RE: [CAD_CAM_EDM_DRO] Rotary phase converter

Thanks Fitch -

Yikes. I was trying to avoid having to set up a phase converter
in order to run a VFD. My lathe has a pretty nice selection of
gearing, and I would be happy with one or the other. The nice
thing about a VFD is that I could easily control the spindle speed
with the IndexerLPT / G-code interface combo. It is my preference
for this reason, although this would require quite a bit of
rewiring to implement the forward reverse and spindle "inching"
controls as they currently exist on the lathe.

The Hitachi specs say that the VFDs are capable of 150% load for
60 seconds. Does anyone know WHY they warn against running a
bigger motor? Couldn't we just pretend the motor was just rated
"conservatively"? The VFD is pretty well protected, and when the
current limit is exceeded, it will fault. It's got enough smarts
to make sure you don't get 5hp out of a 3hp VFD (but 4.5hp for 60
seconds is OK!)

Fitch - could you point me to a 5hp VFD that will run on 1-phase?
It should work on my 20-amp circuit, right?

Another issue - reading up on static phase converters, I found
that they only allow your motor to run at 2/3 rated power because
they only provide 1-phase once the start up circuitry cuts out.
Why don't they do like VFDs and generate 3-phase constantly?

Thanks for the help,

Carlos Guillermo
VERVE Engineering & Design

-----Original Message-----
From: Fitch R. Williams [mailto:frwillia@...]
Sent: Tuesday, January 29, 2002 8:36 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Rotary phase converter

>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.

I would not recommend running a 5hp motor on a 3hp VFD. Hitachi
builds
them, and they know their product, and its limitations. The VFD
market
is very very competitive, cut throat in fact. If they had any
hope that
their 3hp VFD would actually run a 5hp motor with out causing them
to
lose money on warranty claims or create upset customers, they
would sell
it as a 5hp VFD!

You can feed a 5hp 3 phase input VFD from a rotary converter with
a
balanced rotary converter based on a 7.5hp idler motor and it
should
work just fine. You can buy 5hp single phase input VFDs but they
are
more expensive than the three phase input ones at that HP. You
might
get away with feeding it from a 5hp idler if the acceleration
ramps were
programmed for it in the VFD, but it isn't a good idea.

Fitch