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EEE Dept./Mahalakshmi Engineering college,Trichy-621213

MAHALAKSHMI ENGINEERING COLLEGE

TIRUCHIRAPALLI – 621213

QUESTION BANK WITH ANSWER

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Sub. Code : EE2352 Semester : VI

Subject : SOLID STATE DRIVES Unit : IV

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INDUCTION MOTOR DRIVES 1. What are the different methods of braking applied to the induction motor?

Regenerative braking

Plugging

Dynamic braking.

2. What are the different methods of speed control of IM?

Stator voltage control

Supply freq. control

Rotor resistance control

Slip power recovery control.

3. What is meant by stator voltage control?

The speed of the IM can be changed by changing the stator voltage, because

the torque is proportional to the square of the voltage.

4. Mention the application of stator voltage control.

This method is suitable for applications where torque demand reduced with

speed, which points towards its suitability for fan and pump drives.

5. Mention the applications of ac drives.

AC drives are used in a no. of applications such as fans, blowers, mill run-

out tables, cranes, conveyors, traction etc.

6. What are the three regions in the speed-torque characteristics in the IM?

Motoring region(0<=s<=1)

Generating region(s<0)

Plugging region (1<=s<=2) where s is the slip.

7. What is the advantage of stator voltage control method?


The control circuitry is simple

Compact size

Quick response time

8. What is meant by soft start?

The ac voltage controllers show a step less control of supply voltage from zero

to rated voltage is this control is employed in motor then it is called soft start.

9. List the advantages of squirrel cage induction motor?

Cheaper

light in weight

Rugged in construction

More efficient

Require less maintenance

It can be operated in dirty and explosive environment

10. Define base speed.

The synchronous speed corresponding to the rated frequency is called the

base speed.

11. What is meant by frequency control of IM?

The speed of IM can be controlled by changing the supply frequency

because the speed is directly proportional to supply frequency. This method of speed

ctrl is called frequency control.

12. What is meant by V/F control?

When the frequency is reduced the input voltage must be reduced

proportionally so as to maintain constant flux otherwise the core will get saturated

resulting in excessive iron loss and magnetizing current. So maintaining constant

Voltage/Frequency ratio is called V/F control.

13. What are the advantages of V/F control?

Smooth speed ctrl

Small input current and improved power factor at low frequency start

Higher starting torque

14. What are the 3 modes of region in the adjustable-frequency IM drives characteristics?

Constant torque region

Constant power region

High speed series motoring region


15. What are the advantages of induction motors over D.C. motors?

The main drawback of D.C. motors is the presence of commutate and brushes,

which require frequent maintenance and make them unsuitable for explosive and dirty

environments. On the other hand, induction motors, particularly squirrel-cage are

rugged, cheaper, lighter, smaller, more efficient, require lower maintenance and can

operate in dirty and explosive environments.

16. Why the control of a three-phase indication motor is more difficult than D.C.

motors.

The control of a three-phase induction motor, particularly when the

dynamic performance involved is more difficult than D.C. motors. This is due to

a. Relatively large internal resistance of the converter causes voltage

fluctuations following load fluctuations because the capacitor cannot be

ideally

large.

b. In a D.C. motor there is a decoupling between the flux producing

magnetizing current and torque producing armature current. They can be

independently controlled. This is not the case with induction motors.

c. An induction motor is very poorly damped compared to a D.C. motor.

17. What is indirect flux control?

The method of maintaining the flux constant by providing a voltage boost

proportional to slip frequency is a kind of indirect flux control. This method of flux

control is not desirable if very good dynamic behavior is required.

18. What is the purpose of inductance and capacitance in the D.C. link circuit?

The inductance in the D.C. link circuit provides smoothing whereas the

capacitance maintains the constancy of link voltage. The link voltage is a controlled

quality.

19. What are the effects of harmonics in VSI fed induction motor drive?

If the motor receives square wave voltages this voltage has harmonic

components the harmonics of the stator current cause additional losses and heating.

These harmonics are also responsible for torque pulsations. The reaction of the fifth

and seventh harmonics with the fundamental gives rise to the seventh harmonic

pulsations in the torque developed.

20. Where is rotor resistance control used?

Where the motors drive loads with intermittent type duty, such as cranes, ore or

coal unloaders, skip hoists, mine hoists, lifts, etc. slip-ring induction motors with speed

control by variation of resistance in the rotor circuit are frequently used. This method


of speed control is employed for a motor generator set with a flywheel (Ilgner set)

used as an automatic slip regulator under shock loading conditions.

