Printed Pages—4 EAG401

B. Tech.

(SEM. IV) THEORY EXAMINATION 2011-12

ELECTRICAL CIRCUITS AND MACHIENS

Time : 3 Hours Total Marks : 100

Note :— Attempt all questions.

1. Attempt any four parts of the following :— (5×4=20)

(a) Explain the principle of operation of a d.c. motor.

(b) Explain the double revolving field theory.

(c) What are the disadvantages of low power factor ?

Mention some methods of improving power factor.

(d) Explain process of voltage build up of a self excited

d.c. generator.

(e) Explain two wattmeter method of measurement of

three phase power with the help of circuit and phasor

diagram.

(f) What is the need of using starter in :

(i) three-phase induction motor

(ii) Single-phase induction motor ?

2. Attempt any four parts of the following :— (5×4=20)

(a) Mention and explain various electric safety rules.

EAG401/PUR-40960(Re) 1 [Turn Over

(Following Paper ID and Roll No. to be filled in your Answer Book)

PAPER ID : 4308 Roll No.

(ii) Total current,

(iii) Current in each branch,

(iv) Overall power factor,

(v) The three powers.

Figure 2

230 V,50 Hz

I

5 Ω

0.01 H

7 Ω

12 Ω

0.015 H

180 µF

EAG401/PUR-40960(Re) 4 1625

EAG401/PUR-40960(Re) 3 [Turn OverEAG401/PUR-40960(Re) 2

(b) Describe with neat sketch, the star-delta starting of

3-phase induction motor.

(c) A 250 V series motor is developing a torque of

10 N-m when running at 1200 r.p.m. The total armature

and series field resistance is 0·6 Ω. Calculate the motor

current.

(d) Why improvement of power factor is needed ?

Explain.

(e) Explain voltage control in 3-phase induction motor.

For what type of load this method is more suitable ?

(f) Write short note on power factor improvement and

three phase power measurement.

3. Attempt any two parts of the following :— (10×2=20)

(a) Explain the Thevenin’s; Norton’s superposition power

transfer in detail.

(b) Describe the sinusoidal steady state response of

circuits.

(c) Write short notes on :—

(i) Hysteresis and eddy current losses.

(ii) Leakage reactance.

(iii) Voltage regulation.

(iv) EMF and torque equations.

4. Attempt any two parts of the following :— (10×2=20)

(a) Describe the construction, operation, equivalent

circuit and phasor diagram for a polyphase induction

motor.

(b) A long shunt compound generator supplies a load of

20 kW at 500 V. The armature, series and shunt field

resistances are 0·1 Ω and 0·05 Ω and and 100 Ωrespectively. Calculate the induced e.m.f. Also draw

the schematic circuits.

(c) Define form factor and crest factor. Find the form factor

of the triangular waveform shown in Figure 1.

5. Attempt any two parts of the following :— (10×2=20)

(a) What do you understand by Instantaneous and average

power; power factor; reactive and apparent power ?

Describe in detail.

(b) For what purposes transformers are used ? Explain the

principle, construction and working of a single phase

transformer.

(c) Explain the power triangle for electric circuits. In the

network shown in Figure 2; find :

(i) Total impedance,

V

10 V

0 100 t50

Figure 1