There are different techniques to control speed of AC motor. One of the very popular of them is by applying chopped AC waveform – means changing phase angle of applied AC waveform. This method is used in many different devices likeThe firing angle of a thyristor like TRIAC – that gives supply to motor, is delayed to decrease the motor speed or it is fired earlier to increase motor speed. As firing angle is changed, the part of AC waveform applied to motor is chopped more or less. So the average AC voltage applied to motor changes – that changes speed of motor.