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B3D048S Pages: 3

are equal to 950 W. Find (i) emf generated (ii) Copper losses (iii) output of prime
mover (iv) commercial, mechanical and electrical efficiencies. (5)
a. What is armature reaction and explain its effects? Derive expressions for cross
magnetizing and demagnetizing ampere turns per pole (5)
b. A 4 pole, wave wound armature of a DC machine has 880 conductors and delivers
120 A. The brushes are displaced through 3 angular degrees from the geometrical
axis. Calculate (i) demagnetizing ampere turns per pole (ii) cross-magnetizing ampere
turns per pole and (iii) the additional field current for neutralizing the demagnetization
if the field winding has 110 turns per pole. (5)
PART ¢
Answer any 2 questions.
a. Draw and explain the electrical and mechanical characteristics of DC shunt motors.
(5)
b. A 250 V DC shunt motor has an armature resistance of 0.5 Q and a field resistance
of 250 Q. The motor draws 21 A when driving a constant torque load at 600 rpm.
What will be the new speed of the motor if an additional 250 Q resistance is inserted
in the field circuit? (5)
a. Enumerate the losses in a loaded transformer. Derive the condition for maximum
efficiency in a transformer. (5)
b. A 200 / 2000 V transformer is fed from a 200 V supply. The total winding
resistance and leakage reactance as referred to the LV side is 0.15 Q and 0.6 Q
respectively. The resistance representing core loss is 450 Q and magnetizing
reactance is 250 Q. A load of impedance (600+j400) Q is connected across the
secondary terminals. Calculate (i) input current (ii) secondary terminal voltage and
(iii) primary power factor. (5)
a. With neat diagrams, explain the speed control methods in separately excited DC
motors. (5)
b. The efficiency of a 200 KVA, single phase transformer is 98.75% when delivering
full load at 0.8 pf and 99% at 80% of full load at 0.9 pf. Calculate (i) the iron loss and
(ii) the full load copper loss. (5)
PART D

Answer any 2 questions.

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