Examination: V-B.Tech. (EE)
Subject: Electrical Machines-II (EEC308)
Max. Marks: 100
Semester: Monsoon (End-Semester)
Session: 2021-2022
Time: -----
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Attempt All Questions
1. (a) What is auto-transformer method of starting of three-phase induction motor? Find out the [8]
ratio of starting torque to the full load torque, and line current in terms of short circuit
current in this context.
[12]
(b) A 4-pole, three-phase induction motor delivers 37 h.p. at the shaft at a speed of 1425
r.p.m. on 415 V, 50 Hz supply. The total mechanical losses are 3 h.p. and the power
factor is 0.9. Calculate for this load (i) the slip, (ii) the rotor copper loss, (iii) the total
power input if the stator losses are 2.5 kW, (iv) the efficiency, (v) the line current, (vi)
the number of complete cycles per minute of the rotor e.m.f.
2. (a) What is slip-power recovery method of speed control of induction motor? Explain [8]
briefly.
(b) For a three-phase slip-ring induction motor, maximum torque is 2.8 times the full-load [12]
torque and the starting torque is 1.8 times the full-load torque. Determine the percentage
reduction in rotor circuit resistance to get a full-load slip of 3.5%. Neglect stator
impedance.
3. (a) Draw the phasor diagram of an alternator for leading power factor load. Derive an [8]
approximated expression of voltage regulation with standard symbols using the above
phasor diagram.
[12]
(b) A three-phase, star-connected, 415 V synchronous motor having synchronous reactance
of 12 ohm per phase and negligible resistance, draws 5.5 kW at the rated voltage. For an
excitation voltage adjusted to the rated voltage of 415 V, compute the load angle, power
factor and the armature current.
4. (a) Draw the equivalent circuit of a single-phase induction motor at start using standard [8]
symbols.
(b) A single-phase, 230 V, 4-pole, induction motor takes a line current of 60  700 A at [12]
standstill with its main winding excited from 230 V, 50 Hz source. Neglecting stator
impedance, magnetizing current and rotational loss, compute the torque at a slip of 0.05.
5. (a) What is synchronizing torque with respect to the operation of synchronous machine? [8]
Derive its expression for a salient pole machine.
(b) A 6.6 kV star-connected three-phase synchronous motor having armature resistance of [12]
1.2 ohm per phase and synchronous reactance of 10 ohm per phase works at constant
voltage and constant excitation. For an input power of 1.1 MW, the operating power
factor is 0.85 leading. Calculate the power factor when input is increased to 1.5 MW.