Interpoles in a DC motor must be

Interpoles in a DC motor must be

Right Answer is:

Series excited and should have the same polarity as that of the behind main pole in the direction of rotation of the armature.

SOLUTION

Interpoles In DC Machine

  • In DC machine One way to reduce the effects of armature reaction is to place small auxiliary poles called “interpoles” between the main field poles.The interpoles have a few turns of large wire and are connected in series with the armature.
  • One of the disadvantages of armature reaction is brush shifting, therefore a person is always required to adjust the brush position in the machine at every load change. We observe that sparking in the brushes can be avoided if the voltage in the coils undergoing commutation is made zero.
  • This method tries to do just the same. Small poles called commutating poles or interpoles are introduced in between the main poles along the geometrical neutral axis. Brushes are also set on this axis and kept fixed at this position for all the loads. The interpole winding has fewer turns of heavy copper conductor.

  • Interpoles are connected in series with the armature winding so that they carry full armature current, as shown in Fig. As the load on the machine is increased, the current passing through the interpoles also increases, hence the flux produced by the interpoles is very large. Consequently, the large voltage is induced in the conductor that opposes the voltage due to the neutral plane shift and the net result is that they neutralize each other.
  • Note that the interpoles can be used equally effectively in motors as well as in generators. When the mode of operation of the machine changes from the motor to the generator, the currents in the armature and the interpoles is reverses in direction. Therefore, their voltage effects cancel each other out.

Thus, we can conclude that:

  1. In a generator, interpoles must have the same polarity as the next upcoming pole.
  2. In a motor, interpoles must have the same polarity as the previous main pole.

The mmf induced on the interpoles must be sufficient enough to neutralize the effect of armature reaction and to produce enough field in the interpole winding to overcome the reactance voltage due to commutation.

Another important function of the interpole is to neutralize the cross-magnetizing effect of the armature reaction, as shown in Fig. Here, vector FM represents the mmf due to main poles, FA represents the cross-magnetizing mmf due to the armature reaction and FC represents the interpole mmf which is directly opposite to the FA so that they cancel each other out.

It is important to note here that the interpoles do not affect the flux distribution under the pole faces. So, even by using the interpoles in the machine, the flux weakening problem is not completely eliminated. Most medium-sized general purpose motors correct the sparking problems with the interpoles and just live with the flux weakening problems.

Main Functions of the Interpole

  1. The interpole neutralizes the reactance voltage and gives spark-free commutation.
  2. It neutralizes the cross-magnetizing effect of armature reaction so that the brushes are not required to be shifted from their original position for any load.
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