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-----------------------------------------------------------------------Power Semiconductor Drives

POWER SEMICONDUCTOR DRIVES

UNIT I

Control of DC motors by Single phase Converters: Introduction to Thyristorcontrolled Drives, Single Phase semi and Fully controlled converters connected to d.c

separately excited and d.c series motors continuous current operation output voltageand current waveforms Speed and Torque expressions Speed TorqueCharacteristics- Problems on Converter fed d.c motors.UNIT - II

Control of DC motors by Three phase Converters: Three phase semi and fullycontrolled converters connected to d.c separately excited and d.c series motors outputvoltage and current waveforms Speed and Torque expressions Speed Torquecharacteristics Problems.UNIT III

Four Quadrant operation of DC Drives: Introduction to Four quadrant operation Motoring operations, Electric Braking Plugging, Dynamic and Regenerative Braking

operations. Four quadrant operation of D.C motors by dual converters Closed loopoperation of DC motor (Block Diagram Only).UNIT-IV

Control of DC motors by Choppers: Single quadrant, Two quadrant and four quadrantchopper fed dc separately excited and series excited motors Continuous currentoperation Output voltage and current wave forms Speed torque expressions speedtorque characteristics Problems on Chopper fed d.c Motors Closed Loop operation( Block Diagram Only).UNIT V

Control of Induction Motor through Stator voltage: Variable voltage characteristics-Control of Induction Motor by Ac Voltage Controllers Waveforms speed torque

characteristics.UNIT VIControl of Induction Motor through Stator Frequency: Variable frequencycharacteristics-Variable frequency control of induction motor by Voltage source andcurrent source inverter and cyclo converters- PWM control Comparison of VSI and CSIoperations Speed torque characteristics numerical problems on induction motor drives Closed loop operation of induction motor drives (Block Diagram Only).UNIT VII

Control of Induction motor of Rotor side: Static rotor resistance control Slip powerrecovery Static Scherbius drive Static Kramer Drive their performance and speedtorque characteristics advantages applications problems.UNIT VIII

Control of Synchronous Motors: Separate control & self control of synchronous motors Operation of self controlled synchronous motors by VSI and CSI cycloconverters. Loadcommutated CSI fed Synchronous Motor Operation Waveforms speed torquecharacteristics Applications Advantages and Numerical Problems Closed Loopcontrol operation of synchronous motor drives (Block Diagram Only), variable frequencycontrol, Cyclo converter, PWM, VFI, CSI.

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TEXT BOOKS:

1. Fundamentals of Electric Drives by G K Dubey Narosa Publications2. Power Electronics MD Singh and K B Khanchandani, Tata McGraw-HillPublishingcompany,1998

REFERENCES:1. Power Semiconductor Controlled Drives Gopal K Dubey PH InternationalPublications.2. Power Semiconductor Drives - S B Dewan, G R Selmon, A Straughen3. Power Electronic Control of AC drives B K Bose4. Thyristor Control of Electric drives Vedam Subramanyam Tata McGraw HillPublilcations.5. Electric Drives By N K de and P K Sen , Prentice Hall of India Pvt. Ltd.6. A First course on Electrical Drives S K Pillai New Age International(P) Ltd. 2ndEditon7. Analysis of Thyristor Power Conditioned Motors By S K Pillai, Unversity Press

(India) Ltd. Orient Longman Ltd. 1995.8. Fundamental of Electric Drives by Mohd. AEL Sharkawi by VIKAS PublishingHouse.

UNIT I

UNIT OBJECTIVE:

To introduce the thyristor controlled drives. To study the operation of single phase semi and fully controlled converters

connected to DC separately excited motor with neat circuit diagram andwaveforms and also to derive the torque speed characteristics.

To study the operation of single-phase semi and fully controlled convertersconnected to DC series motor with neat circuit diagram and waveforms and alsoto derive the torque speed characteristics.

IMPORTANT POINTS AND DEFINITIOS:

A converter is a static device, which converts fixed AC voltage with fixedfrequency into variable DC voltage.

Afiring angle may be defined, as the angle between the instant thyristor wouldconduct if it were a diode and the instant it is triggered.

A semi (or half) controlled converter is one quadrant converter where as a fullconverter is two-quadrant converter in which, voltage polarity can be reversible

but current polarity cannot be reversible because of the unidirectional propertiesof the SCR.

The average value of DC output voltage of 1 - half controlled bridge convertercan be calculated by,(i) For R load:

VO TH = (Vm/ ) (1 + Cos )


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Where VO TH is the theoretical average value of DC output voltageVm is maximum value of AC input voltage and

is the firing angle.(ii) For RL load:

VOTH = (Vm / ) (Cos - Cos )

The value of Extinction angle can be calculated by, = ( + ); Where = tan-1 (L / R)

The average value of DC output voltage of 1 - full controlled bridge convertercan be calculated by,(i) For R load:

VO TH = (2 Vm/ ) (Cos )Where VO TH is the theoretical average value of DC output voltage

Vm is maximum value of AC input voltage and is the firing angle.

(ii) For RL load:VOTH = (2 Vm / ) (Cos - Cos )The value of Extinction angle can be calculated by, = ( + ); Where = tan-1 (L / R)

In converters if the source inductance is considered the load current will nottransfer immediately from outgoing SCRs to incoming SCRs.

The period during which both outgoing SCRs and incoming SCRs areconducting is known as overlap period.

(Commutation angle) is the angular period of overlap period. The average value of DC output voltage of 1 - full controlled bridge converter

by considering the source inductance for R loadcan be calculated by,

VO TH = (2 Vm/ ) Cos (+) + (Ls / ) I0Where VO TH is the theoretical average value of DC output voltage

Vm is maximum value of AC input voltage and is the firing angle. is the commutation angle.

In DC series motors the torque is directly proportional to the square of armaturecurrent where as in DC separately excited motors torque is directly proportional tothe armature current only.

In converter fed DC series motors the during the current zero period the outputvoltage is equal to the back emf due to residual magnetism in the field, where as it

is equal to zero in DC separately excited drives.OBJECTIVE QUESTIONS:

1. A single-phase half-wave controlled rectifier has 400 sin 314 t as theinput voltage and R as the load. For a firing angle of 60 for the SCR, the average outputvoltage is

(a) 400/ (b) 300/ (c) 240/ (d) 200/


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2. A single-phase one-pulse controlled- circuit has resistance and counter emf load and 400 sin 314 t as the source voltage. For a load counter emf of 200 V, therange of firing angle control is

(a) 30 to 150 (b) 30 to 180(c) 60 to 120 (d) 60 to 180

3. In a single-phase half-wave circuit with RL load, and a freewheelingdiode across the load, extinction angle 13 is more than 1t. For a firing angle a, the BCRand freewheeling diode would conduct, respectively, for

(a) -, (b) -,-(c) - , - (d) - , -

4. In a single-phase one-pulse circuit with RL load and a freewheelingdiode, extinction angle 13 is less than 1t. For a firing angle a, the BCR and freewheelingdiode would, respectively, conduct for

(a) - , 0 (b) - , -(c), - a (d) - ,

5. A single-phase full-wave mid point thyristor converter uses a 230/200

V transformer with center tap on the secondary side. The P.I.V. per thyristor is(a) 100 V (b) 141.4 V(c) 200 V (d) 282.8 V

6. A single-phase two-pulse bridge converter has an average outputvoltage and power output of 500 V and 10 kW respectively. The SCRs used in the two-pulse bridge converter are now re-employed to form a single-phase two-pulse mid-pointconverter. This new controlled converter would give, respectively, an average outputvoltage and power output of

(a) 500 V, 10 kW (b) 250 V, 5 kW(c) 250 V, 10 kW (d) 500 V, 5 kW

7. In a single-phase full converter, for continuous conduction, each pair

of SCRs conducts for(a) - (b) (c) (d) +

8. In a single-phase full converter, for discontinuous load current andextinction angle > , each SCR conducts for

(a) (b)- (c) (d) +

9. In a single-phase semi-converter, for continuous conduction, each SCRconducts for

(a) (b)

(c) + (d) -10. In a single-phase semi converter, for discontinuous conduction and extinction angle > , each SCR conducts for

(a) - (b) - (c) (d)

11. In a single-phase semi converter, for discontinuous conduction and extinction angle < , each SCR conducts for


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(a) - (b) - (d)

12. In a single-phase semi converter, for continuous conduction, freewheeling diode con-ducts for

(a) (b) -

(d) + 13. In a single-phase semi converter, with discontinuous conduction and extinction angle > , freewheeling diode conducts for

(a) (b) - (c) + (d) 14. In a single-phase semi converter, with discontinuous conduction and extinction angle < , freewheeling diode conducts for(a) (b) - (c) - (d) Zero degree15. In a single-phase full converter, if and are firing and extinction angles respective-ly, then the load current is(a) discontinuous if (-) < (b) discontinuous if ( - ) >

(c) discontinuous if ( - ) = (d) continuous if (-) < 16. In a single-phase full converter with resistive load and for a firing angle , the loadcurrent is zero and non-zero, respectively, for(a), - (b) - , (c) , + (d) , 17. In a single-phase semi converter with resistive load and for a firing angle a, each SCRand free wheeling diode conduct, respectively, for(a), 0 (b) - , (c) +, (d) -, 018. In controlled rectifiers, the nature of load current, i.e. whether load current is

continuous or discontinuous.a) does not depend on type of load and firing angle delayb) depends both on the type of load and firing angle delayc) depends only on the type of loadd) depends only on the firing angle delay19. In a single-phase full converter, if output voltage has peak and average values of 325V and 133 V respectively, then the firing angle is(a) 40 (b) 140(c) 50 (d) 13020. In a single-phase semi converter, if output voltage has peak and average values of 325and 133 V respectively, the firing angle is

