Motor Pros: How do you size a start cap???

[Davefr]

Does anyone know how to choose a start cap when the old one is missing and there is absolutely no data to be found anywhere?

Is there any formula based on motor amperage, etc?

This is for an old Milwaukee V-121 7" Bench Grinder with centrifical start switch that energizes a start cap circuit during start up. (cap is missing)

I did some experiments with a Turbo Cap:

At 5 MFD - Won't start
At 10 MFD - sluggish start
At 20 MFD - starts fine
At 25 MFD - starts strong

Should I just go with 25 MFD?? (at 330V rating)

 

[KellyfromVA]

I'd say that if you could fit a 25 MFD 330V cap onto the motor in a way that is safe, that should be fine. I assume this is a 120V motor? If so there is no harm in using a higher-rated voltage for the capacitor.

 

[pattenp]

I just replaced a cap on an old 1/4hp 115V motor on a table saw and the cap it used was a 115V 138 MFD. I bought this one as a replacement..... http://www.galco.com/buy/NTE-Electronics/MSC125V124

I found this basic rule for MFD sizing....

This is not a FIX all, but a basic method for motor capacitors.

(2650 x Full load amps ) divided by supply Voltage = MFD

Example 2650 x 8amps = 21200 divided by 240volts = 88.33 MFD

 

[Davefr]

I just replaced a cap on an old 1/4hp 115V motor on a table saw and the cap it used was a 115V 138 MFD. I bought this one as a replacement..... http://www.galco.com/buy/NTE-Electronics/MSC125V124

I found this basic rule for MFD sizing....

This is not a FIX all, but a basic method for motor capacitors.

(2650 x Full load amps ) divided by supply Voltage = MFD

Example 2650 x 8amps = 21200 divided by 240volts = 88.33 MFD

Thanks! This is helpful. The nameplate says 6 amps and it's 120 VAC so that would imply 133 MFD.

It actually measures 3 amps under no load. Assuming no load at startup, (probably reasonable for a grinder), then that would imply 66 MFD.

Since it starts good at 25 MFD there must be lots of margin in start capacitor matching.

I think I'll try a bench test at close to 133 MFD and if it still starts strong I'll settle for that value.

 

[pattenp]

I just checked my 6" Delta with a 1/4hp motor. It's 120V - 3.5 amps and it has a 125V 100 MFD cap.

 

[nehog]

... there must be lots of margin in start capacitor matching.
...

The way they are made you will often fine a +100/-20% tolerance on values for electrolytic capacitors. So a capacitor that is rated as 50 microfarad may be between 40 and 100 microfarad.