View Full Version : exploding capacitors in RPC

04-12-2010, 07:04 PM
i bought a phase converter from the now defunct des-electric.
unfortunately i didn't wire it up before they went out of business so this malfunction now means i can't ask them how to fix it....

we are talking about a 30hp motor with banks of 7 and 3 start capacitors and 10 run capacitors, all of which was pre-wired and balanced by des-electric when i go it.

all i've done is wire the 240V single phase into the box, and then the three outputs legs to the 30hp motor and my 10hp cnc lathe.

the first couple times i hit the start button the motor hummed loudly but wouldn't spin, and the circuit tripped. then i manually spun the motor shaft while i hit the start button and it spun right up and ran flawlessly for several hours while i cranked up the lathe and got things in order over there....

i stopped and started the converter several times with no issues (except that i had to manually spin the motor as i hit the start button to make sure it would actually spin up).

but then, maybe the 4th or 5th time i attempted to start it up, one of the capacitors blew its cap off, literally like a firecracker.
i was discouraged, but went ahead and reconnected the the remaining bank of 6 start capacitors together, and it started right up again.....several more times.....

then all of a sudden a couple hours later when i hit the start button another of the same bank of capacitors blew it's cap off.

so, i'm sitting here with several replacement capacitors, wondering if there were just a couple bad capacitors in my machine, or if there's more to this.

any thoughts?

when it was running i checked the voltages across each leg and they were all 246V, which jives with my incoming power.

the start capacitors are BMI brand, 270-324uF, 250V rated


04-12-2010, 07:12 PM
also, i never had any load on the converter when i tried to start it.
the lathe was totally off.

04-12-2010, 07:15 PM
i bought a phase converter from the now defunct des-electric.

Maybe they're defunct for a reason!

Start capacitors normally have a very short duration time-rating on AC.
Any replacements should be rated well above the voltage rating of the supply.
If left engaged for too long a time duration will overheat.
Personally I would use 400vac rated ones, 250v is a bit close to the supply voltage.

04-12-2010, 07:55 PM
I have read that unused electrolytic capacitors that have sat around for a time need to be "conditioned". If I remember right, you wire them to a lower voltage to regenerate the electrolyte.

Like Al said, you should always use caps that are rated at least 1 1/2 times the circuit voltage, and go to the next higher voltage if 1 1/2 times isn't available.


04-12-2010, 08:04 PM
I'm sitting here right now reforming capacitors.


04-12-2010, 08:33 PM
cool, thanks for the responses.
sounds like i'll need to find myself a resistor and set aside a few hours tomorrow. very cool link with the explanation, thanks for that.

the caps HAD been sitting for nearly six months!

would these symptoms also explain the motor not starting to spin on its own?
i'm guessing yes, but curious....hoping that with the new/reconditioned capacitors i'll get the "auto-start" i'm looking for (rather than having to nudge it along)

i'm going to plan on replacing these caps with the 2 of the same rating that i already have, and if i have any more explosions i'll replace them with some 400V models.

a quick search of google and i'm having trouble finding a source for 400V, 274-330uF caps....any suggestions?

thanks again!

04-13-2010, 05:09 PM
Try here; https://www.surpluscenter.com/sort.asp?catname=electric&keyword=EMRC


06-05-2010, 11:36 PM
I bought a RPC from DES last year and have not had any problems with it. I wish i would have known about regenerating dormant capicitors, i had to learn the hard way on my Hurco, after sitting dormant for a while on my machine a a bank of noise suppresing capicitors let loose spraying into my servopacks shorting one out. Any way check out http://www.texasconverters.com/ looks like they took over DES judging from the website design, maybe they can help you in the future.