(a) 40 (b) 140(c) 73.40 (d) 8021. For a single-phase phase-controlled rectifier, with a freewheeling diode across theload,a) the instantaneous output voltage V0 is always positiveb) V0 may be positive or zeroc) V0may be positive, zero or negatived) V0 is always zero or negative


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22. In a single-phase full converter, if load current is 1 and ripple free, than averagethyristor current is

(a) 1 / 2 I (b) 1 / 3 I(c) 1 / 4 I (d) I23. In a single-phase full converter, the number of SCRs conducting during overlap is

(a) 1 (b) 2(c) 3 (d) 424. In a single-phase full converter, the output voltage during overlap is equal toa) zerob) source voltagec) source voltage minus the inductance dropd) inductance drop

SUBJECTIVE QUESTIONS:

1. Explain the operation of a single-phase semi converter fed DC separately excitedmotor in continuous current mode and derive the equation relating speed and torque andalso draw their speed torque characteristics.2. Explain the operation of a single phase semi converter fed DC series motor incontinuous current mode and derive the equation relating speed and torque and also drawtheir speed torque characteristics.3. Explain the operation of a single-phase full converter fed DC separately excited motorin continuous current mode and derive the equation relating speed and torque and alsodraw their speed torque characteristics.4. Explain the operation of a single phase full converter fed DC series motor incontinuous current mode and derive the equation relating speed and torque and also draw

their speed torque characteristics.5. A 220 V, 960rpm, 80Aseparately excited DC motor has an armature resistance of0.06ohms. Under rated conditions, the motor is driving a load whose torque is constantand independent of speed. The speeds below rated speed are obtained with armaturevoltage control (with full field), and the speeds above rated speed are obtained by fieldcontrol (with rated armature voltage). Determine (i) The motor terminal voltage when thespeed is 620rpm. (ii) The value of flux as a percentage of rated flux if the motor speed is1200 rpm. Neglect the motor rotational losses.6. A separately excited motor of 220V, 960rpm, 80A with an armature resistance of0.06ohms is coupled to an over hauling load with a toque of 100 Nm. Compute the speedat which the motor can hold the load by regenerative braking. Source voltage is 220V.

Neglect the motor rotational losses.7. A separately excited DC motor is fed from a 230V, 50Hz supply via a single-phasehalf controlled bridge rectifier. Armature parameters are: inductance 0.06H, Resistance0.3ohms. Motor voltage constant is Ka = 0.9 V/A rad/s and the field resistance is Rf104ohms. The field current is controlled by a semi converter and is set to max. possiblevalue. The load torque is 50Nm at 800 rpm. The inductances of armature and field circuitare sufficient enough to make the armature and field currents continuous and ripple free.


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Compute (i) The field current If (ii) The firing angle of converter in armature circuit. (iii)The input power factor of the armature circuit converter. Neglect the system losses.8. The speed of 10HP, 210V, 1000 rpm, separately excited DC motor is controlled by asingle-phase full converter as shown in fig. The rated motor armature current is 30Ampsand the armature resistance is Ra is 0.25 ohms. The AC supply volt is 230V. The motor

voltage constant is K 0.172 V/rpm. Assume that sufficing inductance is present in thearmature circuit to make the motor continuous and ripple free. (a) Rectifier operation(Motoring action) for firing angle alpha = 450 and rated motor armature current,Determine (i) Motor torque (ii) Speed of the motor (iii) the supply power factor (b)Inverter operation. Reversing the field excitation reverses the motor back emf polarity.Determine (i) The firing angle to keep the motor current at its rated value. (ii) The powerfeed back to the supply9. A 210V, 1200rpm, 10A separately exited motor is controlled by a single phase fullycontrolled converter with an AC source voltage of 230V, 50Hz. Assume that sufficientinductance is present in the armature circuit to make the motor current continuous andripple free for any torque greater than 25% of rated torque. Ra = 1.5ohm.

10. What should be the value of the firing angle to get the rated torque at 800rpm?Compute the firing angle for the rated braking torque at 1200rpm.Calculate the motor speed at the rated torque and alpha =165 0 for the regenerativebraking in the second quadrant?11. A small separately excited DC motor is supplied via a half controlled, single phasebridge rectifier .The supply is 240Volts, the thyristors are triggered at 1100, and thearmature current continues for 500 beyond the voltage zero. Determine the motor speed ata torque of 1.8 Nm, Given the motor torque characteristics 1.0 Nm/amps and its armatureresistance is 6ohms. Neglect the all converter losses.12. The speed of 20HP, 210V, and 1000-rpm series motor is controlled by a single-phase(a) semi converter (b) full converter. The combined field and armature circuit resistanceis 0.25ohms. Motor constants are Kaf = 0.03N-m.amp2, Kres = 0.075V-sec/rad, the supplyvoltage is 230V. Assume the continuous ripple free motor current, Determine thefollowing for firing angle alpha = 300 and speed N = 1000rpm; (i) motor torque (ii) motorcurrent (iii) supply power factor

PREVIOUS SUBJECTIVE QUESTIONS:

1) A dc series motor has Ra = 3 , Rf = 3 and Maf = 0.15 H. A phase-controlled bridgevaries the motor speed. The firing angle is /4 and the average speed of the motor is 1450rpm. The applied ac voltage to the bridge is 330 Sin wt. Assuming continuous motorcurrent find the steady state average motor current and torque. Sketch the waveforms for

output voltage, current and gating signals.2) (a) Explain how the speed of a dc series motor is controlled using converters.(b) A series motor is supplied from a full converter whose = 650, 1 supply of 230Vrms, 50HZ frequency. The armature and field resistance together equal 2 . The torqueconstant Maf is 0.23H and the load torque is 20Nm. Neglect damping and find theaverage armature current and speed.3) Two independent single-phase semi-converters are supplying the armature and fieldcircuits of the separately excited dc motor for controlling its speed. The firing angle of


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the converter, supplying the field, adjusted such that maximum field current flows. Themachine parameters are: armature resistance of 0.25 , field circuit resistance of 147 ,motor voltage constant Kv=0.7032 V/A-rad/s. The load torque is T=45 N-m at 1000 rpm.The converters are fed from a 208 V, 50 Hz ac supply. The friction and windage lossesare neglected. The inductance of the filed and armature circuits are sufficient enough to

make the armature and field currents continuous and ripple free. Determine(a) the field current,(b) the delay angle of the armature converter,(c) input power factor of the armature circuit converter..4) A single-phase fully controlled thyristor converter is supplying a dc separately exciteddc motor. Draw the neat waveform diagrams and explain various operating modes of thedrive both in motoring and regenerative braking for(a) < ,(b) > ,Where : is the firing angle, : is the angle at which the source voltage equal to the motorback emf. Assume the armature of the separately excited dc motor can be replaced by

simple R-L and back emf load.5) (a) A DC shunt motor operating from a 1 half controlled bridge at a speed of 1450rpm has an input voltage 330 Sin 314t and a back emf 75V. The SCRs are firedsymmetrically at = /4 in every half cycle and the armature has a resistance of 5.Neglecting armature inductance, find the average armature current and the torque.(b) Sketch the speed-torque characteristics for the above problem.

UNIT II

UNIT OBJECTIVE:

To study the operation of three phase semi and fully controlled converters

connected to DC separately excited motor with neat circuit diagram andwaveforms and also to derive the Torque - Speed characteristics. To study the operation of three-phase semi and fully controlled converters

connected to DC series motor with neat circuit diagram and waveforms and alsoto derive the Torque Speed characteristics.

To derive the Torque Speed expressions for the entire converter fed DC motordrives.

IMPORTANT POINTS AND DEFINITIONS:

The average value of DC output voltage of 3 - half controlled bridge converter

can be calculated by,(i) For R load:

VO TH = (3 Vml / 2 ) (Cos )Where VO TH is the theoretical average value of DC output voltage

Vml is maximum value of AC input line voltage and is the firing angle.

(ii) For RL load:


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VOTH = (3 Vml / 2 ) (Cos - Cos )The value of Extinction angle can be calculated by, = ( + ); Where = tan-1 (L / R)

The average value of DC output voltage of 3 - half controlled bridge convertercan be calculated by,

(i) For R load:VO TH = (3 Vml / ) (Cos )

Where VO TH is the theoretical average value of DC output voltageVml is maximum value of AC input line voltage and

is the firing angle.(ii) For RL load:

VOTH = (3 Vml / ) (Cos - Cos )The value of Extinction angle can be calculated by, = ( + ); Where = tan-1 (L / R)

The average value of DC output voltage of 3 - full controlled bridge converter

by considering the source inductance for R loadcan be calculated by, V0 TH = (3 Vml / ) Cos (+) + (3 Ls / ) I0Where V0 TH is the theoretical average value of DC output voltage

Vml is maximum value of AC input line voltage and is the firing angle. is the commutation angle.

The three phase converter SCRs are triggered at a faster rate when comparedwith single phase converter SCRs, causing the output current to be morecontinuous in the three phase converters.

