Interpoles

Author: E.E. Kimberly

To avoid the commutation difficulty actually encountered in old-style generators, narrow poles called interpoles or commutating poles are used, as indicated in Fig. 10-22. These interpoles, being midway between main poles, are just opposite the armature poles shown in Fig. 10-21 (a). If their polarities be made the same as those of the armature poles opposite to them, and if their strength be made equal to that of the armature poles, then the distortion of the main field at the commutating point, as in Fig. 10-21 (c), may be almost entirety prevented. In practice the interpoles are made stronger than just sufficient to neutralize the armature flux, in order to aid in the reversal of current in the coils being commutated. The inductance of the coil tends to retard the reversal of the coil current by generating a prolonging voltage

Fig. 10-22. Interpole Arrangement and Polarity in a Generator

In a generator the interpoles must be of the same polarity as the main poles which they precede in the direction of rotation.

It is very difficult for a designer to calculate the exact number of turns to be placed on the interpoles, and it is sometimes necessary to connect an adjustable diverter around the interpole windings to permit shunting of a part of the total armature current around those coils. If the ordinary resistance diverter were used, a sudden surge of current caused by a sharp increase in load would not divide between the interpole path and the diverter path in inverse proportion to their respective resistances; but, because of the high reactance of the interpole path, a disproportionately large portion of the surge current would flow through the diverter. Such an improper division would cause the interpole field to fail to rise in proportion to the armature current to be commutated, and would probably cause the commutator to flash over at the brushes. The proper division of surge current can be obtained by inserting, in series with the interpole shunt, a conductor wound around an iron core to form an inductance just equal to that of the interpole coils. Such a diverter is called an inductive shunt.