DC machines - 1

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DC machines - 1


Q.1.
What is the nature (Shape) of the dc armature mmg waveform ?

Ans:
Triangular

Q.2.
What is the nature (Shape) of air gap flux distribution in space at no load in dc machine ?

Ans:
Flat Tapped

Q.3.
What is armature reaction ?

Ans :
Armature reaction is the effect of magnetic field set up by the armature current on the distribution of flux under the main poles of a dc machine.

Q.4.
What are the two unwanted effects of armature reaction ?

Ans :
Demagnetisation or weakening of main field and cross - magnetisation or distortion of field.

Q.5.
Differentiate between geometrical neutral axis and magnetic neutral axis .

Ans :
Geometrical neutral axis is the axis between the magnetic poles and magnetic neutral axis is along zero magnetic field.

Q.6.
Define magnetic neutral plane (MNP)  ?

Ans :
A plane through the axis , along which no emf is induced in the armature conductors,is known as magnetic neutral plane.

Q.7.
In dc machine ,along which one of the following axes the armature mmf is directed  ?
(a)polar axis (b)brush axis (c)interpolation axis.

Ans :
 Brush axis

Q.8.
When the brushes in a dc machine are placed at the interpolar axis , is the armature mmf fully de magnetising, fully cross magnetising or partly demagnetising and partly cross magnetising ?

Ans :
Fully cross magnetising.

Q.9.
Why in dc machines sometimes brushes are shifted from neutral axis by a small angle  ?

Ans :
So as to lie along MNP to provide sparkless commutation.

Q.10.
What is meant by reactance voltage ?

Ans :
The self induced f in the coil undergoing commutation is called the reactance voltage.

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