250+ TOP MCQs on Terms Pertaining To Rotating Machines and Answers

Electrical Machines Multiple Choice Questions on “Terms Pertaining to Rotating Machines”.

1. For a P-pole machine, the relation between electrical and mechanical degrees is given by _____________
A. θ_elec = 2/P θ_mech
B. θ_elec = 4/P θ_mech
C. θ_mech = P/2 θ_elec
D. θ_elec = P/2 θ_mech
Answer: D
Clarification: For a P-pole machine, P/2 cycles of EMF will be generated in one revolution, and thus θ_elec = P/2 θ_mech.

2. What is the relation between ‘ω’, angular speed in electrical radians per second and ‘ω_m‘, angular speed in mechanical radians per second, for a P-pole machine?
A. ω = 2/Pω_m
B. ω = 4/Pω_m
C. ω_m = P/2ω
D. ω = P/2ω_m
Answer: D
Clarification: We know θ_elec = P/2θ_mech ⇒ d/dt(θ_elec) = P/2d/dt(θ_mech) ⇒ ω = P/2ω_m.

3. A pole pitch in electrical machine _____________
A. = 180° electrical
B. = 180° mechanical
C. > 180° electrical
D. < 180° electrical
Answer: A
Clarification: Pole pitch is always expressed in electrical degrees, rather than in mechanical degrees.
pole pitch = 180 electrical degrees or π electrical radians.

4. The part of the coil in which EMF is generated is known as _____________
A. end connection
B. coil sides
C. coil span
D. none of the mentioned
Answer: B
Clarification: The EMF is generated in the active lengths only, and these active lengths are called the coil-sides of a coil.

5. A coil consists of _____________
A. two conductors
B. two coil sides
C. two turns
D. four turns
Answer: B
Clarification: One coil is made up of two coil sides.

6. One turn consists of _____________
A. two coil sides
B. two conductors
C. four conductors
D. four coil sides
Answer: B
Clarification: One turn consists of two conductors, and one coil is made up of two coil-sides.

7. In a full pitch coil, the two coil sides are how many electrical space degrees apart?
A. 180 electrical degrees
B. 90 electrical degrees
C. 45 electrical degrees
D. none of the mentioned
Answer: A
Clarification: A coil, with two coil-sides 180 electrical space degrees apart (or one pole-pitch apart), is called a full pitch coil.

8. A chording angle ε is defined as the angle by which coil span departs from __________
A. 90° electrical space degrees
B. 180° electrical space degrees
C. 360° electrical space degrees
D. all of the mentioned
Answer: B
Clarification: If chording angle is ε°, chorded coil has a coil span = 180°-ε.

9. If B_p= peak value of sinusoidal flux density wave, L = armature core length, D = armature diameter, P = number of poles, then which of the following statements are correct?

(i) flux per pole, Φ=2/PBpLD
(ii) Average flux density, Bav=2/πBp
(iii) Φ=4/PBpLD
(iv) Bav=3/πBp
(v) Bav=PΦ/πDL
(vi) Bav=PΦ/πrL

A. (i), (ii), (v)
B. (ii), (iii), (v)
C. (ii), (iii), (vi)
D. (iii), (iv), (v)
Answer: A
Clarification: Total flux per pole = 4/PB_prL
we know, r=D/2 ⇒ Φ=2/PB_pLD and B_av=2/πB_p.
As Φ = (Average value of constant amplitude flux density wave under one pole)∗ (Area pertaining to one pole of the flux density wave), Φ = (B_av)(2πrL/P)and average value of the positive or negative half cycle of a sine wave = 2/π(peak value of sine wave).

10. In a 4 pole machine, what is the flux per pole produced, if the armature length is l and radius is r and B_p is peak value of sinusoidal flux?
A. 4B_plr
B. 2B_plr
C. B_plr
D. B_plr/2
Answer: C
Clarification: Total flux per pole = 4/PB_plr, since P=4, Φ=B_plr.

11. The equation for flux per pole Φ=4/PB_plr, is valid for _____________
A. for salient pole rotor only
B. for cylindrical pole rotor only
C. for both cylindrical and salient pole rotor
D. none of the mentioned
Answer: C
Clarification: It does not make any difference whether the field winding is on cylindrical pole structure or salient pole structure.