7.3. Split pole motors

Such a motor has pronounced poles with exciter winding in the stator in a similar manner to the direct current machine. Part of the main pole surface has been separated by a split in the pole and enclosed by a copper ring. The rotor features a squirrel cage of aluminium.

Figure 119 - Assembly of a split-pole motor

1 Exciter winding, 2 Short-circuit ring, 3 Squirrel cage rotor, 4 Main pole, 5 Split pole

In principle the split pole motor is a single-phase motor with permanently switched on auxiliary winding (short-circuit ring). The exciter winding establishes an alternating field which also extends to the short-circuit ring. Thereby a voltage is induced in the short-circuit ring capable of driving a powerful current into the ring. This yields an alternating field in the split pole which has not only been spatially displaced against the alternating field of the main pole, but also has a delayed action effect, that is to say is temporally shifted. The preconditions for a rotating field have been met: Interacting with the rotor induction currents, a torque is yielded which is sufficient for motor self-starting. The alternating field of the split pole interacts temporally displaced as compared to the alternating field of the main pole; this yields the rotational field direction from the main to the split pole. The field direction of rotation is thus constructionally conditioned. A directional change in the rotating field and, thereby, rotational direction reversal of the rotor is not possible with split pole motors. In view of the substantial copper loss in the squirrel ring, the efficiency of these motors is extremely limited (20 to 40%). Consequently, the motors can only operate economically up to a power of approx. 2 kW. Their starting current seldom exceeds twofold rated current.