21. Why the static scherbius drive has a poor power factor?

Drive input power is difference between motor input power and the power fed

back. Reactive input power is the sum of motor and inverter reactive power.

Therefore, drive has a poor power factor throughout the range of its options.

22. How is super synchronous speed achieved?

Super synchronous speed can be achieved if the power is fed to the rotor

from A.C. mains. This can be made possible by replacing the converter cascade by a

cycloconverter. A cycloconverter allows power flow in either direction making the

static sherbets drive operate at both sub and supper synchronous speeds.

23. What is static Kramer drive?

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

converted to 60 Hz A.C. and pumped back to the line. The slip power controlled

drive that permits only a sub synchronous range of speed control through a converter

cascade is know as static Kramer drive.

24. What are the advantages of static Kramer 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 below the

synchronous speed. The drive system is very efficient and the converted power rating

is low because t has to handle only the slip power, In fact, the power rating becomes

lower with a more restricted range of speed control. The additional advantages are

that the drive system has

D.C. machine like characteristics and the control is very simple.

25. What are the causes of harmonic currents in static Kramer drive?

The rectification of slip power causes harmonic currents in the rotor, and these

harmonics are reflected to the stator by the transformer action of the machine.

The harmonic currents are also injected into the A.C. line by the inverter. As a result,

the machine losses are increased and some amount of harmonic torque is produced.

Each harmonic current in the rotor will create a reading magnetic filed and its direction

of rotation will depend on the order pf the harmonic.

26. Give the four modes of operation of a Scherbius drive


The four modes of operation of static Scherbius drive are,

Sub synchronous motoring.

Sub synchronous regeneration

Super synchronous motoring

Super synchronous regeneration

27. How is the static Scherbius drive operated in super synchronous motoring mode?

In super synchronous motoring mode, the shaft speed increases beyond

the synchronous speed, the slip becomes negative and the slip power is absorbed by the rotor. The slip power supplements the air gap power for the total mechanical power output. The line therefore supplies slip power in addition to

stator input power. At this condition, the phase sequence of slip frequency is reversed so that the slip current – induced rotating magnetic filed is opposite to

that of the stator

PART-B

1)Explain construction and working principle of three phase induction

motor.(AU/2012/11/)

Construction:

The induction motor consists of two main parts,

Stator

Rotor

Stator:

the stator made up of a stamping with alternate slot and tooth. Stamping are insulated

from each other. Each stamping is 0.4 to 0.5 mm thick. The slot house the three phase winding

just like a three phase alternator. The three phase winding is called stator winding. It may be

connected either in star or delta. The stator winding is made for a fixed number of poles.

Rotor:

There are two type of rotors used in induction motor. They are,

Squirrel cage

Slip ring


Squirrel cage Rotor:

This is made up of a cylindrical laminated core with slots to carry the rotor conductor.

The rotor conductor are heavy bars of copper or aluminum short circuited at both ends by end

rings. Hence the rotor is called a short circuited rotor.

Slip ring or wound Rotor:

The rotor winding may be star or delta connected, distributed winding, wound for as

many number of poles as the stator is wound for. Variable external resistance can be connected

in the rotor circuit, with the help of brushes and slip ring arrangements. By varying the external

resistance in the rotor circuit, the motor speed and torque can be controlled.

WORKING PRINCIPLE:


Three –phase supply is given to the stator winding , due to this current flows through the

stator winding , this current is called stator current , it produces a rotating magnetic field in the

space between stator and rotor , this magnetic field rotates at synchronous speed given by,

Ns=

Where, Ns=synchronous speed

F=supply frequency

P=number of poles for which the stator is wounded

As a result of the rotating magnetic field cutting the rotor conductors , an is emf induced in the

rotor, if the rotor winding is shorted (in cage rotor they are already shorted and in wound rotor ,

they have to be shorted externally)

Then the induced emf produces current , this current produces a rotor field (rotor emf).

The interaction of stator and rotor field developed torque , then the rotor rotates in the same

direction as the rotating maganetic field when the rotor stand still , the frequency of rotor emf is

equal to the supply frequency .

As the rotor speed up , the frequency of the rotor emf and the magnitude of rotor decrease.

