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Sizing start caps on electric motor

Old 03-05-2016, 02:51 PM
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Default Sizing start caps on electric motor

I have come into possession of a 2 HP Air compressor with a bad Start capacitor.
It is an older 1975 year model Quincy wired for 120V and I do not know if the cap is original or not.

It had a 800-872 uf 125V one in it, but all I had laying around was a 500 so I put that in for now.

I would like to rewire to 240V instead of 120V as the unit pulls 26 amps @120V.
The inrush captured on my amp meter shows 84 amps.

Need to know what size it should be using at 240V and 120V incase I want to rewire for 240V or keep it 120V.

Anyone have any info on this?
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Old 03-05-2016, 03:51 PM
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125uf x 360v is my guess
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Old 03-05-2016, 04:20 PM
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Quincy is still in the business of making compressors. Have you asked them for technical help?
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Old 03-06-2016, 06:34 AM
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Originally Posted by yarcraft91 View Post
Quincy is still in the business of making compressors. Have you asked them for technical help?
Thanks for the responses.

I tried but no response, I figured it was because it was so old.
May have to try them again, as I do not know much about the pressure oiling system on the compressor and need to have some idea of what I should be doing to make sure it is set correctly.

I contacted an Electrical Engineer I know for his input, but he said he handed it off to an A/C guy he knows as he deals with single phase start caps a lot more so maybe he would have some info for me.
He deals with mainly large 3 phase high power equipment/systems
But that was a couple of weeks ago and still no response.

I thought the THT group might have someone that had this knowledge just waiting on a question like this.

Seems there are very knowledgeable answers on here most of the times
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Old 03-06-2016, 06:47 AM
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Here's the problem, as I understand it.

There isn't a "right" size for a start capacitor that can be calculated. Instead, companies that make electric motors test different size start capacitors on their motors under various mechanical loads. The goal is to find a capacitor big enough to start the motor effectively, but not larger. The optimal capacitor sizes at 120 VAC and at 240 VAC are usually not the same and knowing the right capacitance value at one voltage isn't always easily translated into the right value at the other.

I have to believe Quincy has the answers you need, you just need to talk to one of their senior techs who has been around long enough to know their older compressors like yours.
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Old 03-06-2016, 07:27 AM
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Did Quincy make the motor? Usually these guys use someone else's motor on their pump.
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Old 03-06-2016, 08:25 AM
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Originally Posted by yarcraft91 View Post
Here's the problem, as I understand it.

There isn't a "right" size for a start capacitor that can be calculated. Instead, companies that make electric motors test different size start capacitors on their motors under various mechanical loads. The goal is to find a capacitor big enough to start the motor effectively, but not larger. The optimal capacitor sizes at 120 VAC and at 240 VAC are usually not the same and knowing the right capacitance value at one voltage isn't always easily translated into the right value at the other.

I have to believe Quincy has the answers you need, you just need to talk to one of their senior techs who has been around long enough to know their older compressors like yours.
this is true. The hp does not mean that much , rather starting load. My guess was based on that type of equipment. After trying a cap you need a good amp tester with high lock so you can see starting amps. then you adjust cap to bring down starting amps of too high.
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Old 03-06-2016, 12:10 PM
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Yea, most of the stuff I found when looking online said to follow the recommendations on name plate, but there is nothing about caps on name plate.

It is a Doerr name brand motor full load amps 27 at 120V and 13.5 at 240V

As I said before,
the inrush my ampmeter records is 84 amps @120V with a 500uf start cap and compressor running at 100 psi it is pulling 26 amps@120V.

I guess I could buy a bunch of different caps to test it myself, but it would be nice if I could find out what it should be instead of spending money on caps I will not use
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Old 03-06-2016, 12:22 PM
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If motor has a centrifugal start switch and cap is on the start winding, then cap is only in play for starting. If it picks up the load quickly, cap is "good enough" Once cent sw opens, cap is out of circuit.
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Old 03-06-2016, 12:52 PM
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Is there only one cap or two?

