Triac Switch Inductive Loads

Hummm someone must have problem's, don't like it don't look at it....

Nice and constructive input than you very much.

Especially from someone that doesn't post their name or email.

Genome wrote:

Is there something wrong with your software that prevents you from drawing nice circuit diagrams.

Like, if you don't care...... why should I give a f*ck?

How do you think Carol might feel?

DNA

Just put him in your killfile. I did, a long time ago; helps the signal to noise ratio quite a bit.

The short answer to your original question (if I understand it correctly) is "not really." Triacs, once triggered, stay on until the trigger is removed AND the current through it drops to zero. You could probably get a PWM-ish effect by selecting 1-of-N half cycles but I doubt that the motor would be entirely happy.

Good info at

particularly

Hi, Chris. Don't mind Genome -- he's a regular contributor to s.e.d. and has a lot of competence in SMPS, as well as high current-high voltage switching and analog designs in general. I guess you just caught him on his rare bad day. But picking on the significant other is usually bad form. We all have our own allotted portion in this vale of tears (note the logical progression -- engagement ring, wedding ring, suffering). And your .jpg file is OK -- just a standard opto-triac setup.

If you're switching an inductive load, you're going to have problems with triacs. They will switch off at zero voltage/zero current, but will tend to stay on for inductive loads because zero current and zero voltage occur at different times in the line cycle.

Also, your zero-crossing opto triac trigger isn't going to cut it for speed control. You can't turn on in the middle of an AC half-cycle at all. Using the MOC3032 for voltage control would work for something like a heater or soldering iron, where the thermal time constant is very long in relation to an AC half cycle. Your motor will probably cog like crazy with this kind of speed control, though. It won't be pretty.

If you want to do something like this, and assuming phase control will work for your AC motor, you're going to first have to replace the zero-crossing opto triac trigger with something like the MOC3011, which will have random phase turn-on. Also, the classic way to overcome the inductive load problem is to use back-to-back SCRs instead of a triac, but that would involve some extensive redesign of your circuit. If you want to keep everything else in place, you might want to look at replacing the triac in your circuit with one of the Teccor "Alternistors", which have many of the characteristics of back-to-back SCRs, but also only have one trigger input. They can be turned on in QI and QIII, and may work for your application. Try the Q60165L6 (TO-220 package, rated for 15A and 600V). It's pin-for-pin drop-in compatible with the standard triac pinout, and will definitely work a lot better than a triac.

Good luck Chris

Hi, Chris. The MOC3022 is alright -- it's actually an upgrade, because the output is rated for 400V instead of 250V. The MOC3022 will work on

If you're wondering, the alternistor is available from Mouser and Digi-Key.

If you run into problems driving the motor, feel free to post again. Please include some information on the motor if you do.

Good luck Chris

Thanks all, is a moc3022 ok? thats all that is available locally i already did the zero detect.

Chris

Chris wrote:

no.

when a triac turns on it doesn't turn off until the current through it drops to zero,

if it turns on at zero crossing it'll remain on until the end of the half-cycle, (or slighly longer with an inductive load)

if you're wanting to do a dimmer by feeding partial cycles to the motor use a regular (non-zero-crossing) triac drive optocoupler like moc3012

you'll also want a zero-crossing detector to give you a starting point for your count...

Yes you can pwm. However, you will most likely turn on 50 cycle half cycles in some repeating sequence. IF you don't then the motor will manifest a beat frequency. To eliminate the speed variations, you need to monitor the ac cycles to synchronize the controls that you are trying to make. The feedback circuit can be a simple opto coupler that turns on every other AC cycle.

Harold