January 6, 2012

ELECTRICAL MACHINES-1 JNTU previous years question papers

ELECTRICAL MACHINES-1 JNTU previous years question papers

Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

1. (a) Show how mechanical energy output can be determined in the multiple excited nonlinear systems.
(b) In a rectangular electromagnetic relay the exiting coil has 1000 turns. Cross sectional area of the core is A = 5 c.m2.neglect reluctance of the magnetic circuit and fringing effects. Find the mechanical energy output when the armature moves such that the air - gap decreases form 1cm to 0.5 cm with flux linkages kept constant at the corresponding Coil current of 2A. [8+8]

2. Draw a detailed sketch of a dc machine and identify the different parts. Briefly explain describe the function of each major part. Explain in detail working of that part which changes ac armature voltage to dc voltage. [16]

3. (a) Explain the following methods of improving commutation
i. Resistance commutation
ii. E.M.F commutation
(b) A 75kw, 4-pole wave wound dc generator has 72 armature conductors. The brushes are given an actual lead of 90 at full load. Calculate
i. Cross magnetizing AT/pole
ii. Demagnetizing AT/pole and
iii. Series turns required to neutralize the demagnetizing effect. [8+8]

4. (a) Distinguish between self excited and separately excited DC generators.
(b) A separately excited generator with constant excitation is connected to a constant resistance circuit. When the speed is 1200 rpm it delivers 120A at 500V. At what speed will be the current be reduced to 60A? Armature resistance = 0.1 ohm. Contact drop per brush = 1V. armature reaction may be ignored. [8+8]

5. In a 110 V compound generator, the resistance of the armature, shunt and series windings are 0.06, 25, and 0.04 ohms respectively. The load consists of 200 lamps each rated at 55 w, 110 V. Find the e.m.f generated and armature current when the machine is connected:
(a) long shunt
(b) short shunt
(c) How will the ampere-turns of series winding be changed if in:i. a diverter of resistance 0.1 ohm be connected in parallel with the series
winding? Ignore armature reaction and brush contact drop. [16]

6. (a) Define speed regulation A 14.92 Kw, 400V, 400-rpm D.C. shunt motor draws a current of 40A when running at full-load. The moment of inertial of the rotating system is 7.5 kg m2. If the starting current is 1.2 times F.L. current calculate the full-load torque.
(b) Derive an expression for armature torque developed in D.C motors.

7. A dc shunt motor takes an armature current of 20A from a 220V supply. Armature circuit resistance is 0.5 . For reducing the speed by 50%. Calculate the resistance required in the series, with the armature:
(a) the load torque is constant
(b) the load torque is proportional to the square of the speed. [8+8]

8. When running on no-load, a 400V shunt motor takes 5A. Armature resistance is 0.5 and field resistance 200. Find the output of the motor and efficiency when running on full load and taking a current of 50A, Also, find the percentage change in speed from no-load to full load.



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