Controlling speed of AC motor is required in industries as well as in domestic use. In industries the required motion control is achieved by controlling speed of big and heavy AC motors. They use variable frequency drives (VFD) for speed control. In some other applications like fan, blowers, heaters etc another method for speed control is used that is phase angle control. By varying phase angle, the current supplied to motor can be varied. And by varying supplied current the speed of motor can be varied.In domestic application the phase angle control method is used. The best example is fan regulator. In domestic fan regulator DIAC and TRIAC are used to vary phase angle of fan motor. This is very easiest method to vary AC motor speed.