Commutation of DC Motors

The information presented here is generally known to dc motor manufacturers but is not common knowledge to dc motor users. As there is a necessity for the equipment to perform beyond its original specifications and adjustment, commutation issues may arise that could increase maintenance requirements or downtime. Understanding the commutation implications of reducing the motor field may direct a user on a course of action, should commutation deteriorate and maintenance costs increase unacceptably. Some applications will tolerate this speed increase and others may not. In some cases, it may be possible to shift neutral slightly in the direction of motor rotation to improve commutation at the higher loads in weak field, with a possible sacrifice in performance at lower loads or with full field. The motor may be readjusted by the blackband method to improve commutation at the weak field settings at all loads, with some sacrifice of performance at base speed. The motor could be returned to the OEM for black-band adjustment with the reduced main field excitation or sent to a motor shop with this capability. Otherwise, it may be necessary to consider the other options of obtaining a higher speed motor, using a different gearbox or using a lower base speed motor and eliminating the gearbox.