1. The armature reaction is produced mainly by __________
a) load current in armature
b) load current in field
c) all of the mentioned
d) none of the mentioned
Explanation: The armature reaction is initiated by the loaded conditions of the machine because an unloaded dc machine does not armature flux to get distorted.
2. The compensating winding in dc generator is provided to __________
a) neutralize cross-magnetizing flux
b) neutralize demagnetizing flux
c) neutralize armature mmf
d) maintain magnetizing mmf
Explanation: The compensating winding functions such that to nullify the cross magnetization effects in the armature.
3.If the magnetic circuit of the dc machine is in the saturation region, the armature reaction __________
a) does not affect flux/pole
b) increases the flux/pole
c) decreases flux/pole
d) affects the flux/pole only when armature current is small
Explanation: In the saturation region the flux will be constant.
4. A dc motor is connected in the short-shunt configuration, but the series and the shunt windings get interchanged by mistake and the motor is connected to the rated voltage?
a) neither of the windings get short-circuit
b) series winding gets overloaded
c) shunt winding gets overloaded
d) both of the winding get overloaded
Explanation: In the short shunt configuration, the winding interchanging does not change the configuration of the set up so none of the winding get affected.
5. What is the main concern before doing hopkinson’s test for finding efficiency?
a) needs one motor and one generator is that it
b) ignores iron and stray losses
c) needs one motor
d) requires identical shunt machines
Explanation: It is must that both the machines are identical as required by the calculations.
6. Retardation test on DC shunt motor is used for finding _______
a) stray losses
b) copper losses
c) friction losses
d) iron losses
Explanation: Retardation test is used for finding the stray losses.
7. The quantities needed to complete retardation test are/is _________
a) dw/dt and moment of inertia
b) dw/dt
c) current
d) any of the mentioned
Explanation: We need the angular acceleration and the moment of inertia of the machine.
8. ______ test is used for determining the efficiency of a traction motor.
a) Field
b) Retardation
c) Hopkinson
d) Swineburne’s
Explanation: Field test is used for finding efficiency of a traction motor.
9. If the field current and armature current are reversed, then the __________
a) direction of rotation remains same
b) direction of rotation reverses
c) stops
d) none of the mentioned
Explanation: When both field current and armature current are reversed then the direction will not change.
10. Mark the possible causes of overheating of commutator in a DC machine.
a) Overload
b) Restricted ventilation
c) Shorted winding
d) Any of the mentioned
Explanation: All the factors can cause the overheating of commutator.