AC Motor speed control

[Ioannis]

If anyone has experience with AC motor control I would appreciate any help.

I want to control the speed of a 240V AC motor (a fan actually) about 70 Watts.

I guess that it is an Induction motor because it has a rotor of aluminium.

Some use classic Dimmers but I fear that this approach is risky for the motor itself (heating-noise).

Is it possible to skip some periods instead of Phase Control? I mean to power the motor in 1st and then the 3rd etc? See picture.



Ioannis

[pedja089]

In theory it should work ... I have not tried but seems like a good idea.
Maybe 1 period on 1 off is not good, because rotor have big inertia. Maybe it will be better to take, eg 5 period ON, 5 period OFF instead of 1 cycle, on 1 cycle off.
For the driver motor you can use zero crossing opto triac driver, and triac eg MCO3042, and BTA10 or similar.
And you must have RC snubber network on triac, because motor is inductive load.

[ronsimpson]

I would not run the motor at 5 on/5 off. That will cause the motor to vibrate at 6 hz. Keep the on/off as fast as possible.

I know of a commercial product that switches at 1/2 cycles. At 50% power it only passes the positive half of the cycles. This causes strange readings on power meters. I don't know why it does not heat the motor. Maybe it does.

It is OK to do this.
on,off,on,off,on,on,off,on,off,on,on,off,on,off,on,

[Ioannis]

Thanks for the replies.

The rate of ON/OFF is to be determined. I need at least 10 steps from stop to full speed.

I am setting up a test circuit to see how it goes. With TRIACS like BTA12/600BW the snubber is not necessary (these are characterized as snubberless).

As for the heat I feel that it is not going to build up since the motor is running with almost normal voltage and is gettin power at zero crossing.

Ioannis

[mister_e]

Your dimmer approach should work without problem.

[Ioannis]

I will test it both with the classic dimmer and the cycle skip idea.

Report will follow.

Thanks,
Ioannis

[moacyr]

Hi

Your dimmer skipping waves is very intersting, what do you think about use with powerhead aquarium pump ? will it works ?

[amgen]

Hi,
I tried the skip cycle a while back and it just would not work, I think the dead times ( no cycle parts) caused the problem. But they have used phase control dimming to control fan speed for larger fan motors than yours. You just limit the phase angle to be above stalling the motor.... the phase range is about 80% to 100%.

Don
amgen

[moacyr]

Please ,
Can you show me a graphic of waves when we change a phase angle 90% to help me to understand. Thanks a lot

[amgen]

http://electronicseverywhere.blogspo...to-dimmer.html
You can also try with a light dimmer and again.... not dimming down to motor stalling