OBJECTIVE QUESTIONS:

1. Each diode of a 3-phase half-wave diode rectifier conducts for(a) 60 (b) 120(c) 180 (d) 90.2. Each diode of a 3-phase, 6-pulse bridge diode rectifier conducts for(a) 60 (b) 120(c) 180 (d) 90.3. In a 3-phase half-wave diode rectifier, if per phase input voltage is 200 V, then theaverage output voltage is(a) 233.91 V (b) 116.95 V(c) 202.56 V (d) 101.28 V

4. In a 3-phase half-wave diode rectifier, the ratio of average output voltage to per-phasemaximum ac voltage is(a) 0.955 (b) 0.827(c) 1.654 (d) 1.169.5. In a 3-phase half-wave rectifier, dc output voltage is 230 V. The peak inverse voltageacross each diode is(a) 481.7 V (b) 460 V(c) 345 V (d) 230 V.


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6. In a 3-phase full-wave diode rectifier, the peak inverse voltage in terms of averageoutput voltage is(a) 1.571 (b) 0.955(c) 1.047 (d) 2.0947. In a 3-phase half-wave diode rectifier, if Vm is the maximum value of per phase

voltage, then each diode is subjected to a peak inverse voltage of(a) Vm (b) (3) Vm(c) 2Vm (d) 3Vm,8. In a 3-phase full-wave diode rectifier, ifVm is the maximum value of line voltage, theneach diode is subjected to. a peak inverse voltage of(a) Vm (b) (3) Vm(c) 2Vm (d) 3Vm9. In a 3-phase full-wave diode rectifier, if V is the per phase input voltage, then averageoutput voltage is given by(a) 0.955 V (b) 1.35 V(c) 2.34 V (d) 3 V

10. A converter which can operate in both 3-pulse and 6-pulse modes is a(a) 1-phasefull converter (b) 3-phase half-wave converter(c) 3-phase semi converter (d) 3-phase full converter.11. In a 3-phase semi-converter, for firing angle less than or equal to 60, each thyristoran diode conduct, respectively, for(a) 60, 60 (b) 90, 30(c) 120, 120 (d) 180, 18012. In a 3-phase semiconverter, for firing angle less than or equal to 60, freewheelingdiode conducts for(a) 30 (b) 60(c) 90 (d) zero degree13. In a 3-phase semiconverter, for a firing angle equal to 90 and for continuous conduc-tion, each SCR and diode conduct, respectively, for(a) 30, 60 (b) 60, 30(c) 60, 60 (d) 30, 3014. In a 3-phase semi converter, for a firing angle equal to 90 and for continuousconduction, free wheeling diode conducts for(a) 30 (b) 60(c) 90 (d) 015. In a 3-phase semiconverter, for firing angle equal to 120 and extinction angle equalto 110, each SCR and diode conduct, respectively, for(a) 30, 60 (b) 60, 60

(c) 90, 30 (d) 110, 3016. In a 3-phase semiconverter, for firing angle equal to 120 and extinction angle equalto 110, freewheeling diode conducts for(a) 100 (b) 200

(c) 50 (d) 11017. In a 3-phase semiconverter, for firing angle equal to 120 and extinction angle equalto 100, none of the bridge elements conduct for(a) 100 (b) 200

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(c)30 (d)6018. A 3-phase semi converter can work asconverter for = 0 to 180 (b) converter for = 0 to 90(c) inverter for =90 to 180 (d) inverter for = 0 to 90.19. In a 3-phase semiconverter, the three SCRs are triggered at an interval of

(a) 600

(b) 900

(c) 120 (d) 18020. In a 3-phase full converter, the six SCRs are fired at an interval of(a) 30 (b) 60(c) 90 (d) 12021. In a 3-phase full converter, three SCRs pertaining to one group are fired at an interval(a) 30 (b) 60(c) 90 (d) 12022. The frequency of the ripple in the output voltage of a 3-phase semiconverter dependsupona) firing angle and load resistance

b) firing angle and load inductancec) the load circuit parametersd) firing angle and the supply frequency23. In a 3-phase full converter, if load current is 1 and ripple free, then average thyristorcurrent is(a) 1/ 2 I (b) 1 / 3 I(c) 1 / 4 I (d) I24. The effect of source inductance on the performance of single-phase and three-phasefull converters is toa) reduce the ripples in the load currentb) make discontinuous current as continuous

c) reduce the output voltaged) increase the load voltage25. In a 3-phase full converter, the output voltage during overlap is equal toa) zerob) source voltagec) source voltage minus the inductance dropd) average value of the conducting-phase voltages.26. The total number of SCRs conducting simultaneously in 3-phaseftill converter withoverlap considered has the sequence of(a) 3, 3, 2, 2 (b) 3, 3, 3, 2(c) 3,2,3,2 (d) 2,2,2,3.

27. A 3-phase full converter has an average output voltage of 200 V for 0 firing angleand for resistive load. For a firing angle of 900, the output voltage would be(a) zero (b) 50 V(c) 100 V (d) 26.8 V28. In a 3-phase full converter, the output voltage pulsates at a frequency equal to(a) supply frequency, f (b) 2 f(c) 3 (d) 6f29. The three-phase ac to dc converter, which requires neutral point connection, is


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(a) 3-phase semi converter (b) 3-phase full converter (c) 3-phase half-wave converter (d) 3-phase full converter with diodes30. A 3-phase full converter can function asa) converter for = 00 to 1800

b) converter for == 00 to 900

c) inverter for = 900 to 1800d) inverter for = 00 to 900

31. In a 3-phase full converter, the number of SCRs working during overlap isa) 1 from positive group, 1 from negative groupb) 2 from positive group, 1 from negative groupc) 1 from positive group, 2 from negative groupd) 2 from positive group, 2 from negative group32. In a 3-phase semiconverter, frequency of the ripple in the output voltage may bea) 3 times the supply frequency/for firing angle < 600b) 3ffor > 600c) 6ffor < 600d) 6ffor > 60033. The peak inverse voltage in ac to de converter systems is highest ina) single-phase full wave mid-point converterb) single-phase full converterc) 3-phase bridge converterd) 3-phase half-wave converter


1. Explain the operation of a three-phase semi converter fed DC separately excited motor

in continuous current mode and derive the equation relating speed and torque and alsodraw their speed torque characteristics.2. Explain the operation of a three phase semi converter fed DC series motor incontinuous current mode and derive the equation relating speed and torque and also drawtheir speed torque characteristics.3. Explain the operation of a three phase full converter fed DC separately excited motorin continuous current mode and derive the equation relating speed and torque and alsodraw their speed torque characteristics.4. Explain the operation of a three phase full converter fed DC series motor in continuouscurrent mode and derive the equation relating speed and torque and also draw their speedtorque characteristics.5. A 80KW, 440V, 800rpm, DC motor is operating at 600rpm and developing 75% ratedtorque is controlled by 3-phase, 6-pulse thyristor converter. If the back emf at the ratedspeed is 410V, determine the triggering angle of converter. The input converter is 3-phase, 415V, 50Hz,AC supply?6. A 3-phase full converter controls the speed of 150HP, 650V, 1750rpm, separatelyexcited DC motor. The converter is operating from a 3-phase 460V, 50Hz supply. Therated armature current of the motor is 170Amps. The motor parameters are Ra= 0.099ohms, La = 0.73mH and Ka = 0.33 v/rpm. Neglect the losses in the converter system.


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Determine (a) No load speed of firing angle alpha = 00, 300. Assume that at no load thearmature current is 10% of rated current and it is continuous. (b) The firing angle toobtain rated speed of 1750 rpm at rated motor current and also computes the supplypower factor. (c) The speed regulation for the firing angle obtained in part.

UNIT III

UNIT OBJECTIVE:

To study the different types of braking methods used for DC motors. To study the operation of dual converter fed DC series motor in both circulating

non circulating current modes. To study the operation of dual converter fed DC separately excited motor in both

circulating non circulating current modes. To introduce the four quadrants operation of DC motors, i.e., motoring operations.

To study the block diagram of closed loop operation of converter fed DC motors.


The main basic procedure for braking of DC motors is to make the current flow inreverse direction during the braking period i.e., from armature to source.

The current flow direction can be reversible by making the back emf of the motorgreater than terminal voltage.

In all the three methods of braking DC motors, regenerative braking is advantagessince the power developed during the braking period is usefully employed to theother loads, which are connected to the source.

Dual converter is a four-quadrant converter, in which two full converters will be

connected in anti parallel and are controlled in such a way that the sum of twofiring angles should be 1800.

The condition (1 + 2) = 1800, implies that if one converter is operating inconversion mode the other one in inversion mode.

In practical dual converter with out circulating current, only one convert is inoperation at a time where as in, dual converter with circulating current twoconverters are in operation at a time.


