JNTU B.Tech I Semester ELECTRICAL TECHNOLOGY, November 2008

(Electronics & Instrumentation Engineering, Bio-Medical Engineering and Electronics & Control Engineering) SET-4

1. (a) Sketch the Magnetisation and Load characteristics of a D.C. shunt Generator?

Explain them.

(b) The open-circuit characteristic of a shunt generator at 800rpm gives:

Field Current (A) 0 0.5 1.0 2.0 3.0 4.0 5.0

Induced emf (V) 10 50.0 100.0 175.0 220.0 245.0 262.0

Find graphically the critical resistance of shunt field circuit. If the field circuit resistance is changed to 75, what will be the critical speed for the machine

to build up. [8+8]

2. (a) Explain various power stages of in a D.C. Motor.

(b) What will happen when a D.C. Series Motor is started without a load connected to it?

(c) A 200V d.c. series motor runs at 750 rpm when taking a current of 30A. The

resistance of the armature is 0.5 and that of field is 0.3. If the current

remains constant, calculate the resistance necessary to reduce the speed to

250rpm. [4+4+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. Describe the exact and approximate equivalent circuit of a single-phase transformer.

Also describe experiments to obtain the parameters of the equivalent circuits. [16]

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) Derive e.m.f equation for an alternator and explain distribution factor and

pitch factor used in e.m.f. Equation.

(b) Write the expression showing the relationship between speed frequency and

no. of poles of a synchronous machine. The speed of rotation of the turbine driving an alternator is 166.7 r.p.m. What should be the no. of poles of the alternator if it is to generate voltage 50HZ. [10+6]

7. (a) Explain the principle of working of synchronous motor.

(b) A 3 phase, 1385 V star connected synchronous motor having synchronous

reactance of 2ohm per phase and negligible resistance takes an input of 207.8

kw with an induced e.m.f of 916.5V per phase. Calculate the motor line current

and its power factor.

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]