Capacitors Parallel or Series Wired?

[markba633csi]

What RJ said: (same as resistors in parallel)

Class dismissed!

 

[CJ5Dave]

Thanks .I do have a couple of analog meters around - always seem to need a battery. I saw a post somewhere on how to convert these to a capacitance meter. They are really 700-830 MFD capacitors, so for ah 7.5 HP motor wired in series would make sense for 100 MFD per HP.

 

[SLK001]

Shameless plug for the Shack

Which has been more or less dead for years.

 

[markba633csi]

The 100 uF per HP is not absolute- I have run across some motors that require as much as 300 per HP
So for example, if your motor needed say, 200 uF per HP you would need about 1500 uF or two 750 uF in Parallel

 

[Weldingrod1]

Keep in mind that ac caps, especially the non-polar electrolytic start caps, have a limited lifespan. Caps on your ac unit probably only last 5 years...

So, it's better to replace than mess around testing

Sent from my SM-S911U using Tapatalk

 

[CJ5Dave]

Yes, I just went to 4 places looking for the start capacitors. Save time to try this first. None in town. Have to go out of town for a haircut, medicine, lumber, doctors, you name it. I found an analog meter and by Mark's test the start capacitors test OK. Headed up there to try the run capacitor the same way. The motor has a thermal cutout, as does the motor starter, a breaker to turn it on, one in the breaker box too, a fuse for the pressure switch circuit, pressure switch, a low oil switch, 3 capacitors, so many things to rule out. Really too hot to mess with at at 95 in a metal building.

 

[markba633csi]

Yeah it makes sense to try to narrow it down some; these caps are not cheap and nobody stocks this stuff as much these days.
Gotta buy more and more stuff online- They are trying to get rid of us DIYers you know. Pesky skinflints LOL
"Just buy a whole new machine/car/tool/appliance" like everyone else

Anyhow, it sounds like your start caps are probably ok- my hunch is a bad switch or wire connection
Stay cool

 

[CJ5Dave]

I am so confused. I checked the run capacitor with the analog meter and it tests as a dead short. Does so with the digital depending on the ohms range setting. So I checked the thermal reset, which was not tripped. No continuity. I reconnected the capacitors, jumpered the thermal reset, and it fired up and ran fine. I turned it off, bolted the capacitors in, and removed the jumper, Switched it on, and it ran without the jumper. Built up to full pressure, and kicked off. I am going to order new capacitors regardless and probably bypass the thermal reset. It has so many resets surely it can do with one less, and I am sure to never find its replacement locally.

 

[CJ5Dave]

My usual source, Surplus Center, does not show that size in either a start or run capacitor. Grainger shows the 700-850 start capacitor for $23. The run capacitor has rust on the surface where the numbers are. There is a 650 - I am assuming 650 MFD. Can't find that size anywhere. Is that a likely size run capacitor for a 7.5 HP motor?

 

[mksj]

Well even as a dead short in one capacitor, there is still the second capacitor that is in the series circuit, if it is a dual voltage motor they sometimes use 120V capacitors in that uF size, but I would be more inclined to get a higher voltage rated one. You also do not know if they are the original capacitors or they were previous replaced w/o looking up the motor specifications. Ones below or the previous link should work fine, as there is a wide +/- range. Check the size to make sure they fit. Run capacitor is usually oil filled and much lower capacitance, wouldn't be 650uF. Rust on the can does not effect the internals.

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