AC electric motors, except universal motors, are constant speed machines. However, it is possible to re-connect the stator coils of an induction motor, in such a way as to double the number of poles, reducing the speed in half. The stators can be constructed with two independent windings, calculated for the number of poles desired, being able to vary the synchronous motor speed by means of reversible poles (pole variation) and with a small number of connections.
With winding rotor induction motors it is possible to obtain any speed from zero to approximately the synchronism speed by varying a simple resistance attached to the rotor winding.
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Another method of regulating the speed of the AC 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.
Another possibility of speed change in the induction motors is through the frequency inverter. In DC motors, speed can be regulated by inserting a rheostat into the field circuit to provide flow adjustments.