start and run Capacitors (again)

[Dany]

Hello.After my last posts about wiring of the two caps on my metal buffing/pollishing machine its been working great doing sterling service nearly every day or a few hours on an off per day..until yesterday when I noticed it felt slightly underpowered.no overheating or burning smells...(just ever so slight.but noticeable when pushing the stainless steel i was buffing into the sisal wheel.. it was slowing down alittle under the load... where before it wouldnt flintch)
Switched off yesterday as normal and this morning i went to start up to find it back on slow start(same as when i first tried wiring with one cap)...just turning slowly.
I disconnected a cap (both caps are on same points) and tried it...wouldnt turn..just made a noise....disconnected other and connected first and it was running slow again.So I figure the one of them isnt healthy..Markings on the smaller one is 30uf...other cap is bigger-but writings rubbed off so I havent a clue what it is...Im assuming its 45uf as the plate on the motor shows 1-75uf C(run ) ...Anyway to cut a long story short after trial and error useing a spare 50uf cap i had(unknown if its 100% functioning also..?)the motor is working with the 3 caps fitted which got me through todays work- but its still not 100% full power.(with the replacement 50uf and the larger cap it would run fast and slow down gradualy after afew secs to 1min - but with all 3 it remains constant speed-but more like 90% power).Question is..Im ordering a pair of new caps for nextweek ..what size should i go for as the motor is still new use but seems the extra load for its application means it needs more juice?I await your advice boys.thanks

 

[Marvo]

Okay, playing around with the values of the caps is a bad idea. Just stick to what the original caps were and go with like for like replacements. If a motor has two caps in parallel to act as one large run cap then they're usually identical caps ie if the run cap value should be 70uF they'd generally use 2 x 35uF caps in parallel rather than say a 40uF and a 30uF. It worries me a little that your motor is an exception to this rule. If you get replacement capacitors make sure the discharge voltage is the same as the originals as well, run caps are usually around 450-500 volts. I'd suggest you replace both caps together even you have reason to think that only one of them is faulty.

The other thing you need to establish is what is the rating of this motor, is it rated for constant use at the power requirement of your machine or is it rated only for intermittent use? Another factor that needs consideration is how many stops and starts per hour is your motor rated for? Enormous amounts of heat are generated in the rotor, the stator and in the capacitors when the motor starts so many motors have a limit to the frequency with which they should be started. The limit is usually lower for single phase motors than 3 phase motors.

Finally given that it sounds like the caps or at least on of them has gone toes-up I'd probably take that as a cue to just drop one of the covers off the motor and visually check the windings. If they're anything less than bright copper coloured and pristine looking I'd take it to a rewinders and get it tested properly.