By means of the application of winding rotor induction electric motors, it is possible to achieve any speed and can change in average from zero to the speed of synchronism, due to the variation of a single resistance connected to the rotor winding.
This factor does not directly imply the heating of the same, considering that the losses in the resistance are external to the motor. Another possible way to regulate the speed of electric motors of alternating current, which guarantees the obtaining in the axis a speed of zero to double the synchronous speed, is made through the system of the rotor with switch, by means of offsets of the brushes.
It is important to note that AC machines are essentially devices capable of operating at a constant speed, and there is always a probability that the stator coils of an induction motor will be re-connected and may result in a doubling of the number of poles.
This can reduce the speed in half, in which the stators have the possibility of being projected with two independent windings, counted for the quantity of poles that is wanted, being obtained by means of reversible poles and with reduced number of connections change the speed synchronous operation of the motor.