On Thu, 10 Jul 2003 06:47:54 -0400, Brian Lawson wrote:
Hey Gary,

Nice little treatise on the VFD versus rotary convertor. Thanks.

Does anyone know what the IDLING current draw is for a rotary where
the idler motor is in the ranges you speak of, typically 5 to 15 HP?
My lathe motor is 5HP , so I expect I'll be needing a 7.5 HP rotary
convertor idler motor, but it turns out a 15 HP wound rotor may be
more readily available to me. I've wondered about which way to go
for long term economy. My hope for the proposed new shop is that if
I go rotary, that I'll "power up" the convertor sometime in the early
part of the day, and it will run for hopefully 5 or 6 hours on a
regular basis. I can see this helping a bit to heat the shop in the
winter, but cost even more to cool it in the summer, but I'm wondering
just about the costs of operating it.

And so my question is, can anyone give us the amperage on their
particular 220/240 line feeding their particular rotary when there is
no "machine" operating from it? And some details as to what sizes
everything is?

Just knowing the current won't tell you the power. That's because
at idle most of the current is reactive, ie the power factor is far from
one. The easiest way to know the actual energy consumption at idle
is to watch the watt-hour meter with everything off but the rotary.
In other words, see how much the reading changes after an hour
with nothing running but the idler.

I think you'll be surprised at how little energy is consumed. System
losses are mainly due to windage, bearing losses, and resistance
losses of the wiring. Windage (air drag) is typically the largest loss.
Using the lowest speed motor you can find will minimize this.

Gary