AC motors, other than universal motors, are machines with a constant speed, but the possibility of reclosing the stator coils of an induction motor Gates A70, in such a way as to double the number of poles and, in this way, reduce speed to half. The stators can be established with two independent windings, calculated for the desired number of poles, achieved by means of reversible poles or pole variation and with a small number of connections varying the synchronous speed of the motor.
In DC motors, the speed may be adequate by the inclusion of a rheostat in the field circuit to provide flow adjustments. Each of these windings may be connected so as to ensure two speeds, in the ratio of 2: 1, thus obtaining four independent synchronous speeds and can not provide any intermediate speeds. With winding rotor induction motors it is possible to achieve any speed from zero to approximately the synchronism speed, by changing a simple resistance attached to the winding of the rotor, and that does not imply heating of the same, therefore, losses in the resistance will be external to the engine.