns in the field windings

This reduces the torque per ampere at zero and low speeds. Presently, separately

excited motors are also used in traction. Because of limitations of a series motor

separately excited motors are now preferred even for traction applications.

Motoring control

A transistor chopper controlled separately excited motor drive is shown in fig

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Fig( 2.12.1) First Quadrant or Type-A or Motoring Chopper

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Fig (2.12.2) First Quadrant or Type-A or Motoring Chopper(waveform)

Current limit control is used in chopper. in current limit control, the load current is allowed to vary between two given (upper and lower) limits. The ON and OFF times of the chopper adjust automatically, when the current increases beyond the upper limit the chopper is turned off, the load current freewheels and starts to decrease. When it falls below the lower limit the chopper is turned ON. The current starts increasing in the load. The load current ‘ia’ and voltage ‘va’ waveform are shown in figure .By assuming proper limits of current, the amplitude of the ripple can be controlled,

The lower the ripple current, the higher the chopper frequency. By this switching losses get increase. Discontinuous conduction avoid in this case, The current limit control is superior one. During ON-period of chopper (i.e.) duty interval,0≤t≤TON, motor terminal voltage Va is a source voltage Vs and armature current increases from ia1 to ia2

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The operation is described by,

iaRa+La 𝑑𝑖𝑎𝑑𝑡 +Eb = Vs ; 0 ≤ t ≤ TON

Chopper is turned off at t = t0N. During off-period of chopper (i.e.) free

wheeling interval, Ton≤ t ≤ T , motor current freewheels through diode FD and motor terminal voltage Va is zero.

This is described by,

iaRa+La 𝑑𝑖𝑎𝑑𝑡 +Eb = 0; Ton ≤ t ≤ T

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2.12.2 Second Quadrant or Type –B or Regenerative braking Chopper

In regenerative mode ,the energy of the load may have to be fed to the supply system.The dc motor works as a generator during this mode.As long as the chopper is ON,the mechnanical energy is converted into electrical by the motor,now working as a generator,increases the stored magnetic energy in armature circuit inductance and remainder is dissipated in armature resistance and transistor.when chopper is switched off,a large voltage occurs across the load terminals.

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Fig (2.12.2) Second Quadrant or Type –B or Regenerative braking Chopper(circuit diagram

This voltage is greater than the supply voltage Vs and the energy stored in the inductance and the energy supplied by the_machine is fedback to the supply system. When the voltage of the load falls to Vs, the diode in the line blocks the current flow, preventing any short circuit of the load can he supplied to the source. Very effective braking of the motor is possible upto the extreme small speeds. Regenerative braking is achieved here by changing the direction of current flow.

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2.12.3. Two Quadrant Chopper Drives

Motoring control and braking control can be achieved by two quadrant chopper

There are two types of two quadrant chopper drives.

1. Two Quadrant type A chopper drive

2. Two Quadrant type B chopper drive

Two Quadrant type A chopper drive

This types of chopper drive provides forward motoring mode and forward braking

mode.

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Fig shows two quadrant type A chopper drive for separately excited dc motor.It

consists of two choppers CH1 and CH2 and two diodes D1 and D2 dc motor.

Fig (2.12.3.)Two Quadrant type A chopper drive

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Fig (2.12.3.)Two Quadrant type A chopper drive ( Quadrant Diagram)

Forward Motoring Mode

When the chopper CH1 is on, the supply voltage is fed to the motor armature

terminals and therefore the armature current increases. Here the voltage and current

is always positive. Therefore the motor rotates in forward direction.

When CH1 is in an off state,ia freewheels through diode D1 and therefore ia decreases.

It is the forward motoring mode. It is first quadrant operation.

Forward Braking Mode

When chopper CH2 is in an ON state, the motor acts as a generator and armature

current ia increases. Due to this energy is stored in the armature inductance.

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Fig (2.12.3.) Wave Form Two Quadrant type A chopper drive

When CH2 is in an off state, diode D2 gets turned on and therefore armature current

ia is reversed. It is the second quadrant operation.

In this mode output voltage is positive and output current is negative. It is forward

regenerative braking mode.

2.12.4 Two Quadrant type B chopper drive

This type of chopper drive provides forward motoring mode and reverse regenerative

braking mode.

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Fig (212.4).Two Quadrant type B chopper drive (circuit diagram)

Fig (2.12.4) –Quadrant Diagram Two Quadrant type B chopper drive

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It consists of two choppers CH1 and CH2,two diodes and dc motor. This type of

chopper operates in the first quadrant and fourth quadrant operation.

Forward motoring mode

When the chopper CH1 and CH2 on, the motor rotates in the forward direction and ia

increases. When CH1 is in an off state ,now the current flows through CH2 and diode

D1.Here the output voltage current is always positive .It gives forward motoring mode

chopper.

Fig (2.12.4 ) Forward Motoring mode, Two Quadrant type B chopper drive

Reverse Braking Mode

When both the choppers CH1 and CH2 are off, the current will flows through the diode Di and D2. Here the output current is positive and output voltage is negative. i.e.,

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power flows from load to source. Here we can achieve the reverse braking mode. It is the fourth quadrant operation. It is shown in fig. Here the motor speed can be controlled by changing the duty cycle of the chopper.

Figure shows r 0.5) waveforrns of two quadrant type B chopper drive.

2.12.4 Four quadrant Chopper or Type E Chopper

Fig (2.12.4) Four quadrant Chopper or Type E Chopper

It consist of four power semiconductor switches CH1 to CH4 and four power diodes D1

and D4 in antiparallel.working of this chopper in the four quadrants is explained as

under,

Forward Motoring Mode

For first quadrant operation of figure CH4 is kept on, Cl-l3 is kept off and CH1 is

operated. when CHI and Cl-I4 are on, load voltage is equal to supply voltage i,e, Va =

Vs and load current ia begins to flow. Here both output voltage va and load current ia

are positive giving first quadrant operation. When CH4 is turned of£ positive current

freewheels through CH-4,D2 in this way, both output voltage va, load current ia can

be controlled in the first quadrant. First quadrant operation gives the forward

motoring mode.

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Forward Braking Mode

Here CH2 is operated and CH1, CH3 and CH4 are kept off. With CH2 on, reverse (or negative) current flows through L, CH2, D4 and E. During the on time of CH2 the inductor L stores energy. When CH2 is turned off current is fedback to source through diodes D1, D4 note that there [E+L di/dt] is greater than the source voltage Vs. As the load voltage Va is positive and load current ia is negative, it indicates the second quadrant operation of chopper. Also power flows from load to source, second quadrant operation gives forward braking mode.

