Electrical Machine Applications & Armature Reaction and Commutation MCQs ( Electrical Machines ) MCQs – Electrical Machines MCQs
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Electrical Machines MCQs – Electrical Machine Applications & Armature Reaction and Commutation MCQs ( Electrical Machines ) MCQs
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Machine Applications
1. Synchronous-induction coupled motors are required for ____________
a) high starting torque applications
b) high pull out torque applications
c) high running torques applications
d) none of the mentioned
Answer: a
Explanation: Induction motor provides high starting torque with proper values of slip ring terminal resistance. Coupled along with the synchronous motor it acts as an independent set and be employed in high torque application.
2. Which of the following can be used for braking purposes in electric trains?
a) Induction generator
b) Induction motor
c) Dc series motor
d) Dc differentially compounded generator
Answer: a
Explanation: Induction generator runs at a speed greater than the synchronous speed. To apply in braking purposes the induction generator can be run in the opposite direction to that of the operating machine direction and stopped.
3. A synchronous motor absorbing 60 kW is connected in parallel with an continuous load of 240 kW operating at 0.8 power factor lag.What is the kVAR supplied by the synchronous motor to obtain a total power factor of 0.9 lagging?
a) 35, leading
b) 35, lagging
c) 145, leading
d) None of the mentioned
Answer: a
Explanation: Total power = 240 + 60 = 300 kW
cos(phi) = 0.90. Total reactive power = P*tan(phi) = 300*tan(25.8) = 180 kVAR Load kVAR = 240*tan(36.87) = 180 kVAR
kVAR supplied by the motor = 145-180 = -35 or k3 kVAR leading.
4. The most widely used application of ac series motor is ____________
a) electric traction upto 1600 kW
b) paper mills
c) welding
d) none of the mentioned
Answer: d
Explanation: Ac series motor is dc series motor connected to ac supply having an unidirectional torque, which is not very high in magnitude. It is widely applicable for small torque requirements like portable drilling machines, mixers etc.
5. Universal motor is used in vacuum cleaners, table fans and portable drilling machine.
a) True
b) False
Answer: a
Explanation: The universal motor is dc series motor with ac supply with smaller torque. So it can be used for lower torque applications.
6. _______ generator is used in arc welding purposes.
a) Differential compound dc
b) Dc series
c) Cumulative compounded dc
d) Shunt
Answer: a
Explanation: The external characteristics of the differentially compound generator have minimum voltage for the high current voltages. This is best harnessed feature for a high current requirement by the welding application.
7. The dc series generator is employed as ____________
a) a booster to maintain constant voltage at the end of the feeder
b) for supplying traction load
c) for battery bank charging
d) none of the mentioned
Answer: a
Explanation: Referring to the external characteristics of dc series generator shows that the terminal end voltage increases as the loading increases from no load to full load. In feeders due to variable requirements the voltage drops while transferring when loading is raised.
8. If it asked by the customer to provide tappings on the transformer, the most efficient and economical way to provide it at ____________
a) middle of hv winding
b) phase end of hv winding
c) neutral end of hv winding
d) middle of lv winding
Answer: a
Explanation: Tappings are provided to tap different levels of voltages at the high voltage end of the transformer. Providing it as low voltage side has no use and it is not economical.
9. In a single phase induction regulator, the voltage boost is maximum when armature mmf and rotor mmf make an angle of ____________
a) zero degrees
b) 45 degrees
c) 90 degrees
d) 180 degrees
Answer: a
Explanation: To obtain a maximum of the voltage in induction regulator, the stator mmf and rotor mmf should be stationary and aligned to each other.
10. An alternator is to be used for generation purpose in an aircraft. The most suited operating frequency of the alternator should be ____________
a) of 400 Hz to remove extra bulk
b) of 400 Hz to reduce losses
c) of 50 Hz to remove extra bulk
d) none of the mentioned
Answer: a
Explanation: The components to be used in aircrafts are made to operate at very high frequencies so that the size of the components to be used is reduced.
