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## Starter capacitor calculation

Can someone tell me how to calculate the starter capacitor i need for any motor?

And in addition, if someone know how to do it for the run capacitor, i'll be glad to know.

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Start caps range from 80-120 MFD per hp.The rating need not be exact anything close will work within a 15%+/- margin.

Run caps,don't hold me to it were 10-15 MFD per hp,these are a little more finicky,too low and you lose toque,too high and the motor heats.

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Thanks guys, i didn't expect you to answer so quickly. I was checking the movements in the forum, and i think that is tha way a forum must be. You're in constant contact. You're great. My congratulations.

By the way, i've got left a doubt. You say the range is from 80-120 MFD per hp for stater capacitor and 10-15 MFD for run caps. But in message in the link there's someone who says 20 MFD for half hp on 240V, that means 40 MFD per hp.

About what cap is he talking about? And you're telling me the capacity but not the voltage, doesn't that matter?

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That is a run cap value I do believe,but he is refering to is a motor that uses the same cap for both start and run.
The value I gave for the run cap is from foggy memory and may indeed be off,but if your motor has a seperate starting cap the run cap value will be lower than what's used on the motr in the link.

Give me a second and I'll look at one of my vendors web pages that might have an accurate value.

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The only thing I find is that the start cap range is +/-10%(little tighter than my number)and the run caps are "tried in different combinations"by the manufacturer to achieve the greatest efficiency and least amount of heat.

You may simply have to get a selection of run caps and experiment,or if the company that made the motor is still in business maybe run the part number of the motor by them.

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Thanks, you have been of a great help. Thanks for your effort.

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Ups, i was forgotting something. How do i know if my motor use the same capacitor for start and run?

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<font face="Verdana, Arial" size="2">Originally posted by wierdscience:
Start caps range from 80-120 MFD per hp.The rating need not be exact anything close will work within a 15%+/- margin.
Run caps,don't hold me to it were 10-15 MFD per hp,these are a little more finicky,too low and you lose toque,too high and the motor heats.
</font>
That looks about right, Wierd. I just took a shifty at the plate on my Chevalier 1.8kw (2.2hp) single phase and the start is 300mfd, the run is 35mfd.

ken

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<font face="Verdana, Arial" size="2">Originally posted by Iodem:
Ups, i was forgotting something. How do i know if my motor use the same capacitor for start and run?</font>
If it uses the same cap for both,it will be a run cap,which most times is an aluminum cased affair and is of the lower MFD ratings.

On rare occasions you will find a cap that looks like a run cap with three terminals and two sperate MFD values.One side of the cap will act as a high value starting cap and the other as a low value run cap.These are most times found on AC compressors.

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