Reverse Motoring Mode

For third quadrant operation of figure, CHI is kept off, CH2 is kept on and CH3 is operated. Polarity of load emf E-must be reversed for this quadrant operation. With CH3 on, load gets connected to source Vs so that both output voltage Va and load current ia are negative. it gives third quadrant operation. lt is also known as reverse motoring mode. When CH3 is turned off, negative current freewheels through CH2, D4. ln this way, output voltage Va and load current ia can be controlled in the third quadrant.

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Reverse Braking Mode

Here CH4 is operated and other devices are kept of£Load emf E must have its polarity reversed, it'is shown in figure . With CH4 on, positive current flows through CH4, D2, L and E. During the on time of CH4, the inductor L stores energy.

When CH4 is turned off; current is feedback to source through diodes D2, D3. Here load voltage is negative, but load current is positive leading to the chopper operation in the fourth quadrant.

Also power is flows from load to source. The fourth quadrant operation gives reverse braking mode.

2.13 Braking

In braking, the motor works as a generator developing a negative torque which

oppose the motion. It is of three types

1. Regenerative braking

2. Plugging or Reverse voltage braking

3. Dynamic braking or Rheostatic braking

2.13.1Regenerative braking

In regenerative braking, generated energy is supplied to the source,for this to happen

following condition should be satisfied

E > V and negative Ia

Field flux cannot be increased substantially beyond rated because of saturation, therefore

according to equation ,for a source of fixed voltage of rated value regenerative braking is

possible only for speeds higher than rated and with a variable voltage source it is also

possible below rated speeds .

The speed –torque characteristics shown in fig. for a separately excited motor.

In series motor as speed increases, armature current, and therefore flux decreases

Condition of equation cannot be achieved .Thus regenerative braking is not possible

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2.13.2 Plugging

The supply voltage of a separately excited motor is reversed so that it assists

the emf in forcing armature current in reverse direction .A resistance RB is also

connected in series with armature to limit the current.For plugging of a series

motor armature is reversed.

A particular case of plugging for motor rotation in reverse direction arises

.when a motor connected for forward motoring,is driven by an active load in the

reverse direction.Here again back emf and applied voltage act in the same

direction.However the direction of torque remains positive.

This type of situation arises in crane and the braking is then called counter –torque braking.

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Plugging gives fast braking due to high average torque,even with one section

of braking resistance RB.Since torque ia not zero speed,when used for stopping a

load,the supply must be disconnected when close to zero speed.

Centifugal switches are employed to disconnect the supply.Plugging is

highly inefficient because in addition to the generated power,the power supplied by

the source is also wasted in resistances.

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2.13.3 Dynamic braking

In dynamic braking ,the motor is made to act as a generator,the armature is

disconnected from the supply ,but it continues to rotate and generate a

voltage.The polarity of the generated voltage remains unchanged if the

direction if field excitation is unaltered.

But if a resistance is connected across the coasting motor,the direction of the

armature current is reversed ,because the armature represents a source of

power rather than a load.

Thus a braking torque is developed ,exactly as in the generator,tending to

oppose the motion.

The braking torque can be controlled by the field excitation and armature

current.

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1

UNIT-III

STATOR SIDE CONTROLLED INDUCTION MOTOR DRIVE

3.1 STATOR VOLTAGE CONTROL

The induction motor 'speed can be controlled by varying the stator voltage. This method of speed control is known as stator voltage control.

The rotor circuit power can be varied either by having a variable resistance or by feeding back to the mains through appropriate power conditioning circuits. In the case of variable resistance, the power associated with the rotor circuit is wasted. This scheme is utilized in the rotor resistance control. This method is used in slip power recovery scheme

Thus the above mentioned various methods of speed control scheme of the induction motor can be listed as follows

1. Slip control a. Stator voltage control b. Rotor resistance control c. Slip power recovery scheme

2. Stator frequency control

Here the supply frequency is constant. Torque equation indicate; that the torque is proportional to the square of its stator voltage i,e., T α V2. For the same slip and frequency, a small change in stator voltage results in a relatively large change in torque, A 10% reduction in voltage causes a 19% reduction in developed torque as well as the starting and maximum torque.

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Fig (3.1 ) speed –Torque Characteristics of Stator voltage control

Figure shows speed torque characteristics of induction motor under stator voltage control. This characteristic is based on the torque equation. This shows two curves for two different values of the' stator voltage. Here the slip at the maximum torque remains unchanged since it is not a function of voltage. For a low slip motor, the speed range is very narrow. So this method is not used for wide range of speed control and constant torque load. This is applicable for requiring low starting torque and a narrow speed range at relatively low slip.

lf the stator copper loss and the friction, windage and core losses are neglected, from equation, the motor efficiency is given by

ηm = Pm/ Pag = (1-S)

This equation, increasing the slip i.e very low speeds the motor efficiency is poor.

It is an excellent method for reducing starting current and increasing the efficiency during light load conditions. The starting current is reduced since it is directly proportional to the input voltage. The losses are decreased, mainly core losses, which are proportional to the square of the voltage. The terminal voltage cannot exceeds rated value to prevent the damage of the winding’s insulation. Thus, this method is only suitable for speed control below the rated speed.

1. Conventional Method 2. Solid State control Method

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3.2 Conventional method

The stator voltage can be controlled by two methods

1. Using auto transformer 2. Primary resistors connected in series with stator winding.

Fig (3.2.1) Conventional method using auto Transformer

The speed of the induction motor can be controlled by using auto transfomer. It is shown in figure . The input to the auto transformer is a fixed ac voltage .

By varying the auto transformer we can get variable ac output voltage without change in supply frequency. The variable voltage is fed to the induction motor. Then the induction motor speed also changes.

4

Primary resistor connected in series with stator winding

Fig (3.2.2) Primary resistor connected in series with stator winding

The primary resistors are connected in series with stator windings as shown in figure, By varying the primary resistance, the voltage drop across the motor terminals is reduced. That is, reduced voltage is fed to the motor. Then the motor speed can be reduced. lt is one method of conventional speed control of induction motor, The control method is very simple. The main disadvantage is that more power loss occurs in the primary resistors

3.3 AC voltage controller for 3-phase induction Motor

The stator voltage is controlled in these speed control systems by means of a power electronic controller. There are two methods of control as follows

(a) on-off control

(b) phase control.

5

Fig (3.3) AC voltage controller for 3-phase induction Motor

In on off control, the thyristors are employed as switches to connect the load circuit to the source for a few cycles of source voltage and then disconnect it for another few cycles Here thyristors acts as high speed switch (contactor). This methodis known as integral cycle control In phase control the thyristors are employed as switches connect the load to the ac source for a portion of each cycle of input voltage. The power circuit configuration for on off control and phase control do not differ in any manner Normally thyristors in phase control modes are used.

