Why will lower voltage capacitor work my pump?

[Albgoblue]

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.

[mas985]

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?

[Albgoblue]

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.

[mas985]

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.

[Albgoblue]

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.

[mas985]

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?