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) all of the mentioned
View Answer
Explanation: All of the above mentioned methods will produce EMF in the armature windings.
2. 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
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 = Emax/√2 = √2πfrNΦ, fr 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) all of the mentioned
View Answer
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
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 kp, is given by ____________
a) kp = cosε
b) kp = cosε/2
c) kp = √2πfrNΦcosε
d) kp = cosε/2cosωrt
View Answer
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 = √2πkpfrNΦ
b) E = 2πkpfrNΦ
c) E = NΦωrkpsinωrt
d) None of the mentioned
View Answer
Explanation: EMF induced = NΦωrkpsinωrt,
Emax = NΦωrkp when sinωrt = 1
RMS value, E = Emax/√2 = √2πfrkpNΦ
7. The effect of short pitched coil on the generated EMF is _____________
a) increasing
b) decreasing
c) either increasing or decreasing
d) none of the mentioned
View Answer
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.
8. A winding is distributed in the slots along the air gap periphery. Which of the following statements are true?
(i) to add mechanical strength to the winding (ii) to reduce the amount of conductor material required (iii) to reduce the harmonics in generated EMF (iv) to reduce the size of the machine (v) for full utilization of iron and conductor materials
a) (i), (iii), (iv), (v)
b) (i), (ii), (iv), (v)
c) (i), (ii), (iii), (v)
d) (i), (iii), (v)
View Answer
Explanation: In rotating electrical machines, the armature turns are usually distributed in slots rather than concentrated in single slot. This is essential from the view point of utilizing the armature periphery completely, and add mechanical strength.
9. In an AC machine, the effect of distributing the turns in different slots, results in a further reduction of generated EMF by the factor kd. This factor is called ____________
a) distribution/speed factor
b) coil pitch factor
c) winding factor
d) all of the mentioned
View Answer
Explanation: E = √2πkpkdfrNΦ, kd is the distribution factor.
10. A polyphase induction motor of the slip ring or wound rotor type can be used ____________
a) for high start-up torque applications
b) as a frequency converter
c) any of the mentioned
d) none of the mentioned
View Answer
Explanation: A polyphase induction motor can be used as a frequency converter (or changer) for changing the supply frequency f to other frequencies sf and (2-s)f at the slip ring terminals.
11. The equation for slip speed is ____________
a) (ω-ωr)/ω
b) ω-ωr
c) (ωr-ω)/ω
d) ωr
View Answer
Explanation: The relative speed between rotor and rotating flux wave, i.e, (ω-ωr) is referred to as the slip speed in rad/sec
slip speed = (ω-ωr) rad/sec.
12. If the rotor of an induction motor is made to revolve in a direction opposite to the rotating flux wave, then RMS value of EMF induced in one phase of rotor E is proportional to ____________
a) 2-s
b) s
c) 1-s
d) 2+s
View Answer
Explanation: Relative velocity between rotor winding and rotating flux wave becomes
(ω+ωr) = ω+ω(1-s) = ω(2-s) and
E = 2πf(2-s)NphrkwΦ.
13. In an alternator, frequency per revolution is equal to ___________
a) number of poles
b) twice the number of poles
c) speed in rps
d) number of pole-pairs
View Answer
Explanation: For a P-pole machine, in n rev/sec, P/2n cycles/second are generated and cycles per second is referred to as frequency f of the EMF wave. Here in this question n=1
⇒ frequency per revolution = P/2
⇒ number of pole pairs.
14. The EMF generated in an alternator depends upon which of the following statements are correct?
(i) speed (ii) maximum flux per pole (iii) series turns per phase (iv) phase spread (v) coil-span (vi) type of alternator
a) (i), (ii), (iii), (iv), (v), (vi)
b) (i), (iii), (iv), (v)
c) (i), (ii), (iii), (v), (vi)
d) (i), (iii), (iv), (v), (vi)
View Answer
Explanation: Eph = √2πfkwNphΦ, here Φ = total flux per pole.
15. If the dimensions of all the parts of a synchronous generator and the number of field and armature turns are doubled, then the generated voltage will change by a factor of ________
a) 1
b) 2
c) 4
d) 8
View Answer
Explanation: E = √2πfrNΦ, Φ = flux per pole = 4/pBplr
dimensions are doubled ⇒ l1 = 2l and r1 = 2r ⇒ Φ1 = 4Φ
No. of turns doubled ⇒ N1 = 2N, fr changes only when the poles are changed ⇒ fr1 = fr
⇒ E1=√2πfr(2N)(4Φ) = 8E
The generated voltage will change by a factor of 8.
16. A 6 pole machine is rotating at a speed of 1200rpm. This speed in mechanical rad/sec and electrical radians per second is respectively?
a) 40π, 40π/3
b) 120π, 40π
c) 20π, 60π
d) 40π, 120π
View Answer
Explanation: N = 1200rpm,P = 6 and f = PN/120 = 60 cycles per sec.
1cycle = 360° electrical degrees = 2π electrical radians
f in electrical radians per sec = 60∗2π electrical radians/sec = 120π elect radians/sec
speed in mechanical radians/sec = 2/P(speed in electrical rad/sec) = 2/P(120π)=40π mechanical rad/sec.
17. The short pitch winding for an alternator gives which of the following statements?
(i) improved wave form of the generated EMF per phase (ii) reduced value of self inductance of the winding (iii) increased rating of machine (iv) reduced tooth ripples (v) increased total generated EMF (vi) saving in winding copper
a) (i), (ii), (iv), (v), (vi)
b) (i), (ii), (vi)
c) (i), (ii), (iv), (vi)
d) (i), (iii), (vi), (iv)
View Answer
Explanation: The effect of short pitched coil is to reduce the generated EMF, and it improves waveform of EMF generated per phase, and as the windings are placed closed to each other, copper will be saved.
18. The DC machines are designed with flat topped flux density waves because _______________
a) average value of brush voltage is more for a flat topped B-wave
b) average value of brush voltage is less for a flat topped B-wave
c) losses for flat topped B-wave are less
d) none of the mentioned
View Answer
Explanation: For the same value of peak flux density Bp, it is easy to see that average value of brush voltage could be more for a flat topped B-wave than for a sinusoidal flux density wave.
Sanfoundry Global Education & Learning Series – Electrical Machines.
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