3 phase converter guru needed ....

[HellinnFrnt]

ok here's my deal. My shop is using a 50hp Des-Co  rotary 3 phase converter and it has recently started burning out the start capacitors (250v 270-324 mfd) 14 cap stack (total) run caps are 370v (not sure mfd) 8 total. The property owners electrician came out and replaced some of the start caps with 330v 270-324 mfd and it worked fine for a few weeks but has blown 2 more 250v out 2 times since ... (each time it has been the 250 next in line after the 330v). The only difference I've been able to see is some have "bleed resistors" some not. I know for sure that the last in each "stack" has one. I don't think the voltage rating is the problem but I AM wondering about the resistors and also what effect the start timer may have on the system. When it doesn't start it seems to stay "engaged" longer than normal (usually about 3 seconds) when it blew Friday I counted to 6 before I pulled the switch on it. I could hear the motor up and running it never seemed to kick out the start circuit like it normally does (loud buzz for a few seconds then quiet smooth motor sound) it stayed at loud buzz for 6 seconds before I shut it down, opened motor room and magic smoke was floating out of the box. OK get the picture ?

Question...

(1)  How do I check the start timer (potential relay) whatever it's called? (seems likely to me it's sticking sometimes)
(2) What is a SAFE way to check the caps in the stacks?
(3) Does anyone KNOW how to work on something like this that I can send pics and talk to?
(4) How do I keep the smoke INSIDE the caps?

We're willing to pay some one that knows what they are doing to come out and fix it. If it's going to cost 3k to fix we can get a whole new one but would need it installed. Our only other option is move to a new building that already has 3 phase and that could be upwards of 25k in moving and installation/setup.

Thanks for your time....

 

[Patchman]

Time for an electrician. When I had problems with mine, it was due to the 10hp motor going bad. think.gif

 

[ChryslerJet]

I am not an electrician nor an electrical engineer but my brother is and so is an uncle.  I sent my uncle your question and waiting on a response.  Would send it to my brother but he and I are not on speaking terms right now.  Now while we are waiting I do dabble with electronics and have had situations where I have had to replace Caps (capacitors) and in my experience once one in a series goes bad its best to replace them all because when one goes out it tends to overload the others and even though they didn't blow they were stressed to the point that they will blow next.  One other thing about replacing them when you get replacements getting ones for a voltage rating higher than you have is fine but since the circuit is designed to work on a certain capacitance the Capacitance rating must be the same as what was replaced.  Typically capacitors that are either blown or about to blow will show outwards signs of bloating either expansion on the top or bottom when they are normally concave or expansion on the sides.  Typically you will see it most on the bottom or at the electrical attachment point since that is the weakest part of the capacitor.

Hope this helps and let you know if I get any more information.

 

[HellinnFrnt]

Well I can add "electrical engineer" to my list of things I can do ... I fixed it myself. It has a magnetic start switch (like some table saws) and one of the contacts in it was burned up causing it to stick ; replaced the contact it's fine now. Of course this was AFTER I paid a "master" electrician to come fix it ... he changed $500 worth of caps out and called it good ... it blew out again the next day so I started taking it apart while one of the crew went to pick up more caps; saw that switch in there and it looked familiar to what I've seen on some 3 phase saws so I said hmmmm... opened it up and voila "magic smoke residue" . Called the electrician back in to show him what the cause was and he said that he didn't look at it because it was "mechanical" ... REALLY ??? if it has wires and contacts in my book that makes it electrical. Anyway if anyone needs a 3 phase converter I know how to build them now!  Thanks for your support!

 

[ChryslerJet]

Good deal glad you got it figured out.  Yeah those contact switches are a lot like the contact switches on a starter motor.