B.TECH. DEGREE EXAMINATION, DECEMBER 2007

Seventh Semester
Branch: Electrical and Electronics Engineering
ELECTRICAL DRWES AND CONTROL (E)
Answer all questions.

1. Field control technique is employed for d.c. motors for getting speeds higher than rated and armature voltage control is employed for getting speeds less than rated. Why?
2. Explain the operation of a half controlled bridge rectifier for d.c. motor control.
3. What is meant by four quadrant operation of chopper fed d.c. drive?
4. Draw and explain the motoring and regenerative braking characteristics of chopper controlled d.c. series motor.
5. What are the applications of static Kramer’s drive?
6. Why stator voltage control is an inefficient method of induction motor speed control ? Why variable frequency control is more efficient?
7. What are the various requirements of an induction motor drive ? What are the various methods of speed control?
8. What is a current source inverter ? Explain its principle of operation.
9. What is the self control mode for variable frequency control of synchronous motor ? What is the basic difference between true synchronous mode and self control mode?
10. Give the principle of vector control.
(10 x 4 = 40 marks)
Part B
Answer all questions.
Each question carries 12 marks.
11. Explain with neat circuit diagram and waveforms the operation of single-phase fully controlled bridge rectifier d.c. separately excited d.c. drive. Obtain the equation for output voltage for continuous conduction. Draw the speed torque characteristics for various values of x.
Or
12. A 220 V, 960 r.p.m., 12.8 A separately excited d.c. motor has armature circuit resistance and inductance of 2 ohm and 150 mu respectively. It is fed from a single-phase half controlled rectifier with an a.c. source voltage of 230 V, 50 Hz. Calculate (a) motor torque for a = 60° and speed = 600 r.p.m. ; (ii) motor speed for a = 60° and T =20 N-rn.
13. The speed of a separately excited d.c. motor is controlled by a class A chopper whose Vg = 100 V, Ra = 0.4 2 and La 10 mH and Ka = 0.05 V/r.p.m. The motor drives a constant torque load requiring an average armature current of 30 A. Compute the range of speed control and corresponding range of duly cycle.
Or
14. Discuss the dual converter control of d.c. separately excited motor. A 220 V. 1500 r.p.m., 50 A separately excited motor with Ra = 0.5 is fed from a circulating. Current dual converter with a.c. source voltage (line) = 165 V. Determine the converter firing angles for motoring operation at rated torque and 1,000 r.p.m.
15. Explain the principle, controller configurations and applications of stator voltage control of an induction motor.
Or
16. What is meant by slip power recovery 7 Explain the various slip power recovery schemes.
17. Explain the application of voltage source inverters to induction motor drives. Explain PWM control.
Or
18. Discuss the application of current source inverter to induction motor drives. Compare it with VSI drives.
19. What are the various methods of speed control of synchronous motors 7 Explain application of VSI drive with open-loop control.
Or
20. Explain self-controlled synchronous motor drive using load commutated thyristor inverter.
(5 x 12 = 60 marks)


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