capacitor sizing

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bill@usps06492

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How do you determine the proper size capacitor for a motor?
This this a single phase motor,(frame 6-56-6, code-k) using 208vac@7.5amps and 1725rpm. The motor is dual voltage, low@115vac/15.8amps, uses a single cap for starting and also has a centrifugial switch. There is no old cap to use as a sample.
Any ideas?
Thanks Bill

[ November 26, 2004, 05:06 PM: Message edited by: bill@usps06492 ]
 
Re: capacitor sizing

If you can read all of that information off the nameplate, you should also be able to locate the manufacturer. Have you tried giving them a call? Or is this one of those old motors for which the manufacturer went out of business long ago?
 
Re: capacitor sizing

If you have a motor shop that repairs and rewinds motors they will be able to help you. just take the motor to them and they should be able to tell you the right starting cap for it. I have had to do this a few times. While you have it there have them make sure the motor is in good shape too.

[ November 28, 2004, 02:00 AM: Message edited by: hurk27 ]
 
Re: capacitor sizing

Is there any rule-of-thumb computation for this problem?

Do you really have to bring the motor to the repair shop?
 
Re: capacitor sizing

Most motors of that size use a capacitor in the range of 100-200 microfarads.

I know of one way to find the correct capacitance, but it requires that you already have a capacitor on hand to begin the tests with.

You need to have access to both capacitor leads as well as the two leads for the start winding. Next lock the rotor in a vice and connect volt meters across the capacitor and start winding. Briefly apply power to the motor and record the voltages. When a correct capacitor is found the voltage across the capacitor should be 10% higher than the voltage across the start winding.
 
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