RPC Start Cap Sizing

gzig5 · Aug 2, 2017

I've got a 5hp RPC I built a few years ago. Works well with the exception of not starting correctly about 5-10% of the time. It sits there and buzzes without turning. Turn it off and then back on and it starts fine. The start cap is a 270-324 uF part. Is this big enough for the 5hp motor? I'm wondering if the poles are in a certain orientation that the cap isn't providing enough phase shift or whatever it is doing to get the rotor turning. Is there a rule of thumb of uF per Hp?

rons · Aug 2, 2017

The times that it just sits there:

A motor overload device should be disconnecting the motor. A manual control mechanism would not be healthy, as in you just let it buzz for a long enough time to cause harm. The start capacitor value is good if when the motor starts the time delay is 1/2 - 1 sec.

gzig5 · Aug 2, 2017

The overload doesn't trip because I hit the stop button as soon as I realize it isn't starting correctly. Time relay to take out the start cap is set to 750ms.

TDegenhart · Aug 2, 2017

It sounds like whatever is being used to connect the start capacitor is not always making contact. First item I would check are the connections and the relay or contactor contacts.

Tom

gzig5 · Aug 2, 2017

TDegenhart said:
It sounds like whatever is being used to connect the start capacitor is not always making contact. First item I would check are the connections and the relay or contactor contacts.

Tom

I was thinking about that this afternoon on the way to pickup a 5hp motor for a second RPC for the basement shop. The contactors had some years on them before I re-purposed them so it is plausible.

Bob-J-H · Aug 2, 2017

Rule of thumb is about 80-100 MFD per HP for start caps. Bob

Newman109 · Aug 3, 2017

Bob-J-H said:
Rule of thumb is about 80-100 MFD per HP for start caps. Bob

Thats right. I have a 5 hp home-built RPC that uses 500 mfd of start caps and it starts very quickly. I use a potential relay and have not had any problems in about 7 years of steady use.

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