Three phase phase-angle control problems

[TOLiS_GR]

I'm stuck waiting for parts at the moment so I've started reading the 3phase motor stuff - mainly from atmel as that's what I'm using.

High voltage tests are done at a friends lab, he's an EE but kinda old (from what he told me, his books had a reference in the final chapter that said "transistors are being developed" heh.. ), as I don't even have a three phase supply in mine.

[Circlotron]

"Proper" 3 phase control is non-trivial at the best of times.

Amen to that. Back in '85 to '90 I worked designing various 3-phase SCR soft starters for large induction motors, up to about 1000hp. When there was less than full load we would reduce the voltage somewhat to save a little power. If the SCRs were outside the delta you would often have motor stability problems if the motor was running at full speed and reduced voltage. The cure took a while to find. No internet in those days.

[TOLiS_GR]

Even with the vast amount of information available nowadays there's still nothing specific for what I'm looking. e.g. there's code around for doing a single phase angle controller even with a cheap arduino. But considering the many many tests and r&d I had to do - as well as the legal stuff I had to read - I might be reluctant to give specific details about my solution (IF I had found one) for fear of lawsuits and health stuff etc.

Now, if anyone is interested and may know anything to help me here's my progress:

I managed to do some tests today using zero-cross opto-isolated triac drivers and snuberless triacs, tested with both outside and inside delta connections. My testing load was near 40Amps. I used a driving pulse based on the AN-3004 Application Note from Fairchild, or at least on how I interpreted it. Trying to cover a full cycle my pulse is around the duration of two half-cycles plus some time to make sure that the triac is fired 2 times and allow a full cycle to conduct.

I took a fixed time base approach of 1 second, and I made a test using 10/20/30-100% settings. The outcome wasn't nice and is nothing like the device I have as a reference that uses the same opto/triac configuration. My amp-meters where bouncing and there was some light flickering on the lamps I have on the same lines, but there was some kind of power control. The bounce was dependant on the setting and was bouncing somewhere near that e.g. not 0-40 on the scale, but 5-15, 20-30, 30-40 (crude example, didn't take notes on that).

I also tried a quick test using variable time base, denser bursts of control pulses and that seemed much much better! The amp-meters where really steady, not light flickering at all, etc. However it was on just a few settings and now I need to better understand the variable timing stuff and re-test.

I've started thinking that maybe I should use a rectifier bridge and pwm the dc after that, but what about rfi and noise then? Is there any commercial product using this way? Apollyon25_ any ideas?

[gadhadar]

I faced the exact same problem. We solved it by using an RC snubber with R=39Ohm and C = 0.01uF.

Hope this helps anyone else trying to get a 3 Phase, Independent Phase angle circuit trigger with TRIACS or SCR's.