# 2006 Mahatma Gandhi University B.Tech Electrical and Electronics Engineering Electrical Machines - III (E) Question paper

** University Question Papers **

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**2006 Mahatma Gandhi University B.Tech Electrical and Electronics Engineering Electrical Machines - III (E) Question paper

Part A

Answer all questions.

Each question carries 4 marks.

1. Compare squirrel-cage induction motor with slip-ring induction motor with respect to its construction and operational aspects.

2. What do you understand by cogging and crawling in a three-phase induction motor?

3. Draw and explain the torque-slip characteristic of a three-phase induction motor.

4. Explain any one method of starting of a three-phase squirrel-cage induction motor.

5. Describe the principle of operation of synchronous induction motor.

6. Draw the stator circuit diagram and the corresponding phasor diagram at starting for the following types of single-phase induction motors:

(a) Capacitor start motor,

(b) Capacitor start-capacitor run motor.

7. Give constructional details of a single-phase series motor.

8. Write short notes on the Hysteresis motor.

9. What is meant by (i) Transverse edge effect and (ii) End effect in linear induction motors?

10. Write short notes on the use of commutator machine as frequency converters and phase advancers.

(10 X 4 = 40 marks)

Part B

Answer one question from each module.

MODULE I

11. (a) A three-phase, 400 V, 50 Hz, slip-ring induction motor with a star-connected rotor has three slip rings brought out to the terminal box. The induced EMF between slip rings is 60V on open circuit at stand still condition with 400V, 50Hz applied to the stator. The resistance and stand still reactance of each rotor phase are 0.6? and 4.0? respectively. Calculate current per phase in the rotor(i) at stand still, when the rotor is connected to a star-connected impedance with a resistance 5? and reactance 2? per phase and (ii) when running, short circuited with a slip of 4%. (6 marks)

11. (b) A three-phase, 6-pole, 580V, 50Hz induction motor develops 20hp at 950 rpm with a power factor of 0.86. The mechanical losses total 1.0 hp. Calculate for this load (i) the rotor copper loss, (ii) Torque output in Nm and (iii) Line current. (6 marks)

Or

12. Determine the parameters of the equivalent circuit of a three-phase, 400V, 50Hz, 4-pole, delta connected, squirrel-cage induction motor with the following data :-

No-Load : 400V 3.0A 300W

Blocked Rotor : 120V 7.0A 500W

Draw the equivalent circuit and calculate the hp output and input when the motor works with a slip of 5%. The stator effective resistance per phase is equal to 4?. (12 marks)

MODULE 2

13. (a) Explain with diagram, rotor resistance control method of speed control of a three-phase slip-ring induction motor.

13. (b) The rotor of a 4-pole, 50Hz, slip-ring induction motor has a resistance of 0.3? per phase and runs at 1440 r.p.m. at full load. Calculate the external resistance per phase which must be added to lower the speed to 1320 rpm, the torque being the same as before. (6 marks)

Or

14. Calculate the steps in a 5-section rotor starter of a 3-phase induction motor for which the starting current should not exceed the full-load current, the full-load slip is 1.8% and the rotor resistance is 0.015? per phase. Also derive the formulae used. (12 marks)

MODULE 3

15. A 150kW, 400V, 50Hz, 4-pole, star connected induction machine is driven as an induction generator supplying power to a three-phase, 400V, 50Hz grid. The rotor of the generator is driven at 1560 r.p.m. The real power supplied to the grid is 100kW, at a power factor of 0.707. What is the value of reactive power drawn from the grid? If the magnetizing current drawn from the supply is 100A and if the generator works with an efficiency of 95% for this load, estimate the generator rotor resistance in terms of stator. Neglect core loss. Draw the phasor diagram of the generator indicating the stator current, magnetizing current and rotor current, taking the stator phase voltage as the reference. Also explain its operation. (12 marks)

Or

16. A 3-phase, 4-pole, star-connected, capacitor excited induction generator works with a capacitor bank of 40µF capacitor/phase connected in delta. The load is star-connected with 10? resistance per phase. At a particular speed, the generator gives a terminal voltage of 400V, 50Hz. Calculate (i) line current of the generator and (ii) power output of the generator. Draw the circuit arrangement. The generator is driven by a wind turbine. Also explain its operation. (12 marks)

MODULE 4

17. Explain with diagrams, the construction and operation of reluctance motor. (12 marks)

Or

18. Derive an expression for the torque developed in repulsion motor. Explain its operation. Also draw its phasor diagram. (12 marks)

MODULE 5

19. Obtain the equivalent circuit of a double cage induction motor. Also draw and explain its torque-slip characteristics. (12 marks)

Or

20. Explain with neat diagram, the construction and principle of operation of a Schrage motor. (12 marks)

[5 X 12 = 60 marks]