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Old 01-10-2011, 09:29 PM   #1
Iroc-Z
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Default Mag Starter for an air compressor?

I just picked up a used air compressor. Its a American IMC two stage compressor. It has a 5 hp electric motor. The compressor date is 1992. I am going to give the compressor a clean up and get it ready for service. My question is how do you know if you need a mag starter? I am not 100 percent sure what they do. Do some electric motors come with internal mag starters? Any info would be great.
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Old 01-10-2011, 09:57 PM   #2
Charles (in GA)
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Default Re: Mag Starter for an air compressor?

Does the motor have a over temp pop out button on the end opposite the pulley?

Some good pics will help.

Being a two stage, the motor probably is a true 5 hp and may very well need a starter. The reason for one is two fold, first, the pressure switch would have to be huge to carry the amperage of the motor, so instead, a small, low amp rated pressure switch pressure is used and it operates the coil of the motor starter (simply a solenoid operated multiple contact switch) and the second need for a starter is that is contains a thermal overload, in the form of a heat element that is sized to the motor, so that if the motor is overloaded (such as the pressure switch sticking and the pressure going way too high) the heat element would get hotter than normal and cause a set of contacts to open, killing the circuit to the motor and stopping it. It simulates the motor overheating.

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Old 01-10-2011, 10:03 PM   #3
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Default Re: Mag Starter for an air compressor?

Charles,
I will take a look tomorrow. I will also snap some good pics of it so you can see what I am dealing with.
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Old 01-12-2011, 09:14 PM   #4
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Default Re: Mag Starter for an air compressor?

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File Type: jpg Giz 001.jpg (139.7 KB, 75 views)
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File Type: jpg Giz 007.jpg (136.3 KB, 65 views)
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Old 01-12-2011, 09:24 PM   #5
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Default Re: Mag Starter for an air compressor?

Single phase motor. Its not using a starter. You sure thats a 5 hp?

Went to their site and I guess it really is. They have it wired through the pressure switch. Another reason not mentioned why you need a starter is to pull in all three legs in one shot when you have three phase.
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Old 01-12-2011, 09:45 PM   #6
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Default Re: Mag Starter for an air compressor?

Another thing I was thinking is this compressor looks to have spent most of its life without a mag starter. I just don't know if its worth the investment.
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Old 01-12-2011, 09:58 PM   #7
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Default Re: Mag Starter for an air compressor?

If it doesn't spec one from the factory you don't need it.
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Old 01-13-2011, 11:44 AM   #8
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Default Re: Mag Starter for an air compressor?

Cool thanks.
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Old 01-13-2011, 10:11 PM   #9
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Default Re: Mag Starter for an air compressor?

That is a "true" 5hp motor rated for compressor duty, with all those capacitors it's an "energy efficient" unit that draws maybe 22 amps. These are common on light commercial compressors like this one. The pressure switch is rated to handle the full current draw of the motor without needing a mag relay. If ever you need to replace the pressure switch, make sure to get one rated for the current draw.
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Old 01-13-2011, 10:16 PM   #10
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Default Re: Mag Starter for an air compressor?

That motor does not appear to be thermally protected so some provision needs to used in case of over current.
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Old 01-13-2011, 10:23 PM   #11
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Default Re: Mag Starter for an air compressor?

Mines similar....just wire those leads to a HD switch box to switch it On/Off. Also be sure to wire a ground wire.



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Old 01-13-2011, 10:25 PM   #12
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Default Re: Mag Starter for an air compressor?

Quote:
Originally Posted by Elroy View Post
That motor does not appear to be thermally protected so some provision needs to used in case of over current.
It likely has an internal - on the other side of the elect motor.
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Old 01-13-2011, 10:32 PM   #13
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Default Re: Mag Starter for an air compressor?

Maybe it does. Maybe it doesn't

Regardless it needs over current protect. Fuses or a stater or a thermal over. You got to have something or you really are playing with fire.
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Old 01-14-2011, 09:45 AM   #14
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Default Re: Mag Starter for an air compressor?

Would that be what the red button on the motor is for?
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Old 01-14-2011, 10:52 AM   #15
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Default Re: Mag Starter for an air compressor?

Quote:
Originally Posted by Iroc-Z View Post
Would that be what the red button on the motor is for?
Yes! Generally most electric motors with multiple capacitors on it (the round humps or caps on top and side) do not need a magnetic starter. The capacitors do all the work. Just wire it up as found and use an appropriate size breaker.
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Old 01-14-2011, 11:26 AM   #16
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Default Re: Mag Starter for an air compressor?

Quote:
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Yes! Generally most electric motors with multiple capacitors on it (the round humps or caps on top and side) do not need a magnetic starter. The capacitors do all the work. Just wire it up as found and use an appropriate size breaker.
Thanks guys. As you can see it needs some love. I will start a thread on its restoration soon.
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