The various schemes are

(1) single phase or 3 phase half wave ac voltage controller (ii) l-φ or 3-φ) full wave ac voltage controller

6

Figure (a) and (b) shows the circuits of three phase half wave and full wave ac voltage controllers for star connected stators. In half wave ac 'voltage controller consists of 3 SCRs and 3 diodes Here one SCR and one diode in antiparallel are connected between the line and motor in a phase., The full wave ac voltage controller consists of 6 SCRs Here two SCRs in antiparallel are connected_ between die line and motor in a phase The main advantage of half wave controller is a 'saving the cost of system The disadvantage is that it introduces more harmonics into the line current. The effective load voltage in three phase ac circuit can be varied by varying the thyristor tiring angles.

Advantages of stator voltage control

1.The control circuit is very simple

2. More compact and less weight

3. lts response time is quick

4. There is a considerable savings in energy and thus it is a economical method

Disadvantages

1. The input power factor is very low.

2. Voltage and current waveforms are highly distorted due to harmonics, which

affects the efficiency of the machine.

3. Performance is poor under running condition at low speed

4. Operating efficiency is low as resistance losses are high.

5. Maximum torque available from the motor decreases with decreases in stator

voltage. 6. At low speeds, motor currents are excessive and special arrangements should be provided to limit the excessive currents.

Applications

1. They are mainly used in low power applications such as fans, blowers and centrifugal pumps, where the starting torque is low.

2. They are also used for starting high power induction motors to limit the in-rush

current.

7

3.4 CLOSED LOOP CONTROL OF ELECTRIC DRIVE

Feedback loops in an electrical drive may be provided to satisfy one or more of the following

requirements,

1. Protection

2. Improvement of speed response

3. To improve steady state accuracy

Closed loop speed control

Fig (3.4) Closed loop speed control

This system consists of a power circuit, 3¢ IM, tachogenerator and control circuits, A DC tacho-generator is coupled with induction motor shaft. This tacho-generator generates a voltage proportional 'to the motor speed. The generated voltage is equal to the actual speed of the motor. This voltage is compared with dc reference voltage (reference speed). The difference between these two voltages are compared in

8

comparator and output voltage is error signal voltage. This voltage is amplified by amplifier and fed to the thyristor gating circuits. This controls the thyristor tiring angles and thereby changes the terminal voltage and hence the motor speed changes.

If the reference speed is greater than actual speed, the conduction periods of the SCRs are increased. The increased stator voltage allows the development of an increased motor torque and hence the motor speed also increases, lf the actual speed is greater than the reference speed, the conduction angle of SCRS are reduced and the motor torque decreases as well as reduces the motor speed. When the motor speed is equal to the reference speed, the conduction angles are just sufficient to allow the development of a motor torque.

This motor torque is equal to the load torque. In a high-gain feedback system the desired speed can be maintained and there is no necessity for the motor to have a flat speed-torque characteristics, since the output speed is determined by the reference (desired) signal rather than the open-loop characteristics of the motor. The stable operation may be obtained at any point of the induction motor speed-torque characteristic.

3.5 STATOR FREQUENCY CONTROL

The stator frequency control is the one of the speed control of 3 phase induction motors. Here we can vary the input frequency of the motor. Under steady state condition, the induction motor operates in the small-slip region, where the speed of the induction motor is always close to the synchronous speed of the rotating flux

The synchronous speed of the induction motor is given by

Ns = 120f / P

Where

f- Frequency of the supply voltage

P – Number of poles

In this equation, synchronous speed of the motor is directly proportional to the frequency of the supply voltage. Here, the supply frequency is changes, the motor speed also changes. Since the emf V1 induced in the stator winding of the induction motor is equal to

V1 = 4.44fφTphKw

Φ – flux /pole

Kw- winding factor

f- frequency of stator (input) supply

Tph – Number of turns in the stator winding

9

lf the frequency of the stator supply IS changes the magnitude of V1 should

also be changed to maintain the same value of flux .Here we consider two cases

1) Low frequency operation at constant voltage

2) High frequency operation at constant voltage

3.6 Low frequency operation at constant voltage

By decreasing the supply frequency at constant voltage V1, the value of air gap flux lncreases and the induction motor magnetic circuits also gets saturated For considering the emf equatlon,

V1 constant ; f decreases; φ increases

Due to this low frequency operation the following effects are given below

l) It draws more magnetizing current

2) Line currents and line voltages are distorted .Increase the core loss and stator copper loss .produce a high pitch acoustic noise.

3) Very low efficiency.

3.7 High frequency operation at constant voltage (field weakening mode)

With the constant input voltage,if the stator frequency is increased ,the motor speed also increases.Due to increase in frequency,flux and torque are reduces.

V1 constant ; f increases; φ decreases

By increasing the supply frequency of the motor ,the following effects are given below,

1. The no load speed increases 2. The maximum torque decreases 3. Starting torque reduces 4. Starting current decreases

The base speed ωb is defined as the synchronous speed corresponding to the rated frequency.The synchronous speed at any other frequency is equal to ωs = K ωb

10

Torque equation for rated voltage and rated frequency is given by

The slip at which the maximum torque occurs is given by

Maximum torque is given by

11

Torque –speed characteristics for stator frequency control at constant voltage.

During this mode of operation of induction 'motor behavior is similar to the working of a dc series motor. It is also known as field weakening mode because the air gap flux gets reduced. Here the maximum torque also reduced.

For K > l, the induction motor is operated at constant terminal voltage, air gap flux is reduced and torque capability of the motor is limited. 'For 1 < K < 1.5, the relation between Tm and K can be approximately linear. For K < 1, the induction motor is normally operated at constant flux by reducing supply voltage along with the supply voltage.

12

3.8 VARIABLE FREQUENCY AC MOTOR DRIVES

The induction motor speed can be controlled by varying the supply frequency This method is mainly applied to the squirrel cage induction motor. The variable frequency control allows good running and transient performance to be obtained from a squirrel cage induction motor.

The variable frequency induction motor drives are very popularly because of

i) Special applications requiring maintenance free operation, such as under ground and under water installations.

ii) Applications involving explosive and contaminated environments such as in mines and the chemical industry.

