RE: [CAD_CAM_EDM_DRO] Rotary phase converter

Hi Fitch -

I appreciate your advice. I'm stuck on this for two reasons:

1.) A 5hp VFD is $400 (3phase; can't find a 1-phase input yet, but
shouldn't be that much), while to run 5hp with a rotary phase
converter will cost twice that (I know, I could easily build one
myself, but I have way too many projects going already)
2.) I my "3hp" lathe turned out to be 5hp (the seller said it was
3, which seemed right, but both the motor nameplate and recently
acquired manual say 5!)

Thanks,

Carlos Guillermo
VERVE Engineering & Design

-----Original Message-----
From: Fitch R. Williams [mailto:frwillia@...]
Sent: Tuesday, January 29, 2002 11:42 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Rotary phase converter

On Tue, 29 Jan 2002 09:20:49 -0500, "Carlos Guillermo"
<carlos@...> wrote:

>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 rotary converter will allow you to use the inching controls
just
lime the lathe was plugged into the wall. Unless you have a
specific
problem with the lathe speed increments as designed, there really
isn't
much need to use a VFD on a manual lathe. The only exception
would be
if the electrical supply is marginal and the slow speed ramp
behavior of
the VFD avoids installing a higher amperage service.

>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"?

If pretending is what you want to base an relatively expensive
decision
on, go for it. Its your wallet. You pick your pony and take your
ride.
As one of my favorite people, Richard Feynman, once said "For a
successful technology, reality must take precedence over public
relations for mother nature can not be fooled."

>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!)

Its your money. I would not do it.

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

The on line sources I know about are:

http://www.kbelectronics.com/ Model KBVF to 1 hp.
www.dealerselectric.com/
www.automationdirect.com

>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?

For the same reason a VW four cylinder doesn't put out the same
torque
and speed as a Cummins N14. The static converter and the VFD are
totally different products. i.e. because static converters aren't
designed like VFDs. There is a reason a 3hp static converter
costs
about $150.00, and a 3hp VFD costs several hundred. If you look
inside
a static converter you will find a circuit that looks for all the
world
like the motor starting circuits on the Houston Metal Shop WEB
page -
although I can't seem to find the URL at the moment. you can't
make a
VFD out of a voltage controlled switch, a resistor, and a
capacitor.
That is essentially what is inside a so called static converter.