View Full Version : Home made RPC, smoke got out today.

06-07-2013, 07:22 PM
I built a RPC almost 20 years ago to operate a No. 2 Reed surface grinder. I haven't operated it in probably 5 years, but needed to do some grinding on some reamers I'm making to swage bullets. I started the RPC, started up the surface grinder and started getting set up to dress the wheel. I heard something like air hissing. I recently ran an air line in to blow chips off of the lathe so I thought the quick connect was leaking. Looked over in the corner at the hose and saw smoke blowing out of the start capacitor on the RPC. I shut down the surface grinder and the RPC which were both still running. Hoping I can just replace the start capacitor and go back to work. Anyone ever seen a capacitor "blow" smoke before???


06-07-2013, 07:27 PM
Yep. Capacitors do that sometimes.

I had one that popped and shot a streamer of oily paper across the desk.


06-07-2013, 07:34 PM
Confetti generators!!

06-07-2013, 08:23 PM
Yep, been-there-done-that. I just reloaded and kept trucking. Caps go bad and go pop or in my case just went smokey/stinky.

06-07-2013, 08:50 PM
Motor start capacitors especially for RPC are large capacity and for that purpose they are often a low-ESR bi-polar aluminum electrolytic capacitor, essentially two caps back to back. Such a capacitor is designed for line ac voltage, high current, short-duty operations.
If powered for any longer than a few seconds, they can start to heat up.
The Run Caps which are usually oil filled paper, can be ran continuously.

06-07-2013, 09:13 PM
I just recently replaced the start cap on our compressor for the second time in a month. This time, a co-worker just happened to be walking by when the cap blew. He yelled, and I looked over in time to see steam shooting out of it.

Often the cause of this is a sticking start switch. I checked it all out- didn't find a problem, replaced the cap and it's been fine since. But the cap itself was a disappointment to me- it's one of the larger diameter ones, a good weight, and looks like a good part. It wouldn't fit within the original metal housing, so I had to modify the housing. No big deal, but with the 'lid' blown off the cap and looking inside- there's a little cap sitting off to one side and the rest of the container filled with tar. It's like a D sized battery- open it up and there's a C sized battery inside.

Rich Carlstedt
06-08-2013, 12:29 AM
My 30 year old phase convertor blew a cap out last fall.
It was a 220 volt 260 mf starting cap ( black plastic)
I replaced it with the same rating cap from Grainger
The new cap is "made in China" and is only 2/3 d's the size of the old one.
Guess that will blow pretty soon ?


06-08-2013, 01:02 AM
The new cap that blew on me was a big casing with a little cap inside, tarred in place- but it wasn't covered in tar. In other words it wasn't completely encapsulated. I took it with me to get a replacement, and the guy looked at it and said they don't make parts like they used to. He's in the business, and didn't know where to get a genuine part. Things are not looking good.

06-08-2013, 01:11 AM
Cap blew in my 20 year old Lincoln welder. Made a god-awful stink. Lincoln wanted $500 for a capacitor bank. I got bigger, higher temp-lifetime caps from digi-key for $68.00.

06-08-2013, 01:26 AM
If you have a potential relay in the rpc, take it part and check the contacts. There a real good chance they have stuck together. If they are.. just replacing the cap will smoke a second.

06-08-2013, 08:24 AM
If you have a potential relay in the rpc, take it part and check the contacts. There a real good chance they have stuck together. If they are.. just replacing the cap will smoke a second.

All I remember about the wiring is I had two switches, one for "start" and one for "run". I would turn on the "start" switch, then the "run", then turn the "start" switch back off. It was simply a 3 ph motor with an extra capacitor wired in to "start" the motor. Got the plans from the electrical supv at my old job. The third "leg" wa always 10-20 volts higher. Everyone said don't worry about it. So I don't.


06-08-2013, 10:31 AM
The problem with manual start is that the start capacitor is in circuit for a period only determined by whoever starts it.
The best methods are by timer and this way the cap is not subjected an overlong period of applied power.
These are usually the ones with shortened life.

06-08-2013, 10:52 AM
I agree.

For the system without a timer it's best to use a non-locking push button. However... if it doesn't drive a contactor, it needs to be " xx HP rated" or similar.