1. A four-quadrant operation requiresa) two full converters in seriesb) two full converters connected back to backc) two full converters connected in paralleld) two semi converters connected back to back2. In circulating-current type of dual converter, the nature of voltage across reactor is(a) alternating (b)pulsating(c) direct (d) triangular


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3. In a dual converter, converters 1 and 2 work as under:a) 1 as rectifier, 2 as inverterb) l as inverter,2 as rectifierc) both as rectifiersd) both as inverters


1. a) Describe the operation of ideal dual converter.b) Describe the operation of dual converter in non-circulating current mode with neatcircuit diagram and waveforms.2. Describe the operation of dual converter in circulating current mode with neat circuitdiagram and waveforms and derive the expression for peak value circulating current.(a). Explain the need of commutation in thyristor circuits. What are the different

commutation schemes? Explain class A commutation with neat diagrams.(b). A circuit employing parallel resonance turn off (class B commutation) circuit has C =50 F, L = 20 H, V= 200 V and initial voltage across the capacitor is 200 V. Determinethe circuit turn off time for main thyristor for load R = 1.5 .3. (a) Distinguish clearly between voltage commutation and current commutation inthyristor circuits.(b) Discuss how the voltage across a commutating capacitance is reversed in acommutating circuit.(c) A circuit employing resonance pulse commutation has C= 20F and L= 3H theinitial capacitor voltage = source voltage, Vs = 230 V DC. Determine conduction time forauxiliary thyristor and circuit turn off time for main thyristor in case constant load currentis 300 A.4. Explain the operation of dual converter fed DC separately excited motor in bothcirculating current and non-circulating modes and also draw their speed torquecharacteristics.5. Explain the operation of dual converter fed DC series motor in both circulating currentand non circulating modes and also draw their speed torque characteristics.6. Explain the operation of single quadrant chopper fed DC series and separately excitedmotor in both circulating current and non-circulating modes and also draw their speedtorque characteristics.7. Explain the operation of two-quadrant chopper fed DC series and separately excitedmotor in both circulating current and non-circulating modes and also draw their speedtorque characteristics.8. Explain the operation of four-quadrant chopper fed DC series and separately excitedmotor in both circulating current and non-circulating modes and also draw their speedtorque characteristics.9. Explain the regenerative braking method used for DC series and separately excitedmotors with neat circuit diagrams.10. Explain the dynamic braking method used for DC series and separately excitedmotors with neat circuit diagrams.


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11. Explain the plugging method used for DC series and separately excited motors withneat circuit diagrams.12. A DC chopper is used to control the speed of DC shunt motor. The supply voltage tothe chopper is 220V. The on time and the off time of the chopper are 10 ms and 12ms,respectively. Assuming continuous conduction of the motor current, and neglecting the

armature inductance, determine the average load current when the motor runs at a speedof 146.60 rad/ sec and has a voltage constant Ka of 0.495 V/ A rad/sec.


1) (a) Explain how four-quadrant operation is achieved by dual converters each of 3 fullwave configuration for d.c. separately excited motor.(b) Distinguish between circulating current and non-circulating current mode of operation2) a) What is a dual converter? Explain the principle of operation of a dual converter incirculating current mode. How the same is used for speed control of dc drive.

b) A 230v separately excited dc motor takes 50A at a speed of 800rpm. It has armatureresistance of 0.4 . A chopper with an input voltage of 230v controls this motor andfrequency of 500Hz. Assuming continuous condition throughout, calculate and plotspeed-torque characteristics for:Motoring operation at duty ratios of 0.3 and 0.6.Regenerative braking operation at duty ratios of 0.7 and 0.4.3) a) Deduce the mathematical expression for minimum and maximum currents for aclass A chopper operated dc motor with back emf.b) A 220v, 24A, 1000rpm separately excited dc motor having an armature resistance of2 is controlled by a chopper. The chopping frequency is 500Hz and the input voltage is230v. Calculate the duty ratio for a motor torque of 1.2 times rated torque at 500rpm.

4) (a) Draw the circuit diagram and explain the operation of closed-loop speed controlwith inner-current loop and field weakening.(b) A single-phase fully controlled double bridge converter is operated from 120v, 60Hzsupply and the load resistance is 10 ohms. The circulating inductance is 40mH. Firingdelay angle for converter I and II are 600 and 1200 respectively. Calculate the peakcirculating current and the current through converters.5) Describe the relative merits and demerits of the following types of braking for dcmotors: mechanical braking, dynamic braking and regenerative braking with neatdiagram.


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UNIT IV

UNIT OBJECTIVE:

To study the operation of single quadrant chopper fed DC separately excitedmotor and DC series motor.

To study the operation of two-quadrant chopper fed DC separately excited motorand DC series motor.

To study the operation of four-quadrant chopper fed DC separately excited motor

and DC series motor. To study the speed torque characteristics of chopper fed DC motors. To study the block diagram of closed loop operation of chopper fed DC motors.


The function of a chopper is to convert Fixed DC voltage in to Variable DCvoltage.

In a chopper output voltage can be controlled by two strategies:(i) TRC (Time Ratio Control)(ii) CLC (Current Limit Control)

In TRC schemes PWM (Pulse Width Modulation) scheme is advantageous thanfrequency modulation scheme.

The output voltage of a step down chopper is given byVO DC = (VI DC)Where VO DC is the average value of the DC output voltage, is the duty cycle and

VI DC is the average value of the DC input voltage Duty cycle, is the ratio of ON time of the chopper to the TOTAL time of the

chopper. The output voltage of a step up chopper is given by

VO DC = (1/(1 - )) (VI DC)Where VO DC is the average value of the DC output voltage, is the duty cycle and

VI DC is the average value of the DC input voltage


1. In dc choppers, ifTonis the on period and fis the chopping frequency, then output


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voltage in terms of input voltage Vs is given by(a) Vs. Ton l f (b) Vs .f / Ton(c) V / f. Ton (d) Vs. f. Ton

2. In dc choppers, the waveforms for input and output voltages are respectively(a) discontinuous, continuous (b) both continuous

(c) both discontinuous (d) continuous, discontinuous3. In PWM method of controlling the average output voltage in a chopper, the on timeis (varied / kept constant) but the chopping frequency is(varied / kept constant).

4. In FM method of controlling the average output voltage in a chopper, choppingperiod is (varied / kept constant) but on time is (varied / kept constant) or off time is(varied / kept constant).

5. For type-A chopper, Vs is the source voltage, R is the load resistance and is theduty cycle. The average output voltage and current for this chopper are respectively(a) Vs, . (Vs / R) (b) (1-) Vs, (1- ) Vs / R(c) Vs / V, V s / R (d) Vs / (1 -), Vs /(1 - ) R

6. A chopper has Vs as the source voltage, R as the load resistance and as the duty

cycle. For this chopper, RMS value of output voltage is(a) Vs (b) () 1/2.Vs(c) Vs/ () (d) (1-) 1/2 Vs

7. For a chopper, Vs is the source voltage, R is the load resistance and a is the duty'cycle. RMS and average values of thyristor currents for this chopper are(a) .Vs / R, ()Vs / R (b) ().Vs / R , () .Vs / R(c) () Vs / R, Vs / R (d) (1-) 1/2.Vs / R, (1-) 1/2Vs / R.

8. In dc choppers, per unit ripple is maximum when duty cycle a is(a) 0.2 (b) 0.5(c) 0.7 (d) 0.9.

9. A voltage-commutated chopper has the following parameters:Vs = 200 V, Load circuit parameter: 1, 2 mH, 50 VCommutation circuit parameters, L = 25 H, C = 50FTon = 500 s, T = 2000 sFor a constant load current of 100 A, the effective on period and peak currentthrough the main thyristor are respectively(a) 1000 s, 200 A (b) 700 s, 382.8 A(c) 700 s, 282.8 A (d) 1000 s, 382.8 A.

10. For the voltage-commutated chopper of Prob. 10, the turn-off times for main andauxiliary thyristors are, respectively,(a) 120 s, 60 s (b) 100 s, 0.5 s(c) 120 s, 55 s (d) 100 s, 55.54s.

11. A load commutated chopper, fed from 200 V dc source, has a constant load currentof 50 A. For a duty cycle of 0.4 and a chopping frequency of 2 kHz, the value ofcommutating capacitor and the turn-off time for one thyristor pair are respectively(a) 25 F, 50 s (b) 50 F, 50 s(c) 25 F, 25 s (d) 50 F, 25 s

12. A dc battery is charged from a constant dc source of 200 V through a chopper. Thedc battery is to be charged from its internal emf of 90 to 120 V. The battery has


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internal resistance of 1 . For a constant charging current of 10 A, the range of dutycycle is.. to......

13. For type-A chopper; Vs, R, Io and a are respectively the dc source voltage, loadresistance, constant load current and duty cycle. For this chopper, average and RMSvalues of freewheeling diode currents are

(a) Io, () Io (b) (1 - ) Io, (1-)1/2

Io(c) Vs / R, () Vs / R (d) (1 - ) Io, () Io14. A step-up chopper has Vs as the source voltage and a as the duty cycle. The output

voltage for this chopper is given by(a)Vs (1+ ) (b) Vs / (l - )(c) Vs (1 - ) (d) Vs / (l + ).

15. A dc chopper is fed from 100 V dc. Its load voltage consists of rectangular pulsesof duration 1 m sec in an overall cycle time of 3 m sec. The average output voltageand ripple factor for this chopper are respectively(a) 25 V, 1 (b) 50 V, 1 (c) 33.33 V, (2) (d) 33.33 V, 1

16. When a series LC circuit is connected to a dc supply of V volts through a thyristor,

then the peak current through thyristor is(a) V. LC (b) V / CL(c) V. (C / L) (d) V. (L / C)

17. In dc choppers, if T is the chopping period, then output voltage can be controlled byPWM by varying(a) T keeping Ton constant (b) Ton keeping T constant(c) Toff keeping T constant (d) T keeping Toff constant.

