Design of DC Machines Interview Question Answer - 8

Important Questions of Design of DC Machines

In this post, some of the important questions of design of dc machines are given. It includes commutation, methods of reduction of armature reactions, function of compensating winding, interpoles, location of interpoles, polarity of interpoles, chamfering of pole faces and limitation of wave winding.

Commutation

What value of flux density at the magnetic neutral axis does not generate emf?

Zero

Define : Commutation

Commutation

• The commutation means reversal of current from one direction to other opposite direction.

Armature Reaction

Describe the different methods to reduce the armature reaction.

Methods of reduce armature reaction

• Use of compensating winding
• Use of inter poles
• Increase the length of air gap at pole tips
• Increase reluctance at pole tips

Chamfering of Pole faces

What is meaning of the chamfering of pole faces?

Chamfering pole tips

• The length of air gap at pole tips is 1.5 to 2 times to that of pole center. This is called as chamfering of pole tips.

Compensating Winding

What is direction of current in the compensating winding? Why?

Compensating winding

• The direction of current in the compensating winding is opposite to that of armature current direction in order to cancel effect of armature reaction. 
• The compensating winding is connected is in series with armature winding in order to cancel armature mmf at all load condition.

At which place the compensating winding is located in the DC machine?

• The compensating winding is located in the axial slots of pole faces.

Whether the number of compensating winding ampere turns affects the armature reaction?

• The compensating winding ampere turns are equal to armature winding ampere turns under the pole shoes in order to compensate completely armature mmf.

Interpoles

Where the inter poles or commutating poles are located in the DC machines?

Location of inter poles

• The inter poles or commutating poles are located mid way between main poles at the geometric neutral axis.

Whether the inter poles neutralize armature mmf at the brush axis?

• The inter poles neutralize the armature mmf completely at all load condition therefore the mmf required for the inter poles must be greater than the armature mmf per pole.

What should be polarity of the inter poles in the DC machines?

Polarity of inter poles

• The polarity of the inter poles is main pole ahead in the case of DC generator and behind for DC motor.

Why the polarity of inter poles is main pole behind in the DC motor?

Inter poles polarity

• The effect of armature reaction shifts the magnetic neutral axis and established a field corresponding to main pole ahead it.
• The inter poles mmf should be equal and opposite to the armature mmf therefore the polarity of the inter poles should be main pole behind it.

Why the inter poles winding is connecting is in series with armature winding?

Inter poles winding

• The armature reaction and reactance voltage is directly proportional to armature winding therefore the inter poles winding is connected is in series with armature winding in order to neutralize the cross magnetizing armature mmf and reactance voltage.

Describe the limitation of wave winding.

Limitation of wave winding

• If the current per parallel path exceeds 200 amp, there is commutation difficulties are experienced. Therefore, the wave winding is used for rated armature current up to 400 amp.

Explain : The maximum voltage between adjacent commutator segments should be high for small rating machines.

• The maximum voltage between adjacent commutator segments should be high for small rating machines because of high internal resistance of small machines.

 

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