JNTU, B.Tech I Semester ELECTRO MECHANICS-I,November 2008

(Electrical & Electronic Engineering)

SET-4

1. (a) Explain the principle of energy conversion of electromechanical system.

(b) Derive an expression for the energy stored in a magnetic field system.

2. The armature core of a 4-pole d. c. machine has 31 slots each designed to accommodate 4 coil sides of a simplex wave winding. The winding has total of 496

conductors. Find

(a) Total number of coils

(b) Turns per coil

(c) Commutator pitch

(d) Back, front and total pitches and

(e) Number of commutator segments.

3. (a) Explain the purpose of laminating the armature core of a d. c. machine.

(b) The diameter of a commutator ring of a lap wound d. c. motor is 25 cms. The

brush width is 1.8 cm and width of mica insulation is 0.2 cm. If the speed of the motor is 200 rpm, determine the time of commutation.

4. The load characteristics of two shunt generators are as follows:

Machine: IArmature current In amperes 0 283 566 850

Machine: IIArmature current In amperes 0 200 400 600

Terminal voltage 500 490 480 470

If the above machine works in parallel to supply a total load of 400 KW, determine

(a) The load taken by each machine and

(b) The amount by which the open circuit emf of the machine must be raised by

field regulation so that the two machines share the 400 KW load equally?

5. (a) Develop the general expression for the speed of a motor in terms of supply

voltage, armature resistance and flux per pole.

(b) Discuss the applications of series motors and compound motors.

6. (a) Explain about series-parallel method of Speed Control of D.C. Series Motors.

(b) A 500V, d.c. shunt motor takes a current of 5 A on no-load. The resistances of the armature and the field circuits are 0.22 ohm and 250 ohm respectively.

Find

i. The efficiency when loaded and taking a current of 100A.

ii. The percentage change of speed. State precisely the assumptions made.

7.(a) Explain how the performance characteristics of a long shunt and short shunt

compound motor can be determined theoretically and experimentally, by drawing the relevant circuit diagram.

(b) A 200V, shunt motor develops 17.158 KW when taking 20.2 KW. The field resistance is 50A and armature resistance is 0.06. What is the efficiency and power input when the output is 7460W.

8. (a) Outline the steps to estimate the efficiency of given two d.c. machines by conducting Hopkinson’s test. Draw schematic diagram to illustrate the method.

(b) In a Hopkinson’s test on a pair of 500V, 100KW, shunt generators, the following data was obtained. Auxiliary supply 30A at 500V

Generator output Current 200 A

Field currents 3.5 A and 1.8A

Armature circuit resistances of each machine 0.075

Voltage drop at brushes (each machine) : 2 V

Calculate the efficiency of each machine acting as a generator.