On 2008-09-29, Vernon wrote:

[ ... ]

Don and everybody.

The motor in the plenum blower is a Baldor "Super E". If I correctly
read the data plate with my head stuck into an awkward backward
position the specs a 7.5 HP, 3ph, 1750rpm. It draws 20 amps at
230v and 10 amps at 460v.

O.K. 20A from a real three phase line -- but about 24A from
single phase.

And this is in the range of what can sometimes be found at
semi-reasonable prices. I have a 30A VFD which can and does run from a
30A single phase 240V line to power thing rather smaller than 7.5 HP.

I got it (by luck) at a hamfest some ten years ago or so for
$115.00 IIRC. Quite reasonable, anyway.

But I've acquired more (more reasonably sized) VFDs to have a
separate one on each machine, because you really don't want to have
switches between the VFD and the motor -- though that big 30A one is
surviving a switch between it and the 1HP motor on my Nichols horizontal
mill.

Actually, I'm somewhat relieved that it's not 15HP. As one of you
mentioned, I was concerned about the power consumption of a 15HP idler
motor.

And the current necessary to spin it up at the start.

Plus, that's bigger than anything I am likely to need to power
in the near future.

And bigger than you could really run from your breaker box
perhaps?

While I like the idea of the VFD converters I need to do this on the
cheap for the moment. Would one of those "motor ready" boxes on the
market be cost effective? It looks like the box wouldn't be more than
about $200 - $250. Even if I could build it cheaper the convenience
would be worth a lot to me right now.

If it is designed for a motor of your 7.5 HP range, why not.
You may still want to add capacitors for tuning and for equalizing the
power factor to keep the current through your circuit breaker panel down
to reasonable levels and avoid nuisance trips.

Good Luck,
DoN.

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