JNTU II B.Tech I Semester Regular Examinations, November 2008

JNTU II B.Tech I Semester Regular Examinations, November 2008

ELECTRICAL ENGINEERING

( Common to Chemical Engineering and Metallurgy & Material Technology)

SET-3

 

 

1. (a) What is meant by Lenz‘s law? Where this law is useful? Explain the concept

of this Law with suitable example.

(b) A steel ring has a mean diameter of 20 cm, and cross section area of 25 cm2

and a radial airgap 0.8 mm cut across it. When excited by a current of 1 Ampere through a coil of 1000 turns wound on the ring core, it produces an airgap flux of 1 mwb. Neglect the leakage and fringing. Calculate the relative permeability of steel and total reluctance of the magnetic circuit.

2. (a) A 3 phase, 150 Kwatt, 400 V load is connected to a three phase supply and the power drawn by this load is measured by using two wattmeters. The two wattmeters read 120 Kwatts and 50 Kwatts respectively. Calculate the input power, power factor of the load and current drawn by the load.

(b) A three phase star connected balanced load is connected to a 3 phase, 400 V, 50 Hz supply system. The current in each phase is 20 Amps and lags behind its phase voltage by an angle 40 degrees. Calculate the readings of the two wattmeters connected to measure the total power, phase voltage and load parameters.

3. (a) Why is the shunt generator characteristic on load is drooping and turns back as it is overloaded? Explain?

(b) Why is it necessary to wind field coils of a medium or large size DC generators with a small number of turns of thick wire?

4. (a) Discuss how Faraday’s laws are useful for production of Torque in DC motors.

(b) Discuss the Ward Leonard method of Speed control of DC motors.

5. Explain OC and SC tests of 1- ? transformer and discuss their advantages.

6. (a) Derive the expression for the torque under running conditions.

(b) A 3-?, 50 Hz, 8 pole induction motor has full load slip of 2%. The rotor resistance and stand still rotor reactance per phase are 0.01  and 0.05  respectively. Find the ratio of maximum to full load torque and the speed at which the maximum torque occurs.

7. The following test results are obtained from a 3-?, 6000 KVA, 6600V, star connected, 2 pole, 50 Hz turbo alternator: with a field current of 125A, the OC voltage is 8000V at the rated speed ; with rated speed and same field current, the short circuit current is 800A. At rated full load, the resistance drop is 3%. Find regulation of alternator on full load and 0.8 pf lagging.

8. Discuss about Moving Iron instruments.

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