18. In dc choppers, for chopping period T, the output voltage can be controlled by FMby varying(a) T keeping Ton constant (b) T keeping Toff constant(c) Ton keeping T constant (d) Toff keeping T constant


1. (a) What is time ratio control of chopper? Explain the operation.(b) A battery is charged from a constant DC source of 220 V through a chopper.The DC battery is to be charged from its internal emf of 90 V to 122 V. The batteryhas internal resistance of 1. For a constant current charging of 10 A, compute therange of duty cycle.

2. (a) Explain class C type of commutation used for thyristors with appropriate currentand voltage waveforms.

(b)Explain the merits and demerits of self-commutation of SCR and its othermethods of commutation.

3. (a) Describe the principle of operation of a step down chopper. Derive anexpression for the average output voltage in terms of input voltage and duty cycle.(b) A chopper circuit is operating on TRC principle at a frequency 1 KHz on a 220V DC supply. If the load voltage is 180 V, calculate the conducting and blockingperiod of thyristor in each cycle



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1) (a) Explain the principle of speed control of a dc motor and show how it can beachieved by a chopper.(b) A 230v, 1200rpm, 15A separately excited motor has an armature resistance of 1.2.Motor is operated under dynamic braking with chopper control. Braking resistance has avalue of 20.

i. Calculate duty ratio of chopper for motor speed of 1000rpm and braking torque equal to1.5 times rated motor torque.ii. What will be the motor speed for duty ratio of 0.5 and motor torque equal to its ratedtorque.2) (a) Explain with neat circuit diagram the basic principle of operation of a class A typeof chopper. The chopper is connected to R-L-E load. Analyze the same for continuouscurrent mode of operation.(b) A dc supply of 200v supplied power to separately excited dc motor via a class Athyristors chopper. The motor has an armature circuit resistance of 0.33 and inductanceof 11mH. The chopper is fully on at the rated motor speed 1200rpm when the armaturecurrent is 20A. If the speed is to be reduced to 800rpm with the load torque constant,

calculate the necessary duty cycle. If the chopper frequency is 500Hz, is the currentcontinuous?3) (a) Discuss with the suitable diagrams I quadrant and II quadrant choppers.(b) A constant frequency TRC system is used for the speed control of dc series tractionmotor from 220v dc supply. The motor is having armature and series field resistance of0.025 and 0.015 respectively. The average current in the circuit is 125A and thechopper frequency is 200Hz. Calculate the pulse width if the average value of back emf is60 volts.4) (a) List the advantages offered by dc chopper drives over line-commutated convertercontrolled dc drives.(b) A dc chopper controls the speed of dc series motor. The armature resistance Ra =

0.04, field circuit resistance Rf = 0.06, and back emf constant Kv = 35 mV/rad/s. Thedc input voltage of the chopper Vs=600v. If it is required to maintain a constantdeveloped torque of Td = 547N-m, plot the motor speed against the duty cycle K of thechopper.5) A class-A chopper, operating in time-ratio control, is supplying the armature of theseparately excited dc motor. Show that the motor speed-torque relationship is m =(.V/K)-(Ra.Ta/K2) , Where V - chopper input voltage, Ra - Armature resistance, Ta -motor torque, K- torque constant.


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UNIT V

UNIT OBJECTIVE:

CONTROL OF INDUCTION MOTOR FROM STATOR SIDE

To study the operation of stator voltage control method of induction motor by ACvoltage controllers both for star and delta connected stator windings. And their

speed torque characteristics.AC VOLTAGE CONTROLLERS:

The function of AC voltage controller is to convert the fixed AC voltage intovariable AC voltage with out changing the frequency.

Varying the firing angle can control the output voltage. The harmonics in the output voltage are minimized by using more and more

number of AC voltage controller units in Synchronous tap changer. The voltage is calculated by using the formula(i) For R load:

V0 rms = Vph {[( - ) + (1/2) Sin 2 ] / } 1/2

Where V0 rms is the theoretical RMS value of the output voltage,Vph is the phase voltage of the input voltage and

is the firing angle

(ii) For RL load:Vo rms = Vph {[( - ) + (1/2)(Sin 2 - (1/2) Sin 2)] / 2} 1/2

Theoretically the value of Extinction angle can be calculated by, = ( + ); Where = tan-1 (L / R)

In AC voltage controllers Self-commutation is used.


The function of inverters is to convert Fixed DC voltage into Variable voltage ACwith variable frequency.

In a Voltage source inverters, the DC source has negligible impedance where asinn Current source inverters, the DC source is having high impedance.

In all the inverters self-commutation is employed except for Mc Murray and McMurray Bedford inverters. For these two inverters forced commutation is used.


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In Mc Murray inverter, Auxiliary commutation is used and in Mc Murray Bedford inverter Auxiliary impulse commutation is used.

In all of the voltage control techniques used for inverters, Pulse Width modulation(PWM) technique is more advantageous one and also it reduces the harmoniccontent in the output waveform.

In single-phase parallel inverter, the capacitor is used for commutation. The function of inverters is to convert Fixed DC voltage into Variable voltage AC

with variable frequency. In a Voltage source inverters, the DC source has negligible impedance where as

inn Current source inverters, the DC source is having high impedance. In all the inverters self-commutation is employed except for Mc Murray and Mc

Murray Bedford inverters. For these two inverters forced commutation is used. In Mc Murray inverter, Auxiliary commutation is used and in Mc Murray

Bedford inverter Auxiliary impulse commutation is used. In all of the voltage control techniques used for inverters, Pulse Width modulation

(PWM) technique is more advantageous one and also it reduces the harmonic

content in the output waveform. In single-phase parallel inverter, the capacitor is used for commutation.

OBJECTIVE QUESTIONS :

1. A single-phase voltage controller feeds an induction motor (A) and a heater (B)(a) In both the loads, fundamental and harmonics are useful(b) InA only fundamental and inB only harmonics are useful(c) InA only fundamental and inB harmonics as, well as fundamental are useful (d)InA only harmonics and inB only fundamental are useful.

2. A load resistance of 10 is fed through a I-phase voltage controller from a voltage

source of 200 sin 314 t. For a firing angle delay of 90, the power delivered to loadin kW, is(a) 0.5 (b) 0.75(c) 1 (d) 2

3. A single-phase voltage controller is employed for controlling the power flow from260 V, 50 Hz source into a load consisting ofR = 5 and L = 12 . The value ofmaximum RMS load current and the firing angle are respectively

(a) 20 A, 0 (b) 260/10.91A, 0

(c) 20 A, 90 (d) 260/10.91 A, 90

4. A "load, consisting ofR = 10 and L = 10 , is being fed from 230 V, 50 Hz

source through a I-phase voltage controller. For a firing angle delay of 30, the

RMS value of load current would be(a) 23 A (b) 23/2 A(c) > 23/2 A (d) < 23/2 A

5. In a single-phase voltage controller withRL load, ac output power can be controlledif(a) firing angle > (load phase angle) and conduction angle =


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(b) > and < (c)

6. A single-phase voltage controller feeds power to a resistance of 10 .0. The sourcevoltage is 200 V rms. For a firing angle of 90, the RMS value of thyristor current

in amperes is(a) 20 (b) 15(c) 10 (d) 5

7. A single-phase voltage controller is connected to a load of resistance 10.0 and asupply of 200 sin 314t volts. For a firing angle of 90, the average thyristorcurrent in amperes is(a) 10 (b) 10 / (c) 52 / (d) 52

8. A single-phase voltage controller, using two SCRs in anti parallel, is found to beoperating as a controlled rectifier. This is because(a) load isR and pulse gating is used

(b) load isR and high-frequency carrier gating is used(c) load isRL and pulse gating is used(d) load isRL and continuous gating is used

9. A single-phase ac voltage controller (or regulator) fed from 50 Hz system supplies aload having resistance and inductance of2.0 .0 and 6.36 mH respectively. Thecontrol range of firing angle for this regulator is(a) 0 < < 180 (b) 45 < < 180(c) 90 < < 180 (d) 0 < < 45

10. If, for a single-phase half-bridge inverter, the amplitude of output voltage is Vs andthe output power isP, then their corresponding values for a single-phase full-bridgeinverter are(a) Vs, P (c) 2 Vs, 2P(b) V/2, 2P (d) 2 Vs, P

11. In voltage source inverters(a) load voltage waveform Vo depends on load impedance Z, whereas load currentwaveform io does not depend onZ(b) Both Voand iodepend onZ(c) Vodoes not depend onZwhereas iodepends onZ(d)both Voand iodo not depend upon Z

12. A single-phase full bridge inverter can operate in load-commutation mode in caseload consists of(a) RL (b) RLCover damped

(c) RLCunder damped (d) RLCcritically damped13. A single-phase bridge inverter delivers power to a series connectedRLCload withR = 2 , L = 8 Q. For this inverter-load combination, load commutation ispossible in case the magnitude of l/C in ohms is .(a) 10 (b) 8(c) 6 (d) zero

14. For a 3-phase bridge inverter in 180 conduction mode, Fig. A-33, the sequence ofSCR conduction in the first two steps, beginning with the initiation of thyristor 1, is


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(a) 6, 1,2 and 2,3, 1 (b) 2,3, 1 and 3, 4, 5(c) 3, 4, 5, and 5, 6, 1 (d) 5, 6, 1 and 6, 1, 2

