On Wed, 22 Aug 2007 21:13:25 -0700, Grant Erwin
wrote:

I use my Hand switch when I just want to turn it on for a minute to show
someone how quiet my compressor is, or to see if the belts are flapping,
or just to check things out. Does seem a little strange now that you
mention it, though, but I've seen a bunch of 'em done that way.

Grant

It's NOT the normal way of doing it - you know to only use it for
testing, but someone else wandering in off the street could do very
bad things playing with the "Hand" setting on an air compressor.

The only practical reason I can see is if the compressor has been
rigged with a constant-run unloader system, and you manually switched
it over from the 'pressure switch' mode to the 'unloader' mode to do a
lot of high air demand work. The motors are only rated for so many
starts an hour, and short-cycling can and will burn up the windings.

Unloader mode operation is more useful in commercial applications
where you pay a demand charge on the power usage, and the idling
current of the unloaded compressor motor is more than offset by not
making start surges all day and bumping up your demand multiplier.
But it can also be useful if your power supply is marginal and you
need to weld/plasma cut or run the lathe at the same time you use air
- the start surge on top of the welder current will trip breakers.

They usually use a Hand-Off-Auto switch on things like water wells,
pressure boost or sewer lift pumps - for when the floats fail, or you
want to avoid short cycling and just let it run.

Oh, and a wiring issue for the Original Poster - When you wire the
motor starter coil, you also have to loop that circuit through the
overload block switch contacts - otherwise the motor won't stop if it
single-phases and trips out an overload heater, and the Magic Smoke
will escape from the motor.

And it's REAL expensive to get fresh Magic Smoke installed in an
electric motor, it's usually cheaper to get a new one.

-- Bruce --