The variable frequency AC drives applications are in

I) Pumps 2) Fans 3) Mill run out tables

4) Blowers 5) Compressors 6) Spindle drives

7) Conveyors 8) Machine tools and so on

Due to availability of power semiconductor devices such as power transistors, power MOSFETs, IGBTS and GTOs with improved ratings and characteristics, general purpose medium and high power variable frequency drives are available. The cost of the equipment is less compared to the drives. The variable frequency conversion can be made by using,

1. Voltage source inverter 2. Current source inverter 3. Cycloconverter

3.8.1 Voltage source inverter fed AC drives

An inverter is defined as converter that converts DC into AC. An inverter called voltage source inverter, if viewed from the load side, the AC terminals of the inverter function as a voltage source i.e`., the input voltage should be constant. The VSI has low internal impedance. Because of this the terminal voltage of a VSI remains constant with variations in load. The VSI are capable of supplying variable frequency variable voltage for the speed control of induction motors.

13

Fig (3.8.1)VSI employing IGBT’s

VSI allows a variable frequency supply to be obtained from a DC supply. MOSFET is used in low voltage and low power inverters. Power transistors and IGBTS are used for medium power level of inverters.

For high power level of inverters thyristors,-GTOs and IGCTs (insulate Gate Commutated Thyristor) are`used.

Voltage source inverter can be operated as a stepped wave inverter or a Pulse Width Modulated

(PWM) inverter, Inverter operated as a stepped wave inverter IGBTS are switched in the

sequence of their numbers with a time difference of T/6 and each IGBT is kept an for the

duration T/2 where T is the time period of the one cycle.

14

Figure shows stepped wave inverter line voltage waveform. In the stepped wave inverter the

output frequency can be varied by varying T. Output voltage can be varied by varying input DC

voltage. When the input voltage is DC, variable DC input voltage is obtained by connecting a

chopper between DC supply and inverter. It is shown in figure

Here DC supply is given to the chopper. The DC chopper converts fixed DC to variable DC voltage, This voltage fed to the filter. It is used to filter out harmonics in DC link voltage. The DC voltage is fed to the six step inverter. The inverter output voltage is variable frequency variable voltage. It is fed to the 3¢ induction motor.

When the input voltage is AC, variable DC input voltage is obtained by connecting a controlled rectifier between AC supply and inverter, It is shown in above figure. Here 3¢ AC supply is fed to the controlled rectifier, It converts fixed AC into variable DC. This voltage is fed to the filter. Filter reduces the harmonics.

The filtered output is fed to the inverter. The inverter output voltage can be varied by varying DC voltage, It is done by controlled rectifier. The output frequency can be varied by time period of the inverter, The main disadvantages of stepped wave inverter is the large harmonics of low frequency in the output voltage. A stepped wave inverter fed induction motor drives suffers from the following disadvantages.

1. Due to low frequency harmonics, the motor losses are increased at all

15

speeds causing derating of the motor.

2. Motor produces pulsating torques because of fifth, seventh, eleventh and

thirteenth harmonics which cause jerky motion of the rotor at low speeds.

3. Harmonic content in an induction motor current increases at low speeds. The

machine saturates at low speeds due to high (V/f) ratio. These two effects

overheat the motor at low speeds, thus limiting lowest speed to around 40%

of base speed.

The above drawbacks are eliminated by using of Pulse Width Modulated inverter

(PWM). The advantages PWM inverters are

l. Harmonies are reduced.

2. Losses are reduced.

3. Smooth motion is obtained at low speeds,

Fig (3.8.1.) PWM inverter

Figure shows output voltage waveform for sinusoidal pulse width modulation. By using this method, the inverter output voltage and frequency can be controlled. There is no need of external control. Voltage and frequency can be controlled inverter itself.

16

When the input voltage is DC, it is directly connected to the PWM inverter. It is shown in figure

When the input voltage is AC ,DC supply is get from a diode bridge rectifier,shown in below fig.

Here 3φ supply is fed to the diode bridge rectifier. It converters fixed AC voltage into fixed DC voltage, This voltage is fed to the filter and then PWM inverter. PWM inverter gives variable an voltage and frequency. By changing the voltage and frequency the motor speed can be controlled.

1

UNIT-IV

ROTOR SIDE CONTROLLED INDUCTION MOTOR DRIVE

4.1 ROTOR RESISTANCE CONTROL

The induction motors are widely used in industrial applications. The stator side control is applicable to both squirrel cage and slip ring induction motors .Because of more advantages, squirrel cage motor is always preferred. the speed control of slip ring induction motor i.e. rotor side control. The slip ring induction motor has a number of disadvantages compared to squirrel cage motor such as

1. Wound - rotor machine is heavier

2. Higher cost

3. Higher rotor inertia

4. Higher speed limitation

5. Maintenance and reliability problems due to brushes and slip rings

However, a wound-rotor (SRIM) machine speed control method is very simplest and oldest method. The speed can be controlled by mechanically varying rotor circuit rheostat.

The main feature of this machine is that slip power becomes easily available from the slip rings, which can be electronically controlled to control speed of the motor. For limited range speed control applications,where, the slip power is only a fraction of the total power rating of the machine, the_converter cost should be reduced. The main applications of slip power recovery drives are,

1. Variable speed wind energy systems.

2. Large - capacity wind energy systems.

3. Shipboard VSCF (Variable - Speed/Constant - Frequency) systems.

4. Utility system flywheel storage system.

5. Variable- speed hydropumps / generators.

4.2 Types

1. Conventional rotor resistance control

2. Static rotor resistance control

3. Slip power recovery scheme (Energy efficient drives)

2

4.2.1 Conventional Rotor Resistance Control

Fig (4.2.1) conventional Rotor Resistance Control

This method is only applicable for slip ring or wound-rotor induction motor. Here, 3-phase ac supply is fed to the stator and a variable resistance R2 is connected in the rotor side. Here r2 is rotor resistance.

Fig(4.2.2.) -Speed –torque characteristics of rotor resistance control

3

By varying the rotor circuit resistance R2 the starting torque and starting current can be controlled. Figure show the speed - torque characteristics and speed - stator current characteristics.

In this curve, by increasing the rotor circuit resistance, the maximum torque remains constant but the starting torque increases and the speed decreases.

Fig(4.2.3.) -Speed –Stator current characteristics of rotor resistance control

From this curve, by increasing the rotor circuit resistance, the stator current decreases and the speed decreases.

Advantages of this method

1. Absence of in-rush starting current

2. Availability of full-rated torque at starting

3. High live power factor

4. Absence of line current harmonics

5. Smooth and wide range of speed control

4

Main drawbacks of this speed control

1. Reduced efficiency because the slip energy is wasted in the rotor circuit resistance.

2. Speed changes very widely with load variation

3. Unbalance in voltage and current if rotor circuit resistance are not equal.

4.2.2 Slip ring induction motor speed control with rotor circuit chopper or static

rotor resistance control

Fig (4.2.2) Slip ring induction motor speed control with rotor circuit chopper or static

rotor resistance control.

