Cochin University of Science and Techology (CUST) 2011-6th Sem B.Tech Electrical and Electronics Engineering BTech ,il - EE 604 ELECTRICAL MACHINES III ( 2002 Scheme) - Question Paper
BTS (C) VI -11 - 014-J
B. Tech Degree VI Semester Examination April 2011
EE 604 ELECTRICAL MACHINES III
(2002 Scheme)
Time : 3 Hours Maximum Marks : 100
I. (a) Draw and explain the torque slip characteristics of a 3 - phase induction
motor. Clearly indicate the effect of change in rotor resistance. (10)
(b) Explain the working principle of a 3 phase induction motor. Why does
an induction motor never run on synchronous speed.? (10)
OR
II. (a) A 3 phase 6 - pole 50 Hz induction motor develops 3700 Watts at
950 rpm. What is the stator input, if the stator losses is 300 W? (8)
(b) Draw the circle diagram from no load and blocked rotor test of a 3 phase 14.92 KW, 400 V, 6 - pole induction motor from the following test results:
Noload : 400 V 11A pf=0.2
Blocked rotor : 100 V 25 A pf=0.4
Rotor copper loss at standstill is half the total copper loss. From the diagram find line current, slip, efficiency and pf at fiill load. (12)
HI. (a) Why a starter is necessary to start an induction motor? Mention various methods of starting. Explain in detail auto transformer method of starting a squirrel cage induction motor. (10)
(b) A 11.2 KW, 3 phase, 6 - pole, 50 Hz, 400 V delta connected induction motor runs at 960 rpm on full load. If it takes 86.4 A on direct starting, find the ratio of starting torque to full load torque with a star delta starter.
Full load efficiency and power factor are 88% and 0.85 respectively. (10)
OR
IV. (a) At standstill the equivalent impedance of inner and outer cages of a double cage rotor are (0.4 + j2) Ohms and (2 + j 0.4) Ohms respectively.
Calculate the ratio of torques produced by the two cages (i) at standstill
(ii) at 5% slip. (10)
(b) Differentiate between harmonic induction torque and harmonic synchronous torque developed in an induction motor. What are their effects? (10)
V. (a) Explain the cascade arrangement for controlling the speed of three phase
induction motor. (10)
(b) Discuss the method of speed control of induction motor by changing the
number of poles. (10)
OR
VI. (a) Explain with diagram Leblanc system of speed control of 3 - phase
induction motor. (10)
(P.T.O.)
(b) Describe with a neat diagram the principle of operation of induction
generator. (10)
VII. (a) Explain the working principle and applications of a shaded pole
induction motor. (10)
(b) Explain why a single phase induction motor does not self start. Discuss
its operation based on double field revolving theory. (10)
OR
VIII. Write short notes on the following :
(i) Torque speed curve of a single phase induction motor
(ii) Capacitor start induction run motor
(iii) Repulsion induction motor (20)
IX. (a) Mention the problems usually encountered when a d.c series motor is
operated on ac. What design modifications are to be incorporated for its satisfactory operation on ac? (14)
(b) Explain the principle of working of a universal motor and mention its
applications. (6)
OR
X. (a) How does a 3 phase ac shunt commutator motor differ from a 3 phase ac
series commutator motor? (10)
(b) Explain with neat diagram the working principle of a schrage motor. (10)
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