This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “MMF Produced by Distributed Windings”.
1. The winding MMF in rotating machines depends on
a) winding arrangement
b) winding current
c) air gap length,slot openings etc
d) both winding arrangement and winding current
Explanation: The winding MMF depends only on the winding arrangement and the winding current.
2. A knowledge of the air gap flux distribution in a machine helps in determining the
a) generated EMF waveform and its magnitude
b) electrical torque
c) winding MMF
d) both generated EMF waveform and electrical torque
Explanation: The winding mmf depends only on the winding arrangement and the winding current.
3. If the current in the coil is DC, then MMF doesn’t vary with ____________
c) both space and time
d) None of the mentioned
Explanation: If the coil current is DC, the magnitude of MMF wave does not vary with time and space.
4. If the current in a coil is AC, the amplitude of MMF
a) varies with time but not with space
b) varies with space but not with time
c) varies with both space and time
d) doesn’t vary with both space and time
Explanation: For AC in the coil, the air-gap MMF wave is time variant but space invariant.
5. Peak value of fundamental component of MMF produced by one N-turn coil carrying a current ‘i’ is ____________
Explanation: The fundamental component of rectangular wave is found to be Fα1=(4/Π)(Ni/2)cosα, for 2-pole machine where α=electrical space angle
The peak value of the sine MMF wave for a 2 pole machine is given by F1p=(4/π)(Ni/2)AT’s per pole= 2/π(Ni) AT.
6. The armature MMF wave in a DC machine is ____________
a) sinusoidal and depends on the speed
b) square and independent of speed
c) triangular and depends on speed
d) triangular and independent of speed
Explanation: The armature MMF for a DC machine remains constant in amplitude and does not depend on the armature speed, also the MMF wave produced is seen to be a triangluar MMF wave.
7. Which of the following statements are correct regarding the current sheet?
a) it is a thin strip of conducting material carrying the current in the dot
b) it is wrapped around stator or rotor
c) it develops MMF identical to that produced by the winding it replaces
d) Any of the mentioned
Explanation: The MMF produced by uniformly distributed current sheet is identical with the MMF produced by uniformly distributed winding. If the distributed winding in the slots is on the rotor, then the uniformly distributed current sheet would be wrapped around the rotor accordingly.
8. A current sheet with sinusoidal current produces?
a) sinusoidal MMF wave lagging it by 90°
b) sinusoidal MMF wave leading it by 90°
c) trapezoidal MMF wave leading it by 90°
d) trapezoidal MMF wave lagging it by 90°
Explanation: A uniform current sheet produces trapezoidal MMF wave and sinusoidal current sheet produces sinusoidal MMF wave leading by 90°.
9. A uniformly distributed winding on the stator has three full pitched coils, each coil having N turns and each turn carrying a current i. The MMF produced by the winding is
a) sinusoidal in waveform with an amplitude 3Ni
b) sinusoidal in waveform with an amplitude 3Ni/2
c) trapezoidal in waveform with an amplitude of 3Ni
d) trapezoidal in waveform with an amplitude of 3Ni/2
Explanation: When the machine has more than 3 slots per pole per phase, the steps in the MMF wave are neglected and MMF variation is taken as smooth over these slots and as a consequence, stepped MMf wave changes to trapezoidal MMF wave.
The trapezoidal MMF wave amplitude =3Ni/2.
10. MMF produced by one N-turn coil carrying a current i is ____________
a) rectangular of amplitude Ni/2
b) trapezoidal of amplitude Ni/2
c) rectangular of amplitude Ni
d) trapezoidal of amplitude Ni
Explanation: The MMF variation due to one N-turn coil is seen to be a rectangular wave of amplitude +1/2(Ni) or -1/2(Ni).
11. A winding of 20 full pitched series turns, distributed over a band of 60° under each pole, carries a current of 3A. The winding produces a uniform current sheet of density (in AT’s per electrical radiangs) of
Explanation: Uniform current density J=(20∗3)/(Π/3)=180/Π.
12. A sinusoidal current sheet in a rotating electrical machine has peak value along q-axis. The peak value of sinusoidal MMF would be
a) in phase with current peak
b) 90° lagging the current peak
c) 90° leading the current peak
d) may lag or lead depending upon the type of machine
Explanation: Sinusoidal current sheet produces sinusoidal MMF wave leading by 90°.
13. In a DC machine, z,p,Ia and a are respectively the number of conductors, number of poles, armature current and number of parallel paths. The peak value of fundamental component of armature MMF wave is ____________
Explanation: The peak value of the fundamental sine component Fa for a DC machine is Fa=8/&Pi2;[(z/p)(Ia/a)] AT/pole.
14. The armature MMF wave in a DC machine is triangluar and
(i) rotates with respect to stator
(ii) is stationary with respect to stator
(iii) rotates with respect to armature
(iv) is stationary with respect to armature
Which of the above statements are correct?
a) (ii) only
b) (iii) only
c) (i) and (iv)
d) (ii) and (iii)
Explanation: In a DC machine, armature current doesn’t vary with time. Therefore, the armature MMF wave for a DC machine remains constant in amplitude and doesn’t depends on the armature speed. Hence the armature MMF wave is stationary with respect to stator and armature.
Sanfoundry Global Education & Learning Series – Electrical Machines.
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