I have a 3 phase generator which is powering some system. (50Vrms @3A, with variable frequency from 50-5KHz).

I've used mechanical relays to control each phase to the load. However, there was no real protection on the contact side, and I would imagine that the repetitive on/off over time is eating away at the contacts due to arcing. I've had to add some thermystors to control the inrush current as well in the past.

What I would like to do, is replace the mechanical relay with an SSR or non mechanical part, that would allow me to allow the outputs to go to their loads, at or near the zero crossing point to control inrush.

The problem I am facing is, the frequency of operation. I was looking into SSRs with built in zero crossing, however, they appear to be tailored for the common house hold mains frequency ranges. I considered Triacs as well, and from the few data sheets I've seen, they do have a relatively low frequency of operation as well.

What are component or components can I use, that will allow me to switch on/off at near zero ?

  • \$\begingroup\$ Zero crossing may be a problem with 3 Phases. Consider quickly reducing current/voltage before opening the circuit. (Dimmer switch) \$\endgroup\$ – Optionparty Jun 10 '15 at 1:24
  • \$\begingroup\$ This is outside my area of experience. But three phase doesn't really have a zero crossing. I mean, each phase could be turned off when IT crosses, but that would create a major imbalance which might have undesirable consequences. Again, outside my domain of experience. \$\endgroup\$ – mkeith Jun 10 '15 at 4:09

I would say that even for arbitrary wave-form the firmware for this should be entirely feasible if you simply do:


simulate this circuit – Schematic created using CircuitLab



The optocoupler can be an opto-triac like the ones in the MOC series, but more aimed at the 48V-ish regions, or just an Opto-MOST rated correctly. For the latter you might have to increase R4, but that's all down to datasheets.

You can even put the entire system between the rectifiers to trigger a SCR (in stead of a triac) with just a simple MOST, but that will cost you 1.8V to 4V of the top of your 50VAC wave, since the load will also trigger through the rectifier (for mains that's not mush of an issue, but here it's an actual percentage).

Of course, you would be best off (least noise and interference) to handle each phase separately, but with the right uC math, if you do this for one phase, you can also control the other phases, if they come from the same generator with known phase angles (presumably 120 degrees).

Also, don't forget snubberage for inductive loads. Snubberage is a cool word, for that alone you should just always do it!

The interrupt in a 8MHz RC-running 8-bit MCU can be handled quick enough, if done right, so even with software lagging in reactive triggering, you'll turn on in microseconds, but if the frequency change is not in 100's of Hz per seconds, you can also design a very good predictive triggering scheme that can get you right on the 0 every time, apart from the first 3 crossings in a frequency change or so. But that's harder on the FW side.

Obviously: keep the resistors such that the current dumped into VCC through R2/D1 is always smaller than what R1 drains when the MOST is on (since the MOST turns on well before its gate reaches VCC+Vf, R1 will always be draining current when power is being dumped into VCC). Keep the resistors as small as is safely allowable, for faster switchy-times. Of course, you can increase R1 to 10k if you know the MCU will ALWAYS drain more than the dump into VCC from your 50VAC through R2/D1, which makes the signal even sharper (though, how noticeable?)

Anyway, enough for you to puzzle on, me thinks.

EDIT: Of course, your MCU has some sort of input somewhere from the main control, likely optically isolated.

EDIT: If you do the SCR inside the rectifier trick, you can do it all with two or three transistors again and no MCU, but as said before: voltage loss.

  • \$\begingroup\$ It's definitely given me some places to focus my attention on. \$\endgroup\$ – efox29 Jun 10 '15 at 2:11
  • \$\begingroup\$ This might work for single phase AC but the three phases in AC are not crossing thru zero together and if this was applied independently on each phase I can imagine all sorts of problems. \$\endgroup\$ – Andy aka Jun 10 '15 at 7:15
  • \$\begingroup\$ @Andyaka I do talk about single to three phase. If you want to treat each phase separately you can. With three phase direct rectification the problem can be tuned to only use the top 12%, or can look at peaks. If the generator is a coupled generator, as stated, looking at one phase will give enough information to fire below 10% on each other phase. All in all the bigger problem will always be the inductance of the load, generating phase shift in 0 current points, which the uC will have to estimate for perfect behaviour, which is way harder than a 3 phase problem. \$\endgroup\$ – Asmyldof Jun 10 '15 at 9:23

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