Re: [CAD_CAM_EDM_DRO] Rotary phase converter

In a message dated 29-Jan-02 00:05:00 Central Standard Time,
carlos@... writes:

> Are rotary phase converters compatible with equipment that also runs other
> systems (such as coolant pumps, brakes, CNC controls, etc) off of the 3
> legs,? Would the phase converter be trying to start and stop every time
> the spindle motor was powered on and off, and in doing so affect the other
> systems? I've never used one, and I'm trying to figure out how and when
> they "self-start". Can someone please explain? Also, can I run a 5hp
> motor at 3hp with a 3hp VFD? The Hitachi literature warns against running
> a motor rated higher than the VFD's rated HP, but I can't understand why.

Someone else in this group will surely be willing to confuse you with more
information/detail that I have on this topic, but I will nose-in a bit,
hoping at the outset that I do not LIE about something, and confuse you worse
than I should:

Think of a rotary converter as a "rotary transformer". It does have a
rotating armature "in there", but that does no "outside work" like a motor,
except the little bearing-friction that heats up the bearings and end-bells.
And a little "electrical heating" due to the fact that copper and magnetic
parts are not 100% efficient. If "perfect", you could connect up a
phase-converter, "turn it on", and let it run 24/7. But this is kinda akin
to those old turn-of-the-Century factories with overhead line-shafts and flat
leather belts hanging down for each machine in the buildings. That is, in
practice, one should have some kind of electrical lashup that would start the
converter only when needed, not leaving it running when not actually using
the supplyied 3-phase. Now, if you had rigged it when you got it to ONE
three phase machine, and had some "came with" switchery to turn it on ONLY
when that original 3-phase load wanted to run, and then, later, bought a new
3-phase machine, it would be silly/inefficient to hafta turn ON that original
3-phase machine just to spin the phase-converter, so you could run your NEWER
3-phase machine! That is, if you have more than the one 3-phase load, you
should have a separate switch to turn-on the phase-converter ONLY and
WHENEVER needed. Common sense. Right?

Of course, too, if you have a phase-converter that is rated for powering a 5
hp motor, you must never turn-on TWO 3-hp motors at the same time, and
loading both to their full 3-hp capacity, as this clearly "presents" a 6-hp
load to the po' struggling li'l ol' phase converter! And/or will pop its
breakers and/or rivets, fuzes, etc., if present. They ARE there, arent'
they?

MY solution to all this was to PAY the power-company to drop 3-phase lines to
my new shop in the backyard when we moved in here in '85. Whew. I learned
later that same year that, pay or not, they would no longer supply
three-phase to a residence, no matter IF the transformers on the pole were
already there (as they were in my case, due to a private school with about 50
tons of HVAC running on 240 V.-3-phase "open-delta").

What is "open delta"? Is there sufficient interest/need-to-know in this
group to ask, or allow me to respond? TELL me. (USA, only!!!)

Jan Rowland, Old Troll with Attitude



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