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[Fwd: Re: phase converters]

To: "" <>
Subject: [Fwd: Re: phase converters]
From: Dave Dahlgren <>
Date: Thu, 24 May 2001 06:40:39 -0400
being a machinist of sorts LOL I monitor the CNC machining newsgroup. For all of
you that have shops and are not up to speed on this info I thought I would pass
it along. It is a good collection of sources for getting a new or used 3 phase
machine running when you only have single phase power.
Dave Dahlgren

-------- Original Message --------
Subject: Re: phase converters
Date: Sat, 19 May 2001 06:44:03 -0700
From: Fitch R. Williams <>
Newsgroups: alt.machines.cnc
References: <1103_990220932@sheridan>

<sheridanREMOVETHE@CAPSgrapevine.netPLEASE> wrote:

>I may purchase an engine lathe and bridgeport from the company that I work at. 
>I need enough three phase power to run 3hp@8 amps. Have any of you used the 
>phase converters and /or rotary phase converters on the market? I see lots of 
>plans out there too, has anyone built a converter from scratch?

I have built several from 3hp to 20 hp.  The machines you are
discussing are not listed as CNC machines which means the input
voltage tolerances are not going to be particularly fussy.  If the
lathe is a 3hp gear head lathe with out a clutch, you may want to use
a 7.5hp motor if you intend to plug reverse it for metric threading
purposes, or want it to start easily when set to its top speed.  If
the lathe is 2hp and the mill is 3hp, you will get along fine with a
rotary converter based on a 5hp idler motor.

Rotary converters are very forgiving - if you do something as simple
as spin the shaft to start the idler, then plug it into 220V single
phase, you will get enough three phase out of it to allow your tools
to run at around 75% of their rated power.  If you tune it up
carefully, not hard to do, I can talk you through it via e-mail, you
won't be able to notice any substantive difference between running on
a rotary and from commercial three phase power.  

There is another option which is to buy your self a couple of the
readily available single phase input, three phase output Variable
Frequency Drives (AKA VFDs).  I have a Hitachi SJ100 on my Lagun mill
and it works well (Programming them is like dealing with a mugger in a
dark alley though!).  If you don't intend to do metric threading on
the lathe, the VFD is a nice way to go because it gives you
essentially continuously variable speed from 20% of the lowest normal
speed to about 150% of the highest normal speed - although caution is
urged when running the machine faster than it was originally geared to

The mill has more options.  If it is a 16 speed belt drive type (not
variable speed) you can use:

1)  A so called "static converter" which is in reality a motor starter
that starts the motor by phase shifting, and then runs it on single
phase.  You will get around 60% of the usable motor hp with out over
heating.  This is inexpensive, and works well if all of the following

        a) you don't want to plug reverse for power tapping
        b) you don't load the mill heavily
        c) it isn't a variable speed drive which can be harder to start.

2)  A single phase in, three phase out VFD (sources for obtaining
these are listed below.  This is really nice if the machine is a 16
speed machine because it gives you the ability to cover a large speed
range on one of the middle belt settings.  It will allow the motor to
develop full torque at nearly all rpm settings from around 25% to 100%
of normal, and full hp from 100% of normal rpm up to around 175% of
normal rpm.  Note, full torque and full hp are only the same thing at
the designed rpm of the motor.  The only caveats are that these drives
do not work well if you want to plug reverse for power tapping.  You
may have to do some rewiring of your mill - you must turn the machine
on and off with the converter, not the normal stock switch.

3)  A rotary converter with an idler motor hp of at least 1.5 times
the mill motor power rating.  The rotary converter, if it is set up
correctly, will be for all practical purposes indistinguishable from
running your mill on commercial three phase power.  It will plug
reverse for power tapping, and all the normal wiring and switches will
work fine.

For excellent information on making your own rotary converter see: and

If you don't want to make one, see:

GWM's web page is:

ARCO Roto-Phase is:

Phas-A-Matic is:


ZZZAP converters are at:

MPC (Martin Phase Converters) is at:

An on line directory to makers of rotary converters is at:

There are a number of articles in DEJA News also.

Sources that some on RCM have used to buy VFDs are:  Model KBVF to 1 hp.

If you have questions after reviewing the above information, I will
try to answer them.  Of use in understanding your application is the
combined hp of the motors you need to run, the hp of the largest motor
you need to start - probably the spindle motor - whether or not you
have any voltage regulation requirements for CNC, etc.

In So. Cal.

The FAQ for RCM is:
Metal Web News at      
The "Drop Box" is at

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