On Tue, 7 Oct 2008 01:28:51 -0400, "DrollTroll" wrote:
wrote in message
...
On Oct 6, 5:57 pm, Don Foreman wrote:
On Mon, 6 Oct 2008 11:43:28 -0400, "DrollTroll" wrote:

No, I hadn't. But it is an idea. About 5 hp.

Certainly caps will be cheaper, but wiring relays, start/run is going
to be a pain as well. Plus, I will have to de-rate the motor, and
possibly put a smaller pulley on it.

Keep in mind that this is a scenario where the motor is starting under
near-full load, which might be some electrical wear/tear on a VFD.

At least I *think* it is starting under full load (tank pressure).
Is there a way to set up an air compressor so that the motor starts
under no load? Only with a clutch, I would imagine, or "almost
no-load", where some vent bypasses the compressor head to air, or
sumpn, and then closes. So much drama....

Many (if not most) compressors do not start under full load. There is
an "unloader" in either the pump output or in the controller.

Methinks that an 'unloading check valve' is a standard commercial
item, useful for smaller compressors. This valve is placed between
the compressor and air receiver. After compressor shut-off this valve
bleeds off the pressure between it and the compressor, allowing the
compressor to start against atmospheric, instead of tank, pressure.

Is the loud air exhaust noise when the compressor shuts off this "unloading"
valve at work?

Yes, that hiss is the unloader, it's usually built into the pressure
switch and has an 1/8" tubing to the output check valve right at the
tank. The check valve keeps the air from coming back from the
receiver, and the unloader valve relieves the pressure in the output
tubing between the compressor and the check valve when the pressure
switch cuts the motor.

The length of the output tubing gives a time delay before the
compressor starts working against the main receiver pressure - and the
delay can be lengthened with longer output tubing or a small
accumulator chamber made from large pipe.

Or if your VFD has auxiliary contacts, you can put a solenoid valve
that vents the output tubing until the motor is up to speed.

-- Bruce --