5

The speed of a wound rotor induction motor can be varied by varying the rotor circuit resistance, The rotor resistance can be varied steplessly by using a diode bridge rectifier and chopper as shown in fig

This method of speed control is very inefficient because the slip energy is wasted in rotor circuit resistance. However, advantages are that high starting torque is available at low starting current and improved power 'factor is possible with wide range of speed control.

The stator of the machine is directly connected to the line power supply and in the rotor circuit, slip voltage is available across the slip rings. This slip voltage is rectified by the three phase diode bridge rectifier. The dc voltage is convened to current source Id by connecting a large series inductor Ld. It is then fed to shunt chopper with resistance R as shown figure . The chopper circuit may use IGBT, GTO, thyrister or any other power semiconductor devices. Here the dc chopper circuit consists of an IGBT.

Fig (4.2.2.1) Slip ring induction motor speed control with rotor circuit chopper or static

rotor resistance control

The chopper periodically connects and disconnects the resistance R. When the IGBT

chopper is on, the resistance is short-circuited and the current Id is passed through

it. i.e. Vdc = Vd., = 0 and R = O. It is indicated as shown in figure.

6

When the lGB'I` chopper is off; the resistance is connected in the circuit and the ‘ dc

link current I, flows through it. i.e. Vdc = Vd, and resistance in the rotor circuit is R.

lt is indicated as shown in figure

7

The effective external resistance Re is

During on time of the chopper R= 0 i.e

The effective resistance between terminals A and B is given by

8

The effective rotor circuit resistance Re can be varied by varying the duty cycle of the

chopper .Therefore the developed torque and speed of the machine can be controlled

by the variation of the duty cycle of the chopper.

Power consumed by effective resistance Re is

The rotor current waveform is shown in fig,when the ripple is neglected,the RMS value

of of rotor current is

9

Current waveform

The per phase power consumed by resistance Re*

10

-(4)

Hence effective value of resistance per phase is given by

4.2.3 Analysis of Induction motor with chopper control

The equivalent circuit for 3 phase induction motor, diode bridge rectifier and chopper-circuit is shown in figure . The stator and rotor leakage impedances are neglected as compared to inductor Ld. It is indicated as shown in the equivalent circuit of figure.

The stator voltage V1 when referred to rotor circuit gives slip-frequency voltage as

11

sE2 = rotor induced emf per phase at stand still

V1 =stator voltage per phase

b= rotor effective turns N2

Stator effective turns N1

= per phase turns ratio of rotor to stator

Voltage sE2 = sbV1 is applied to the three phase diode bridge rectifier and the

rectified output voltage is,

12

13

14

15

4.3 Closed loop control for static rotor resistance control

Fig (4.3) Closed loop control for static rotor resistance control

For satisfying the transient and steady state performance of induction motors, a

closed loop control is normally used. The IGBT chopper circuit allows the external

rotor resistance to be varied statistically and steplessly, and provides a low cost

variable speed drive with a good dynamic response. Figure shows closed loop control

for rotor resistance chopper circuit control. The rotor slip power is converted into dc

by using diode bridge rectifier and fed through a smoothing inductor Ld to a resistor

R. A single IGBT chopper is connected across the resistor. The IGBT chopper is on and

off by control circuit. When the chopper is on, the resistance is short circuited. When

16

the chopper is off, the resistance is included in the circuit. By varying the duty cycle

Ton / T of the IGBT chopper the effective resistance can be varied. Due to the

variation of the rotor resistance the motor speed also varied. The control signal (pulse)

can be obtained from sensing of speed and current. The actual speed is fed back from

a tachogenerator coupled to the slip ring induction motor and compared with a

reference voltage (set speed). The error voltage is amplified by the speed amplifier and

set the desired current reference.

The actual current can be obtained from current sensing circuit and compared with

actual current and set current. The error output goes to the current amplifier and

driver circuit. The current feedback loop adjusts the current of the system by

controlling the IGBT chopper. By controlling on and off times of chopper, the effective

value of rotor resistance can be determined and thus controls the motor speed by

altering its torque-speed characteristics. By connecting a capacitor in series with the

external resistance,it is possible to obtain a variation in the effective resistance from

zero to unity, thus permitting a wider range of speed control. The rotor resistance

control is used in the high-torque range.

4.4 SLIP POWER RECOVERY SYSTEM

This system is mainly used for speed control of slip ring induction motor. The speed of

slip ring IM can be controlled either by varying the stator voltage or by controlling the

power flow in the rotor circuit.

It has been discussed earlier that the power delivered to the rotor across the air gap

(Pag) is equal to the mechanical power (Pm) delivered to the load and the rotor copper

loss (Pcu). Thus

17

T = electromagnetic torque developed by the motor

ωs = Synchronous angular velocity

The air gap flux of the machine is established by the stator supply and it remains

practically constant if the stator impedance drops and supply voltage fluctuations are

neglected. The rotor copper loss is proportional to slip.

The speed control of a slip ring induction motor by connecting the external resistance

in the rotor side.

This slip power can be recovered to the supply source can be used to supply an

additional motor which is mechanically coupled to the main motor. This type of drive

is known as a slip power recovery system and improves the overall efficiency of the

system.

4.5 Types of Slip Power Recovery System

The slip power recovery system can be classified two types

1. Kramer system

2. Scherbius system

These two systems can further be classified two methods

18

1. Conventional method

2. Static method

4.6 Kramer System

The Kramer system is only applicable for sub synchronous speed operation. The

classification of Kramer system is,

1. Conventional Kramer system

2. Static Kramer system

4.6.1 Conventional Kramer system

Fig (4.6.1) conventional Kramer system

Fig shows conventional Kramer system .The system consists of 3 phase rotary

converter and dc motor. The slip power is converted into dc power by a rotary

converter and fed to the armature of a dc motor.

The slip ring induction motor is coupled to the shaft of the dc motor .The slip rings

are connected to the rotary converter. The dc output of rotary convener is used to

drive a dc motor. The rotary converter and dc motor are excited from the dc bus bars

or from an exciter .The speed of slip ring induction motor is adjusted by adjusting the

speed of dc motor with the help of a field regulator.

This system is also called the electromechanical cascade, because the slip frequency

power is returned as mechanical power to the slip ring induction motor shaft by the dc

motor.

19

If the mechanical losses in cascade system are neglected the shaft power output

of the SRIM motor is

Pin - input power to the stator

The slip power Ps = sPin is added to Pm by converting it to mechanical power by the

dc motor.this mechanical power is fed to the slip ring induction motor shaft.this

method is used for large motor of 4000KW or above.

Advantages

l. The main advantage of this method is that any speed, within the working range can

be obtained.

