Electrical Machines - Dec 2014

Electronics Engineering (Semester 4)

TOTAL MARKS: 60
TOTAL TIME: 2 HOURS (1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Use suitable data wherever required.
(4) Figures to the right indicate full marks. 1 (a) Explain significance of back emf in DC motor.(5 marks) 1 (b) Draw the block diagram and explain v/f control using converter inverter scheme for 3 phase induction motor.(5 marks) 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 marks) 2 (a) Draw and explain 3-point starter of DC shunt motor.(7 marks) 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 marks) 3 (a) Explain construction and working of 3 phase squirrel cage induction motor.(8 marks) 3 (b) Draw and explain torque speed characteristics of 3 phase induction motor.(7 marks) 4 (a) Explain in detail different starting methods of single phase induction motor.(7 marks) 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 marks) 5 (a) Classify the brushless DC motor and explain in detail unipolar brushless DC motor.(7 marks) 5 (b) Explain construction, working and control requirements of switched reluctance motor.(8 marks)

Write short notes on :-

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