Here is a link that might help

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Old 03-06-2016, 03:19 PM
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Originally Posted by Corndog38 View Post
If motor has a centrifugal start switch and cap is on the start winding, then cap is only in play for starting. If it picks up the load quickly, cap is "good enough" Once cent sw opens, cap is out of circuit.
I understand, but that does not help sizing if I rewire to 240V.
If I leave it wired as 120 v ,I guess I could try something closer to the 800 I pulled out that was bad, I just thought I could save money if I found out what it was suppose to have when new as I have no idea who or when that 800 was put in
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Old 03-06-2016, 03:32 PM
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All the motors I've rewired from 120 to 240 or reverse, it did not require changing the cap. Just follow the reconnection diagram. That older motor, not sure.

I think there would be no harm in reconnecting it for 240 and trying it with a cap on hand. If it has a cent start sw and it starts ok, then you are good to go.
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Old 03-06-2016, 03:41 PM
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Originally Posted by mikeloew View Post
Is there only one cap or two?

Here is a link that might help

http://www.youtube.com/watch?v=daWoXnajbAU

Edit to comment on voltage.

the start cap is rated at 125VAC, so hate to put it on 240v


Only a start cap,
and I have been a electrician for most of my working life.
well for the last 20 years of it anyway, a mechanic and then plant operator before that
I have been retired now for the last 10 years
I was dealing mostly with high voltage 3 phase equipment/systems just like my EE friend, but was exposed to other systems and equipment through the years too.
We did have at one time a motor shop in the plants that took care of big and small motors that I could have contacted, but they went away years ago too.
Most motors when small were cheaper to by new if you could, but special motors had to be rewound by specialty shops some times.
Not a lot of single phase stuff was used when 3 phase was so much more efficient
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Old 03-06-2016, 04:29 PM
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Originally Posted by Corndog38 View Post
All the motors I've rewired from 120 to 240 or reverse, it did not require changing the cap. .
Exactly..if the motor is dual (or multi) voltage full voltage wire it appropriately and the cap shouldn't matter.
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Old 03-06-2016, 04:52 PM
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Voltage ratings on the caps matter,
and the higher the voltage the less uf is needed from everything I have seen
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Old 03-06-2016, 05:21 PM
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Microfarad rating changes the phase shift of the start coil, which helps starting. Volt rating is a limit, does not change electrical characteristics other than how long cap lasts. The actual voltage at cap terminals depends on how the motor windings are connected and tapped. Voltage at cap may or may not change when you switch from 120 to 240. Only way to tell for sure is to meter the cap during a start with it configured for 120 and again for 240.

Cap volt rating will not change start performance, unless it fails by being too low a rated voltage.
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Old 03-06-2016, 07:00 PM
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Originally Posted by 99yam40 View Post
I have come into possession of a 2 HP Air compressor with a bad Start capacitor.
It is an older 1975 year model Quincy wired for 120V and I do not know if the cap is original or not.

It had a 800-872 uf 125V one in it, but all I had laying around was a 500 so I put that in for now.

I would like to rewire to 240V instead of 120V as the unit pulls 26 amps @120V.
The inrush captured on my amp meter shows 84 amps.

Need to know what size it should be using at 240V and 120V incase I want to rewire for 240V or keep it 120V.



Anyone have any info on this?
26A @ 120V?

Thats not a motor its a space heater!
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Old 03-06-2016, 07:27 PM
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If that motor is pulling 26Amps it will overheat very quickly. No wonder the starter cap. is shot. 26 amps it more like 4hp. I bet that compressor had a 5hp motor on it and someone put a 2hp motor on it then sold it because the motor kept burning out the capacitor and/or overheating,
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Old 03-06-2016, 08:41 PM
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If motor is overloaded, and it is at 26A as noted above, you can put a smaller dia pulley on motor shaft and that will reduce motor load.
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Old 03-07-2016, 11:13 AM
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FLA on motor name plate is 27 at 120V so it is within the specs on nameplate while running at load.

I did some switching around in breaker panel and have it running on a 30 amp breaker right now, but would prefer to get it wired to 240V, if I can come up with a spec for start cap.

I sent a message to Quincy again and I may send one to Emerson electric motors. I believe they are part of them or bought them(DOERR) out, from what I gather on line
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