2. If the rotary converter is over excited, it will take a leading current which

compensates for the lagging current drawn by SRIM and hence improves the power

factor of the system.

4.6.2 Improved modified Kramer system

20

Fig (4.6.2) Improved modified Kramer system

21

Fig (4.6.2.1) Improved modified Kramer system using three phase controlled rectifier.

22

4.6.3 Static Kramer System

Fig (4.6.3) Static Kramer system

In rotor resistance control method the slip power is wasted in the rotor circuit

resistance. Instead of wasting the slip power in the rotor circuit resistance, it can be

converted to 50 Hz ac and pumped back to the line. Here, the slip power can flow only

in one direction. This method of drive is called static Kramer drive. It is shown in

figure . The static Kramer drive offers speed control only for sub-synchronous speed.

i.e. speed can be control only less than the synchronous speed is possible.

In this method, the slip power is taken from the rotor and it is rectified to dc voltage by

3-phase diode bridge rectifier. Inductor Ld smoothens the ripples in the rectified

voltage Vd. This dc power is converted into ac power by using line - commutated

inverter. The rectifier and inverter are both line commutated by alternating emfs

appearing at the slip rings and supply bus bars respectively. Here, the slip power flows

from rotor circuit to supply, this method is also, called as constant - torque drive.

The static Kramer drive has been very popular in large power pump and fan type

drives, where the range of speed control is limited near, but less than the synchronous

speed. This method of speed control is economical because the rectifier and inverter

23

only have to carry the slip power of the rotor ,which is considerably less than the input

power to the stator.

4.7 Closed loop control for Kramer system

Fig(4.7) Closed loop control for Kramer system

The actual speed is fedback from a tachogenerator, which is coupled to the SRIM. This

actual speed is compared with a reference voltage (ref Speed). The error voltage is

amplified by the speed amplifier and set the desired current reference. The current

feedback loop adjusts the current of the system by controlling the firing angles of the

inverter. This current determines the motor torque. The current signal is proportional

to the ac current of the inverter. This is compared with the current reference set by the

speed amplifier.

The error voltage is amplified by the current amplifier and fed to the firing angle

control circuit of the inverter. Thus, in this system, speed error produces a motor

torque which again reduces the error. The maximum current limit can be set to any

desired value by setting the current reference through the speed - error amplifier.

Thus the current can be limited to any desired value even under the stalled condition.

24

The acceleration and deceleration is fairly smooth. The cascade drive control system is

much simpler and stable than any other variable - speed slip ring induction motor

drive system in which the rotor slip is measured and controlled.

4.8 Scherbius System

The scherbius system is similar to Kramer system but only the difference is that in

the Kramer system the feedback is mechanical and in the scherbius system the

return power is electrical. The different types of scherbius system are

1. Conventional scherbius System

2. Static Scherbius drive

4.9 Conventional Scherbius Drive

Fig (4.9) Conventional Scherbius Drive

Here the rotary converter converts slip power into dc power and the dc power fed to

the dc motor. The dc motor is coupled with induction generator. The induction

generator converters the mechanical power into electrical power and return it to the

supply line. The SRIM speed can be controlled by varying the field regular of the dc

motor.

4.10 Static Scherbius System

25

For the speed control of SRIM both below and the above synchronous speed,static

scherbius drive system is used.This system can again be classified as

1. DC link static scherbius drive

2. Cycloconverter static scherbius drive

4.10.1 DC link static scherbius drive

This system consists of SRIM,2 no of phase controlled bridges,smoothing inductor and

step up transformer.this system is used for both sub-synchronous speed and super-

synchronous speed operation.

4.10.1.1 Sub-Synchronous speed operation

In sub-synchronous speed -control of SRIM, slip power is removed from the rotor

circuit and is pumped back into the ac supply. Figure shows the dc link static

Scherbius system. ln the Scherbius system, when the machine is operated at sub-

synchronous speed, phase controlled bridge l operates in the rectifier mode and bridge

2 operates in the inverter mode. In other words, bridge l has firing angle less than 90°

whereas bridge 2 has firing angle more than 90°. The slip power Flows from rotor

circuit to bridge l, bridge 2, transformer and returned to the supply i.e.

26

Fig (4.10.1.1) Sub-Synchronous speed operation

4.10.1.2 Super Synchronous Speed operation

In super synchronous speed operation, the additional power is fed into the rotor circuit at slip frequency. Figure shows super synchronous speed operation ofa DC link static Scherbius system. In the Scherbius system, when the machine is operated at super synchronous speed, phase controlled bridge 2 should operate in rectifier mode and bridge l in inverter mode.

In other words, the bridge 2 has firing angle less than 90° whereas bridge l has tiring angle more than 90°. The slip power flows from the supply to transformer, bridge 2 (rectifier), bridge l (line commutated inverter) and to the rotor circuit.

27

Fig (4.10.1.2) Super Synchronous Speed operation

Near synchronous speed, the rotor voltage is low, and forced commutation must be employed in the inverter, which makes the scheme less attractive. The replacement of sirt diodes by six thyristors increases the converter cost and also necessitates the introduction of slip frequency gating circuit.

Difficulty is experienced near synchronous when the slip frequency emfs are insufficient for line or natural commutation and special connections or forced commutation methods are necessary for the passage through synchronism Thus, the provision of super synchronous speed control unduly complicates the static converter cascade system and nullifies the advantages of simplicity and economy.

4.10.2 Cycloconverter Static Scherbius Drive

28

Fig (4.10.2) Cycloconverter Static Scherbius Drive

The Kramer drive system has only a forward motoring mode (one quadrant) of operation But this system is applicable 'for both motoring and regenerating in both subsynchronous and super synchronous ranges of speed.

The dual bridge convener system in figure can be replaced by a three phase controlled line commutated cylcoconverter, as shown in figure Here the slip power flow in either direction.

The various modes of operation shown in figure can be explained as follows. Assuming motor shaft torque is constant and the losses in the motor and cyclo converter are negligible.

Mode –I Sub Synchronous motoring

This mode, shown in figure (a) is similar to that of the static Kramer system. The stator input or air gap power Pag remains constant and the slip power sPag, which is proportional to the slip (which is positive), is returned back to the line through the cycloconverter.

29

Therefore the line supplies the net mechanical power Pm = (1-s) Pag consumed by the shaft.

The slip frequency power in the rotor creates the rotating filed in the direction as in the stator and the rotor speed ωr corresponds to the difference (ωs-ωsl) between these two frequencies.

At slip is equal to zero,the cycloconverter supplies dc excitation to the rotor and the machine behave like a standard synchronous motor.

