Triac Switch Inductive Loads

No. It's a zero crossing, it will only turn on at the "base" of the wave. IOW, you can't PWM it. Actually, for AC, it's more usually called "phase control". Use something from the 3020 series.

Good Luck! Rich

I'm afraid not. Dimming (phase control is accomplished by delaying the turn on point in each half cycle, since, once triggered, thyristors stay on till the current passes through zero. Note that with an inductive load, the current zero does not coincide with the voltage zero, but is delayed.

The zero crossing firing delay will prevent you from turning the TRIAC on anywhere except at the very beginning of each voltage half cycle.

Is that circuit ok for a 3022? the best would be to start at the base and shut it off, which means i'd have to be in synch ? will do some reading thanks..

Rich Grise wrote:

Right. You delay the trigger pulse starting from each voltage zero crossing. And you will almost certainly need an RC voltage snubber across either the motor or TRIAC, to prevent the TRIAC from being re triggered each time the motor current goes through zero. Because of the phase shift, at that moment there will be some voltage, and its sudden appearance will try to re trigger the TRIAC through its internal capacitance to the gate. An RC snubber will slow the rate of rise of that voltage.

Asfaik, dimming will change the motor "strength". But frequency alteration is usually "the right way" (for ac motor).

revised circuit

John P>

Yeah, only the tic's are available here, would have to hit digi key's min order lol

Phil Allis> "Chris"

I don't see any means that the micro will have of knowing when the line goes through zero volts.

Zero cross is take through a 1k resistor to rectified 6v with a 10k pulldown, the posting was only on the triac switch

On Fri, 23 Sep 2005 18:26:49 -0400, Chris top-posted:

Reading is good, yes. :-)

BUT! Don't think you can start at zero-cross and turn the device _off_ somewhere in the middle of the sine wave - what you do is wait a certain number of nanoseconds after zero-cross, _then_ turn the device _on_, to conduct for the remainder of the half-cycle. It stops conducting at the next zero-crossing of current (which is why power factor correction is so important). Then, on the "negative" half-cycle, do it again, either by triggering the same triac, or the other SCR, depending. :-)

And yes, I think the MOC3022 is the right part for this job.

And top-posting is frowned upon in the NG, just a word to the wise. :-)

Cheers! Rich

Fine, except the MOC3032 should be a MOC3022. :-)

Cheers! Rich

YEah i know forgot to change that :)

Ahh i thought you switched on at zero crossing then simply skip waves, so what you are saying if i understand is to clip the begining or each half cycle?

Yep it may but gonna check it anyways :) the main use for this is simply an on/off but i'm wondering if i can modulate the speed without the motor heating up / bruning out basically i want to bring a single phase motor up to normal voltage the start dropping phases to see what the motor does...

where could i find some info on duty cycle modulation? is that simply dropping frames?

What is this motor used for? The induction run types do not like anything less than rated voltage, you may just cause an internal thermal overload to trip out.

The induction motors are designed to handle a specific loading as well as voltage. They will suffer premature failure on underloading as well as overloading. When you run the motor at low RMS voltage, you are overloading it. The failure is not instantaneous, it just means a rated lifetime of like 40,000 hours can be reduced to as little as 1,000. Instead of PWM you might try integer cycle duty modulation over a 1/4 minute period, or 900 line cycles. This is a valid technique depending on application. If your motor is shaded pole, and the label says "impedance limited" then you can do your original idea.

"Chris"

See figs 7 & 8.

The TIC226D triac is not suitable, since its Igt is 50 mA.

Try the BTA08-600TW ( Igt = 5 mA )

or the BTA16-600SW ( Igt = 10 mA)

............. Phil