# Electrical Machines Questions and Answers – MMF and Flux Density Waveforms in DC Machines

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Mmf and Flux Density wave forms in dc machines”.

1. The armature mmf affects __________
a) commutation
b) generated voltage
c) torque
d) any of the mentioned

Explanation: Armature reaction thus armature mmf affects all of the mentioned above.

2. The space distribution of flux produced in a DC machine is?
a)
b)
c)
d) None of the mentioned

Explanation: Distorted triangular waveform shape is that of flux distribution under poles, which is due to the armature reaction.

3. Armature reaction in a dc machine is __________
a) cross magnetizing in nature
b) magnetizing in nature
c) demagnetizing in nature
d) none of the mentioned

Explanation: It is cross magnetizing in nature which happens due to interaction between stator and field mmf.
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4. The air-gap flux density waveform has decreased flux under one pole tip and measured under the other is due to __________
a) cross magnetization
b) magnetization
c) demagnetization
d) saturation of iron magnetic circuit

Explanation: The flux density increases under one pole tip while it decreases under the other due to the saturation of iron magnetic circuit.

5. The flux distortion caused by cross magnetizing armature reaction is more pronounced in a __________
a) dc shunt motor
b) dc series motor
c) any of the mentioned
d) differential Compound Motors

Explanation: Since field excitation remains substantially constant, while aarmature mmf reaches higher values at loading.

6. Which method needed to limit cross-magnetizing effect?
a) by increasing reluctance of cross flux path
b) by chamfering the pole faces
c) by compensating winding
d) any of the mentioned

Explanation: By ultimately increasing the reluctance of the magnetic circuit at the pole tips so that the armature reaction effect is neutralized.

7. The brushes of a dc motor, are shifted by 5′, from the main field axis, then __________
a) demagnetizing or magnetizing effect may occur depending on machine
b) cross-magnetization will be pronounced
c) both cross-magnetization as well as magnetization and demagnetization will occur
d) none of the mentioned

Explanation: When the brushes are shifted from GNA, there will either be magnetizing or demagnetizing effect will be observed.

8. A 100KW, 250 V, 400 A, a long shunt compound generator has an armature resistance of 0.025 ohms. There are 1000 shunt fields turns per pole and 3 series field turns per pole. The series field is connected in a such a fashion that positive armature current produces direct-axis MMF which adds to that of the shunt field. Compute the gross MMF at the rated terminal current when shunt field current is 4.7A and speed is 1150 rpm.
a) 5.9AT
b) 3.5AT
c) 4.7AT
d) 1.2AT

Explanation: Series field current = Is = I(l)+I(f)
= 400+4.7
= 405 A(approx)
Main field mmf = I(f)+(Ns/Nf)*I(s)
= 4.7+(3/1000)*405
= 5.9 AT.

9. Compensating windings are embedded in pole faces and having the same polarity as that of adjoining armature winding.
a) True
b) False

Explanation: Compensating windings are embedded in pole faces and having an opposite polarity as that of adjoining armature winding.

10. Compensating winding has an advantage of __________
a) eliminating flux distortion
b) importing speed of response
d) any of the mentioned

Explanation: Any of the corrective measures will be applicable.

11. The correct positioning of the windings in the below diagrams is?
a)
b)
c)
d) Any of the mentioned

Explanation: Commutating and compensating fields act along armature axis. Shunt and series fields act along main field axis.

12. A dc shunt motor is connected to the source through 3-point starter. If the field id kept open and starter handle is moved from off to on position, then __________
a) motor will not start
b) armature will draw large current from source
c) no sparking would occur
d) all of the mentioned

Explanation: Leaving the field winding open, the machine will not start as the exciting flux is zero.

13. A dc shunt motor is running at 1000 rpm at the rated load torque. If we reverse the filed winding terminals, then the __________
a) direction of rotation reverses and commutation will be unaffected
b) direction of rotation will be same and commutation will be unaffected
c) direction of ration reverses and commutation will be affected
d) direction of ration will be same and commutation will be affected

Explanation: Reversing the field will change the direction of rotation but will not affect the commutation.

14. A dc shunt motor is running at 1000 rpm at the rated load torque. If we reverse the supply terminals, then the __________
a) direction of rotation reverses and commutation will be unaffected
b) direction of rotation will be same and commutation will be unaffected
c) direction of ration reverses and commutation will be affected
d) direction of ration will be same and commutation will be affected

Explanation: Reversing the supply in dc machine will not change the direction of rotation and will not affect the commutation too.

15. A dc shunt motor is running at 1000 rpm at the rated load torque. If few of the field windings get shot circuited then the __________
a) motor speed will increase and more armature current is drawn
b) motor speed will decrease and more armature current is drawn
c) motor speed will increase and lesser armature current is drawn
d) motor speed will decrease and lesser armature current is drawn

Explanation: If field winding get short circuited, field excitation reduces causing rise in speed and more armature current.

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