Bypassing inter poles in a DC motor....

 

 

 

What are inter poles ?

 

As the name suggests, these are poles which are in between something. These are magnetic poles placed in between main magnetic poles in a DC motor. Normally these are present only in bigger motors.

 

Look at the sketch below...

 

 

What is the purpose of these poles?

 

These poles are used for reducing Armature Reaction in a DC motor. Armature Reaction is nothing but an undesirable effect seen in DC motors when the armature starts carrying current. Armature current in a DC motor is supposed to produce a magnetic flux, which reacts with the magnetic flux produced by Field winding, to create a mechanical torque. For optimum creation of torque, these two fluxes should be at Right Angles to each other. In practice, when the armature starts carrying current, it has a tendency to distort the flux produced by the field winding. This distortion results in reducing the 90 degree angle between the two fluxes. This is called ‘shifting of the neutral axis’ of the main field. Neutral axis is the axis which is perpendicular to the North – South axis of the main field. When the neutral axis is shifted by a few degrees, its effect is that the carbon brushes which are supposed to be along the neutral axis,  no longer fall on the neutral axis. This results in sparking between the commutator segments and the brushes. Sparking takes place because the brushes which short two consecutive segments no longer have zero voltage across them. This voltage causes sparking.

 

In order to nullify this ill effect of the armature current, Inter poles are placed between main poles to counter the distortion of main filed. For making the compensation effective at all levels of armature current, the inter poles are connected in series with armature. Thus, the magnetic nullification effect produced by the inter poles is always proportional to the distorting effect. Thus sparking is avoided at all levels of armature current. Polarities of Inter poles should be made dependent on the direction of rotation of the motor. Effect of Inter poles is adverse in other direction. Therefore, if there is reversal of armature is involved, it should reverse the inter pole connections also.

 

If during trouble shooting it is seen that the inter pole windings are short to body they can be removed temporarily till the shut down is planned and motor is spared for repairs. This is normally possible in medium sized motors.

 

 

 

 

 

Common problems & symptoms

 

 

 

Bypassing interpoles

 

Rocker adjustment

 

Motor okay? Check Electronic parts

 

 

 

Drop Test for Armature

 

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