11. A 3-phase synchronous alternator is connected to the infinite bus is over excited, that corresponds to ______________
a) capacitor
b) inductor
c) variable inductor
d) any of the mentioned
Answer: a
Explanation: As the machine is over excited, it will act as source of reactive power and it is positive.
Q=V (Ecos (delta)-V)/X.
Action of Commutator
1. What is Commutator?
a) rotating segment to which brushes are attached
b) stationary segment to which rotating brushes are attached
c) rotating segment to which rotating brush segment is attached
d) any of the mentioned
Answer: a
Explanation: Commutator is a rotating segment to which brushes are attached.
2. Commutator action is analogous to _________ in analog electronics.
a) full wave rectifier
b) half wave rectifier
c) comparator
d) amplifier
Answer: a
Explanation: Commutator converts the alternating voltage induced at the armature to the unidirectional voltage at the slip rings.
3. What is Inductive kick?
a) reactance voltage in commutator segments
b) reactance voltage in shorted commutator segments
c) impulsive voltage in commutator segments
d) none of the mentioned
Answer: a
Explanation: Inductive kick is the name given to the reactance voltage which occurs at the commutation taking place in the commutator. It occurs due to the high circulating current during the commutation.
4. Inductive kick causes ________
a) sparking at the brushes of the machine
b) similar problems when neutral phase shift takes place in the armature
c) large current to flow in the equalizer rings
d) none of the mentioned
Answer: a
Explanation: Inductive kick is the name given to the reactance voltage which occurs at the commutation taking place in the commutator. It occurs due to the high circulating current during the commutation.
5. Operation of a dc machine will ________
a) not be affected by commutating poles
b) be affected by commutating poles
c) be affected significantly upto 30% spread of commutating poles
d) none of the mentioned
Answer: a
Explanation: Commutating poles are placed under the armature poles and it compensates only poles under these regions.
6. A 200 V dc machine working at rated speed. Voltage across the commutating terminals should ideally be ________
a) zero
b) 2 V
c) 1 V
d) 200 V
Answer: a
Explanation: The commutating poles undergoing commutation should have zero voltage across it so that there is no reactive voltage induced.
So it will not cause large circulating current in the winding.
7. For a four pole, 16 slot armature with two coil-side/slot having lap connected progressive winding has single turn coils. Then number of commutator segment is?
a) 16
b) 32
c) 4
d) 8
Answer: a
Explanation: Commutator segment is equal to number of coil slots.
8. For a four pole, 16 slot armature with two coil-side/slot having wave connected progressive winding has single turn coils. The commutator pitch for the machine should be ________
a) 1
b) -1
c) +1 and 1
d) none of the mentioned
Answer: a
Explanation: Commutator pitch for the wave connected progressive winding should be 1.
9. The brushes in dc machine are normally placed electrically in interpolar region to ________
a) make an angle of 90 deg. electrical
b) make an angle of 90 deg. mechanical
c) make an angle of 0 deg. electrical
d) none of the mentioned
Answer: a
Explanation: To obtain proper commutation without any sparks, the magnetic neutral axis and geometrical neutral axis should be aligned to each other, such that coils undergoing commutation experience zero emf across them.
10. The armature rotation in the dc machine and the magnetic field are independent of each other and obey laws of conservation of energy.
a) True
b) False
Answer: a
Explanation: The armature rotation does not affect the stored magnetic energy at the steady state of operation.
11. Under commutation results ________
a) sparking at the trailing edge of brush
b) sparking at the leading edge of brush
c) sparking at the middle of brush
d) any of the mentioned
Answer: a
Explanation: Under commutation takes place when the commutation speed is slow and it takes more time than critical defined time for commutation. Because of this the trailing edge of the brush still remains under the commutation.
12. While the operation of dc motor, there are sparkings at the commutation observed. These can be happening due to ________
a) under commutation
b) over commutation
c) too rapid reversal of the current
d) any of the mentioned
Answer: d
Explanation: The over commutation, over commutation or rapid reversal of the current, any of these reasons can lead to sparking at the brushes.