Multiple Choice Questions on DC Machines
1- Function of interpoles in the DC generator is to
(A) neutralize the reactance voltage
(B) make commutation sparkless
(C) neutralize the cross magnetizing effect of armature reaction
(D) all of the above
2- The mechanical power developed by a DC motor is maximum when the back e.m.f is
(A) equal to the applied voltage
(B) equal to zero
(C) equal to half of the applied voltage
(D) approaching the value of the applied voltage
3- The commercial efficiency offered by DC shunt generator is maximum when
(A) variable loss=iron loss
(B) variable loss=stray loss
(C) variable loss=constant loss
(D) variable loss=friction and windage loss
4- Lap winding is suitable for
(A) Low voltage and large current DC generator
(B) High voltage and small current DC generator
(C) Low voltage and small current DC generator
(D) High voltage and large current DC generator
5- The series field of short-shunt generator is excited by
(A) shunt field current
(B) armature current
(C) load current
(D) external current
6- In a DC generator, the effect of armature reaction on the main field is to
(A) reduce it
(B) distort it
(C) reverse it
(D) Both (A) and (B)
7- The friction and windage losses in a DC motor depends upon
(A) speed
(B) flux
(C) armature current
(D) field and armature resistance
8- In a DC generator, generated E.M.F is proportional to the
(A) field current
(B) number of dummy coils
(C) pole flux
(D) both (A) and (B)
9- Laminated yoke is used in DC motor to reduce the
(A) iron loss
(B) temperature rise
(C) sparking at carbon brushes
(D) speed regulation
10- The sole purpose of commutator in DC generator is to
(A) increase output voltage
(B) reduce sparking at brushes
(C) provide smooth output
(D) convert generated AC into DC
11- Which of the following motor has the poorest speed regulation?
(A) shunt motor
(B) series motor
(C) differential compound motor
(D) cumulative compound motor
12- The residual magnetic field of DC shunt generator is destroyed accidentally. It can be restored
(A) by connecting the shunt field winding to earth
(B) by connecting the shunt field winding to DC source
(C) by reversing the terminals of the DC
(D) by making the shunt field winding open circuited
13- The critical resistance of DC generator is resistance of the
(A) field circuit
(B) armature circuit
(C) load circuit
(D) brushes
14- The breakdown torque of a synchronous motor varies as
(A) 1/V
(B) 1/V2
(C) V
(D) V2
ANSWERS:
1-(C), 2-(C), 3-(C), 4-(A), 5-(C), 6-(D), 7-(A), 8-(C), 9-(A), 10-(D), 11-(B), 12-(B), 13-(A), 14-(C)