Why will lower voltage capacitor work my pump?

I am having an issue with my pumps impeller turning. The pump turns on and hums but the impeller does not turn. I read on-line that it may be a issue with the capacitor. So I took another capacitor and plugged it in and the pump worked and the impeller turned perfectly. However this other capacitor is a lower voltage capacitor (165 VAC) and it causes the pump to shut off in a few minutes. So I took the original capacitor (370 VAC) to an electrical supply store and they tested it and they said it was working fine. So I plugged it back in and the same thing occurs with just the hum. So I got a new capacitor (370 VAC) and I still just get the hum and the impeller does not move. Why would the impellor work with a lower voltage capacitor? Any ideas? Thanks.

If the motor has a centrifugal switch, it could be the switch causing the issue. The contacts can get dirty so they don't conduct.

Also, is the motor wired for 120v or 240v?

Thanks for responding. If the switch is bad, why does it work with the lower voltage capacitor? It appears that the motor is wired for High voltage based on the pump label and the wire colors on the corresponding jumpers.

An under voltage capacitor will have more internal resistance than the correct sized capacitor and might have a different effect on the phase shift between the winding's. It could also be a coincidence where some fiddling around the switch makes it work occasionally. But I would not use the under voltage capacitor in the circuit because it could explode.

Is this a pump motor that was working ok but has since failed or is this a pump motor that was just installed but never worked? A little history might be useful.

This pump is about 3 years old. It was working well until about a month ago and then started cutting off after about ten minutes of use. I figured something was causing it to overheat. It would turn back on after 10 minutes. I played around with the pump and it started to run longer. But then it started cutting off after 10 minutes again. Then a week ago it just started to hum and the impellor stopped turning as stated above. Its not an issue of working sometimes as it works every time with the lower voltage capacitor.

That sounds more like a bearing issue than a capacitor issue. When bearings start to fail, they create extra load on the motor which tends to make the motor overheat and trip the thermal limiter switch. After the motor cools off, the thermal limiter will then reset and the motor can then be turned on again. When the bearings get really bad, the motor will have difficulty spinning up to speed. Of course, any mechanical interference within both the motor and pump wet end can cause the same symptoms.

As to why the lower voltage capacitor seems to work, that could be due to extra torque created by this particular capacitor. Does it have the same micro-farad rating as the original?

The original 370 vac capacitor has a 30 MFD. The lower voltage capacitor says 124-149 MFD on the side. Does the higher mfd create greater torque? If the bearings are going, is there anything to do other than getting a new pump? Should they go in less than 3 years? Also could the possibility of all of this be happening because I'm not getting enough electricity to the pump? Could that be the reason the lower capacitor works but then the pump is shutting off due to not enough power? I'm just trying to think outside the box.

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Does the higher mfd create greater torque?

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Yes, it creates a larger phase shift between the primary and start windings so start torque is higher but so is start current so you risk burning out the starter winding's.

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If the bearings are going, is there anything to do other than getting a new pump? Should they go in less than 3 years?

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If the pump seal fails, which they do, the water can leak into the motor and cause the bearings to rust. But 3 years is a short time although I have helped people with shorter times too. There are several things you can do.

1-Replace the bearings. You would need a bearing puller to do this.

2-Replace just the motor. Not very hard to do.

3-Replace the entire pump. More expensive than 1 & 2 and just about as hard as #2.

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Also could the possibility of all of this be happening because I'm not getting enough electricity to the pump?

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If the voltage is too low on an induction motor, it will draw more current and the thermal limiter switch should shut it off so it will cycle on and off which are similar symptoms to a bad bearings but the voltage would have to be very low. Have you measured the voltage?

I have a tester and it showed it is pulling in 120 volts. The pump appears to be wired for high voltage. If the case is that the pump is wired for high voltage but only low voltage is going to the pump, how did it work for 2 years? I have a friend who is better at electricity than I am to confirm that only 120 is coming into the pump.

Most single speed pump motors can be wired for either 120v or 240v. Normally, a motor will pop a breaker not run at all if wired incorrectly.

It appears my pump is fixed and the issue solved. There is indeed only 120 volts coming into the pump and the pump was wired for high voltage. I switched the pump wiring to low voltage and everything is now running great with the original 370 vac capacitor. The pump ran all night and has not shut down at all. The question is why did it run for 2 years wired incorrectly and why did the pool people who installed the pump not switch the wiring to the correct setting? I guess we do not need to answer that. Anyway, thanks for your responses and help. I am happy I have a running pump again.

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Originally Posted by Albgoblue

There is indeed only 120 volts coming into the pump and the pump was wired for high voltage. I switched the pump wiring to low voltage and everything is now running great with the original 370 vac capacitor. ---- The question is why did it run for 2 years wired incorrectly and why did the pool people who installed the pump not switch the wiring to the correct setting?

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For the record -- and for all the other people who may read this thread in the future -- I'm very doubtful that the pump actually DID run for years wired 240V, but supplied 120V. As far as I know, with most pool motors, this is not possible.

I'd guess that somewhere in the assembly and repair process, some wiring got switched around.

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