1. For an inductor made from magnetic core, with two air gaps of equal length g, exciting coil with 1000 turns, A=5cm∗5cm and g=1cm, what will be the coil inductance?
a) 0.314H
b) 0.157H
c) 0.078H
d) 0.628H
Explanation: Reluctance Rl=2g/μ0A and coil Inductance L=N2/Rl = N2μ0A/2g = (1000)2(4π∗10-7)(0.05)2/(2∗0.01) = 0.15708H.
2. For a P-pole machine, the relation between electrical and mechanical degrees is given by _____________
a) θelec = 2/P θmech
b) θelec = 4/P θmech
c) θmech = P/2 θelec
d) θelec = P/2 θmech
Explanation: For a P-pole machine, P/2 cycles of EMF will be generated in one revolution, and thus θelec = P/2 θmech.
3. What is the relation between ‘ω’, angular speed in electrical radians per second and ‘ωm‘, angular speed in mechanical radians per second, for a P-pole machine?
a) ω = 2/Pωm
b) ω = 4/Pωm
c) ωm = P/2ω
d) ω = P/2ωm
Explanation: We know θelec = P/2θmech ⇒ d/dt(θelec) = P/2d/dt(θmech) ⇒ ω = P/2ωm.
4. A pole pitch in electrical machine _____________
a) = 180° electrical
b) = 180° mechanical
c) > 180° electrical
d) < 180° electrical
Explanation: Pole pitch is always expressed in electrical degrees, rather than in mechanical degrees.
pole pitch = 180 electrical degrees or π electrical radians.
5. 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
Explanation: The EMF is generated in the active lengths only, and these active lengths are called the coil-sides of a coil.
6. A coil consists of _____________
a) two conductors
b) two coil sides
c) two turns
d) four turns
Explanation: One coil is made up of two coil sides.
7. One turn consists of _____________
a) two coil sides
b) two conductors
c) four conductors
d) four coil sides
Explanation: One turn consists of two conductors, and one coil is made up of two coil-sides.
8. 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
Explanation: A coil, with two coil-sides 180 electrical space degrees apart (or one pole-pitch apart), is called a full pitch coil.
9. 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
Explanation: If chording angle is ε°, chorded coil has a coil span = 180°-ε.
10. If Bp= 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)
Explanation: Total flux per pole = 4/PBprL
we know, r=D/2 ⇒ Φ=2/PBpLD and Bav=2/πBp.
As Φ = (Average value of constant amplitude flux density wave under one pole)∗ (Area pertaining to one pole of the flux density wave), Φ = (Bav)(2πrL/P)and average value of the positive or negative half cycle of a sine wave = 2/π(peak value of sine wave).