Electrical Machines MCQs – Constructional Features of Rotating Electrical Machines MCQs ( Electrical Machines ) MCQs
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Electrical Machines MCQs – Constructional Features of Rotating Electrical Machines MCQs ( Electrical Machines ) MCQs
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Constructional Features of Rotating Electrical Machines-1
1. Which of the following two windings produces/induces the working flux and working EMF respectively?
a) field winding and armature winding
b) armature winding and field winding
c) both are produced in field winding
d) both are produced in armature winding
Answer: a
Explanation: Field winding produces the working flux and armature winding has the working EMF, which is induced by the working flux.
2. Armature winding carries which of the following current?
a) magnetizing current only
b) load current only
c) both magnetizing current and load current
d) none of the mentioned
Answer: b
Explanation: The winding on the machine that carries only load current is called armature winding.
3. Field winding carries which of the following current?
a) exciting current only
b) load current only
c) both exciting current and load current
d) none of the mentioned
Answer: a
Explanation: The winding that handles only exciting current is called field winding.
4. Which of the following statements are correct regarding exciting current?
(i) exciting current varies with the load
(ii) exciting current doesn’t vary with load
(iii) exciting current produces only a working magnetic flux
(iv) exciting current is not responsible for the production of working magnetic flux
a) (i) & (iv)
b) (ii) & (iii)
c) (i) & (iii)
d) (ii) & (iv)
Answer: c
Explanation: The current that produces only a working magnetic flux, and does not vary with the load on the machine is called exciting current, magnetizing current or field current.
5. Current in the field winding is ___________
a) AC always
b) DC always
c) Both AC and DC
d) Either AC or DC
Answer: b
Explanation: Field winding is always given DC supply and hence current is also DC.
6. The rating of the armature winding is ___________
a) 1/2 to 2% of the rated power of the machine
b) less than the power rating of the machine
c) greater than the power rating of the machine
d) equal to the power rating of the machine
Answer: d
Explanation: The armature winding handles all the power that is being converted or transformed.
7. The power rating of the field winding is ___________
a) equal to the power rating of the machine
b) 1/2 to 2% of the rated power of the machine
c) greater than the power rating of the machine
d) less than the power rating of the machine
Answer: b
Explanation: The power input to the DC field winding is dissipated as i2R loss in the field winding (once the required field current is established).
8. What is the reason behind armature structure for both DC and AC machines being laminated?
a) reduce i2R losses
b) reduce the leakage flux
c) reduce the eddy current losses
d) for better operating power factor
Answer: c
Explanation: The armature windings of both the DC and AC machines have to deal with alternating current only; this is the reason why the armature structure of all rotating machines are laminated in order to reduce the eddy current losses.
9. Armature winding is one in which working ___________
a) flux is produced by field current
b) flux is produced by the working emf
c) emf is produced by the working flux
d) emf is produced by the leakage flux
Answer: c
Explanation: In an electrical machine, armature winding is one in which the working emf is induced by the working flux.
10. Why is the air gap between stator and rotor should be kept as small as mechanically possible in induction motor?
a) to reduce the leakage flux between stator and rotor
b) as it leads to better operating power factor of the induction motor
c) reduce the eddy current losses
d) both, to reduce leakage flux between stator and rotor, and also for better operating power factor
Answer: d
Explanation: The leakage flux and power factor depend on the air gap and it should be as small as possible.
Constructional Features of Rotating Electrical Machines-2
1. Which type of slots are used in the construction of large size and small size induction motors respectively?
a) open slots and semiclosed slots
b) semiclosed slots and open slots
c) open slots and open slots
d) semiclosed slots and semiclosed slots
Answer: a
Explanation: Large size motors use open slots, so that already prepared and properly insulated coils can be inserted in open slots. Small size induction motors use semiclosed slots, so as to reduce the effective gap length between stator and rotor.
2. In which of the following applications, wound rotor type of induction motor is used?
a) where the driven load requires speed control
b) where high starting torque is required
c) when external resistance is to be inserted
d) any of the mentioned
Answer: d
Explanation: Only in wound rotor type of induction motor, external resistance can be inserted in series with the rotor winding, for speed and starting torque control.
3. For an induction motor, Which of the following statements are correct?
(i) squirrel cage type is simpler and more economical in construction
(ii) wound rotor type requires less maintenance
(iii) squirrel cage type is more rugged and requires less maintenance
(iv) no external resistance can be inserted in the rotor circuit of squirrel cage induction motor
(v) no external resistance can be inserted in the rotor circuit of a wound rotor induction motor
a) (ii), (v), (iii)
b) (ii), (iii), (v)
c) (i), (iii), (iv)
d) (i), (ii), (iv)
Answer: c
Explanation: The squirrel cage type is simpler and more economical in construction, requires less maintenance than wound rotor type, since the former doesn’t require slip rings and carbon brushes.
4. What are the advantages of providing the field winding on rotor and armature winding on the stator?
a) more economical
b) more efficient
c) efficient cooling
d) all of the mentioned
Answer: d
Explanation: If the armature winding is on the stator, only two slip rings are required which implies less losses, more efficiency and more economy. Also, stationary armature winding can be cooled more efficiently.
Rating And Loss Dissipation MCQs
5. The stator frame and end covers in synchronous and induction machines are designed to ___________
a) carry the magnetic flux
b) to serve as a mechanical support
c) to provide cooling or to carry induced EMF
d) any of the mentioned
Answer: b
Explanation: The stator frame and end covers serve merely as a mechanical support to the stator core, and are not designed to carry the magnetic flux.
