What will happen if one of three rectifiers wired in parallel fails

[RickR]

My 12ft wind gen has been running pretty well for about one and a half year. In the mean time my brother has been building one similar trying to improve on my design.

One area in question is the rectifier set up.

I have 3, 3phase rectifiers wired in parallel to convert the AC to DC and handle the load.  I figured parallel wiring would give some safety factor in case one would fail.  Each rectifier is 35 amp rated and my system is only 40 amp total at 24 volts.

It's working well but recently someone told me if one fails it could wreck other things in my system?

Is this setup OK or should I scrap it?  Or is there a better way?

Thanks, Rick

[Flux]

Picture doesn't show much.

You say you have 3, 3 phase rectifiers in parallel that is a bit unusual as most people seem to build 3 phase bridges from single phase blocks.

You can parallel rectifiers for increased current handling but they don't fully share current so 3 in parallel may only double the rating.

Any rectifier failure will cause some sort of trouble whether it is a big single unit or many small ones in parallel so it really doesn't matter much. The main thing is to have lots of spare capacity.

I find that decent stud mounted diodes properly heatsinked are cheaper better and work with less losses than loads of the block bridge units in parallel but the stud diodes need a lot of mounting and insulating of heat sinks so the crappy insulated block things seem to be more popular.

Much depends on whether a diode fails to open or short circuit. An o/c diode in a parallel stack may not even show but will reduce safety margin. Any short circuit diode will prevent correct operation even if it is only one of many in parallel.

Flux

[SparWeb]

I'd echo Flux's comments, and I'll add my own personal experience.  I had a rectifier fail last year.  And, just to rub it in, I had the rectifiers hooked up in parallel when it happened.  Lucky for me it was a short-circuit failure, causing the windmill to slow down somewhat.  If it had been an open-circuit failure, the mill would have accelerated.  The wind was strong at the time.  The rectifier blew at a load much lower than the rated 35A, so I learned another lesson from that, too.  

When in parallel, the little rectifier blocks won't share the load perfectly.  You can expect that one of the diodes in the parallel circuit will turn ON before the others.  It will take the spike and full current for an instant before the other one turns ON.  This may only last a fraction of a milli-second, but over time, that may kill it if the imbalance is big enough.  Unless you have expensive equipment to test and measure, you'll never know if it's happening or not.

[hiker]

[RickR]

So would you stick with the parallel wiring or is there a better way to do this?

Also, is there a simple circuit, mechanical or electrical for a safety brake if diodes or a controller would fail when im not around?

[SparWeb]

Well, actually what I was saying is that parallelling cheap rectifier didn't work out so well.  Now I use a big rectifier rated for 4x more current than I've ever seen.  I must give credit to a very generous forum member for that gift.  (You know who you are :^)

Three-phase, high current rectifiers can get expensive unless you know where to look.  There may be some part numbers in the Digikey catalog for 3-ph bridges (can't find them today, for some reason).

As for the shut-down, you can either use a mechanical switch, or a relay.  The relay is necessary if you want it to be automatic.  Say the sense circuit detects an AC over-voltage, indicating a run-away.  The relay kicks in, shorting everything.

[RickR]

OK, Thanks for the input

Rickr