On 2010-03-01, Dave H. wrote:

"Richard W." wrote ...
"john" wrote...

[ ... ]

I would not recommend using an underrated VFD. I would spring for a
single phase motor of about 5 hp or so and eliminate all your other
electrical problems. It would probably be cheaper in the long run and
would also make the lathe easier to sell.

I would run a rotary phase. My P&W has a 7.5 HP motor and it works just
fine.

I was thinking of a VFD as a measure of future-proofing as at some point I'd
like to add a smallish mill to the toolkit, and rotary convertors take a
fair amount of real-estate - not something I'm particularly blessed with
(SWMBO would rather have other luxuries and space for our motorcycles in the
Big Shed!) - and it looks like there's not a lot of difference on price. A
single-phase motor in that power bracket is probably going to cost more than
the VFD, too, and would still leave me with around half the original
horsepower and needing a rewire for the "shop" power...

You're thinking of a "static" converter for the reduced
horsepower. Make a rotary converter and take the time to balance it
with tuning capacitors, and you can get full horsepower from the tool's
motor.

And it looks as though you are looking at *new* prices for the
idler motors used in the rotary converter. Get a used motor from
someplace like an industrial HVAC contractor who pulls out used
equipment while installing new, and will usually be glad to sell you the
pulls. You don't even care if the shaft keyway is messed up, because
all you want out of this is *electrical* power, not mechanical.

The primary place that the VFDs win is in giving you variable
speed when you don't already have it (or smooth variation when otherwise
you have to stop the machine and shift belts or gears.

Good luck,
DoN.

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