15. For a 3-phase bridge inverter in 120 conduction mode, Fig. A-33, the sequence ofSCR conduction in the first two steps, beginning with the initiation of thyristor 1, is(a) 6, 1 and 1, 2 (b) 1,2 and 2,3

(c) 1, 6 and 5, 6 (d) 1, 3 and 3, 416. In single-pulse modulation of PWM inverters, third harmonic can be eliminated ifpulse width is equal to(a) 300 (b) 600

(c) 1200 (d) 150

17. In single-pulse modulation of PWM inverters, fifth harmonic can be eliminated ifpulse width is equal to(a) 300 (b) 720

(c) 36 (d) 10818. In single-pulse modulation of PWM inverters, the pulse width is 120. For an input

voltage of 220 V dc, the RMS value of output voltage is

(a) 179.63 V (b) 254.04 V(c) 127.02 V (d) 185.04 V19. In single-pulse modulation used in PWM inverters, Vs is the input de voltage. For

eliminating third harmonic, the magnitude of RMS value of fundamentalcomponent of output voltage and pulse width are respectively

(a) 2 2 Vs,120 (b) 4 Vs, 60

(c) 2 2 Vs, 60 (d) 4 Vs, 120

20. In multiple-pulse modulation used in PWM inverters, the amplitudes of reference

square wave and triangular carrier wave are respectively 1 V and 2 V. Forgenerating 5 pulses per half cycle, the pulse width should be(a) 36 (b) 24 (c) 18 (d) 12

21. In multiple-pulse modulation used in PWM inverters, the amplitude and frequencyfor triangular carrier and square reference signals are respectively 4 V, 6 kHz and 1V, 1 kHz. The numbers of pulses per half cycle and pulse width are respectively(a) 6, 90 (b) 3, 45(c) 4, 60 (d) 3, 40

22. In sinusoidal-pulse modulation used in PWM inverters, amplitude and frequency fortriangular carrier and sinusoidal reference signals are respectively 5 V, 1 kHz and 1V, 50 Hz. If zeros of the triangular carrier and reference sinusoid coincide, then the

modulation index and order of significant harmonics are respectively(a) 0.2,9 and 11 (b) 0.4,9 and 11(c) 0.2, 17 and 19 (d) 0.2, 19 and 21

23. Which of the following statement/statements is/are correct in connection with in-verters:(a) VSI and CSI both require feedback diodes(b) Only CSI requires feedback diodes(c) GTOs can be used in CSI


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(d) Only VSI requires feedback diodes24. In a CSI, if frequency of output voltage isfHz, then frequency of voltage input to

CSI is(a) f (b) 2f (c) f/2 (d) 3f

25. In sinusoidal-pulse modulation used in PWM inverters, amplitude and frequency oftriangular carrier and sinusoidal reference signals are respectively 5 V, 1 kHz and 1V, 50 Hz. If peak of the triangular carrier coincides with the zero of the referencesinusoid, then the modulation index and order of significant harmonics are(a) 0.2,9 and 11 (b) 0.4,9 and 11(c) 0.2, 17 and 19 (d) 0.2, 19 and 21

26. In sinusoidal PWM, there are 'm'cycles of the triangular carrier wave in the halfcycle of reference sinusoidal signal. If zero of the reference sinusoid coincides withzero/peak of the triangular carrier wave, then number of pulses generated in eachhalf cycle is respectively(a) (m - 1)/m (b) (m - 1)/(m - 1)

(c) m/m (d) m/(m - 1)27. In an inverter with fundamental output frequency of 50 Hz, if third harmonic iseliminated, then frequencies of other components in the output voltage wave, in Hz,would be(a) 250, 350, 450, high frequencies (b) 50, 250, 350, 450(c) 50,250,350,550 (d) 50, 100, 200, 250

28. A single-phase CSI has capacitor C as the load. For a constant source current, thevoltage across the capacitor is(a) square wave (b) triangular wave(c) step function - (d)pulsed wave

29. A single-phase full bridge VSI has inductorL as the load. For a constant sourcevoltage, the current through the inductor is(a) square wave (b) triangular wave(c) sine wave (d)pulsed wave

SUBJECTIVE QUESTIONS


1) A pump has a torque-speed curve given by TL = (1.4/103)N2 Nm. It is proposed to usea 240V, 50 Hz, 4 pole, star connected Induction motor with the equivalent circuitparameters (referred to stator turns) R1 = 0.25, R2 = 0.6, X1 = 0.36, X2 = 0.36,Xm = 17.3. The pump speed N is to vary from full speed 1250 RPM to 750 RPM byvoltage control using pairs of inverse-parallel connected thyristors in the lines. Calculate

the range of firing angles required.2) A 3 phase, 4 pole, 50 Hz squirrel cage Induction motor has the following circuitparameter.r1 = 0.05ohm, r2 = 0.09ohm,X1 + X2 = 0.55ohm.The motor is star connected and rated voltage is 400V. It drives a load whose torque isproportional to the speed and is given as T1 = 0.05 Nw-m. Determine the speed andtorque of the motor for a firing angle of 450 of the AC Voltage Controller on a 400V, 50Hz supply.


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3) What is an AC Voltage Controller?Explain with suitable diagrams the various types of solid state 3 phase AC VoltageControllers that can be used for speed control of 3-phase Induction motors from statorside. Mention the advantages of the AC Voltages Controllers over the other methods ofsolid-state speed control techniques of 3-phase Induction motor.

4) (a) For stator voltage control scheme of a 3-phase Induction motor discuss about speedrange, regeneration, harmonics, torque pulsating, power factor, cost, efficiency andapplications.(b) Draw a block schematic diagram for automatic speed control of 3 phase cageInduction motor using solid state AC Voltage Controller on stator side.5) (a) Using 3 phase solid state AC Voltage Controllers explain clearly how it is possibleto achieve 4 quadrant operation of 3 phase Induction motors.(b) Draw a closed loop block schematic diagram for the above speed control technique.Mention the merits of the above method of speed control.


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UNIT VI

UNIT OBJECTIVE:

CONTROL OF INDUCTION MOTOR THROUGH STATOR FREQUENCY

To study the operation of stator supply frequency control method of induction

motor by VSI (voltage source inverter), CSI (current source inverter) and Cycloconverters. And their speed torque characteristics. To know about the PWM (pulse width modulation) control of induction motors To study the comparisons between VSI and CSI.To study the block diagram of closed loop operation of induction motor drives.

IMPORTANT POINTTS:

The function of inverters is to convert Fixed DC voltage into Variable voltage ACwith variable frequency.

In a Voltage source inverters, the DC source has negligible impedance where as

inn Current source inverters, the DC source is having high impedance. In all the inverters self-commutation is employed except for Mc Murray and Mc


Bedford inverter Auxiliary impulse commutation is used. In all of the voltage control techniques used for inverters, Pulse Width modulation

(PWM) technique is more advantageous one and also it reduces the harmoniccontent in the output waveform.

In single-phase parallel inverter, the capacitor is used for commutation. The function of inverters is to convert Fixed DC voltage into Variable voltage AC

with variable frequency. In Voltage source inverters, the DC source has negligible impedance where as inn

Current source inverters; the DC source is having high impedance. In all the inverters self-commutation is employed except for Mc Murray and Mc


Bedford inverter Auxiliary impulse commutation is used.


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In all of the voltage control techniques used for inverters, Pulse Width modulation(PWM) technique is more advantageous one and also it reduces the harmoniccontent in the output waveform.

In single-phase parallel inverter, the capacitor is used for commutation.

CYCLO CONVERTERS:

Cyclo converters are single stage frequency conversion devices. Their function is to convert Fixed AC voltage with fixed frequency into Variable

AC voltage with variable frequency. Cyclo converters are classified into mid point type and Bridge type cyclo

converters.


1. If, for a single-phase half-bridge inverter, the amplitude of output voltage is Vs andthe output power isP, then their corresponding values for a single-phase full-bridgeinverter are(a) Vs, P (c) 2 Vs, 2P(b) V/2, 2P (d) 2 Vs, P

2. In voltage source inverters(a) load voltage waveform Vo depends on load impedance Z, whereas load currentwaveform io does not depend onZ(b) Both Voand iodepend onZ(c) Vodoes not depend onZwhereas iodepends onZ(d)both Voand iodo not depend upon Z

3. A single-phase full bridge inverter can operate in load-commutation mode in caseload consists of(a) RL (b) RLCover damped(c) RLCunder damped (d) RLCcritically damped

4. A single-phase bridge inverter delivers power to a series connectedRLCload withR = 2 , L = 8 Q. For this inverter-load combination, load commutation ispossible in case the magnitude of l/C in ohms is(a) 10 (b) 8(c) 6 (d) zero

5. For a 3-phase bridge inverter in 180 conduction mode, Fig. A-33, the sequence ofSCR conduction in the first two steps, beginning with the initiation of thyristor 1, is

(a) 6, 1,2 and 2,3, 1 (b) 2,3, 1 and 3, 4, 5(c) 3, 4, 5, and 5, 6, 1 (d) 5, 6, 1 and 6, 1, 26. For a 3-phase bridge inverter in 120 conduction mode, Fig. A-33, the sequence of

SCR conduction in the first two steps, beginning with the initiation of thyristor 1, is(a) 6, 1 and 1, 2 (b) 1,2 and 2,3(c) 1, 6 and 5, 6 (d) 1, 3 and 3, 4

