ELECTRICAL MACHINES-3 JNTU previous years question papers

ELECTRICAL MACHINES-3 JNTU previous years question papers
 Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

1. (a) Explain the various winding factors? Explain the effects of each of them.
(b) Determine the slot distribution and the pole phase group sequence for a 45 slot, 6 pole, 3-winding. [6+10]

2. (a) Explain the operation and effect of load power factor on the performance of alternator.
(b) The effective resistance of a 2200 V, 50 Hz, 440 kVA, single phase alternator is 0.5 . On short circuit, a field current of 4 A gives the full load current. The EMF on open circuit for the same field current is 1160 V. Find Synchronous impedance, Synchronous reactance and % regulation of 0.6p.f lagging. [8+8]

3. (a) Explain the various tests to be conducted on an alternator to find the voltage regulation of an alternator.
(b) Explain the effect of ‘Saturation’ on the performance of an alternator. How the effect of saturation can be overcome in calculation. [8+8]

4. (a) Explain all the necessary conditions for successful parallel operation of alternators.
(b) A 2 MVA, 3-, star connected, 4 pole, 750 RPM alternator is operating on 6000 V bus bars, xs is 6 /ph. Find synchronizing power and torque for full load 0.8 power factor lagging. [8+8]

5. A 3- , 30 kW, 400 V, star connected synchronous motor operates on full load at 0.8 pf lagging. The machine has synchronous reactance of 4  & negligible armature resistance. Calculate the new value of current & power factor if the excitation is increased by 50%. [16]

6. The data for no load saturation curve of a 6.6 kV, 1.8 MVA. 3-, 50 Hz, star connected synchronous motor is given below:
V (k V) 3.6 5.9 7.4 7.9 8.4
If Amp 45 91 130 160 210
The effective resistance & synchronous reactance per phase of the motor are 0.35  & 7 respectively. Plot the V curves for this machine when the input is maintained constant at 450 kW. [16]

7. (a) Explain, the speed of single phase Induction motor can be controlled by supply voltage where as them is not possible with 3 - IM, why?
(b) The name plate of single phase IM, 4 pole Induction motor gives the following data:
Output = 410 W, Supply voltage = 230 V, Frequency = 50 Hz,
Input current = 3.2 A, Power factor = 0.7, speed = 1410 RPM.
Calculate:
i. The efficiency of the motor
ii. The slip of the motor when delivering rated output. [8+8]

8. Write short notes on following:
(a) AC series motor
(b) Universal motor
(c) Variable reluctance stepper motor
(d) Permanent magnet stepper motor.

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