How do I change the speed of an AC motor?

The speed of the AC motor depends on the number of poles and power frequency. The AC motor speed change is not available when the number of poles and power frequency are fixed. The input voltage change will result in motor output torque and speed change, however, the speed does not change much. Moreover, if the voltage is decreased too much, the motor may possibly stall due to unstable operate situation. The motor may burn out because of overheat after continuous operation. The motor speed can be changed by adding a speed reducer or choosing an speed controlled motor.