Start Capacitor Value for Compressor Motor

Thread Starter

SLK001

Joined Nov 29, 2011
1,548
I have an air compressor whose motor is missing the start capacitor. I need to get a capacitor to see if it is worth fixing or just salvaging the parts. The motor is a DOERR 3HP, 1-PHASE, 230V motor mounted on a Craftsman twin cylinder compressor. The compressor was originally purchased in 1972. Here is a picture of the entire motor plate:




MOTOR_PLATE.jpg
Now DOERR was bought by GRAINGER in 1969 and later sold to EMERSON. Where it went from there is anyone's guess. I thought that the LR-22132 number on the plate represented the motor model number, but it doesn't - that number appears on quite a few different motor types and horsepowers.

I thought that the capacitor value would be printed on the motor plate, but it is not. Does anyone know what value to put in?
 

Thread Starter

SLK001

Joined Nov 29, 2011
1,548
Is the circuit a resonant circuit, or will it hurt to go over the recommended value?

Why "Run Rated"? It's not a "run" capacitor, but a "start" cap.
 

MaxHeadRoom

Joined Jul 18, 2013
20,860
There are two types of motor start capacitors, motor start, and motor start-run, the former is only in circuit for a few seconds and is typically a large uf value.
The start run is in circuit the whole time the motor is running, and is quite a bit smaller value.
If experimenting with values, it pays to have a current meter on hand.
If there is only one capacitor for the compressor, it is most likely a start-run version.
Max.
 

BobaMosfet

Joined Jul 1, 2009
1,117
I have an air compressor whose motor is missing the start capacitor. I need to get a capacitor to see if it is worth fixing or just salvaging the parts. The motor is a DOERR 3HP, 1-PHASE, 230V motor mounted on a Craftsman twin cylinder compressor. The compressor was originally purchased in 1972. Here is a picture of the entire motor plate:




View attachment 211912
Now DOERR was bought by GRAINGER in 1969 and later sold to EMERSON. Where it went from there is anyone's guess. I thought that the LR-22132 number on the plate represented the motor model number, but it doesn't - that number appears on quite a few different motor types and horsepowers.

I thought that the capacitor value would be printed on the motor plate, but it is not. Does anyone know what value to put in?
It will probably be a capacitor like you see on HVAC- they are quite large, as this is a current issue. Some other info I found:

1594752046021.png

Grainger has all kinds:

https://www.grainger.com/category/motors/motor-supplies/capacitors-and-accessories/capacitors
 
Last edited:

Thread Starter

SLK001

Joined Nov 29, 2011
1,548
This capacitor is a start only cap - it is switched out once the motor is up to speed.

So, using 12A as the full load amps and multiplying by 2,650, then dividing by 230, the rated supply voltage, I get 138μF as the value. So a 130-158μF cap should do the trick. Do you guys agree with my calculations?
 

Thread Starter

SLK001

Joined Nov 29, 2011
1,548
Okay, I'll give it a try.

Another question, is what is the cover for the capacitor on the motor called? Any place someone could pick up a match using dimensions? I'm afraid that I'm going to have to fab my own.
 

MaxHeadRoom

Joined Jul 18, 2013
20,860
Usually made by the manuf. at build time, I know of no source for them in particular.
Generally not a 'Lost' item!
Use/adapt a metal can of some kind, cocoa tin etc. ;)
Max.
 
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