EMF Polygon - Electrical Machines Questions and Answers for Entrance Exams

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “EMF Polygon – 2”.

1. The winding’s for a 3-phase alternator are:

(i) 36 slots, 4 poles, span 1 to 8 
(ii) 72 slots, 6 poles, span 1 to 10
(iii) 96 slots, 4 poles, span 1 to 21

The winding’s having pitch factor of more than 0.97 are __________
a) (i) and (ii) only
b) (ii) and (iii) only
c) (i) and (iii) only
d) (i),(ii) and (iii)
View Answer

Answer: c
Explanation: K_p=cosε/2
(i) Slots per pole = 36/4=9, for a coil span of 8 slots, the coil is short pitched by 1 slot and the chording angle is ε=γ = 20° ⇒ K_p=cos10°=0.985
(ii) Slots per pole = 72/6=12, for a coil span of 10 slots, the coil is short pitched by 2 slots and the chording angle is ε=2γ and γ=180/12 ⇒ ε = 30° ⇒ K_p=cos15°=0.9659
(iii) Slots per pole = 96/4=24, for a coil span of 21 slots, the coil is short pitched by 3 slots and the chording angle is ε=3γ and γ=180/24 ⇒ ε = 24.5° ⇒ K_p=cos12.25°=0.97723.

2. In 48 slot, 4-pole,3 phase alternator, the coil-span is 10. Its distribution and pitch factors are respectively ____________
a) 0.9717,0.966
b) 0.9822, 0.9814
c) 0.9577, 0.9814
d) 0.9577, 0.966
View Answer

Answer: d
Explanation: Slots per pole = 48/4=12, for a coil span of 10 slots, the coil is short pitched by 2 slots and the chording angle is ε=2γ and γ=180/12 ⇒ ε = 30° ⇒ K_p=cosε/2=0.9659.
We know, K_d=(sin(qγ/2))/qsin(γ/2), here q=48/4∗3 = 4 ⇒ K_d=0.957662.

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3. A 3-phase, 4-pole alternator has 48 stator slots carrying a 3-phase distributed winding. Each coil of the winding is short chorded by one slot pitch. The winding factor is given by ______________
a) (cos7.5)/16
b) (cot7.5)/8
c) 1/(8sin7.5)
d) (cot7.5)/16
View Answer

Answer: b
Explanation: Slots per pole = 48/4 = 12 ⇒ γ=180/12=15° and q=48/4∗3=4
coil of the winding is short chorded by one slot pitch ⇒ ε=γ=15°, K_w=K_p∗K_d = cos7.5(sin(4∗15/2))/(4∗sin(15/2)) = (cot7.5)/8.

4. The chording angle for eliminating 5^th harmonic should be ____________
a) 30°
b) 34°
c) 36°
d) 35°
View Answer

Answer: c
Explanation: To eliminate 5^th harmonic k_p₅ must be zero, k_{p_n} = cosnε/2 ⇒ k_p₅ = cos5ε/2=0=cos90° ⇒ 5ε/2=90° ⇒ ε = 36°.

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5. Which of the following statements are true?

(i) breadth factor for third harmonic k_d₃ is more than that for fundamental k_d₁
(ii) k_d₃ < k_d₁
(iii) k_d₃ may be less or more than k_d₁ depending upon the number of slots and poles
(iv) coil-span factor for third harmonic k_p₃>k_p₁ (coil span factor for fundamental)
(v) k_p₃ < k_p₁
(vi) k_p₃ may be less or more than k_p₁ depending upon the number of slots and poles

a) (ii), (v)
b) (i), (iv)
c) (iii), (vi)
d) (i), (iii), (iv), (vi)
View Answer

Answer: a
Explanation: Examples will show that k_{d_n} < k_d₁ and has the effect of reducing the n^th harmonic EMF in comparison with the fundamental EMF, similarly k_{p_n} < k_p₁.

6. A 6-pole alternator with 36 slots carries a 2-phase distributed winding. Each coil is short pitched by one slot. The winding factor is given by _____________
a) cot15°/3√2
b) cot15°/4
c) cot15°/2√2
d) cot15°/4
View Answer

Answer: a
Explanation: Slots per pole = 36/6=6, q=6/2=3 ⇒ γ=180°/6=30°, coil is short pitched by one slot ⇒ ε=γ=30° and K_p=cosε/2=cos15° and K_d=sin(γq/2)/q(sinγ/2)=sin45/(3sin15)
K_w=K_p∗K_d=cot15°/3√2.

7. For eliminating n^th harmonic from the EMF generated in the phase of a 3-phase alternator, the chording angle should be ______________
a) n∗full pitch
b) (1/n)∗full pitch
c) (2/n)∗full pitch
d) (3/n)∗full pitch
View Answer

Answer: a
Explanation: k_{p_n}=cosnε/2=0 ⇒ nε/2=90° ⇒ ε=180°/n=full pitch/n.

8. Which among the given harmonics are called belt harmonics?
a) 5, 7, 11, 13
b) 3, 6, 9, 12
c) 5, 6, 11, 12
d) 7, 11, 13, 15
View Answer

Answer: a
Explanation: The odd harmonics of the order 5, 7, 11, 13 etc are called belt harmonics.

9. Machine A has 60° phase spread and machine B has 120° phase spread. Both the machines have uniformly distributed winding. The ratio of distribution factors of machine A to machine B is ____________
a) 0.866
b) 1.1
c) 1.55
d) 1.155
View Answer