# GTU last year question papers -BE- Sem-VIIth -Power Electronics & Industrial Drives -May/June- 2012

GUJARAT TECHNOLOGICAL UNIVERSITY

BE- VIIth SEMESTER–EXAMINATION – MAY/JUNE- 2012

Subject code: 170801

Subject Name: Power Electronics & Industrial Drives

Instructions:

1. Attempt all questions.

2. Make suitable assumptions wherever necessary.

3. Figures to the right indicate full marks.

Q.1 (a) With neat sketch describe the operation of half wave uncontrolled rectifier

(b) A single phase full wave rectifier with R-L-E load type has R=2.5 ohm, L=7

mH, E=10 V, It is fed by a voltage source of 240 V,50 Hz through a

transformer with transformation ratio of 2:1.Draw neat sketch and wave form.

Determine (i) Steady state load current I1 at wt=0 (ii) the average diode current

Id (iii) the rms diode current Ir.

Q.2 (a) Derive the equation of output voltage for on/off a.c. voltage controller. Draw

neat sketch of circuit diagram and waveform.

(b) A single phase bidirectional a.c. controller supplies a resistive load of R=10

ohm and the input voltage of 150 Vrms, 50 Hz. The delay angle of thyristors

T1 and T2 are equal: _1= _2= _=_/2.Determine (i) the rms output voltage

Vo,(ii) the input power factor (iii) the average current of thyristor Ia(iv) rms

current of thyristor Ir.

OR

(b) Briefly describe the operation of single phase cycloconverter with necessary waveform.

Q.3 (a) Briefly discuss the operation of three phase inverter for 120 degree mode of

conduction. Draw necessary waveforms.

(b) Briefly discuss the principle of single pulse width modulation with necessary waveform.

OR

Q.3 (a) Briefly describe the performance characteristics of induction motor.

(b) Briefly describe the rotor voltage control method for induction motor using thyristor.

Q.4 (a) Discuss the role of snubber circuit from protection aspect.

(b) Describe voltage protection phenomena by selenium diodes and metal oxide varistors.

OR

Q.4 (a) Briefly discuss the excitation angle control method for power factor improvement

with neat sketch and necessary waveforms.

(b) Briefly discuss with neat sketch the operation of semi converter rectifier circuit with

R-L-E type load. Draw necessary waveforms.

Q.5 (a) Briefly discuss the operation of single phase semi converter d.c. drives.

(b) The speed of a separately excited d.c. motor is controlled by a single phase

bidirectional converter. The field circuit is also controlled by a full converter and the

field current is set to the maximum possible value. The a.c. supply voltage to the

armature and field converter is one phase , 440 V rms , 50 Hz . The armature

resistance s Ra=0.25 ohm , the field circuit resistance is 100 ohm. The motor voltage

constant is Kv= 1.4 V/A-rad/s. The armature current corresponding to load demand is

45 A. The viscous friction and no load losses are negligible. The inductances of the

armature and field circuit are sufficient to make the armature and field currents

continuous and ripple free. If the delay angle of the armature converter is 60 degree

and armature current is 45 A. Determine (i) the torque developed by the motor (ii)

the speed of the motor.

OR

Q.5 (a) Briefly discuss the operation of current source inverter with necessary waveform.

(b) A resistive load of value 2.5 ohm is being controlled by thyristor T1 that takes its

input from the transformer. The transformer is having the turns ratio of 2:1 and its

primary is connected to a.c. source of 215 v rms, 50 Hz. The delay angle is _ = _/2.

Determine (i) the rectification efficiency. (ii) the form factor (iii) Ripple factor (iv)

Transformer utilization factor. Draw neat sketch of circuit and its waveform.