Question

A DC shunt motor is designed without compensating windings. If there is no compensating winding then the effect of armature reaction will exist. What effect does armature reaction have on the torque-speed characteristic of a shunt DC motor? Explain mathematically in detail. What can be done to remove this problem completely?

Answer 1

A.) For shunt motor the effect of armature reaction on speed torque curve is given as :-

W= (V/Km ) - [×R_a /{( K_{​​​​​​m}​​​​​ )²}]

As mentioned above the mathematical relation between speed and torque for the shunt motor. Now as we can see in the graph the blue line indicates the variation of speed w.r.t. torque under normal conditions and the red line indicates the the variation of speed with respect to torque under the influence of armature reaction. This is because due to armatur reaction flux distortion takes place because of which as the load increases the flux in field winding reduces that causes the slope of the line in the graph to become more negative.

B.) In order to remove or neutralise the armatur reaction completely we use compensation winding. The role of compensation winding is to cancle the effect of armature reaction under the poles. The compensation winding is embedded in the pole face and since it's task is to cancle Armature reaction it has to be connected in series with the armature so that it also carries armature current. As the armature reaction are most dominant at the edge of the pole and the polarity of the compensating winding is such that it cancle out the armature reaction at the pole edge.