# GTU Electric Machines Question Paper Mar 2010

GUJARAT TECHNOLOGICAL UNIVERSITY

B.E. Sem-III Remedial Examination March 2010

Subject code: 131701

Subject Name: ELECTRICAL MACHINES

Total Marks: 70

Instructions:

1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.

Q.1 (a) A d.c. shunt generator delivers 450 Amp at 230V and the resistance of the shunt 07 field and armature are 50Q and 0.03Q respectively. Calculate the generated emf.

(b) Explain following characteristic of separately excited d.c. generator.

(ii)  Internal and External characteristic

Q.2 (a) Explain armature reaction of the d.c. machine. Give its remedies also.

(b) Explain in brief various methods of speed control of d.c. shunt motors.

OR

(b) Explain working principle of d.c. motor. Derive the condition for maximum Power.

Q.3 (a) Explain the conditions of parallel operation of three phase transformers.

(b) How three phase to two phase transformation of transformer is obtained?

OR

Q.3 (a) In no load test of single phase transformer, the following test data were obtained: 07 Primary voltage: 220V; Secondary voltage: 110V;

Primary current: 0.5A; Power input: 30W.

Find the following:

(i)                 The turns ratio

(ii)               The magnetizing component of no-load current

(iii)             It’s working (or loss) component

(iv)             The iron loss Resistance of primary winding =0.6Q.

(b) Write advantages and applications of auto transformer.

Q.4 (a) Drive torque equation for three phase induction motor. Also derive condition for  maximum torque and equation for maximum torque.

(b) Draw torque-speed characteristic of induction motor. Also explain change in it with

change in rotor resistance and frequency.

OR

Q.4 (a) Explain rotating magnetic field theory. Also explain how three phase induction 07 motor

starts.

(b) Explain speed control of three phase induction motor.

Q.5 (a) Compare star-delta starter with auto transformer starter for tree phase induction  motor.

(b) Explain why single phase induction motor is not self started? Explain the starting  methods for single phase induction motor in brief.

OR

Q.5 (a) What is commutation? Give remedies for commutation.                                               (b) Explain various types of losses occurring in a d.c.generator.