JNTU B.Tech I Semester ELECTRICAL TECHNOLOGY, November 2008

(Electronics & Instrumentation Engineering, Bio-Medical Engineering and Electronics & Control Engineering) SET-2

1. (a) What is the difference between the short-shunt and long-shunt Compound

Generators?

(b) What is the function of an armature in a D.C. Generator.

(c) Enumerate the conditions necessary for self excitation of a D.C. Generator.

(d) A 20 kw, 200V shunt generator has an armature resistance of 0.05 and a

shunt field resistance of 200. Calculate the power developed in the armature

when it delivers rated output. [3+3+4+6]

2. (a) Explain the various methods of speed control of a D.C. Shunt Motor.

(b) A 460V d.c. series motor runs at 1000rpm taking a current of 40A. Calculate

the speed and percentage change in torque if the load is reduced so that the

motor is taking 30A. Total resistance of the armature and field circuits is 0.8.

Assume flux is proportional to the field current. [8+8]

3. (a) Explain the principle of operation of transformer. Derive its e. m. f. equation.

(b) A 1-phase transformer has 180 turns respectively in its secondary and primary

windings. The respective resistances are 0.233 and 0.067.

Calculate the equivalent resistance of

i. the primary in terms of the secondary winding,

ii. the secondary in terms of the primary winding, and

iii. the total resistance of the transformer in terms of the primary. [8+8]

4. Write short notes on:

(a) OC and SC tests on transformers.

(b) Losses in transformers. [10+6]

5. (a) Explain the principle of rotating magnetic field and hence prove that it is of

constant magnitude and rotates at synchronous speed.

(b) A 3-phase, 4 pole 50 Hz induction motor has a full-load speed of 1440 r.p.m.

For this motor, calculate the following

i. full-load slip and rotor frequency

ii. speed of stator field with respect to

A. stator structure and

B. rotor structure and

iii. speed of rotor field with respect to

A. rotor structure

B. stator structure and

C. stator field. [8+8]

6. (a) Explain the tests to be conducted for determining synchronous impedance.

(b) A 100KVA, 6.6KV, 3phase star connected synchronous generator has a syn-

chronous reactance of 25 ohm per phase. It supplies full load current at 0.75 p.f lagging and a rated terminal voltage. Compute the terminal voltage for the same excitation when the generator supplies full load current at 0.8 p.f leading. [8+8]

7. (a) Explain the principle of operation of synchronous motors.

(b) A 3-phase alternator is rated at 5 KVA, 110V, 26.3A, 50 Hz and 1200 r.p.m.

The stator resistance between terminals as measured with dc is 0.2 ohm. With no load and rated speed the stator line voltage is 160V for a field current of 4A.At rated speed, the short circuit stator current per terminal is 50A for a field current of 4A.compute voltage regulation of alternator at 0.8 p.f. Lagging. Using synchronous impedance method. [8+8]

8. (a) Single phase induction motors are not self starting. Explain Why?

(b) How is single-phase induction motors made self started? Explain one method.