suggestion - buy a VFD and an analog ( variable resistance) pressure gauge.
follow the VFD circuit example to have a "reference" pressure - the VFD will
then act as a servo amp of sorts and as the pump comes to the ref pressure
it will slow it down, and if it exceeds it, it will slow it more - so it
will keep running, but stay near the ref pressure. this will save lots of
wear and tear and also save you the wasted energy (and noise) of an unloader
dumping air into the, well, "air" making lots of hissing noises. your cost
will be modest, any VFD supplier can help you choose a suitable control




"Tom Gardner" wrote in message
et...
My air supply is in stages. One 7.5 hp 3-phase 230v Quincy can usually
carry the load. It come on at 90 and runs to 120, or there about. If the
demand is bigger, another Quincy gets put on line. I also have a 10 hp.
Hydrovane as a back-up. I have yet another Quincy in reserve. I hate not
having air! We're looking at #1 that seems to be about 50% duty cycle
with just less than 1 minute off time. We're wondering if we would be
better off financially running the motor continuously and unloading the
valves at high pressure.

One of my ideas was for the lowest mechanic to catalog every air leak and
repair them...they add up quick. It seems we can go about a year before
they become an issue.




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