Thread: Of induction motors and starting capacitors

Let me get the formalities out of the way first

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So, that should shorten this thread appreciably!

The other day I was using my 8" long bed jointer, a Woodman with 2hp motor (made in Taiwan 1991), bought s/h, when without warning it stopped dead.

No strange noises, no magic smoke, nor expensive smells. On checking the RCD (this thing has a 15A plug) it had tripped.

Time to investigate, putting the multimeter across the input plug revealed a dead short between active and neutral. Inspection for physical damage to the cord, both outside and inside the cabinet, revealed no problems.

Time to further investigate. The 2hp motor is an induction motor with a big capacitor, again putting the multimeter across the active and neutral input terminals on the motor revealed a dead short.

Opening the cover over the, rather large (85mm long x 45mm diam), capacitor pointed to a very likely cause. The top of the capacitor had come off completely, revealing a smelly mess. It was a 400mfd, 125V capacitor, first phone call to an electric motor place struck gold, they had one in stock same electrical specs and same physical dimensions, mine for $20.
When I queried the guy on the 125V bit, he told me that it was quite normal for start capacitors to have this voltage rating, in fact he showed me a brand new 2hp motor on the shelf that had the same capacitor.

Coming home, installed new capacitor (after changing connectors from lug to female spade), fingers crossed switched on and bingo, worked like a bought one.

At least I didn't have to buy a new motor