Mode 2 : Super Synchronous Motoring

30

In this mode, as shown in figure(b), the shaft speed increases beyond the synchronous speed, the slip becomes negative and the slip power is a absorbed by the rotor. The slip power sPag supplements the air gap power Pag for the total mechanical power output (l + s) Pag. The line therefore supplies slip power in addition to stator input power.

During this condition, the slip voltage is reversed, so that the slip frequency- induced rotating magnetic field is opposite to that of the stator.

Mode 3: Sub –synchronous Regeneration

In regenerative braking condition, as shown in figure (c), the shaft is driven by the load and the mechanical energy is converted into electrical energy.

With constant negative shaft torque, the mechanical power input to the shaft Pm = (l-s) Pag increases with speed and this equals the electrical power fed to the line.

31

In the subsynchronous speed range, the slip s is positive and the air gap power Pag is negative. The slip power sPag is fed to the rotor from the cycloconverter so that the total air gap power is constant. The slip voltage has a positive phase sequence.

At synchronous speed, the cycloconverter supplies dc excitation current to the rotor circuit and the machine behaves as a synchronous generator. The main application in this is a variable-speed wind generation system.

Mode 4: Super synchronous regeneration

The super synchronous regeneration is indicated as shown in figure (d). Here, the stator output power remains constant, but the additional mechanical power input is reflected as slip power output Now the rotor field rotates in the opposite direction because the cycloconverter phase sequence' is reversed.

Power distribution as a function of slip in subsynchronous and supersynchronous speed ranged is summarized for all four modes in figure below, where the operating speed range of ± 50 percent about the synchronous speed is indicated.

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Advantages of static Scherbius Drive

1.In this method,the problem of communication near synchronous speed disappears.

2.The cycloconverter can easily operates as a phase controlled rectifier ,supplying dc current in the rotor and permitting ture synchronous machine operation.

3.the near-sinusoidal current waves in the rotor,which reduce harmonic loss ,and a machine over excitation capacity that permits leading power factor operation on the stator side so the line’s power factor is unity.

4.The cycloconverter is to be controlled so that its output frequency tracks precisely with the slip frequency.

Disadvantages

1.The cycloconverter cost is increases

2.The control of the scherbius drive is some what complex.

Applications

1.Multi-MW,variable-speed pumps / generators

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2.Flywheel energy storage systems.

Unit V Synchronous Motor Drives

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UNIT V

SYNCHRONOUS MOTOR DRIVES

5.1 Introduction

Synchronous motor is an AC motor which rotates at synchronous speed at all loads.

Construction of the stator of synchronous motor is similar to the stator of an induction motor.

But the rotor has a winding.

5.2 Types of synchronous motors

5.2.1 Wound field synchronous motor

Rotor of this motor has a winding for which a dc supply is given.

Rotor may have either cylindrical structure or salient pole structure. Motors with

cylindrical construction are used for high power and high speed applications.

Salient pole construction is used for low power and low speed applications due to low

cost.

5.2.2 Permanent magnet synchronous motor

It is similar to a salient pole synchronous motor without field winding on the poles.

Field flux is produced by permanent magnets mounted on the rotor.

Ferrite magnets are used to construct the permanent magnets.

Cobalt – samarium made magnets may be used if the volume and weight of the motor is

to be reduced.

The motor losses are less because of the absence of field winding and two slip rings.

For the same size, a PMSM has higher pull-out torque and more efficiency as compared

to salient pole motor.

These motors are used in medium and low power applications like robots and machine

tools.

The main disadvantage in this motor is the inability to adjust the field current.

5.2.3 Synchronous reluctance motor

It has salient poles. But there is no field winding or permanent magnet.

A salient pole synchronous motor connected to a voltage source runs at synchronous

speed.

If its field current is switched off, it continues to run at synchronous speed as a

reluctance motor.

The motor is operated by the reluctance torque. This torque is produced by the

alignment of the rotating flux with the stator flux at synchronous speed.

These motors are used for low power drives where constant speed operation is required.

5.2.4 Hysteresis synchronous motor

These motors are employed in low power applications requiring smooth start and noise

less operation.

The motor has low starting torque and hence it is suitable for high inertia loads.

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5.3 Variable Frequency Control

We know that, the synchronous speed is given by,

From the above equation, it is clear that the speed of a synchronous motor can be

controlled by varying the frequency of the supply.

As in the case of induction motors, the stator flux is maintained constant by keeping the

(v/f) ratio constant in this motor also. Constant flux operation ensures that the

maximum torque at all frequencies is same.

v/f ratio is increased at low frequencies to increase the torque producing capability of

motor.

Above rated speed, the stator voltage is kept constant and the frequency alone is

increased. In this case, the torque produced by the motor may be reduced.

Variable frequency control may be achieved by any one of the methods listed below.

1. True synchronous mode (or) separate controlled mode.

2. Self synchronous mode (or) self controlled mode.

5.3.1 True synchronous mode (or) Separate controlled mode

In this mode of speed control, the stator supply frequency is controlled from outside by

using a separate oscillator.

The frequency is changed from one value to the other gradually so that the difference

between synchronous speed and rotor speed is small during any speed change.

This gradual change in frequency helps the rotor to follow the stator speed properly at

all operating points.

When the desired speed is reached, the rotor gets locked with the stator flux speed (rotor

pulls into step) after hunting oscillations.

The block diagram of self control of multiple synchronous motors is shown in Fig. 5.1

Fig. 5.1

Here a voltage source inverter is used to feed the

synchronous motors. It may be either a stepped

wave inverter or a PWM inverter.

A rectifier is used to supply dc voltage to the

inverter. The rectifier will be a full converter if a

six step inverter is used.

If a PWM inverter is used, then a diode rectifier is

sufficient at the input side.

A smoothing inductor is used to filter out the

ripples present in the dc link voltage.

The frequency command f* is applied to the VSI

through a delay circuit. This delay circuit ensures

that the rotor follows the stator speed.

5.3.2 Self control mode of synchronous motor drive

In self control, the stator supply frequency is changed proportional to the rotor speed.

Hence the stator rmf rotates at the same speed as the rotor speed.

This ensures that the rotor moves in synchronism with stator at all operating points.

Consequently a self controlled motor will never come out of synchronism or step.

It does not suffer from hunting oscillations.

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Disadvantages of open loop control

Hunting of motor

Problems of instability

Poor dynamic behavior

Harmonic distortion

All the above disadvantages except harmonic distortion may be completely eliminated by

using the motor in self control mode.

The block diagram of a self controlled motor fed from a 3 phase inverter is shown in Fig.

5.2.

The inverter may be a CSI or VSI. Depending on the type of inverter, the input dc source

may be a controllable current source or controllable voltage source.

Fig. 5.2 Self Controlled Synchronous Motor

The inverter output frequency is determined by the rotor speed.

