On Mon, 10 Sep 2007 13:38:58 -0700, Bruce L Bergman wrote:
On Mon, 10 Sep 2007 11:21:13 -0500, Ignoramus2883
wrote:

On Mon, 10 Sep 2007 09:11:36 -0700, Bruce L Bergman wrote:

Hey, that helps the power situation if they are put in constant-run
mode and cycle on the unloaders, then there's only one start surge per
shift. And it's those essentially locked-rotor start surges that kick
the demand factor through the roof, and trip that Main Breaker that's
teetering on the edge.


An electronic soft start drive (or a VFD) could help here, right?

i

When you start a motor from a dead stop it draws the full rated
locked rotor current for a few hundred milliseconds till it gets
started rotating, and then the current ramps down over the next three
to five seconds as the motor gets itself and the load moving and up to
speed.

When you pay for commercial power with a demand factor, there's a
demand meter that records the highest peak momentary amperage you
draw. Looks like a big speedometer dial on the meter face, that's
what they use the reset arm in the middle of the glass for... And
they add a multiplication factor to your overall KWH billing rates
based on the peak current you have drawn - this is to compensate the
utility for supplying the oversized transformers needed to accommodate
those current spikes, the transformers still draw current to energize
the windings even if you only draw loads for a short period per day.

A motor drive can only help so much - it can regulate the LRA
current to an extent, but it still takes a big grunt to get the load
spinning. And the heating problem is still there.

Soft Starts are practical for motors that are expected to have short
starts, like elevator pumps - but it's just as effective and less
money to use a "Delta-Wye" reduced voltage starter on those motors.
They get the motor spinning on Wye 277V and switch to Delta 480V
running when up to roughly 80% speed.

Short cycling kills motors fast because they don't run long enough
to cool off from the start surge heating, then they stop and the air
cooling stops, then they are asked to start again with the windings
still hot. Do that a few times, and the Magic Smoke (TM) escapes.

The trick on a constant-run compressor is there's only one start
surge when you turn it on at the beginning of the day, usually with
nothing else running. After that the compressor just opens the valves
and spins idle, and the current draw drops off. (Not to zero, but the
lack of start surges makes up for it.)

Bruce, forgive my ignorance, but let's say that a compressor has an
unloader valve (even better, electrically controlled). Then the motor
can be slowly ramped up to speed using a electronic drive, with the
unloader engaging at the proper moment, right?

My compressor is single phase, but my Bridgeport is 3 phase, on a VFD,
set to accelerate over 1 second when turned on. I am sure that it does
not draw anywhere close to the LRA when accelerating that slowly.

What am I missing?

i