Rated speed is the speed (rpm) of the electric motor running at nominal power, under rated voltage and frequency. The mechanical speed will depend on the slip, the number of poles and the frequency of the mains. The speed of the induction motor changes very little between the vacuum and full load condition, typically around 5% and in motors with high slip up to 10%. In this way, the induction motor fed from the utility grid is not very correct where variable speed is required.
Since the motor speed depends on the frequency and number of poles, there will be a limited number of speeds for the induction motors fed directly from the network. However, when fed by means of a static converter, the speed variation is permissible over a very wide range. There are engines that can be operated at two or more speeds from the reconnection of their windings, such as the engine known as Dahlander. This type of motor generally offers a degree of utilization of the active material, lower than standard motors.