8 pole 1/4 hp single phase induction motor - capacitor sizing

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Feb 13, 2012
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Hi folks

Inspired by our friend on the 'bench grinders' thread, I have been digging into a pile of induction motors I was given to sort out and sell for a charity I support.

There are a couple of nice old Brook Gryphon jobs, all the way from Huddersfield, which are 1/4hp 8 pole 710-ish rpm things. There is no centrifugal switch. One came with a 10uF capacitor lashed-up by a previous owner, and another with none at all. With no cap they won't start - or rather they might wobble a bit and run off in either direction! With 2uF again things are iffy, but 4uF seems to get them going reliably, again on no load.

The start windings are apparently terminals Z1 and Z2, and the supply goes to terminals marked A1 and A2. The capacitor is connected across terminals A1 and Z1 all the time, and A2 and Z2 are joined by a brass link. Swapping the internal winding wires to Z1 and Z2 (but leaving A1 and A2 alone) reverses the motor, as you might expect.

Most of the motors I have dealt with are those with no cap, and start windings which are switched out by a centrifugal switch. So I am unsure how to size what seems to serve as both a 'start' and 'run' cap which will be in service all the time - I imagine it might also influence full load torque. Any suggestions from folks with more experience than me?


(Edited to sort out my terminal labelling - I made a pig's ear of it first time around.)
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