Electrical Machines Questions and Answers – Constructional Features of Rotating Electrical Machines-3

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “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
View Answer

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.
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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
View Answer

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
View Answer

Answer: d
Explanation: Cylindrical rotor synchronous generators are used commercially at high speed, driven by steam and gas turbines.
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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
View Answer

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
View Answer

Answer: c
Explanation: f = PN/120 Hz
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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
View Answer

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
View Answer

Answer: b
Explanation: Fabricated steel has higher permeability than cast iron.
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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)
View Answer

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.
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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
View Answer

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
View Answer

Answer: b
Explanation: A DC motor is equivalent to an AC machine plus a mechanical inverter, i.e, commutator.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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