The rotor tries to catch up with the rotating magnetic field however, the rotor cannot really catch

up the synchronous speed , because if it does so the relative speed would becomes zero and

then ther is no rotor induced emf , no current and hence no torque. therefore, the rotor runs at a

speed slightly less then the synchronous speed , in an induction motor the rotor speed is always

less than the synchronous speed , therefore this machine is called as synchronous machine .

The difference between synchronous speed and rotor speed is called as slip speed.

(Slip speed) = Ns - Nr

Slip, s=

So, Nr=Ns (1-s)

% slip =

At no-load, the difference between synchronous speed and rotor speed is only about 1%.

Advantages of squirrel cage induction motor

1. Cheaper


2. Light weight

3. Higher efficiency

4. Required less maintenance

Disadvantages of squirrel cage induction motor

1. Moderate starting torque

2. External resistance cannot be connected to rotor circuit

Applications of squirrel cage induction motor

Squirrel cage induction motors are used in lathes, fans, blowers water pumps, grinder

etc..,.

Advantages of slip ring induction motor

1. The starting torque can be controlled by varying the rotor circuit resistance.

2. The speed of the motor can be controlled by varying the rotor circuit resistance.

Disadvantages of slip ring induction motor

1. Wound – rotor machine is heavier.

2. High cost

3. High rotor inertia

2)Analysis and performance of three phase induction motor:

Per phase equivalent circuit of a three phase induction motor is shown in the fig. and

are the stator referred values of rotor resistance Rr rotor reactance Xr. Slip is defined by

ms

mmss

_____________ (1)

p

4ππms Rad/sec _______________ (2)

Since, stator impedance drop is generally negligible compared to terminal voltage V, the equivalent circuit can be simplified to that shown in the fig:


a) b)

from equation ( 1 )

s)(1ms _______________( 3)

From fig

'

rI =

)Xj(Xs

RR

V

'

rs

'

rs

_______________( 4)

Power transferred to rotor (or air gap power)

/sR3IP '

r

'2

rg __________(5)

Rotor copper loss is

'

r

'2

rcu R3Ip _____________(6)

Electrical power converted into mechanical power

s

s1R3IPpP '

r

'2

rcugm ______________(7)

Torque developed by motor

m

mpT

____________(8)

Substituting equation (3) and (7) yields

s

RI

3T

'

r'2

r

ms _______________(9)

Substituting in from eq (4) gives

2'

rs

'

rs

'2

r

2

msXX

s

RR

/sRV3T

_______(10)

A comparison of eqn (5) and (9) suggests that


ms

gPT

_________(11)

The developed torque is a function of slip only and slip for maximum torque is given by

2'

rs

2

s

'

rm

XXR

RS

___________(12)

Substituting from eqn (12) into (10) yields an expression for maximum torque

2'2

rs

2

ss

2

ms )X(XRR

V

2

3T

______________ (13)

Dividing eqn (10) by (13) and substituting in from (12) yields

m

'

r

sm

m

m'

s

s

max

SR

R2

s

S

S

S

sR

R12

T

T

______________ (14)

Fig: Speed –Torque characteristics of induction motor

The term m'

s

s sR

R is very small compared to I and dominating the denominator. Therefore, it can

be dropped from eqn (14)

maxT

T

S

S

S

S

2

m

m

__________________ (15)


3)Operation with unbalanced source voltage and single phasing

As supply voltage some time become unbalanced, it is useful to know the effect of unbalanced voltages on motor performance.

As three – phase set of unbalanced voltages (Va,Vb and Vc) can be resolved in to three phase balanced positive sequence, negative sequence and zero sequence voltages, using

symmetrical components relations:

Vp= 1/3(Va+αVb+2 Vc)

Vn= 1/3(Va+2 Vb+αVc)

V0= 1/3(Va+Vb+Vc) ______________ (1)

Where α= 120sinsin120cos120 je j ___________ (2)

For positive sequence voltages, equivalent circuits are same, except that V is replaced by Vp. The positive sequence rotor current and torque are obtained by replacing V by Vp. Thus

2'

rS

2'

rs

p'

rp

)Xj(XS

RR

VI

__________________(3)

2'

rS

2'

rs

2

r

2

P

ms

P

)X(XS

RR

/SRV3T

_______________(4)

The slip is

ms

mmsnS

_____________ (5)

Sn= (2-S) ______________(6)

When S is replaced by (2-S), and V is replaced by Vn, Then the following expressions are

obtained for rotor current and torque:

