Author Topic: Electric fan motor run capacitor voltage?  (Read 232 times)

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Offline wilfred

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Electric fan motor run capacitor voltage?
« on: June 14, 2018, 08:29:03 pm »
I have an electric fan that has started to become slow to start. Sometimes it can take 5 minutes to get going. Twisting the shaft without the blades attached seems to spin easily so I don't think it is in need of lubrication. I did that two years ago.

I took out the motor run capacitor (1.5uF 450VAC) and measure it as 1.24uF. Could that be a cause of it being slow to start? BTW it starts fine if it has just been running.

Looking in my local electronics shop I notice some motor run caps are 450VAC 425VAC and some are 250VAC. Are there varying means of specifying the voltage rating? Should I avoid the 250VAC one? And will an increase to 2uF cause much of a problem in an electric fan application. I have read that it will cause a loss of torque and run hotter but is that mostly for higher power applications?

 

Offline ArthurDent

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Re: Electric fan motor run capacitor voltage?
« Reply #1 on: June 14, 2018, 10:08:52 pm »
A motor run capacitor should have a voltage rating of at least 1.5 times the mains voltage so a 450VAC capacitor is reasonable if you have 240 volt mains. As far as measuring the capacity, using a low voltage tester will probably give readings that are on the low side so your reading of the value looks o.k.. The capacitor is probably oil filled and could outlast the life of the motor so I would dismantle the motor and really clean the bearing surfaces and reoil. The gradually slowing down is probably caused by the lubrication turning to a coating on the shaft over time cause by temperature. Just reoiling without cleaning probably won't cure the problem.
 

Offline wilfred

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Re: Electric fan motor run capacitor voltage?
« Reply #2 on: June 14, 2018, 10:39:57 pm »
Thanks @ArthurDent. When I re-lubed it I did give it a thorough clean. When It needed lubrication it was clear due to the fact it would come to a stop within a few revolutions. Now once it does get started it spins down eventually. I don't think it is lubrication that's needed.


Edit: The replacement capacitor I ordered from China on Ebay has sorted out the slow startup. The fan was 9 years old and operated from 253VAC. The old cap measured 0.8uF down from the 1.5uF spec.
« Last Edit: July 28, 2018, 09:23:02 am by wilfred »
 

Offline Gyro

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Re: Electric fan motor run capacitor voltage?
« Reply #3 on: June 15, 2018, 04:38:59 am »
Self-healing of a Polypropylene film cap (which is almost certainly what it is) over time can increase its impedance (and therefore reduce AC current handling) without necessarily showing up as significantly in a simple capacitance measurement. It certainly wouldn't be the first time that replacement of an apparently 'just a bit low' motor run capacitor had completely solved a poor starting problem.

You really want to look for a 1.5uF one, it's a fairly common motor run value. I believe the rule of thumb is to keep the capacitor marked value within 10% or so of the motor indicated value.
Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline Gregg

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Re: Electric fan motor run capacitor voltage?
« Reply #4 on: June 15, 2018, 12:52:55 pm »
It could be that the shaft has worn into the bronze bushings making a fairly tight fit until the fan gets up to speed and the shaft tends to climb out of its worn groove.  If feasible, try turning the fan upside down to see if it starts normally; i.e. so the shaft is initially only making a point contact at the bottom of the sleeve resulting in minimal initial friction.  If so, maybe you can unmount the motor and turn it upside down for a few more years of service.
 


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