Your best bet is to go to the manufacturer's Internet site and hunt down their favorite wiring schemes.
Emerson Electric has an entire page dedicated to every variant of their designs.
The thing to remember with one-phase motors that are cap-start and cap-run is that the cap-run is in the design not to improve power factor at all -- but to suppress harmonics. (torque, noise and vibration)
There are two basic schemes that have been employed.
1) Is the 'either--or' scheme: The motor starts up on the start cap ONLY; then when the motor spools up - - the centrifugal switch cuts out the start cap and then engages the run cap.
2) Is the 'both-- then one' scheme: The motor starts up with BOTH caps in the start winding circuit. Then, when the motor spools up, the start cap is cut out.
The run-cap is always dinky -- 7.5 mfd 10 mfd 12.5 mfd are typical values -- and can be back calculated when missing JUST BY THEIR (footprint) SIZE. Go to ANY capacitor distributor/ manufacturer Internet page and pull off the dimensions. The key one will be the LENGTH. The NEMA players have at most three values in a given length.
Because the run-cap stays in the START circuit, always, it's not only dinky -- it's not critical to the motor, which would run if the start circuit (the run cap) were wire nutted out of the motor running circuit. It's there merely to improve torque and suppress harmonics. Its dinky rating is there to CHOKE back the current flow into the start windings down to modest levels -- lest the wiring be burnt up straight away.
So, for test purposes -- and NO LOADS -- you can power up such a motor with the leads to the run cap nutted off/ taped off in isolation.
A properly working motor would do its thing whether it was design #1 or design #2.
[Keeping in mind that one can conceive of schemes that route critical conductors for the start cap through the run cap location. When this is done, they should become obvious as one can either tug on them physically (direct and short taps) -- and such a link would be a dead givaway that it's a 'jumper.']
The start cap is sized based upon current more than any other factor. So a low voltage motor at low horse power may need the exact same cap as motor with twice the HP at twice the voltage.
All caps are rated by voltage limits -- which are NOT based upon the RMS we're all used to. The figures used are the PEAK voltage availble from AC power... plus some insurance.
Run caps will be marked with a value and a % tolerance on the case.
Start caps will be marked without a % tolerance -- instead they will show a capacitance RANGE right on the body.
So it's a snap to spot one versus the other even if you knew nothing else.
The dinky values used by run caps does not make their caps fantastically smaller than a start cap... though they will always be smaller; say half-off. ( not 93% off )
Like the run caps, the start caps have a limited range -- for a given voltage -- for a given size -- NEMA standards and all.
Because they're only in the circuit for the starting... even being off a tad in the start cap will not be a crippler... though it's always best to get the factory's spec installed. If the motor is older, your best bet is to pick the medium value -- for the cap size that best fits the 'foot print' still evident on the motor.
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You've not listed your tool suite, so I don't know if you can meg out the (motor) circuits. I'd never want to put an abused motor back into service until it was megged.
You might even be looking at a ruined motor that has been salvaged for its caps.
That may mean sluggish bearings, a rotten centrifugal switch, etc.
BTW, if you've got the ability to spin the motor -- even chuck it in your drill motor -- then you can dope out bearings -- and if you can get it up to speed -- safely -- you can test for the centrifugal switch, too.
The second you are dealing with a 'salvaged' motor you need to take it through a fair amount of testing -- at low (instrument testing level) power (no load) -- lest you let the smoke out.
As a practical matter, older motors have only scrap economic value.
Why? They can't compete with the latest units which are so much more efficient that they pay for themselves in less than two-years in active service.
If you're just a hobbyist -- then it's a whole different equation.
I'm not in Alaska, but hereabouts, if it's not factory perfect -- we don't install it. We're not set up as a motor repair shop. Even defective caps are a 'tell' that the motor is at the end of its design life -- as far as we see it.
Last edited by Tesla; 04/24/15 03:19 PM.
