# Electric Motors :: Not For Air Compressor Use



## abrace (Dec 24, 2016)

All,

            I was looking th the Grizzly catalog today and their electric motors. They all have a disclaimer in the description that says 'Not for Air Compressor Use'. I know a compressor puts some unique strains on a motor, especially when the tank is partially full and the motor fires on and the compressor doesn't have an unloader valve, or it is stuck. That said, I am no motor expert and am curious. What is it about air compressor use that makes these Grizzly motors unsuitable? As long as you pick a motor with the right horsepower, why wouldn't it work?


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## markba633csi (Dec 24, 2016)

Good question and I believe it has to do with starting torque, especially when the pressurestat is set to restart with only a slight drop in tank pressure.
Mark S.


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## Ulma Doctor (Dec 24, 2016)

another consideration is operation temperature.
air compressor motors are usually rated for continuous use


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## Keith Foor (Dec 25, 2016)

Two biggest issues with a compressor are high starting load and run time.  
For the average Joe and his garage compressor.  He airs up bike tires for his kids.  Occasionally will add air to a car tire and once in a while he will use his handy blow gun things as he refers to it to dust off his garage floor when his buddies are coming over to drink beer.

then you have clowns like us.  
We fire up a compressor and will connect air die grinders to it and run it for hours as we knock off welds, polish and surface parts we have just machined.  Blow the swarf (bad idea) off our mill.  Connect it up to our plasma cutter and cut out parts. Then its back to the die grinder to clean up the edges.  And the hole time the compressor is pumping away.  If not continuously very close to it.  Machinists use a ton of air in the shop.  We really don't even think about it if we have a big enough compressor.  If we don't, then we notice as we are waiting on the compressor to recover enough to go back to work.  I burnt up a craftsman compressor twice (pump failure) porting and polishing a set of heads.  Mind you it was one of the oil-less designs but it still didn't live long running it that hard.  I still have it and still use it but I am mindful of how hard I run it and how long.  

So basically their motors are not rated for continuous duty.  Figure if you put that motor on ANY other piece of machinery, how long and often will it get run?  We fire up our lathes and mills and maybe run them for 5 or 10 minutes and then shut them off while we change tools, run in the house to grab something to drink.  Walk away to answer a phone.  Point is that no other machine in the shop save a fan for air circulation gets run as much as a compressor.  And those motors are not up to the task.  Buy a Dayton or a Baldor and be done with it.  Yes they are expensive, but sometimes you get what you pay for.


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## jim18655 (Dec 25, 2016)

I read through the listings for the Grizzly motors and couldn't find a reason. Continuous duty motors were and weren't listed for compressor, capacitor start and capacitor start/run-  same way and then reversible and non-reversible some were OK and some not OK. Go figure. Might have something to do with inflated HP ratings on some of them. Starting torque might also be a factor.


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## Keith Foor (Dec 25, 2016)

jim18655 said:


> Might have something to do with inflated HP ratings on some of them. Starting torque might also be a factor.




Yeah, I swear they are rating motors now buy power consumption and not shaft RPM and torque.  
And the reading they use is with a stalled shaft at startup and it's a measurement of inrush current to boot.


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## JimDawson (Dec 25, 2016)

I think most compressor rated motors have a service factor rating of 1.25.  They also may have a better centrifugal switch than lesser motors due the the expected high cycle rate.  Baldor, Dayton, or Leeson would be my choice for a replacement motor.


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## Downunder Bob (Dec 26, 2016)

Starting torque is the main problem plus duty cycle. If you're going to run your compressor a lot get the better motors as recommended by others. however if your compressor is only going to be used for occasional short term use, then buy the cheap motor and fit an unloader valve for starting.


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## cvairwerks (Dec 26, 2016)

According to a Leeson book I glanced thru, compressor duty means that it will supply up to 300% at start, of the running torque.


