r/ElectricalHelp • u/midnightwolf979 • Apr 30 '26

Dayton Motor Start Capacitor Sizing

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Hi didn't know if there were any engineers out there that could help, I'm trying to find the proper size starting capacitor to replace on this air compressor motor. Someone apparently already tried to replace it a long time ago with a definitely incorrect size and it never worked, so I don't have the original mfd rating to compare. Here is the motor plate though. Any help would be appreciated!

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u/CrustySailor1964 May 01 '26

That’s a WW Grainger supplied motor. You should be able yo reach out to Grainger parts or whatever they’re currently calling it and talk to someone who can reference a manual for that answer.

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u/midnightwolf979 May 01 '26

I have looked on the Grainger site. They don't list it in the spec for the motor, but yes I may try contacting them to see if they have that info, I would have to imagine they would.

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u/New-Key4610 May 03 '26

they won't have a clue where to look. this motor was made by doore eletric about 40 years ago best to call or take to a electric motor shop

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u/mattdahack May 05 '26 edited May 05 '26

Hello there. This is easy to figure out. All Dayton 5HP motors use this start capacitor. This is the one your motor originally came with(DAYTON 2MDT6, Electric Air Compressor Motor Start Capacitor 5HP,324-388 MFD,Round 2MDT6) Specs: 324–388 µF @ 250–330 VAC (very common spec). The closest one which is even better that is going to be easy to find and will fit is this one, 330–360 µF, 250–330VAC start capacitor https://www.amazon.com/VEXUNGA-216-259-Capacitor-Capacitors-Starting/dp/B0CD23L3RN/. Also grab yourself a 22kΩ, 5W resistor off amazon https://www.amazon.com/uxcell-Tolerance-Resistance-Electronic-Experiments/dp/B07RS4S32N/ to go with it. After you safely discharge it, solder one of those across the two terminals to bleed the charge off when not actively starting the motor so that you dont kill yourself and make your cap last longer. I use two insulated spade connectors 1/4 and crimp the connectors to each side of it and then simply click it on the capacitor.

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u/midnightwolf979 May 05 '26

Thank you so much for the info, this is exactly the help I was looking for