MU Electronics Engineering (Semester 4)
Electrical Machines
December 2014
Total marks: --
Total time: --
INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary


1 (a) Explain significance of back emf in DC motor.
5 M
1 (b) Draw the block diagram and explain v/f control using converter inverter scheme for 3 phase induction motor.
5 M
1 (c) A 230V D.C. motor has an armature circuit resistance of 0.6Ω if the full load armature current is 30A and no-load armature current is 4A. Find th change in back emf from no load to full load.
5 M

2 (a) Draw and explain 3-point starter of DC shunt motor.
7 M
2 (b) A 6-pole lap wound shunt motor has 500 conductors the armature and shunt Filed resistance are 0.5 Ω and 25 Ω respectively, find the speed of the motor if it takes 120 A from a DC supply of 100V. Flux per pole is 20mWb.
8 M

3 (a) Explain construction and working of 3 phase squirrel cage induction motor.
8 M
3 (b) Draw and explain torque speed characteristics of 3 phase induction motor.
7 M

4 (a) Explain in detail different starting methods of single phase induction motor.
7 M
4 (b) A 600W, 115V, 60Hz capacitor start motor draw 13.8 A from the supply at rated load if the efficiency is 65% and rated speed is 1750 RPM, calculate
i) Input power at rated load
ii) Power Factor at rated load
iii) Rated motor horse power.
8 M

5 (a) Classify the brushless DC motor and explain in detail unipolar brushless DC motor.
7 M
5 (b) Explain construction, working and control requirements of switched reluctance motor.
8 M

Write short notes on :-
6 (a) Star-Delta starter of 3 phase induction motor.
5 M
6 (b) Different speed control methods of DC shunt motor.
5 M
6 (c) Explain the double field revolving theory in single phase induction motor.
5 M



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