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

«
»

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
View Answer

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

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

Answer: d
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
View Answer

Answer: a
Explanation: It is cross magnetizing in nature which happens due to interaction between stator and field mmf.
Sanfoundry Certification Contest of the Month is Live. 100+ Subjects. Participate Now!
advertisement
advertisement

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
View Answer

Answer: a
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
View Answer

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

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
View Answer

Answer: a
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
View Answer

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

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
View Answer

Answer: a
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
View Answer

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

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

Answer: a
Explanation: Any of the corrective measures will be applicable.

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

Answer: a
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
View Answer

Answer: d
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
View Answer

Answer: a
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
View Answer

Answer: b
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
View Answer

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

Sanfoundry Global Education & Learning Series – Electrical Machines.

To practice all areas of Electrical Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

advertisement
advertisement
Subscribe to our Newsletters (Subject-wise). Participate in the Sanfoundry Certification contest to get free Certificate of Merit. Join our social networks below and stay updated with latest contests, videos, internships and jobs!

Youtube | Telegram | LinkedIn | Instagram | Facebook | Twitter | Pinterest
Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

Subscribe to his free Masterclasses at Youtube & technical discussions at Telegram SanfoundryClasses.