(2010-2011)

1. a Explain the basic principle of thyristor. Explain forward conduction mode of thyristor.

b. Explain two transistor model of thyristor. Explain holding and latching current.

c. Explain thyristor turn-on methods. What are the applications of thyristor?

2. a Explain the basic principle of phase control. Define single phase half wave circuit with RL load.

b. A single phase 230 V, 1KW heater is connected across 1-phase, 230 V, 50 Hz supply through an SCR

for firing angle delay of 450 and 900 . Calculate the power absorbed in the heater circuit.

c. Explain single phase full wave bridge converter. Also explain three phase bridge inverter.

3.a A step-up chopper has input-voltage 220 V and output voltage of 660 V. If the non- conducting time

of thyristor chopper is 100 s, compute the pulse width of output voltage. In case pulse width is halved

for constant frequency operation, find the new output voltage.

b. Explain the basic principle of operation of step up and step down chopper with V-I characteristics.

c. Define the basic principle of operation of cycloconverter. Explain the working of 1- and 3- half wave

cycloconverter.

4.a Explain the basic machine equations. Define DC motor speed control.

b. Explain 1- dual converter drives; also explain two-quadrant chopper drive.

c. Explain four quadrant chopper drive. Define 3- semiconductor drives.

5.a. Define speed control of induction motors. Define method of resistance control.

b. A 3-, 400V, 15KW, 1440 rpm, 50Hz star connected induction motor has rotor leakage impedance of

0.4 + j1. 6. Starter leakage impedance and rotational losses are assumed negligible. If this motor is

energised from 120 Hz, 400 V, 3- source, then calculate:

(i) the motor speed at rated load

(ii) the slip at which maximum torque occurs and

(iii) the maximum torque.

c. What are ac drives? Give the merits and demerits of ac drives with respect to dc drives.

(2011-2012)

1. a Explain the various terms involved in turn on and turn-off time of SCR in dynamics characteristics.

Also explain why circuit turn-off time is kept larger than the device turn-off time.

b. Define di/dt and dv/dt protection of SCRs. What are the components used to protect SCR from dv/dt

and di/dt large values.

c. Give the various triggering method in SCR. Explain in detail pulse triggering method with request

diagram.

2a. Derive the expression of a 1- full wave bridge rectifier fully controlled for RLE load.

An armature (RLE load) of a separately excited dc motor fed from 1- full wave fully controlled

converter. The supply ac voltage V = 220 V, Ra= 0.06 , La= 0.85 mH, Ia= 150A, speed (rated)= 875

rpm, Va(rated) is 200 V. Find out voltage and for a rated load when:

(i) E = 160 V(speed=733 rpm, Ist quadrant).

(ii) E= - 109 V (speed = -500 rpm, 4th quadrant).

b. Explain the operation of 3- half controlled full wave rectifier with desire with desired waveforms.

c. Explain the control strategies of chopper.

In a 100- V dc chopper drive using current limit control scheme, the maximum possible value of the

acceleration current is 425A, lower limit of the current pulsation is 180 A. The length of on and off period

is 14 ms and 11 ms, respectively. Determine the limit of current pulsation of the chopping frequency, the

duty cycle ratio and output voltage.

3a. Give the difference between 1800 and 1200 mode of 3- inverter with relevant waveform of phase and

line voltages.

b. Explain the concept of step down cycloconverter. Enumerate the advantages and application of

cycloconverter.

c. Derive the expression for a 1- ON-OFF ac voltage controller. State the difference between integral

cycle control and phase control.

4a. The speed of a separately-excited dc motor is controlled through 1- half wave controlled converter

from 230 V mains. The motor armature resistance is 0.5 and motor constant is K = 0.4 V-s/rad. For

load torque of 20Nm at 1500 rpm and for constant armature current, calculate:

(i) firing angle of the converter.

(ii) rms value of thyristor current

(iii) I/P power factor of the motor

b. Describe the regenerative braking of a chopper-fed separately excited dc motor. Illustrate answer with

circuit diagram and relevant wave forms.

c. Enumerate the various method of speed control of a 3- induction motor when fed through

semiconductor devices.

d. Describe static Kramer drive and show that the steady state torque is not influenced by whether a

transformer is used or not. Derive appropriate expression to obtain torque-speed characteristics of static

Kramer drive.

(2012-2013)

1a. Explain the characteristics and principle of operation of a thyristor.

b. What are the characteristics of a thyristor during turn ON and turn OFF? Explain the gate recovery and

reverse recovery.

c. What are the various types of gate triggering circuit? Explain.

2a. Explain the circuit and wave form of a 3- fully controlled rectifier with a firing angle of 300.

b. What are the two main configuration of a 3- bridge inverter? Explain the operation of a 1200 mode 3-

phase inverter.

c. Explain the term (i) Input power factor and (ii) Total harmonics distortion with reference to a two pulse

converter.

3a. Derive the equation for the output voltage of a step-up chopper.

b. Explain the operation of a single phase to single phase cycloconverter.

c. With a neat diagram, explain the operation of Jones Chopper.

4a. what are the various braking method of DC motor? Explain.

b. Does the armature current change in the armature speed control method in DC motor? Explain

c. Write short notes of DC chopper Drives.

5a. Draw the torque-slip characteristics of a 3- induction motor. How does the shape of the

characteristics change in stator? How does the shape of the characteristics change in stator voltage

control? Explain.

b. Explain the slip power recovery scheme of a three phase Induction Motor.

c. Write a short note on synchronous motor drives.

(2013-2014)

1a. Explain why the inner two layer of an SCR are lightly doped & wide.

b. What is meant by commutation of SCR? What are the different types of forced commutation methods?

c. Draw the turn-off characteristics of an SCR & explain its mechanism.

2.a Explain the operation of a three-phase, half controlled bridge converter with suitable waveform.

b. A three phase half-wave controlled rectifier has a supply of 200 V/ phase. Determine the average load

voltage for firing angle of 00 & 600, assuming a thyristor voltage drop of 1.5 V & continuous load current.

c. Justify the statement freewheeling diode improve the power factor of the system.

d. Explain the operation of single-phase bridge inverter with the help of load voltage &load current wave

form.

3a. Explain the principle of chopper operation what are the types of control strategies in chopper? Explain

any one of them.

b. A chopper circuit is operating on TRC principle at a frequency of 2 KHz on 220 V dc supply. If the

load voltage is 170 V, compute the conduction of blocking period of thyristor in each cycle.

c. Why does a three phase to single phase cycloconverter required positive & negative group phase

controlled converter? Explain

4.a Explain in brief any one method of breaking of dc motor.

b. Draw & explain the torque speed characteristics of a separately excited dc motor at different firing

angles for a full converter feeding.

c. Explain with the associated wave form, how power factor can improved by symmetrical angle control

scheme.

5a. Explain the variable frequency control method of an induction motor. Also explain operation below

the rated frequency mode.

b. An inverter supplies a 4-pole, thee phase cage induction motor rated at 220 V, 50 Hz. Determine the

approximate output required of the inverter for motor speed of (i)900 (ii)1200 (iii) 1500 (iv) 1800 rpm.

c. Draw a suitable diagram & explain working of slip power recovery system using commutator- less

Kramer drive.