The accurate speed of the rotor is tracked by using rotor position sensors.

The output of rotor position sensor is used to produce firing pulses for the semi

conductor switches used in the converter which feeds the motor.

It means that the instants at which the switching devices operate to turn the stator

windings ON and OFF is determined by the rotor position sensors.

The switches are fired at a frequency proportional to the motor speed.

With the increase of load if the rotor slows down, then the stator supply frequency

automatically changes so that the rotor remains synchronized with the rotating field.

When the motor starts from rest, the motor current will be large at first and then will

decrease with increase of speed.

The speed of the motor is controlled by varying the dc link voltage to the inverter.

This dc link voltage is controlled by varying the firing pulses of the controlled rectifier.

Four quadrant operation is possible if the inverter is fed from a full converter.

5.4 Self controlled synchronous motor fed from a load commutated thyristor inverter

A self controlled synchronous motor employing a load commutated thyristor inverter is

shown in Fig. 5.3

The drive employs two converters. One is called the side converter and the other is

called the load side converter.

Source side converter

It is a line commutated thyristor converter. It works as a line commutated controlled

rectifier in the firing angle range of

Its output voltage Vds and the output current Id are positive.

Source side converter works as a line commutated inverter in the firing angle range of

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Now the voltage Vds is negative and the output current Id are positive.

Load side converter

When synchronous motor operates at leading power factor, the thyristors of the load

side converter can be commutated by the motor induced voltages.

It is called load commutation. This converter operates as an inverter and delivers a

negative Vdl and positive Id in the firing angle range of

It operates as a rectifier and delivers a positive Vdl and Id in the firing angle range of

Fig. 5.3

Fig. 5.2 Self controlled synchronous motor drive employing a load commutated inverter

The synchronous motor can be operated at leading power factor by adjusting the field

excitation of it.

In this condition, the inverter operates as line commutated inverter.

When source side converter is operated as rectifier and load side converter as inverter,

then the power flows from ac source to the motor which gives motoring operation.

When source side converter is operated as inverter and load side converter as rectifier,

then the power flows from the motor to ac source which gives regenerative braking

operation.

The torque produced by the motor depends on the difference in voltages Vds & Vdl. i.e (Vds

– Vdl).

The speed of the motor is changed by changing the voltage Vds which in turn is changed

by varying the firing angle of source side converter.

When the source side and load side converters are working as inverters, the firing angle

of each thyristor switches should be less than 1800 to avoid the short circuit of the dc

supply.

It may happen if two devices in the same leg conduct when firing angle is 1800. So care

should be taken for commutation overlap and turn off of thyristors.

Let the commutation lead angle for load side converter as βl. Then,

If commutation overlap is neglected, then the input ac current will lag the input dc

voltage by an angle l.

As the motor current is opposite to converter input current, the motor current will lead

the terminal voltage by an angle βl. Hence the motor operates at leading power factor.

For low values of βl, the power factor will be high and the inverter rating will be low.

The value of βl may be reduced by reducing the sub transient inductance of the machine.

It is done by using damper windings.

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When the load side converter acts as inverter, it is operated with a fixed commutation

lead angle βlc and when it acts as rectifier, it is operated with β = 1800.

At high power factor, the rating of the converter required is reduced. This is achieved by

operating the load side converter with constant margin angle control.

If µ is the commutation overlap of thyristor under commutation, then the duration for

which reverse bias applied is,

For successful commutation,

Where tq is the turn off time of thyristors.

The commutation overlap is proportional to the dc link current Id. Keeping a minimum

value of , the value of can be calculated.

Keeping , the value of will be reduced and hence power factor will improve.

This control scheme is called constant margin angle control.

At low speeds, motor voltage will be less and not enough for commutating the thyristors.

Hence force commutation is used when the motor speed is below 10% of rated speed.

5.5 Closed loop speed control of load commutated inverter fed synchronous motor

drive

Close loop control shown in Fig. 5.4 employs outer speed control loop and inner current

control loop with a limiter

The terminal voltage sensor generates reference pulses whose frequency is same as that

of the induced voltages in the rotor.

These reference signals are shifted suitably by phase delay circuit to produce a constant

commutation lead angle.

Based on the speed error, the value of βlc is set to provide either motoring or braking

operation.

Motoring operation is required to increase the speed and braking is required to reduce

the speed.

Actual speed of the rotor is sensed either from terminal voltage sensor or by using a

separate tachometer.

Fig. 5.4

Increasing the speed

If the speed is to be increased, then it is given as reference speed ωm*.

Actual speed and reference speed are compared at the comparator and it produces a

positive speed error.

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Now the firing circuit produces βlc corresponding to motoring operation.

The speed controller and current controller set the dc link current reference at the

maximum allowable value.

Now the machines starts accelerating and when rotor speed reaches the reference speed,

the current limiter de-saturates and the acceleration stops.

Hence the drive runs at constant speed at which motor torque is equal to load torque.

Decreasing the speed

If the speed is to be decreased, then it is set as reference speed ωm*.

Actual speed and reference speed are compared at the comparator and it produces a

negatitive speed error.

Now the firing circuit produces βlc corresponding to braking operation.

The speed controller and current controller get saturated and set the dc link reference

current at the maximum allowable value.

Now the machines starts decelerating (braking operation) and when rotor speed reaches

the reference speed, the current limiter de-saturates and the deceleration stops.

Hence the drive runs at constant speed at which motor torque is equal to load torque.

Advantages of this drive

High efficiency

Four quadrant operation with regenerative braking is possible

Drives are available for high power ratings up to 100 MW

High speed operation is possible. (up to 6000 rpm)

Applications of this drive

High speed and high power drives for compressors, blowers, pumps, fans, conveyers etc.

5.6 Power factor control of Synchronous Motor

By varying the excitation of a synchronous motor, it can be made to operate at lagging,

leading and unity power factor.

The V curve of a synchronous machine shows armature current as a function of field

current. With increasing field current, the armature current at first decreases, then

reaches a minimum, then increases.

The minimum point is also the point at which power factor is unity.

Excitation at which the power factor is unity is termed normal excitation voltage. The

magnitude of current at this excitation is minimum.

The current drawn from by the motor will be minimum at unity power factor. Hence

power losses will be minimum and the efficiency increases.

Excitation voltage more than normal excitation is called over excitation voltage.

Excitation voltage less than normal excitation is called under excitation.

Power factor is varied by varying the field current of the synchronous motor. This is

possible in wound field machine.

Motor voltage and current are sensed and given to power factor calculator where the

phase angle between the current and voltage is computed.

An over-excited synchronous motor has a leading power factor. This makes it useful

for power factor correction of industrial loads.