Speed control for PSC induction motor
The speed of a PSC induction motor with a fan load can be adjusted while the motor is running. Changing the voltage provides a continuous range of adjustment. The diagram below, shows the full-voltage torque vs. speed curve and three reduced-voltage examples. If the voltage is reduced, the torque capability that will be provided at any given slip is proportion to the square of the voltage. If the voltage is reduced to half, the torque at a given slip will be 25% of the previous torque. The slip at any given load torque will increase accordingly as shown.



The motor will operate at the intersection of the torque capability curve (blue) and the torque demand characteristic curve of the load (red). The torque characteristic curve of a fan is shown because this type of speed control is sometimes used for fans and centrifugal pumps. Loads that require the torque to remain constant at reduced speeds or have less reduction in torque requirement are rarely operated this way. This method may work to some extent with those type of loads but it is generally not recommended. Even with variable-torque loads, a three-phase motor with a variable-frequency drive (VFD) would be preferable.

The losses due to slip are proportional to the load torque multiplied by the percent slip.

This can not be done with a single-phase, capacitor-start motor because the motor needs to be operating near full speed for the centrifugal switch to disconnect the capacitor.