)Xj(XS

RR

VI

'

rS

'

rs

'

rn

_________________ (7)


2'

rS

2'

rs

2

r

2

n

ms

P

)X(XS

RR

S)-/(2RV3T

_____________ (8)

Torque has a negative sign because for negative sequence voltages the synchronous speed is the

synchronous speed is (- ms )

The rms rotor current and torque are given by

2'

rS

2'

rs

2

r

2

n

2'

rS

2'

rs

2

r

2

P

ms

nP

1/2'2

rn

'2

rp

'

r

)X(XS

RR

S)-/(2RV

)X(XS

RR

/SRV3

TTT

)I(II

______________ (9)

Positive sequence, negative sequence and the resultant speed – torque characteristics are shown

in fig:

Operation with unbalanced rotor impedance

Earlier unbalanced rotor impedance was employed in starting and speed control. It is useful to examine the effect of unbalance on motor performance. Unbalalance rotor impedance

causes the unbalance in rotor currents. The unbalanced rotor currents can be resolved in to positive and negative sequence components.


For (0≤S≤0.5), the speed of negative sequence rotor field is positive, therefore interaction between this field and stator current induced by it, produce a positive on the stator, and the fig

shows the nature of the speed torque characteristics.

Speed – torque curves with unbalanced rotor resistance

4)Explain Various method of Starting in three phase Induction Motor.

Starting arrangement is chosen based on the load requirements and nature of supply.

The following methods are employed for starting of induction motor.

Star-Delta Starter

In this method motor designed run in delta connection and connected in star during

starting, this allows reduction in stator current and voltage by 1/√3.

Circuit breakers CBr and CBs are closed to start the motor in star connection and when

the motor started to run in steady state speed, the circuit breakers CBs and CBr are closed run the motor in Delta connection


Auto- transformer starter

Reduced voltage for starting can be obtained from autotransformer.

For a secondary to primary turns ratio of aT , motor terminal voltage and current are reduced by aT . This reduces the current drawn from supply aT

2

Since the torque is proportional to square of the motor terminal voltage, it is

also reduced to2

Ta .


Rotor Resistance Starter

Wound – rotor motors are generally started by connecting rotor resistance in the rotor circuit.

The highest value of resistance is chosen to limit the current at zero speed

within safe values.

C1, C2, C3 are the closing contacts to close step by step to reduce the current

when the motor accelerates and also between the maximum and minimum for specified values.

5)Explain with a neat circuit diagram, the Braking of InductionMotor.

Following methods are employed for braking of an induction motor:


The power input to an induction motor is given by

sCOSφVI3P Sin

Where ФS is the angle between the stator phase voltage and stator phase current.

For motoring operation ФS<90 , if the rotor speed greater than the synchronous, relative

speed between the rotor conductor and air gap field is reverse.


Consequently, ФS becomes greater than the 90 and power flow reverse, giving regenerative braking

When fed from a source of fixed frequency, regenerative braking is possible only for

speed greater than the synchronous speed.


Plugging or Reverse Voltage Braking

When phase sequence of the supply with respect to terminal is changed means, the motor

also start to rotate in reverse. Then the motor shift it operation from motoring to plugging.

The slip for plugging is given by S

2S

ms

mmsn


Since the instant switchover slip can be up to 2 and motor current is large and braking

torque is low,

A special case of braking occurs when an induction motor connected to positive sequence

voltage is driven by an active load in reverse direction. Crane hoist is one such application.

Zero Sequence braking

In this case, three phases are connected in series across either single phase ac or dc, Such connection is known as zero sequence connection.

With an ac supply, resultant field is stationary in space and pulsating at frequency of supply.

However braking torque produced by this connection larger than the motoring. Motor

essentially works in regenerative braking.

TRANSIENT ANALYSIS

A rigorous analysis of operation induction motor can be done with help of d-q axis model

involving long calculation. Simple method of analysis with satisfactory accuracy for most application obtained by using steady state torque relations.

The equation for transient operation of an induction motor drive is

)(T)T(dt

dJ mlm

m


Starting and plugging

For starting and plugging torque equation is given by

)(T

SS

SS

2T

dt

dJ ml

m

m

maxm

For no load operation

SS

SS

2T

dt

dJ

m

m

maxm

max

msm

m

mm

msm

T

Jτ

dsS

S

S

S

2

τdt

dt

ds

dt

d

mτ is the mechanical time constant of the motor. It is defined as the time taken by the motor to

reach the synchronous speed from stand still under constant accelerating torque equal to the mechanical

torque of the motor.

