With the electric motors of induction of coiled rotor, it is feasible the reach of any speed, being able to vary from zero to the speed of synchronism, according to the variation of a simple resistance, connected to the winding of the rotor, and that does not imply in the heating since the losses in the resistance are external to the motor. AC motors, with the exception of the universal type, consist of constant speed machines. In this way, it is possible to re-connect the stator coils of an induction motor, doubling the number of poles.
In this way, the speed must be reduced by half, allowing the stators to be designed with two windings independently, counted for the number of poles required, achieving by means of reversible poles and with a small number of connections, change the synchronous speed of the electric motor expansion tanks bell gosset. Each of the existing windings can be operated in a way that allows two different speeds in the ratio of 2 to 1, thus reaching up to four independent synchronous speeds, however, failing to provide velocities considered as intermediate.