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What Size Start Capacitor Do I Need?

bobinyelm

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I was given a supposedly good 220v single phase 5 hp compressor motor but it has the start capacitor missing.

What value capacitor and voltage rating should I try for it?

This is the label on the motor if it helps:

motor.jpg
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Thanks,
Bob
 
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Dagny

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I'm in the business so it would be easy for me. I keep around 90 in my service truck. I would start with a 250 volt 3 to 4 hundred micro farad.
 
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bobinyelm

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As memory serves I called IR a couple of years ago when I got the compressor. The pump itself was toast and had no compression at all.

I'd picked up 2 IR compressors, one a T-30 80gal that weighed like 600# with a 3 phase 7.5hp monster motor, dead (the pump was fine) and this 60 gal 5hp single stage for which I am seeking a capacitor. I rebuilt the 7.5hp unit w/ a new 5hp Century motor and a new Harbor Freight cast iron V-Pump on the 80 gal tank.

I plan to use 60gal tank w/ the existing IR motor and another HF V-Pump, which seem to be pretty decent pumps for the $$.

They told me that the 5hp unit was discontinued because though the motors and tanks were fine, the pumps all self destructed so they no longer offer parts or literature on them. From the pulley sizes, it appeared they ran the pumps at something like 1600-2000rpm and they were all aluminum (no mention of sleeved cylinders). I know I can pick up the pump with the pulley using my pinkie finger.

It apparently only used one capacitor because the single metal can for it (missing) would only hold one, and there are only 2 wires coming out of the case into where the capacitor can was located (a somewhat surface-rusted area outlining where the can sat).

Worth calling again, though. I get WAGs of anywhere from 175mf to 400mf, so I guess it's not a simple "formula", but depends just how the motor was wound.

If I can't get an answer from IR, is it terribly critical? In a compressor, the motor starts basically unloaded, since the pressure switch unloads the pump pressure line initially, unlike some motors that have to start under load, like a pool pump it would seem.

Bob
 
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bobinyelm

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No wiring diagram on case unfortunately.

Just the label shown and 2 wires coming through the case for the capacitor.
 
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Bert_

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This is really a 3hp motor.

I looked up a baldor motor and it lists a 216-259mfd capacitor for a 3hp 3450 1ph motor. Seems like a good place to start.
 
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LS6 Tommy

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I'm in the business so it would be easy for me. I keep around 90 in my service truck. I would start with a 250 volt 3 to 4 hundred micro farad.

How do you know what size start cap the motor requires? You can't oversize too much without causing damage to the motor and undersizing will definitely kill the motor.

Tommy
 
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bobinyelm

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This is really a 3hp motor.

I looked up a baldor motor and it lists a 216-259mfd capacitor for a 3hp 3450 1ph motor. Seems like a good place to start.

You say the motor is a 3hp, but the decal on the case says 5hp, though with the 1/2" motor shaft and only one capacitor provided for, while most 5hp have a start and a run capacitor.

Could you tell me how you determined it's a 3hp?

Thanks,Bob
 
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bobinyelm

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Here is a guide on selecting the right size Start and run capacitors. It also has links for different motor brands. https://inspectapedia.com/electric/Motor_Capacitor_Selection.php

My 5hp IR motor is newer than yours and was made in Mexico by Emerson.

I can post photos of mine as well as cap sizes if it helps. I put new capacitors in mine early this year.

Sure, thanks.

You mention that yours has "capacitors" as in 'more than one' while mine has only one capacitor and one set of wires.

Another poster said mine is a 3hp, though the decal says 5hp SPL (not sure what SPL stands for).

Bob
 

wyliesdiesels

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Yours has an inflated HP rating.

Spl means special

The way Bert figured 3HP is by the 15a FLA rating. A 5HP motor would have a higher FLA.
 

Bert_

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Most common 5hp 1 phase motors will pull 22-25 amps. There are some premium efficiency motors made in the last few years that are as low as 19A for a real 5hp motor. Your motor ain't one of those.

15A is a 3hp motor. Spl is just a way they can put a BS rating on it to make it look better than the competition. Kind of like the 6.5hp shop vac that runs on 120v...
 
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