[Shop-talk] Air Compressor ?

[Randall]

> If I run 
> it down to 150psi and the compressor attempts to start up the 
> magnetic relay trips on the compressor. If I let it deplete 
> to 25 or 50 lbs it will start right up and cut off at 175psi.
<snip>
> The compressor is always free wheeling, in other words there 
> is never a load on the compressor when it starts up.

If you are certain the unloader & check valve are working (meaning the tube
between the compressor and tank has zero pressure when the compressor
starts); then my first guess would be excessive resistance in the supply
line to the compressor.  The current draw while the motor is starting will
be considerably more than 40 amps, probably more like 70-80.  And if it
doesn't get enough current while starting, it won't be up to speed when the
load picks up.

Try hooking an AC voltmeter to the supply at the motor, and watch what
happens just before the relay trips out.  If it's sagging by more than 15
volts or so, I'd say you need to upgrade some wiring (or find the bad
connection).

Another possibility might be a starting capacitor that has lost part of its
capacitance, especially if the compressor was purchased used.

One workaround, if you can't find another solution, would be to add some
more tubing to the run from compressor to tank.  That wastes a bit more
energy every time the compressor stops, but gives it more of a chance to get
up to speed.

Randall