7. In single-pulse modulation of PWM inverters, third harmonic can be eliminated ifpulse width is equal to


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(a) 300 (b) 600

(c) 1200 (d) 150

8. In single-pulse modulation of PWM inverters, fifth harmonic can be eliminated ifpulse width is equal to(a) 300 (b) 720

(c) 36 (d) 1089. In single-pulse modulation of PWM inverters, the pulse width is 120. For an input

voltage of 220 V dc, the RMS value of output voltage is(a) 179.63 V (b) 254.04 V(c) 127.02 V (d) 185.04 V

10. In single-pulse modulation used in PWM inverters, Vs is the input de voltage. Foreliminating third harmonic, the magnitude of RMS value of fundamentalcomponent of output voltage and pulse width are respectively

(a) 2 2 Vs,120 (b) 4 Vs, 60

(c) 2 2 Vs, 60 (d) 4 Vs, 120

11. In multiple-pulse modulation used in PWM inverters, the amplitudes of referencesquare wave and triangular carrier wave are respectively 1 V and 2 V. Forgenerating 5 pulses per half cycle, the pulse width should be(a) 36 (b) 24 (c) 18 (d) 12

12. In multiple-pulse modulation used in PWM inverters, the amplitude and frequencyfor triangular carrier and square reference signals are respectively 4 V, 6 kHz and 1V, 1 kHz. The numbers of pulses per half cycle and pulse width are respectively(a) 6, 90 (b) 3, 45(c) 4, 60 (d) 3, 40

13. In sinusoidal-pulse modulation used in PWM inverters, amplitude and frequency fortriangular carrier and sinusoidal reference signals are respectively 5 V, 1 kHz and 1V, 50 Hz. If zeros of the triangular carrier and reference sinusoid coincide, then themodulation index and order of significant harmonics are respectively(a) 0.2,9 and 11 (b) 0.4,9 and 11(c) 0.2, 17 and 19 (d) 0.2, 19 and 21

14. Which of the following statement/statements is/are correct in connection with in-verters:(a) VSI and CSI both require feedback diodes(b) Only CSI requires feedback diodes(c) GTOs can be used in CSI(d) Only VSI requires feedback diodes

15. In a CSI, if frequency of output voltage isfHz, then frequency of voltage input toCSI is(a) f (b) 2f (c) f/2 (d) 3f

16. In sinusoidal-pulse modulation used in PWM inverters, amplitude and frequency of


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triangular carrier and sinusoidal reference signals are respectively 5 V, 1 kHz and 1V, 50 Hz. If peak of the triangular carrier coincides with the zero of the referencesinusoid, then the modulation index and order of significant harmonics are(a) 0.2,9 and 11 (b) 0.4,9 and 11(c) 0.2, 17 and 19 (d) 0.2, 19 and 21

17. In sinusoidal PWM, there are 'm'cycles of the triangular carrier wave in the halfcycle of reference sinusoidal signal. If zero of the reference sinusoid coincides withzero/peak of the triangular carrier wave, then numbers of pulses generated in eachhalf cycle are respectively(a) (m - 1)/m (b) (m - 1)/(m - 1)(c) m/m (d) m/(m - 1)

18. In an inverter with fundamental output frequency of 50 Hz, if third harmonic iseliminated, then frequencies of other components in the output voltage wave, in Hz,would be(a) 250, 350, 450, high frequencies (b) 50, 250, 350, 450

(c) 50,250,350,550 (d) 50, 100, 200, 25019. A single-phase CSI has capacitor C as the load. For a constant source current, thevoltage across the capacitor is(a) square wave (b) triangular wave(c) step function - (d)pulsed wave

20. A single-phase full bridge VSI has inductorL as the load. For a constant sourcevoltage, the current through the inductor is(a) square wave (b) triangular wave(c) sine wave (d)pulsed wave

21. A cyclo converter is a(a) frequency changer (fc) from higher to lower frequency with one-stateconversion(b) fc from higher to lower frequency with two-stage conversion(c) fc from lower to high frequency with one-state conversion(d) either(a) or (c).

22. The cyclo converters (CCs) require natural or forced commutation as under:(a) natural commutation in both step-up and step-down CCs(b) forced commutation in both step-up and step-down CCs(c) forced commutation in step-up CCs(d) forced commutation in step-down CCs.

23. For converting 3-phase supply at one frequency to single-phase supply at a lowerfrequency, the basic principle is to_______(vary/keep) the firing angle__________ (constant/gradually).

24. Three-phase to three-phase cyclo converters employing 18 SCRs and 36 SCRs havethe same voltage and current ratings for their component thyristors. The ratio of VArating of 36-SCR devices to that of 18-SCR devices is(a) 2 (b) 1 (c) 2 (d) 4.

25. Three-phase to 3-phase cyclo converters employing 18 SCRs and 36 SCRs have thesame voltage and current ratings for their component thyristors. The ratio of powerhandled by 36-SCR devices to that handled by 18-SCR devices is


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(a) 2 (b) 1 (c) 2 (d) 4.26. The number of thyristors required for single-phase to single-phase cyclo converter

of the mid-point type and for three phase-to three-phase 3-pulse type cycloconverters are respectively -.(a) 4, 6 (b) 8, 18 (c) 4, 18 (d) 4, 36.

27. A 3-phase to single-phase conversion device employs a 6-pulse bridge cycloconverter. For an input voltage of 200 V per phase, the fundamental rms value ofoutput voltage is

(a) 600/ V (b) 3003/ V (c) 300/ V (d) 6003/ V.


1. A 3 , squirrel cage induction motor is developing a torque of 1500sync. Watts at50GHz and 1440rpm (Ns = 1500rpm). If the motor frequency is now increased to

75Hz using constant power mode, determine the new value of torque developed bymotor at constant slip.2. A 3 star connected, 50Hz, 4-pole induction motor has the following parameters

in per phase referred to the stator:R1= R2= 0.034 and X1=X2=0.18The motor is controlled by the variable frequency control with a constant (V/ f).Determine the following for an operating frequency of 15Hz.

(a) The brake down torque as a ratio of its value at the rated frequency for motoring andbraking.

(b) The starting torque and rotor current in terms of their values at the rated frequency.(c) The frequency, stator current and the voltage at rated braking torque and 1200rpm.

3. An inverter supplies a 4 pole, 3 cage induction motor rated at 220V,50Hz.Determine the approximate output required of the inverter for motor speeds of(i) 900rpm (ii) 1200rpm (iii) 1500rpm (iv) 1800rpm

4. A 3 400V delta connected induction motor has the following parameters at50 Hz.R1= 0.5, R2= 1.5 and X1= X2 =2.5, Xm= 130This motor is fed from a square waved inverter. The voltage waveform is such thatits fundamental is equal to the rated voltage of the motor. Determine input currentwave from corresponding to a rotor frequency of 2Hz. When the supply frequencyis 50Hz and 10Hz and the voltage applied is proportional to frequency. Whatwaveform do you expect at 10Hz if the voltage is varied to keep air gap flux

constant.5. A 3 400V, 50 Hz, 4 pole; 1450rpm star connected squirrel cageinduction motor has the following parameters per phase referred to the stator.R1= 0.11, R2= 0.09 and X1=0.4, X2 =0.6, Xm= 12, 6-stepinvertercontrols the motor. The DC input to the inverter is provided by a 6 pulse fullycontrolled rectifier.

6. What should be the rectifier firing angle for getting the rated fundamental voltageacross the motor if the rectifier is fed by an AC source of 400V, 50Hz.


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If the machine is operated at a constant flux then determinea. The inverter frequency at 560rpm and rated torque.b. The inverter frequency at460 rpm and half the rated torque.

Also determine the motor current.7. A 3 , 400V, 50Hz 980rpm, 6pole star connected squirrel cage induction motor

has the following parameter. per phase referred to the statorR1= 0.20, R2= 0.12 and X1= 0.18, X2 =0.4, Xm= 10.3The current source inverter controls the motor at the rated value, flux maintain constant.

Compute the following(a) The stator current and DC link current, when the machine operates at rated torque and

50Hz.(b) The inverter frequency and DC link current for a speed of 500rpm and rated torque .The motor speed stator current and DC link current for half of the rated torque andinverter frequency of 25Hz.


1) With the help of circuit diagram and waveforms explain the induction motor withcurrent source inverter. Draw the circuit diagram of the Auto-sequentially commutatedinverter.2) A 460V, 100-HP (74.6 KW), 1775 RPM, three-phase, squirrel cage Induction motorhas the following equivalent circuit parameters.Rs = 0.060 ohm Rr = 0.0302 ohmL1s = 0.638mH, Lms = 23.3 mH, L1r = 0.957 mHThe motor is to be driven from a current source inverter with the rotor frequencycontrolled at the rated value. Maximum output power is to be limited to 80% of the ratedvalue. Motor friction, windage, and core losses may be neglected. The load is to consist

of a pump presenting a load characteristic described by the equation. T = m2

/110 N.m.Determine the maximum values of motor speed, inverter frequency, rms motor linecurrent, and fundamental line-to-line motor terminal pd at maximum power output.3) Explain the operation of voltage source inverter (180-degree conduction mode), usedfor induction motor speed control. Draw neat waveforms of line voltages (Vab, Vbc,Vca) and hence show that the phase voltage, Van, is six-step voltage waveform.4) Discuss in detail the role of Cyclo converters for speed control of Induction motor.Draw neat circuit diagram for speed control of 3-phase Induction motor usingCycloconverters. Mention the merits and limitations of the above scheme.5) A three phase star connected 50 Hz, 4-pole induction motor has the followingapproximate per-phase equivalent circuit parameters referred to stator side:

Rs=Rr=0.024, Xs=Xr=0.12. The motor is controlled by the variable frequency controlwith constant (V/f) ratio. For an operating frequency of 12 Hz, calculate(a) The breakdown torque as a ratio of its value at the rated frequency for the motoringoperation,(b) The starting torque and rotor current in terms of their values at the rated frequency.6) While explaining the principle of varying the speed of 3 phase Induction motor by v/fmethod discuss if for the following two different modes.(a) Operation below rated frequency


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(b) Operation above rated frequency.

