This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “EMF Polygon – 1”.
1. The angular slot pitch, γ is given by __________
a) γ = πP/Total number of slots Elect radians
b) γ = 180P/Total number of slots Elect degrees
c) γ = 180/Slots per pole Elect degrees
d) Any of the mentioned
Explanation: Angle γ is the angle between adjacent slots.
2. Which of these statements are correct regarding the design of distributed armature winding in a 3-phase alternator?
(i) it reduces the phase-belt harmonics (ii) it increases the utilization of the armature iron and copper (iii) it increases rigidity and mechanical strength of the winding (iv) it reduces copper in the overhang of the winding
a) (i), (iii), (iv)
b) (ii), (iii), (iv)
c) (i), (ii), (iii)
d) (i), (ii), (iii), (iv)
Explanation: Distributed armature winding does not reduce copper in the overhang of the winding, also kdn=(sinqnγ/2)/(qsinnγ/2), kdn
3. What is the coil span for a 2 pole 18 slot machine?
Explanation: Coil span = number of slots/number of poles = 18/2 = 9.
4. In a machine with coil span 9, if a coil has its one coil-side in slot 1, then its other coil-side must be in ____________
a) slot 9
b) slot 8
c) slot 10
d) slot 2
Explanation: The other coil-side must be in slot 10=1+coil span=1+9.
5. The belt factor is defined as the ratio of __________
a) arithmetic sum of coil EMF to the phasor sum of coil EMFs
b) phasor sum of EMF per coil to the arithmetic sum of EMF per coil
c) phasor sum of coil EMFs to the arithmetic sum of coil EMFs
d) phasor sum of coil EMFs to per phase voltage
Explanation: Belt factor or distribution factor or breadth factor = kd= phasor sum of coil EMFs/arithmetic sum of coil EMFs.
6. The armature winding of a 2-pole, 3-phase alternator for each phase is distributed in a number of slots per phase. The RMS value of the voltage per phase is less than the RMS value of the voltage per coil multiplied by the number of coils in series because the _________
a) RMS value of the voltage in different coils of the phase is different
b) equal RMS voltages in different coils of the phase has mutual phase difference
c) maximum value of the induced voltage in different coils of the phase are different
d) different coils of the phase pass through different saturated regions of the magnetic circuit
Explanation: The phase displacement will be given by slot angular pitch γ.
7. A 3-phase machine has integral slot winding with fundamental distribution factor kd1, the distribution factor for nth harmonic kdn is ____________
a) less than kd1
b) more than kd1
c) equal to kd1
d) depends upon the number of slots and poles
Explanation: kdn=(sinqnγ/2)/(qsinnγ/2) and examples will show that kdn is less than kd1. This has the effect of reducing the nth harmonic EMF in comparision with fundamental EMF. This is certainly an advantage of distributing the winding in slots.
8. If a stator has 48 slots, 6 poles and 3 phase with narrow spread winding, then the third harmonic belt factor is ____________
d) None of the mentioned
Explanation: q= (s/m)/p=k.Sk/k.Pk=Sk/Pk for fractional slot winding where k is the highest common factor between s/m and P, here q= 48/3*6 =16/6= 8/3 = Sk/Pk
The distribution factor for fractional slot winding’s is obtained by replacing q by Sk and kdn = (sinnγ/2)/(Sksinnγ/2Sk), if n=3, kd3=0.6407.
9. A fraction pitch winding is used to reduce which of the following?
(i) amount of copper in the winding (ii) size of the machine (iii) harmonics in the generated EMF (iv) cost of the machine
a) (i), (ii), (iii), (iv)
b) (i), (ii), (iii)
c) (ii), (iii), (iv)
d) (i), (iii), (iv)
Explanation: The fractional pitch winding results in reduction of copper in overhang and it results in less cost of the machine. kpn=cos(nε)/2 implies reduces harmonics and thereby rendering the output EMF wave almost a sine wave.
10. The pitch factor, in rotating electrical machinery, is defined as the ratio of resultant EMF of a _________
a) full-pitched coil to that of a chorded coil
b) full-pitched coil to the phase EMF
c) chorded coil to the phase EMF
d) chorded coil to that of a full-pitched coil
Explanation: Kp = Resultant EMF of a chorded coil/Resultant EMF of a full pitched coil = cosε/2.
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