SOLUTION

In a differential compound motor, because of the opposition of the series field to the shunt field, the flux decreases as the load and armature current increase.  As we know that the decrease in flux causes an increase in speed. However, because the speed is proportional to E_B/φ, if both factors vary in the same proportion, the speed will remain constant. This action may occur in the differential compound motor. If more turns are added to the series coil, it is possible to cause the motor to run faster as the Load is Increased.

•  With over compounding, a slight increase in speed is possible with an increase in load. This speed characteristic is achieved only with a loss in the rate at which torque increases with load.
• Because the field decreases with a load increase, a differential compound motor has a tendency for load instability.
• When starting a differential motor, it is recommended that the series field is shorted because the great starting current in this field may overbalance the shunt field and cause the motor to start in the opposite direction.