Three Phase AC Series Commutator Motor:

This machine has a 3-phase stator winding similar to that of an ordinary induction motor and may be wound for high voltage if required. The rotor carries a winding similar to that of a dc machine and its commutator is provided with three brush sets per pair of poles spaced 120 electrical degrees apart. Connection diagram of three phase AC series commutator motor are shown in Fig. 1.36.

In effect, the stator and rotor windings are in series, so that the working flux is dependent on the current providing series speed characteristic. The speed can be controlled by moving the brush gear—the backward movement (i.e., in a direction opposite to that of rotation of the rotor) of brushgear increases the speed and vice versa. For any given position of brushes, the speed falls with the increase in load, as in case of a dc series motor.

The power factor is high approaching unity for speeds near and above synchronous speeds. The speed range is limited between 40 and 150 per cent of synchronous speed owing to commutation difficulties. Such a motor is useful when large starting torque is required such as in haulage and hoisting work if only ac supply is available.