AC motors, other than universal motors, are constant speed machines, but there is a possibility that the stator windings of an induction motor can be reclosed in order to double the number of poles, reducing the speed by half. Another possibility to change the speed in the induction motors is leeson ac motors through the frequency inverter. In DC motors, the speed can be regulated by the inclusion of a rheostat in the field circuit, to provide adjustments in the flow.
The stators can be constructed with two independent windings, calculated for the number of poles that are desired, being able to change the synchronous speed of the motor by means of variation of reversible poles and with small number of connections. With winding rotor induction motors it is possible to obtain any speed from zero to approximately the synchronism speed, by the variation of a simple resistance attached to the rotor winding. Another method of regulating the speed of the alternating current motors, which allows to obtain in the axis a speed that can go from zero to twice the synchronous speed, is by the known system of the rotor with switch, by means of the brushes.