Electric Motors :: Not For Air Compressor Use

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.
 
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
 
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.
 
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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