. The EMF equation e = NωrΦsinωrt is applicable to
a) AC systems with time variant field flux
b) DC systems with time variant field flux
c) both AC and DC systems with time invariant field flux
d) both AC and DC systems with time variant field flux
View Answer
Answer:c
Explanation: e = N�...
, YOMI- ELECT/ELECT (UNILAG).
SOLUTION FOR OLUSEYI TEST AND EXAM.EEG202.
This set of Electrical Machines Multiple Choice Questions & Answers
(MCQs) focuses on “Generated EMF”.
1. Which of the following statements is/are correct regarding the generation
of EMF in rotating electrical machines in the armature winding? EMF is
generated
a) by rotating armature windings through a magnetic field
b) by rotating magnetic field with respect to the armature windings
c) by designing the magnetic circuit to have variable reluctance with rotor
rotation
d) any of the mentioned
View Answer
Answer:d
Explanation: Any of the above mentioned methods will produce EMF in the armature windings.
2. The EMF equation e = Nω Φsinω t is applicable to r r
a) AC systems with time variant field flux
b) DC systems with time variant field flux
c) both AC and DC systems with time invariant field flux
d) both AC and DC systems with time variant field flux
View Answer
Answer:c
Explanation: e = NωrΦsinωrt-NdΦ/dtcosωrt, for time invariant field flux dΦ/dt = 0 ⇒ e = NωrΦsinωrt, is the general equation, and is
applicable to both AC and DC systems.
3. In the equation for RMS value of the generated EMF in a full pitched coil
of an AC machine,
E = E /√2 = √2πf NΦ, f depends on
max r r
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
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, Answer:c
Explanation: fr 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=Emaxcos(ωrt-π/2)
ψ=NΦcosωrt
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 , is given by p
a) k = cosε
p
b) k = cosε/2
p
c) k = √2πf NΦcosε
p r
d) k = cosε/2cosω t
p r
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 f NΦ p r
b) E = 2πk f NΦ p r
c) E = NΦω k sinω t r p r
d) none of the mentioned
View Answer
Answer:a
Explanation:
EMFinduced=NΦωrkpsinωrt,
Emax =NΦωrkp when,sinωr=1
RMS value, E = Emax/√2 = √2πfrkpNΦ
7. The effect of short pitched coil on the generated EMF is
a) increasing
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