When changing the connection of one stator winding, magnetic pole rotating magnetic field generated is changed. This method of variable pole speed only applies to three-phase cage induction motor, does not apply to the wound rotor asynchronous motor. Because the number of poles of cage rotor of the motor can be changed with the stator pole number changes, while the rotor winding rotor motor in rotor winding has been determined the number of pole pairs, in general it is difficult to change the number of pole pairs.

    The advantages of variable pole adjustable speed is the required equipment is simple, the disadvantage is the motor winding head more, speed too few. In order to avoid the difficulties of rotor winding pole changing wound rotor asynchronous motor, without the use of variable pole speed, variable speed control for cage asynchronous motor.