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The Shunt Motor

Author: E.E. Kimberly

Let Fig. 11-2 (a) represent a shunt generator. If the top brush is positive, all currents will be in the directions shown. By definition, the positive terminal of a generator is the one from which the current is said to emerge, and the positive terminal of a motor is the one at which the current is said to enter. If the load be replaced by an electric battery or by another shunt generator G2, with the same polarity as G1 but with slightly greater emf, all currents will be as shown in (6). Then, the currents are related as follows:


The direction of IF1 is unchanged because the voltage applied to the field of G1 is not reversed, and so the field polarity of G1 is unchanged. How ever, the direction of IA1 is reversed because of the higher voltage of G2, and hence the armature torque is reversed. When the machine G1 operated as a generator, the torque with which the armature resisted the clockwise rotation by the prime mover was counter-clockwise; while the torque on a motor with the same brush polarity is clockwise, and hence the machine runs as a motor in the same direction as when driven as a self-excited generator.

Fig. 11-2. Driving a Generator as a Motor

Last Update: 2010-10-05