Oil-filled Capacitor capacitance question

[hudstr]

It seems the oil-filled run capacitor in my rotary phase converter spilled its guts. When I went to start it, it started to spin but never got up to speed. There was oil splattered everywhere when I opened it up.

Unfortunately the info printed on the capacitor has slightly worn off. It looks like it says 50/15 uF 330VAC. I think the model number is 97F4747 FB but I wasn't able to find anything with that.

From my quick research, dual capacitance caps are mainly used in HVAC where one terminal is used for the compressor and the other the fan, with the third being common. Only the "C" and "HERM" terminals were hooked up, the "FAN" wasn't used. Can I replace it with a regular oil capacitor with one capacitance? Which capacitance do I need, 15 uF or 50 uF?

 

[Eddyde]

Is that the only capacitor in the RPC? 50 uF sounds low for a start cap but could be right for a run cap. Some units use the same capacitor for both start and run, which might explain the failure.

 

[hudstr]

No this is the run capacitor as I said. There is a start capacitor, and also what I believe is another small capacitor attached to the relay circuitry. There is no brand or markings anywhere I can find so I don't have a wiring diagram. It is possible it was home made because I got the machine from an estate where someone might have been an electrical engineer or something, they had a lot of electrical related stuff.

 

[Eddyde]

Sorry missed that.
Yeah 50 uF would be fine for a run cap but only one? Usually, there is a cap between each hot leg and the ghost leg.

 

[markba633csi]

I'm nearly 100% certain the "fan" is the 15 uF and the "Herm" is the 50 uF
says Herman Munster
Gotta be home-brew since a factory build would not leave one side of a cap unused

 

[cathead]

It appears it was connected to the 50 microfarad side according to the photos. I would try find what the motor requires instead
of relying on what was in there.

 

[hudstr]

The motor doesn't have a data plate unfortunately. It doesn't have anything other than a sticker saying it passed QC. Whats weird is the motor seems like it was purpose built to be a phase converter because there is no output shaft so its not like someone took an old motor and cut the shaft off.

I'm going to buy a 50uF cap and see what happens. I imagine the voltages between the phases will be imbalanced. I didn't measure that when I got the RPC along with the lathe. I think it will still work, just be inefficient. If it continues to be a problem it will probably be cheaper to buy a used RPC locally or a VFD rather than buy a new enclosure and rewire everything.

 

[BaronJ]

Hi Hudstr

That capacitor is a dual 50uf + 15uf one. It says on the can "no PCB's" but that isn't always the case. PCB's are carcinogenic, so I wouldn't get The oil on my skin.

 

[hudstr]

Replacing the run capacitor worked, but I'm concerned by some arcing that is happening with the homemade relay thingy.

https://imgur.com/GnlPSp9

I'm not really sure what is going on, circuits aren't my strong suit. Going into that homemade assembly is one leg which goes through a diode and a bunch of resistors in series before connecting to a Potter and Brumfield KCP11, which is a socket mounted relay. There is also a DC capacitor in parallel with some of the inputs. I made a messy and probably wrong diagram of the wiring. When I bought the lathe, the PO was using a 120v circuit and somehow with a mess of relays, wires, and I think a buck transformer to get 240v for the RPC. I wired it up temporarily when I got it home just to test it but it tripped the GFCI outlet. I assumed it was the mess of wiring which I took out when I wired it for 240v, but now I think it might have been this relay thingy in the RPC leaking to ground.


At this point I think I'll just rewire the RPC. The enclosure on top of the motor is pretty small so I'd like to be as compact as possible. I was looking at this Simple Rotary Converter design. Is this as simple as it gets or is there something better that uses a timer or something?

Thanks for the concern about PCB oil. I looked it up when I first took it apart and I washed my hands really well afterwards. I plan on cleaning the enclosure before I put it back together.

 

[mksj]

It is arching to ground which is not a good thing, it may be a carbon track, but not a good thing in either case. The area under the relay looks darkened from heat. I would probably recommend just replacing the RPC control box, as the whole thing looks iffy and other parts may be questionable.