Time required to start the induction on no load is

m

m

m SS

5.14

1t

dsS

S

S

S

2

τt

s

0.05

1

m

m

ms

Thus starting time is a function of Sm. Starting time has a minimum value of 1.22 m at Sm=0.4, When Rs

is negligible, rotor resistance required to start the motor in minimum time is

( '

rs

'

rm XX0.4()R S


Time required to stopping by plugging, when initially running at synchronous speed, can be

expressed as

1

2

75.0345.0

2 m

mm

m

mmb

SSds

S

S

S

St

Stopping time function of Sm. It has a minimum value 1.027 m at Sm =1.47 corresponding value of rotor

resistance is

'

rs

'

rm XX1.47()(R b

Time required for speed reversal by plugging when running on load is given by

m

mmm

m

mt

SSds

S

S

S

St

169.3

2

05.0

2

Minimum time for reversal is thus 2.88 m corresponding values of Sm is 0.52. Rotor resistance

required for speed reversal by plugging in minimum time is

'

rs

'

rm XX0.52()(R r

Calculation Of Energy Losses


The rotor winding loss for the starting can be written

as

'

r

s2

msss

t

0

s

'2

rss

2

ms

0

1

212

mssr

ms

ms

m

t

0

mssr

t

0

'

r

'2

rsr

R

RJ

2

1E

RIE

current gmagnitizin neglecting ding,stator win in the lossEnergy

JsdsJE

dsJTdt

Tdt

dsJ

Tdt

dJ

load nounder operating machine theAs

TdtE

dtR3IE

s

s

s

Hence, total winding loss during starting under no – load

'

r

S2

msSR

R1J

2

1E

Rotor winding loss during stopping by plugging under no load can be written

as

1

2

2

ms

2

ms

'

sr J2

3sdsJE

6) Explain the stator voltage control of Induction motor drives.

Stator Voltage control

By reducing stator voltage, speed of a induction motor can be reduced by an amount which is sufficient for speed control of pump drives.


If stator copper loss, core loss and friction loss are ignored, then the motor efficiency is

given by S1P

Pη

g

m

Stator voltage control

7) Explain the closed loop speed control of VSI fed & CSI fed induction motor

drive.

VSI fed Induction Motor Drives

Voltage source inverter allows a variable frequency supply to be obtained from a dc supply.

VSI can be operated as a stepped wave inverter or PWM inverter. When operated as stepped wave inverter, transistors are switched in sequence with time difference.

Frequency of the inverter operation varied by varying the T and output of the inverter

also varied accordingly.

Inverter output line and phase voltage are given by the following Fourier series:

t....Sin13

13

1tSin11

11

1tSin7

7

1-t5Sin

5

1tSinV

π

32V dAB


VSI fed induction motor

Stepped wave inverter line voltage waveform

CSI fed Induction Motor Drives

CSI consist of diodes D1-D6 and capacitors C1-C2 to provide commutation for the

thyristor T1- T, which are fired in with a phase difference of 60˚ in sequence of their numbers.

Inverter behaves as current source because of presence of large inductance Ld in dc link.


The fundamental component of motor phase current is dS Iπ

6I

CSI fed induction drive

Static Rotor Resistance Control


Rotor resistance can also vary steplessly using the above circuit. The ac output voltage of the rotor can be converted with Diode Bridge and it is given to parallel combination of

the resister and transistor switch.

The resistance value can be varied with help of the duty ratio of the transistor.

The RMS rotor current will be

dr I3

2I

Therefore average value of resistance between the terminals is given by

RAB= Rδ1

Power consumed by RAB is

PAB= δ)R(1IRI 2

dAB

2

d

Power consumed per phase= 2

rAB δ)I0.5R(1/3P

Thus total rotor circuit resistance per phase will be δ)0.5R(1RR TrT

8)Explain the voltage\frequency control of Induction motor drives.

Open Loop Volts/Hz Control

Open loop volts/Hz control of an induction motor is very familiar in the industry.

For adjustable speed applications, frequency control is natural.

The phase voltage command Vs* is generated from frequency command and the flux is

remain constant.


9)with necessary diagrams explain the vector control of induction motor

drives.

Vector Control of Induction motor

Vector control of induction can be controlled like a separately excited DC motor. Vector

control applicable for both induction as well synchronous motor.

