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Capacitor Start Motor Question

Capacitor Start Motor Question

Capacitor Start Motor Question

(OP)
I have a fairly new 5hp belt+disc sander. Was using it last week when it started to smoke. Went to shut it off and it kept running, so I pulled the power cord. Opened the cover up for a look see and noticed an oily residue. Concluded that a capacitor blew. There is a start cap, a run cap and a black mystery box with connections to the capacitors and the power switch. The sander was fixed under warranty (start capacitor replaced) but I remain curious as to why the thing kept running and would not shut off with the power switch.

RE: Capacitor Start Motor Question

Is the black mystery box perhaps a relay that stuck?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

RE: Capacitor Start Motor Question

If the start cap blows, and does not short out and cause a breaker to trip, the motor will slow, until the "start switch" (a centrifugally-operated switch typically) turns on and switches in the start cap. If the run cap is missing/fried the start cap is not always enough on its own to get the motor speed above the speed to cause the start switch to switch out. Sometimes it is, and the motor will cycle on and off, as the speed goes above and drops below the start switch tripping point.

RE: Capacitor Start Motor Question

Could be it has an external start relay and it stuck on. Was it the start or run cap that toasted? Hence it not shutting off.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Capacitor Start Motor Question

I have seen quite a few failure modes of small motors, but given the information supplied in this instance, I would have to see the motor before venturing an opinion.
The information given may be correct, but it is pushing the envelope of coincidence way out of shape. Some information may be missing.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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