AC Motor Speed Control using Triac & Freewheeling


Thread Starter


Hello Everyone,

I was trying to control the speed of an AC motor using a Triac (BT139). I am triggering the Triac using an optocoupler (MOC3021)through a microcontroller. I am detecting the ac zero crossing and then deciding the firing angle to control the average voltage which is to be provided to the motor for speed control. My question is I came to know about the concept of freewheeling in DC motor circuits and wanted to know whether it is required in AC motor speed control circuits or not? If yes then how it can be implemented? and If not then why it is not required?

Thanks in advance.

Robert Scott

The reason you need a free-wheeling diode in a DC motor control circuit is that when you cut off the current suddenly the inductance of the motor will cause the voltage to rise to destructive levels to try to keep the current flowing. But the nature of a triac is to keep conducting until the current goes to zero naturally (and gradually) at the end of the half-cycle of AC power. So when the triac "turns off" the current is already zero and no further protective action is required.

Robert Scott
Embedded Systems Consultant
Hopkins, MN

Curt Wuollet

As you know by now, controlling the speed of an AC motor by triac firing angle is not all that great. You operate the motor past it's breakdown torque and are really controlling that torque. That's because most AC motors are synchronous and speed in any normal mode is proportional to frequency not voltage or current. And freewheeling diodes do not apply here because a triac is a bilateral AC device and a
freewheeling diode would render the triac ineffective, essentially shorting across it for one quadrant.