In a DC machine neglecting the armature reaction effect and field saturation, the

developed torque is given by

fa

'

te IIKT


fIIKψψKT a

'

tfate

Torque Component Field Component

dsqs

'

tqsrte IIKIψKT


10) What are the various methods of speed control of 3ф induction motors and neat

diagram explain the slip power recovery scheme?

The slip power recovery system can be classified into two types

Kramer system

Scherbius system

These two systems can further be classified into two methods


Conventional method

Static method

Kramer system:

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

of Kramer system is

Conventional Kramer system

Static Kramer system

Conventional Kramer system

Fig. shows conventional Kramer system. The system consists of three phase rotary converter

and a 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 dc motor. The slip rings are connected to

the rotary converter. The dc output of rotary converter 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 field

regulator.

This system is also called the electrochemical cascade, because the slip frequency power is

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

mechanical loses in cascade system are neglected the shaft power output of the SRIM motor is,

Pm = (1-s) Pin

Where,

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. Thus, irrespective of


the value of the slip and consequently the speed of the SRIM, the power output remains more or

less constant. Hence, it is also called the constant power cascade. This method is used only for

large motors of capacity 4000KW or above.

Advantages

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

obtained.

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.

The slip power is converted into dc by a three phase diode bridge rectifier fig. this dc power

is fed to the dc motor. This dc motor is mechanically coupled to SRIM. The slip power is

converted to mechanical power and fed back to the SRIM shaft.

The torque supplied to the load is shared by SRIM and DC motor. The SRIM speed can

be controlled by controlling the field regulator (field current) of the dc motor. In this method

the speed control range is synchronous speed to around half of the synchronous speed.

In the fig. the diode bridge rectifier is replaced by thruster bridge rectifier. The speed of

SRIM can be controlled from zero to around synchronous speed, by varying the firing angle

of thirstier rectifier.


Static Kramer system:

In the rotor resistance controlled 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 50Hz ac and pumped back to the line.

Here, the slip power flows only in one direction. This method of drive is called static

Kramer drive. It is shown fig. the static Kramer drive offers speed control for sub

synchronous speed only. i.e. speed can controlled less than the synchronous speed only.

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

three phase diode bridge rectifier. Inductor Ld smoothens the ripples in the rectified voltage

Vd. this DC power is converted in to 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 constant – torque drive.


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

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

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

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

power to the stator.

SCHERBIUS SYSTEM

The scherbius system is similar to Kramer system but only 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:

Conventional scherbius drive

Static scherbius drive

CONVENTIONAL SCHERBIUS DRIVE:

Figure shows conventional method of scherbius drive. This method consists of SRIM,

rotary converter, DC motor and induction generator. 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 the electrical power and returns it to the supply line. The SRIM speed

can be controlled by varying the field regulator of the DC motor.

STATIC SCHERBIUS DRIVE:

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

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

DC link static scherbius drive

Cycloconverter static scherbius drive

DC LINK STATIC SCHERBIUS DRIVE

This system consists of SRIM, two numbers of phase controlled bridges, smoothing

inductor and step up transformer. This system used for sub synchronous and super

synchronous speed operation.

I. 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. Fig. shows the dc link static scherbius system.


In the scherbius system, when a machine is operated at sub synchronous speed, phase

controlled bridge 1 operates in the rectifier mode and bridge 2 operates in the inverter

mode. In other words, bridge 1 has firing angle less than the 900 whereas bridge 2 has firing

angle more than 900.

The slip power flows from rotor circuit to bridge 1, bridge2, transformer and return to the

supply.

Slip power→ rectifier (bridge1) → inverter bridge2→transformer→supply

II. Super synchronous speed operation

In super synchronous speed operation, the additional power is fed into the rotor circuit at

slip frequency. Fig. shows super synchronous speed operation of a 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 bridge1 in inverter

mode. In other words, the bridge2 has firing angle less than 900 and bridge 1 has firing

angle more than 900. The slip power flows from the supply to transformer, bridge2 (rectifier),

bridge1 (line commutated inverter) and to the rotor circuit.


Supply → transformer →rectifier (bridge2) → Bridge 1 (inverter) →rotor circuit

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 6 diodes by 6

thyristors increases the converter cost and also necessitate the introduction of slip frequency

gating circuit.

Difficulty is experienced near synchronous speed when 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 prevision of super

synchronous speed control unduly complicates the static converter cascade system and nullifies

the advantages of simplicity and economy.

mahalakshmimahalakshmiengineeringcollege.com/pdf/eee/visem/ee2352/unit 4.pdfgive the four modes of...

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