On Dec 28, 2:20*am, wrote:
On Fri, 28 Dec 2007 13:27:12 -0800 (PST), willray


Looking for some advice on running a piece of equipment with a 3-phase
motor. *It's tagged at 200V, with a nameplate that says "rerated to
above specifications by G.E."
...
P.S. *Any suggestions on "soft starting" a 15HP idler, such that I
could realistically spin it up on my new shop's 200A feed
(and preferably on a 100A branch circuit), would also be delightful.


* * * * * *Reasonable overvoltage is no problem. The no load
losses will increase but it will draw slightly less full load
current and run cooler.
...
* * * A useful insurance is to check that the measured full load
line currents are not significantly higher than the rated full
load current.

I knew there was a reason I liked this place :-) Of course, I can
do that - or at least approximate it - not sure I want to get close to
this thing in an "as designed' full-load context - it's one of those
"toss you across the shop without even slowing the cutter down"
kinds of tools, but I certainly can easily test any sane and
reasonable
load that I'll ever apply.

* * * A sure fire idler soft start system is to use a small pony
motor to prerun it up to half speed.

I've been thinking in that direction (minor difficulty in that my 15HP
idler is a purpose-built idler, and doesn't have a stub shaft outside
the housing, but I expect that I can fix that), but I'm a bit worried
about idiot-proofing the setup. Any thoughts on preventing the
phase-converter electronics from _trying_ to start, unless the
idler is spun up? Not like it's a huge problem if it just blows the
breaker on start up if some idiot (most likely me) hits the start
button
without spinning the idler up with the pony motor, but the purist in
me thinks that using that as a safety is inelegant.

Can't use a potential relay to cut over, since it won't reliably
generate
much until L1 and L2 are provided... No centrifugal starter switch
in the idler... I feel like I'm overlooking something trivially
obvious, but
currently apparently have turned stupid...

I briefly considered setting up a VFD to spin the idler up,
then rigging it to power off without braking, and power up the
converter electronics, but by the time I can afford a 15HP VFD,
I might as well just use it to pretend that it's the utility service,
and
run the whole shop from that - excepting the fact that I don't know
if 15HP is oversize enough to start the 5HP Rivett, nor whether VFDs
do well pushing step-up transformers for the 440 stuff, and blowing
up a 15HP VFD would be, umm, painful...

Thanks again,
Will Ray