Conversion of Shunt Machine

Author: E.E. Kimberly

A shunt machine used as a motor with line polarity as shown in Fig. 14-1 will have currents in its armature and field windings as indicated. If the motor be disconnected from the power supply, but its armature speed be maintained by an external driver, the back emf E_g will maintain I_f in the same direction and almost as great as before. Hence, a d-c shunt motor will operate as a self-excited generator in the same direction of rotation without changing the direction of the field flux and without changing any internal connections. If it were necessary to reverse the direction of rotation from that when used as a motor, it would also be necessary to reverse the field-coil connections to the armature in order that the voltage might build up as explained on page 113.

Fig. 14-1. Direction of Currents in a Shunt Motor