In article et,
Karl Townsend remove .NOT to reply wrote:


I'd first check the no-load power consumption of the idler. If it's 7
amps at near 0 power factor, it doesn't cost much to run. Just
watch the electric meter for a little while with the idler being the
only load running. That (and a bit of calculation) will tell you
cost per hour of the idler's overhead.

I get 15 cents per hour(7amp*220Volt*$0.10/KwHr), or about $54 per month, or
$270 per season as my estimate.

Do you have a power-factor correction capacitor on the rotary
converter? What you do is connect AC run capacitors between the two
sides of the 220V line as it enters the rotary converter, measuring the
current in one of those legs (between the circuit breaker and where you
add the capacitors), until you get a minimum current. Past a certain
amount of added capacitance, you will start to increase the current
again.

Anyway -- the current at the minimum will be pretty much the
"real" current, and any above that will be "imaginary" current. (They
add at 90 degrees, so you can't really just add them, but for most power
services, the "real" current is all that you are charged for, and the
benefits of tuning the power factor are mostly less heating of the wires
and less chance of nuisance trips at the circuit breaker (if your "real"
+ "imaginary" current takes it close to the trip point.)

I've got no way to check power factor. Just
watching the meter, you can see it turns way more than the load from all the
lights being on in the barn.(1200 watts total). If the VFD has virtually 0
power use at no load, there is enough savings to install it here.

It does, indeed.

The problem with using a single VFD for driving several machines
is that you are then tempted to switch the power to the individual
machines between the VFD and the machine, instead of setting things up
so the machine commands the VFD to apply power or not to the machine.

Switching between the VFD and the machine increases the risk of
damaging the output circuits of the VFD with the switching spikes. I've
gotten away with it when using a 1 hp milling machine on a 7-1/2 hp VFD,
but I'm not sure how long I will get away with it even with that big a
difference between capacity and load.

Enjoy,
DoN.

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