Electric motors of alternating current, with the exception of motors of the universal type, are machines of constant speed. However, it is possible to have the stator coils of an induction motor re-connected in such a way that it is possible to double the number of poles and thus reducing the speed by half.
The present stators can be composed of two independent windings, calculated for the specific number of poles that are expected, by means of reversible poles, to vary the poles, and with a small number of connections, the synchronous motor speed can be varied.
Each of these windings can be connected in a way that allows two different speeds, two-for-one ratio, thus obtaining up to four synchronous speeds independently, therefore, will never be able to provide speeds at the intermediate level.
With winding rotor induction motors ECP4107T-4 it is possible to obtain any speed from zero to approximately the synchronism speed, by varying a simple resistance attached to the winding of the rotor, and which does not imply heating of the rotor, since losses Resistance are external to the motor.