# Compressor



## Beden469 (Oct 11, 2009)

Have a air compressor with 30 amp breaker and 10 wire 70 feet. Compressor rating 25 amp start. Have 233 volts to compressor. When kicks on drops to 176 volts. Draws 34 amps trips breaker. Is this because motor is going bad or is there a possible other issue? Any thought would be appreciated.


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## Big John (May 23, 2010)

What size compressor? Check to see if you've got a stuck unloading valve that is causing a hard start and abnormally high starting current. Otherwise, having a 27% voltage drop might be the cause of the hard start, in which case you'd have to work on getting a stiffer source.

The #10 isn't long enough to be the cause of the voltage drop by itself, so you can check the voltage drop possibility by looking at the source that feeds this circuit. If turning on other high current loads causes similarly severe VD, that's your problem.


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## Beden469 (Oct 11, 2009)

Have buck boost transformer that is putting out 266 volts. Nameplate on compressor is 208-230. 5 h.p. Motor


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## Cow (Jan 16, 2008)

What is the buck boost for? What's your service voltage?


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## Beden469 (Oct 11, 2009)

Incoming getting 233 volts. Just saying drops under load of compressor to 176 at compressor.


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## wendon (Sep 27, 2010)

Beden469 said:


> Have buck boost transformer that is putting out 266 volts. Nameplate on compressor is 208-230. 5 h.p. Motor


What's the KVA rating of your transformer?

What's the amp rating? Is it 25 amps @230 volts or 208?
Like John was stating, it's probably an issue further downstream. I'd check the voltage on both sides of the buck-boost when it's starting and also when it's running. I'd also upsize the wire. 

Just some rough figures 25x230=5750 watts
5750/176=33 amps


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## Big John (May 23, 2010)

I assume this is 3 phase? 

A) Has it ever worked?
B) Is it belt driven?
C) Can you check the unloader?
D) Why is there a buck-boost on it?


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## Beden469 (Oct 11, 2009)

Your right big john. Stuck valve. Thanks to all of you for the replies


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## Ultrafault (Dec 16, 2012)

Wait don't go. Where is this buck boost?


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## Beden469 (Oct 11, 2009)

I took buck boost off. They put one on because they had 176 volts at compressor after start up. I metered it it was 233 volts before trans. Questioned why the trans. They said because volt drop at start up. i checked amps and thought it was little high so reset and tried for better reading and it was insane. Took belt off and worked perfect. had to look up relief valve big john posted about and checked it. Was stuck put back together and is good.


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## Beden469 (Oct 11, 2009)

They had at compressor.


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## Beden469 (Oct 11, 2009)

Messed up part is when I mentioned unloader big John mentioned he said I just changed that.


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## Awg-Dawg (Jan 23, 2007)

Big John said:


> I assume this is 3 phase?
> 
> A) Has it ever worked?
> B) Is it belt driven?
> ...



For those of who dont know, what does the belt driven matter?

Edit, I should have kept reading. I understand now.


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## John Valdes (May 17, 2007)

What size is the breaker?


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## JRaef (Mar 23, 2009)

Beden469 said:


> Your right big john. Stuck valve. Thanks to all of you for the replies


I think that probably 90% of the time someone tells me that a compressor is not starting or causing too much of a voltage drop, it's the damned unloader valve.


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