UNIT VII

UNIT OBJECTIVE:

CONTROL OF INDUCTION MOTOR FROM ROTOR SIDE

To study the performance Static rotor resistance control of the induction motordrives.

To study the performance Slip power recovery scheme of the induction motordrives.

To study the performance Static Scherbius drive. To study the performance Static Krammer drive. To derive the Torque Speed characteristics of all the induction motor drives. To know the advantages, disadvantages and applications of all the induction

motor drives.


1. Explain the Static rotor resistance control scheme of induction motor drive with neatcircuit and waveforms and draw their speed torque characteristics.

2. Explain the Slip energy recovery scheme of induction motor drive with neat circuitand waveforms and draw their speed torque characteristics.

3. Explain the operation of Static Scherbius Drive with neat circuit and waveforms anddraw their speed torque characteristics.

4. Explain the operation of Static Krammer drive with neat circuit and waveforms anddraw their speed torque characteristics.

List out the advantages and applications of any three rotor side controlled induction

motor drives.


1) A 3-phase, 420V, 50Hz, star connected induction motor has the following parameters:R1 = 2.95, R2 = 2.08, X1 = 6.82, X2 = 4.11 per phase. Neglect core loss. Themotor draws a current 6.7A at no load and controlled by rotor resistance controller. Aresistance Re has been controlled by chopper. Determine the value of Re to get a speed


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range of 1500 to 500 rpm, assuming a turns ratio of two between stator and rotor. Thetorque and speed of the load are related by T N. Determine the characteristics givingthe speed Vs time ratio of the chopper.2) A 3-phase, 400V, 50Hz, 4 pole, 1400rpm, star connected wound rotor induction motorhas the following parameters referred to the stator R1 = 2, R2 = 3, X1 = X2 = 3.5.

The stator to rotor turns ratio is 2. The motor speed is controlled by static Scherbiusdrive. The inverter is directly connected to the source. Determine.(a) The speed range of the drive when max = 1650

(b) The firing angle for 0.4 times the rated motor torque and speed of 1200 rpm.(c) Torque for a speed of 1050rpm and firing angle of 950.3) The speed of a 3-phase slip ring induction motor is controlled by variation of rotorresistance. The full load torque of the motor is 50Nm at a slip of 0.3. The motor drivesload having a characteristics T N2. The motor has 4 poles and operates on 50Hz, 400Vsupply. Determine the speed of the motor for 0.8 times the rated torque. The operatingcondition is obtained with additional resistance in the circuit. Chopper in the rotor circuitcontrols the resistance. Determine the average torque developed for a time ratio of 0.4.

4) A 3-phase 400V, 4 pole, 50Hz, Star connected induction motor has the followingparameters referred to the stator: R2 = 0.2, X2 = 0.35. Stator impedance and themagnetizing branch can be ignored. When driving a load with its torque proportional tospeed, the motor runs at 1450rpm. Calculate the magnitude and phase of the voltage(referred to the stator) to be impressed on the slip rings in order that the motor mayoperate at 1200 rpm and unity power factor.5) A 3-phase, 50Hz Star connected, 970rpm, 6-pole induction motor has the followingparameters referred to the stator. R1 = 0.2, R2 = 0.15, X1 = X2 = 0.4. Stator torotor turns ratio = 3.5. The motor is controlled by the static Kramar drive. The drive isdesigned for a speed range of 30% below the synchronous speed. The maximum value offiring angle is 1700. Calculatea) Turns ratio of the transformerb) Torque for a speed of 750rpm and = 1400.c) Firing angle for half the rated motor torque and a speed of 850rpm.6) A 600V, 50Hz, 30kW, 3-phase induction motor is used as the drive motor in an SERsystem. It is required to deliver constant (rated) motor torque over the full range from100rpm to the rated speed of 1000rpm. The motor equivalent circuit parameters are:R1 = 0.05,R = 0.07,R0 = 53,X0 = 23, X1+ X2 = 0.153.Stator to rotor turns ratio is 1.3. Calculate the motor currents, efficiency and power factorat 300 rpm.7) (a) Draw and explain a closed loop operation for a static Kramer controlled drive.(b) In which way static Kramer Control is different from static Scherbius drive?


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UNIT VIII

UNIT OBJECTIVE:

CONTROL OF SYNCHRONOUS MOTORS

To know about the separate control of synchronous motors. To know about the self-control of synchronous motors. To study the operation of self controlled synchronous motor drives by VSI. To study the operation of self controlled synchronous motor drives by CSI. To study the operation of self controlled synchronous motor drives by Cyclo

converters. To study the operation of load commutated CSI fed synchronous motor drives and

to draw the Speed Torque characteristics.


1. Explain the difference between separate and self-control of synchronous motor.2. Explain the operation of VSI fed self-controlled synchronous motor drive with

neat circuit and waveforms and also draw their speed torque characteristics.3. Explain the operation of CSI fed self-controlled synchronous motor drive with

neat circuit and waveforms and also draw their speed torque characteristics.4. Explain the operation of Cyclo converter fed self - controlled synchronous motor

drive with neat circuit and waveforms and also draw their speed torquecharacteristics.

5. Explain the closed loop control of synchronous motor drives with neat blockdiagrams.

List out the advantages, disadvantages and applications of self controlled synchronous

motor fed by VSI, CSI and Cyclo converter.


1) A 6 MW, three phase, 11 kV, 50 Hz, unity power factor, 6-pole, star-connectedsynchronous motor has the following parameters: armature resistance = 0, synchronousreactance = 9 ohms, rated field current = 60 A. The machine is controlled by variablefrequency at constant V/f ratio up to base speed and at constant V above base speed.Calculate the torque and field current for rated armature current, 750 rpm and 0.8 leading


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power factor. Draw motor characteristics and waveforms under the above method ofcontrol.2) Describe self-controlled and load-commutated inverter controlled synchronous motordrives in detail and compare them.3) Describe the open loop and closed loop methods of speed control of a synchronous

motor using VSI.4) Discuss the VSI method of speed control of synchronous motor describe the operationof the converter with waveforms.5) A 500 kW, 3-phase, 6.6 kV, 60 Hz, 6-pole, Y-connected wound-field synchronousmotor has the following parameters: Xm = 78, Xsf = 3, rated pf = 1, n = 5, Rs =negligible. The motor speed is controlled by variable frequency control with a constantV/f ratio up to base speed and rated terminal voltage above base speed. Calculate and plotT, Pm, V, Im, and IF versus speed for the motor operation at rated armature current andunity pf. What is the range of constant power operation? Neglect friction, windage andcore loss. Draw motor characteristics and waveforms under the above method of control.6) Describe the converter and control systems used for

(a) constant air gap flux density and(b) constant V/f operation of a synchronous motor. Draw the characteristics of the drivefor the two cases.7) Draw the block diagram of a closed loop synchronous motor drive fed from VSI andexplain.

Old Question Papers

Co de No: 07A70201 R07 Set No. 2

IV B.Tech I Semester Examinations,MAY 2011

POWER SEMICONDUCTOR DRIVES

Electrical And Electronics Engineering

Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

? ? ? ? ?

1. (a) Generally the stator voltage control is suitable for speed control of Induction

motor in fan and pump drives. Discuss in detail why the above method is

useful.

(b) Explain why the stator voltage control is not an efficient method of control.

[8+8]

2. Draw the circuit diagram and explain the operation of rotorresistance control

using chopper. Mention the advantages and disadvantages of the above method of

control. [16]

3. Derive the Speed, Torque Equations of a fully controlled converter connected to

separately excited D.C motor with continuous current operation with necessary

waveforms. [16]

4. Explain the basic operational aspects of three phase fully controlled converters

with


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neat sketches of the waveforms and the circuit diagram. What is the e

ect of free

wheeling diode. [16]

5. What is continuous current operation of chopper fed DC motors? Explain with

relevant waveforms. [16]

6. With suitable circuit diagrams discuss in detail the principle of operation of Selfcontrolled Synchronous motor drive employing a Cyclo converter. [16]

7. A 50 hp 440 V, 50 Hz six pole Star connected Induction motor has the following

equivalent circuit parameters:

R1 = 0.1 Ohm, R2 ' = 0.12 Ohm,

X1 = 0.3 Ohm, X2 ' = 0.3Ohm . Xm = 15 Ohm. For a slip of 0.03 at rated frequency

determine the torque developed. [16]

8. (a) With neat circuit diagram and waveforms, explain dynamic braking of sepa-

rately excited motor by single phase converter.

(b) A dc shunt motor has the armature resistance of 0.04 oms and the field winding

resistance of

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Psd Course File 2011-12