ELECTRICAL MACHINES-2 JNTU previous years question papers

ELECTRICAL MACHINES-2 JNTU previous years question papers

Time: 3 hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

1. (a) In detail give the advantages & disadvantages of core & shell type construction.
(b) A 25 kVA, 2400 / 240 V, 50 Hz, single phase distribution transformer operates at no-load in step down mode, draws 138 W at a pf of 0.21 lagging. Determine the magnetising current components, magnetizing reactance & core loss resistance. [8+8]

2. (a) Derive the condition for maximum efficiency of a transformer.
(b) A single phase 150 kVA transformer has efficiency of 96 % at full load, 0.8 pf and at half load, 0.8 pf lagging. Find maximum efficiency of transformer and corresponding load. [8+8]

3. With neat diagram, explain the various tests conducted on transformer to obtain its equivalent circuit. Derive all related equations. [16]

4. (a) With neat phasor diagram, explain the voltage regulation of three-phase transformer.
(b) An ideal 3-step down transformer connected in delta/star delivers power to a balanced 3-load of 120 kVA at 0.8 pf. The input line voltage is 11 kV and the turn’s ratio of transformer (phase to phase) is 10. Determine the line voltage line currents, phase voltages, phase currents on both primary & secondary sides. [8+8]

5. (a) Explain the terms Slip, Slip speed, Rotor frequency, Rotor EMF.
(b) A 3-, 50 Hz slip ring IM gives a standstill open circuit voltage of 500 V between slip rings. Calculate the current and power factor at standstill when the per phase rotor winding resistance and inductance are 0.2  & 0.04 H and slip rings are short circuited. Repeat the calculation when slip is 4 %. [8+8]

6. (a) With neat diagram the explain Torque-Slip characteristics of IM.
(b) A 3-, 50 Hz, 4 pole slip ring IM gives a reading of 120 V across slip rings on open circuit, when at rest and supplied with normal supply voltage. The rotor impedance per phase is 0.3 + j1.5
. Find the rotor current and torque when machine is running at 5 % slip. [8+8]

7. (a) The short circuit line current of a 6 HP IM is 3.5 times its full load current, the stator of which is arranged for star- starting. The supply voltage is 400 V, full load efficiency is 82 % and full load power factor is 0.85. Calculate the line current at the instant of starting. Neglect magnetising current.
(b) Why are the adverse effects of high starting current? What are the methods by which we can reduce the starting current of IM? [8+8]

8. (a) Compare the speed control of 3-IM by rotor resistance control & variable frequency control.
(b) Two slip ring IMs having 10 & 6 poles respectively are mechanically coupled.
i. Calculate the possible speed when first motor is supplied from a 50 Hz supply line.
ii. Calculate the ratio of power shared by the two motors.
iii. If the smallest possible speed is to be attained independently by each machine,
calculate the frequency of the voltage to be injected in the rotor circuit.
[8+8]

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