JNTU B.Tech I Semester ELECTRICAL TECHNOLOGY, November 2008

(Electronics & Instrumentation Engineering, Bio-Medical Engineering and Electronics & Control Engineering) SET-3

1. (a) List out applications of D.C. generators and motors.

(b) A 400V shunt generator has full load current of 200A, its armature resistance

is 0.06, field resistance 100; the stray losses are 2,000W. Find the horse

power of its prime-mover when it is delivering full load and find the load for

which the efficiency of the generator is maximum. [8+8]

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) Sketch torque-speed characteristics of an induction motor working at rated

voltage and frequency, deriving necessary expressions.

(b) A 3-phase, 50 Hz, 400 V, wound-rotor induction motor runs at 960 r.p.m. at full-load. The rotor resistance and standstill reactance per phase are 0.2 and1 respectively. If a resistance of 1.8 is added to each phase of the rotor at standstill, what would be the ratio of starting torque with full voltage and the added resistance to the full-load torque under normal running conditions? [8+8]

6. (a) Draw the open circuit and short circuit characteristics of a synchronous generator. Explain the shape of the characteristics.

(b) Determine the voltage regulation of a 200V per phase alternator at 0.8p.f lag

giving a current of 100A is produced on short circuit by a field excitation of 2.5A.An e.m.f of 500V is produced on open circuit by the same excitation. The armature resistance is 0.8 ohm. [8+8]

7. (a) A 500V, 50 HZ, single-phase synchronous motor take 50 A current at a power

factor of 0.8 lagging. The motor has a synchronous reactance of 2 ohm and neglible resistance. The armature has 120 full pitch coils in series, with a distribution factor of 0.95. Assuming a sinusoidal variation flux in the air gap.

Calculate the flux per pole.

(b) Draw the phasor diagram of a synchronous motor. [8+8]

8. (a) What is a stepper motor? Enumerate its advantages and applications.

(b) With neat sketch, explain the working principle of shaded-pole single-phase

induction motor. [8+8]