GTU Exams papers
GUJARAT TECHNOLOGICAL UNIVERSITY
B. E. Sem. – V – Examination – June- 2011
Subject code: 150802
Subject Name: Electrical Machine – II
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
Q.1 (a) Define the following terms using supporting diagram
Pole pitch, Coil pitch, Front pitch, Back pitch and Commutator pitch
(b) Explain in brief about the techniques to improve the commutation in DC machine.
(c) Explain the construction, operation and equivalent circuit of double cage
Q.2 (a) Using double field revolving theory, discuss in detail why single phase
induction motor is not self starting.
(b) Draw the developed view diagram of lap winding for 4 poles and 12 slots
armature with two coil sides per slot.
(b) Discuss in detail about the cogging and crawling phenomena for three
phase induction motor.
Q.3 (a) Using circle diagram method, determine
(i) Full load current & power factor (ii) Maximum possible output
power, for the given data.
3 phase, 20 HP, 440 Volt, 50Hz, 4 pole Induction motor
No load test: 440 V, 10 A, 1.5 kW
Blocked rotor test: 120 V, 30 A, 2.25 kW
(b) Discuss construction, phasor diagram and torque-speed characteristics
for the following single phase motors:
i. Split phase
ii. Capacitor start
iii. Shaded pole
Q.3 (a) Discuss the construction and operation of permanent magnet brush less
(b) Draw the following vector groups of three phase transformer winding
Yy0, Dd6, Dy1, Yd11, Dd0
Q.4 (a) Two shunt machines connected for Hopkinson’s test take 15A at 200 V
from the supply. The motor current is 100 A and shunt currents are 3 A
and 2.5 A. If the armature resistance of the each machine is 0.05 Ω.
Calculate the efficiency of each machine for this particular load condition.
(b) Discuss the construction, operation and speed control for the Universal motor
Q.4 (a) A 400 kVA load at 0.7 power factor lagging is supplied by three single
phase transformers connected in Δ-Δ each. Each transformer is rated at
200 kVA, 2300/230 V. If one defective transformer is removed from
service, calculate for the V-V connection,
(a) The kVA load carried by each transformer. (b) Percent rated load
carried by each transformer. (c) Total kVA ratings of transformer bank in
V-V. (d) Ratio of V-V bank to Δ-Δ bank transformer ratings. (e) Percent
increase in load on each transformer when one transformer is removed.
Q.4 (b) Draw the equivalent circuit of the synchronous motor and explain its
operation under the various excitation conditions using phasor diagram
Q.5 (a) Derive the expression for the power developed by the synchronous motor
(b) What are the desirable starting conditions for a three phase induction
motor? With the neat schematic arrangement discuss star-delta starter.
Q.5 (a) Explain the operation of single stack variable reluctance type stepper
motor. Also discuss about the micro stepping.
(b) A 1000 kVA, 11000 V, 3-phase star connected synchronous motor has
an armature resistance and reactance per phase of 3.5 Ω and 40 Ω
respectively. Determine the induced e.m.f. and angular retardation of
rotor when fully loaded at (i) unity power factor, (ii) 0.8 power factor
lagging (iii) 0.8 power factor leading.
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