MCQs on Electrical Machines
01․ If field current is decreased in shunt dc motor, the speed of the motor
(A) remains same.
(B) increases.
(C) decreases.
(D) none of the above.
Right answer :- (B) Increase
Hints:- As a shunt field current If decreases, φ also decreases and the speed rises as speed is inversely proportional to flux.
02․ What is the mechanical power developed by a DC series motor is maximum?
(A) Back emf is equal to half the applied voltage.
(B) Back emf is equal to applied voltage.
(C) Back emf is equal to zero.
(D) None of above.
Right answer:- (A) Back emf is equal to half the applied voltage.
Hints:-
03․ In Ward-Leonard system, the lower limit of the speed imposed by
(A) Field resistance.
(B) Armature resistance.
(C) Residual magnetism of the generator.
(D) None of above.
Right answer:- (C) Residual magnetism of the generator.
Hint:- In Ward-Leonard method of speed control, the lower limit of speed is imposed by residual magnetism of the generator.
04․ Ward-Leonard control is basically a ___________ control method.
(A) Field control.
(B) Armature resistance control.
(C) Armature voltage control.
(D) Field diverter control.
Right answer:- (C) Armature voltage control.
Hint:- Ward-Leonard speed control method consists a motor with a constant excitation and applying a variable voltage to its armature to provide the required speed. Hence it is armature voltage control method.
05․ For very sensitive and wide speed control, the preferable control method is..
(A) Armature control.
(B) Ward-Leonard control.
(C) Multiple voltage control.
(D) Field control.
Right answer:- (B) Ward-Leonard control.
Hint:- In ward-Leonard method, very fine speed control over the whole range from zero to normal speed in both directions can be obtained. The motor-generator set can provide speed both below and above the rated speed and in both direction.
06. Eddy current loss will depends on
(A) Frequency
(B) Flux density
(C) Thickness
(D) All of the above
Right answer:- (D) All of the above
Hints:-
So eddy current loss will depend upon frequency, flux density and the area of the eddy current loop.
07. Hysteresis loss will depends on
(A) f
(B) L f²
(C) f³
(D) f¹.⁶
Right answer:- (A) f
Hints:-
So, hysteresis loss will depends on frequency.
08. Thin laminations are used in a machine in order to reduce
(A) Eddy current losses
(B) Hysteresis losses
(C) Both 1 and 2
(D) Copper losses
Right answer :- (A) Eddy current losses
Hints:- Thin laminations are used in order to reduce the eddy current losses only. Due to laminations the area of the eddy currents loops are minimized and the losses due to eddy current losses are minimized.
09. Hysteresis loop represents the area of
(A) copper loss
(B) eddy current loss
(C) hysteresis loss
(D) total iron losses
Right answer :- (C) hysteresis loss
Hints:- Hysteresis loop will represents only hysteresis losses. It is found out by area of B-H loop curve of a magnetic material.
10․ What is the shunt resistance component equivalent circuit obtained by no load test of an induction motor representative of ?
(A) windage and friction loss
(B) core loss only
(C) both 1 and 2
(D) Copper loss
Right answer:- (B) core loss only
Hints :- No load loss consists of core loss, friction and windage loss and no load copper loss. But shunt branch resistance Rc in the induction motor equivalent circuit does not represent the no load losses, it represents only the core loss of the induction motor.