6. What is the advantage of connecting two coils in parallel?
a) reduce the amount of copper required
b) increase the current per parallel path to double the value
c) increase the voltage capacity
d) all of the mentioned
Answer: b
Explanation: The parallel connection of two coils will increase the current per parallel path to double the value.
7. In a synchronous machine, salient pole construction has been found to suit best for ___________
a) low speed prime movers
b) high speed prime movers
c) medium speed prime movers
d) any of the mentioned
Answer: a
Explanation: For accommodating large number of poles, the rotor diameter must be comparatively increased, and from mechanical considerations, salient pole construction has been found to suit best for low speed prime movers.
8. What is the equation for frequency of generated EMF?
a) f = PN/120 Hz
b) f = 120/PN Hz
c) f = P/120 Hz
d) f = N/120 Hz
Answer: a
Explanation: f = PN/120 Hz in India and many other countries, alternators feeding the power system generates at a frequency of 50 Hz.
9. How many poles should the alternators driven by the oil engines and hydraulic turbine prime movers must have, for better operation?
a) fewer number of poles
b) larger number of poles
c) medium number of poles
d) any of the mentioned
Answer: b
Explanation: Oil engines and hydraulic turbines operate best at relatively low speeds, therefore, the alternators driven by these prime movers must have relatively large number of poles, as f ∝ 1/N.
10. Large synchronous machines are constructed with armature winding on the stator because stationary armature winding. Which of the following statements are correct?
(i) can be insulated satisfactorily for higher voltages
(ii) can be cooled more efficiently
(iii) would lead to reduced slip ring losses
(iv) would have no slot harmonics
(v) would have reduced armature reactance
a) (i), (ii), (iii), (iv)
b) (i), (ii), (iii), (v)
c) (i), (ii), (iii), (iv), (v)
d) (i), (ii), (iv), (v)
Answer: b
Explanation: If the armature winding is on the stator, only two slip rings are required, which implies less losses, more efficiency and more economy. Also, stationary armature windings can be cooled more efficiently.
Constructional Features of Rotating Electrical Machines-3
1. How many poles should the alternators, driven by the steam and gas turbines prime movers must have, for better operation?
a) fewer number of poles
b) large number of poles
c) medium number of poles
d) any of the mentioned
Answer: a
Explanation: Steam and gas turbines have best operating characteristics at relatively high speeds, therefore, alternators driven by these must have a fewer number of poles, say 2 or 4, as f ∝ 1/N.
2. In a synchronous machine, cylindrical pole construction has been found to suit best for ___________
a) low speed prime movers
b) high speed prime movers
c) medium speed prime movers
d) any of the mentioned
Answer: b
Explanation: For high speed prime movers, if salient pole construction is used, the rotor structure may not be able to withstand the enormous centrifugal forces, developed by the salient poles. Therefore, cylindrical rotor construction is best suited from mechanical considerations.
3. Commercially, which of the following generators is called turbo-alternators or turbo-generators?
a) salient pole rotor synchronous generators
b) cylindrical pole rotor induction generators
c) salient pole rotor induction generators
d) cylindrical rotor synchronous generators
Answer: d
Explanation: Cylindrical rotor synchronous generators are used commercially at high speed, driven by steam and gas turbines.
4. For relatively large sizes of synchronous machines, which of the following are used for providing closed circuit cooling?
a) air
b) water
c) hydrogen/helium
d) any of the mentioned
Answer: d
Explanation: For synchronous machines of large sizes, adequate cooling arrangements must be incorporated to dissipate the heat produced by various losses.
5. In a synchronous generator, the frequency of the generated EMF depends on ___________
a) rotational speed
b) poles
c) both rotational speed and poles
d) none of the mentioned
Answer: c
Explanation: f = PN/120 Hz
6. A synchronous machine can be connected to an energy system only when ___________
a) frequency/rotational speed matches
b) voltage matches
c) current matches
d) power matches
Answer: a
Explanation: For successful operation, the rotational speed/frequency of the synchronous machine must synchronize with the frequency of the energy system, and this is the reason for calling it a synchronous machine.
7. In larger DC machines, the reason behind using fabricated steel instead of cast iron in the manufacture of yoke is?
a) economical considerations
b) permeability considerations
c) mechanical considerations
d) insulation considerations
Answer: b
Explanation: Fabricated steel has higher permeability than cast iron.
8. Which of the following statements are correct in a DC machine, when the pole core is usually of smaller cross section than the pole shoe?
(i) smaller cross section requires less copper
(ii) larger pole shoe increases the air gap reluctance
(iii) larger pole shoe increases the flux per pole entering the armature
(iv) larger pole shoe reduces the mechanical strength
(v) larger pole shoe supports the field winding
a) (i), (ii), (v)
b) (ii), (iii), (v)
c) (i), (iii), (v)
d) (ii), (iv), (v)
Answer: c
Explanation: Larger pole shoe area increases the flux per pole entering the armature, due to the reduction in air gap reluctance, and it also provides mechanical strength.
9. In a DC generator, the commutator serves as a _________
a) mechanical rectifier for alternating EMF to direct EMF
b) mechanical inverter to invert the direct applied voltage to alternating voltage
c) all of the mentioned
d) none of the mentioned
Answer: a
Explanation: A DC generator is equivalent to an AC machine plus a mechanical rectifier, i.e, commutator.
10. In a DC motor, the commutator serves as a ___________
a) mechanical rectifier for alternating EMF to direct EMF
b) mechanical inverter to invert the direct applied voltage to alternating voltage
c) all of the mentioned
d) none of the mentioned
Answer: b
Explanation: A DC motor is equivalent to an AC machine plus a mechanical inverter, i.e, commutator.