Single phase motor woes

Looking for some advice on a motor/pump issue. We have a hot tub,The main pump is driven by a single phase two speed motor. The symptoms are that the motor intermittently stalls in low speed. Sometimes it will run for hours and sometimes just for a few minutes before stalling. The pump seems free to turn by hand and it will always restart OK in either low or high speed mode. So I am suspecting something resulting in lower torque on the motor side, although I am not ruling out a worn-out pump. The motor wiring plate is marked 4P for low speed and 2P for high speed. It is a dual capacitor unit. The caps look in good condition with no signs of bulging. The motor doesn't seem to be hot. Would an intermittent run cap cause reduced torque resulting in a stall? Any advice from the motor experts on here would be appreciated.

Hi Stuart,

I presume you have done the appropriate checks on bearings/bushes/fuzz and gunge clogging the pump impeller and that the motor shaft turns freely? In which case:

I have a similar pump both in my Solar heated water system and in my solar heated under-floor heating system in my house - both pumps ( they are 2 speed as well) developed running/stall problems late in the 10 years of operation - both problems were due to the motor capacitors - they looked fine, but when measured with a capacitance meter, they had reduced from what should have been around 5uF, the one down to 3uF, the other to a shade below 2uF - replaced both and all is fine again...maybe the electrolyte dried out internally, and they dropped in value as a result

Swap them with new and I am sure they will work fine again.

Happy New Year!

Joe

Thanks Joe, good advice

Concur with Joseph's advice, would add however - ensure that you get the correct voltage when buying the new capacitors.

Same happened on my brother's fan motor on his pool heat exchanger. Both caps replaced, all fine again. Maplin sell motor caps at reasonable price. Get prope r motor caps, same working voltage as existing.

Sorry, forgot to mention, apart from correct voltage and uF value caps, you must use motor RUN capacitors, not Starting Capacitors - the capacitors normally indicate this on the data label. Start caps are not designed to carry continuous ripple current.

Also, don't use Lighting caps - sometimes used for Sodium or mercury vapour lamps for power factor correction - they do not have the same rating as motor run caps either. All the mentioned caps tend to look very similar.

Joe

Maplin is a convenient place to get motor run caps, either in the shop if there is one near you or through the post. The other place to look would be CPC who are generally cheaper than the likes of Farnell (their sister company) and RS Compts.

Caps that use a wet electrolyte will eventually dry out after a few years, depending on how hot they run (local ambient temp and load). But most motor run caps are dry metalised film caps. How old are they?

Certainly sounds as if the caps are not caps any more.

Murray

Edited By Muzzer on 29/12/2017 17:50:04

Pleased to be able to report that all is now working.
For the the small price of £16, 40uF and 10uf motor run caps have been replaced. Hot tub is now back in action.
CPC had the best selection. Interestingly the old caps had a hand written date label on them, 2007, so 10 years service isn't that bad I guess.

A couple of years ago I replaced the motor on my lathe with a VFD system, the motor that was replaced was manufactured by BTH, no idea who made the start capacitor but I do know that the motor and capacitor were at least 55 years old. In that time the motor had been stripped twice, once for a sticky centrifugal switch and the other for a repaint as it's colour didn't match the lathe, (I know I am a sad person). I think you could call that good service.

Phil