Electrical Machines Questions and Answers_pag36

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3. In the equation for RMS value of the generated EMF in a full pitched coil of an AC machine, 
E = E​max​/√2 = √2πf​r​NΦ, f​r​ depends on 
a) rotating speed of the armature coil 
b) rotating speed of the flux density wave 
c) relative velocity between the flux density wave and armature coil 
d) any of the mentioned 
View Answer 
Answer: c 
Explanation: f​r​ is called the rotational or speed frequency, since its value depends upon the 
relative velocity between the flux density wave and the armature coil. 
4. In AC rotating machines, the generated or speed EMF 
a) leads Φ by 90° 
b) lags Φ by 90° 
c) is in phase with working flux 
d) lags Φ by 180° 
View Answer 
Answer: b 
Explanation: e = E​max​cos(ω​r​t-π/2) 
ψ = NΦcosω​r​t 
It reveals that the speed or generated EMF lags by 90° the flux that generates it, and is true 
when flux is time invariant and is sine distributed in space. 
5. In a short pitched coil, the coil pitch factor k​p​, is given by ____________ 
a) k​p​ = cosε 
b) k​p​ = cosε/2 
c) k​p​ = √2πf​r​NΦcosε 
d) k​p​ = cosε/2cosω​r​t 
View Answer 
Answer: b 
Explanation: The effect of short pitched coil is to reduce the generated EMF, and this reduction 
factor is cosε/2, and is referred to as coil pitch factor. 
6. Which of the following equations represents the RMS value of the generated EMF in a 
short-pitched N-turn armature coil of an AC machine ____________ 
a) E = √2πk​p​f​r​NΦ 
b) E = 2πk​p​f​r​NΦ 
c) E = NΦω​r​k​p​sinω​r​t 
d) None of the mentioned 
View Answer 
Answer: a 
Explanation: EMF induced = NΦω​r​k​p​sinω​r​t, 
E​max​ = NΦω​r​k​p​ when sinω​r​t = 1 
RMS value, E = E​max​/√2 = √2πf​r​k​p​NΦ 
7. The effect of short pitched coil on the generated EMF is _____________ 
a) increasing