DC MACHINES
Q.The speed of a d. c series motor at no load is
a) Zero
b) Infinity
c) Q00 r. p. m
Ans. b
Q.A sinusoidal voltage of frequency 1 Hz is applied to the field of a d.c generator. The armature voltage will be
a) 1 Hz square wave
b) 1 Hz sin e wave
c) d. c voltage
Ans. b
Q.The function of the commutator in a d. c machine is
a) to change ac to dc
b) to change alternating voltage to direct voltage
c) for easy control of voltage
Ans. b
Q.The direction of rotation of a dc series motor can be reversed
a) by interchanging the supply terminals
b) by interchanging the field terminals
c) either a) or b) .
Ans. b
Q.A dc shunt generator driven at normal speed in the normal direction fails to build up armature voltage because
a) the armature resistance is high
b) the field current is not sufficiently high
c) there is no residual magnetism.
Ans. c
Q.If a dc motor is connected across an ac supply , the dc motor will
a) burn as the eddy currents in the field produce heat
b) run at its normal speed
c) run at a lower speed
Ans. a
Q.What would happen if the field of dc shunt motors is opened?
a) the speed of the motor will be reduced
b) it will run at its normal speed
c) the speed will be enormously high , and the motor will destroy itself.
Ans. c
Q.Which one of the following motors is used in a locomotive?
a) Induction motor
b) dc series motor
c) Synchronous motor
Ans. b
Q.The efficiency of a dc shunt generator has a maximum value when
a) magnetic losses are equal to mechanical losses
b) armature copper loss is equal to the constant loss
c) field copper loss is equal to the constant loss.
Ans. b
Q.The dummy coil in the dc machine is used to
a) eliminate armature reaction
b) bring about mechanical balance of armature
c) eliminate reactance voltage
Ans. b
Q. The sparking at the brushes in a dc generator is attributed to
a) quick reversal of current in the coil under commutation
b) armature reaction
c) reactance voltage
Ans. . c
Q.Interpoles in the armature of a dc generator are used to
a) neutralize the reactance voltage
b) neutralize the reactance voltage and the cross-magnetizing effect of the armature reaction
c) neutralize the cross-magnetizing effect of the armature reaction
Ans. b
Q.The mechanical power develop by the dc motor is maximum when
a) back emf = applied voltage
b) back emf = 0
c) back emf = 0.Q times the applied voltage.
Ans. c
Q.In series –parallel control method when two dc series motors are connected in series , the speed of the set is
a) half of the speed of he motors when connected in parallel
b) ¼ th of the speed of the motors when connected in parallel
c) same as in parallel.
Ans. b
Q.The armature torque of the dc shunt motor is proportional to
a) armature current only
b) field flux only
c) armature current and flux both
Ans. c
Q.The reversal of speed of motors used as electric braking is known as
a) regenerative braking
b) plugging
c) dynamic braking.
Ans. b
Q.The method used for braking in rolling mills is
a) plugging
b) regenerative braking
c) dynamic braking
Ans. a
