# SYNCHRONOUS AND INDUCTION MACHINES S4 KTU QUESTION PAPERS II

PART A

1. List any four advantages of adopting rotating field system over rotating armature in a synchronous generator.

2. A 3 phase star connected alternator is rated at 1500kVA, 12000V.The armature effective resitsance and synchronous reactance are 2Ω and 35Ω respectively per phase.

Calculate the percentage regulation for a load of 1200 kW at a power factor of 0.8 leading.

3. What is synchronising power? Explain its operation in alternator operation.

4. Draw and explain the power angle curve of a synchronous machine.

5. Explain the starting methods of slip ring induction motor.

6. What is crawling? How does it affect the working of a three phase induction motor?

7. Explain the operation of single phase induction motor on the basis of double field revolving theory.

8. Compare induction generators with synchronous generators.

9. Derive from the fundamental, generalised equation of the EMF generated in a non salient pole synchronous generator, taking into account the effect of 5th and 7th harmonic components.

10. Calculate the rms value of the induced emf per phase of a 10 pole, 3-phase, 50Hz alternator, 2 slots per pole per phase and 4 conductors per slot in 2 layers.

The coil span is 150 deg. The flux per pole has a fundamental component of 0.12 Wb and a 20% third harmonic component

12. Explain the determination of direct axis and quadrature axis synchronous reactance by slip test.

13. With the help of a neat diagram explain the dark lamp method of synchronisation. What are the draw backs of this method.

14. A 3 phase 400V induction motor gave the following test readings:

No load : 400V, 1250W, 9A

Short circuit : 150V, 4 kW, 38 A

Draw the circle diagram. If the normal rating is 14.9 kW, find from the circle diagram the full load value of current, p.f. and slip

15. Explain the following starting methods of a single phase Induction motor.

(i). Split phase starting

(ii)Capacitor start- capacitor run starting

16. Draw and explain the method of regenerative braking as applied to polyhase induction motor.

17. A squirrel cage Induction motor when started by means of a star delta starter takes 200% of full load line current and develop 45% of full load torque at starting.

Calculate the starting current and starting torque in terms of full load values, if an autotransformer with 80% tapping is employed.

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