Terminal characteristics of machines with thyristor-assisted commutation

Machines with thyristor-assisted commutation(t.a.c.) have some unusual output characteristics. The small number of commutator sections and the consequently wide interpoles cause voltage and torque pulsations of appreciable amplitude, although the current ripple is usually small. A change in the commutating time with current causes an effective brush swing, resulting in an interpole-generated component of the armature voltage that is current dependent, and also changing the main-pole flux. This current-dependent brush-swing is very advantageous in lapwound machines, as it automatically acts to compensate unbalance of the generated voltages to an extent that makes equalising connections unnecessary. The development of a t.a.c. machine and the commutating-time calculations are described in companion papers.