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## master53yoda (Dec 27, 2016)

the Start capacitors on a compressor duty motor are substantially higher MFD and some also drop out the start winding at a higher RPM.
Both these items require a different start winding.  The do deliver substantially higher torque on startup. conventional motors will start a empty compressor but will have trouble with one that is at operating pressure. The only piping that gets relieved by the pressure switch is the  inlet pipe to the tank and about 5 strokes of the compressor has it to full pressure so the motor is truly starting under load.

Art B


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## Downunder Bob (Dec 27, 2016)

master53yoda said:


> the Start capacitors on a compressor duty motor are substantially higher MFD and some also drop out the start winding at a higher RPM.
> Both these items require a different start winding.  The do deliver substantially higher torque on startup. conventional motors will start a empty compressor but will have trouble with one that is at operating pressure. The only piping that gets relieved by the pressure switch is the  inlet pipe to the tank and about 5 strokes of the compressor has it to full pressure so the motor is truly starting under load.
> 
> Art B


That is why unloader valves are fitted to most large and some small compressors, if setup correctly it will completely eliminates the problem

Mostly it's only cheap compressors, "made to a price ", that don't use unloader valves. 

A simple unloader valve can be fitted to any compressor by using a solenoid valve in the pipe from the compressor to the tank that opens on startup and remains open for a few seconds, whatever it takes to reach full speed. This valve dumps the air to atmosphere for a few seconds while the compressor runs up to full speed. Make sure you also have a non return valve at the tank inlet.


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## master53yoda (Dec 27, 2016)

that is a good explanation of the unloader valves,  the compressors that are used like the large commercial  Quincys have mechanical unloaders that port enough air that they don't close until the compressor is up to speed.  but even the newer lower priced Quincys don't have them.  What i have done before is tee in a piece of 2 inch pipe about a foot long into the pipe between the compressor tank and the compressor it gives a large enough reservoir for the motor to get up to speed, it does make a fair amount of noise when they shut off and vent the connection piping.

Art B


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## Downunder Bob (Dec 27, 2016)

master53yoda said:


> that is a good explanation of the unloader valves,  the compressors that are used like the large commercial  Quincys have mechanical unloaders that port enough air that they don't close until the compressor is up to speed.  but even the newer lower priced Quincys don't have them.  What i have done before is tee in a piece of 2 inch pipe about a foot long into the pipe between the compressor tank and the compressor it gives a large enough reservoir for the motor to get up to speed, it does make a fair amount of noise when they shut off and vent the connection piping.
> 
> Art B



Yes large air compressors in commercial installations have unloaders, usually built into their inlet valves, In multi stage setups, they will also have unloaders of the solenoid valve type at the intercoolers between stages. Quite frequently these larger systems will have two or more compressors ganged together, the lead unit will not shut down when the system is up to pressure, but will keep running with the unloaders opened, until the system pressure drops enough to load them up again, this is because of the very high starting current even with unloaders. The other machines will stop and start as required using the unloaders.

Your 2"x 1 foot pipe will help some. but unless you have a way to release the pressure from the pipe when it shuts down, you run the risk of it starting again with a high pressure already in the pipe, especially if it's short cycling. It's also a waste of energy, you've already used the energy to compress the air and then you vent it, much easier to vent it at atmospheric pressure.


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## master53yoda (Dec 27, 2016)

bobshobby said:


> Your 2"x 1 foot pipe will help some. but unless you have a way to release the pressure from the pipe when it shuts down, you run the risk of it starting again with a high pressure already in the pipe, especially if it's short cycling. It's also a waste of energy, you've already used the energy to compress the air and then you vent it, much easier to vent it at atmospheric pressure.



the 2" pipe is teed in between the tank check valve and the compressor so it gets vented by the pressure switch,  some of the small compressors have what looks like a muffler or cooler attached to the discharge it serves the same purpose as a startup reservoir.  agreed it is a loss in startup efficiency, but for those compressors without any start up reservoir it works.


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