running 3 phase motors from single phase...

From: Jim Arnott <>
Date: Tue Mar 30 08:20:49 2004

I KNEW I saved this for a reason...

Everybody knows that 3 phase power is expensive to get, and most homes
have single phase (110 or 220) power. But 3 phase motors run smoother,
power is constant, so they would be nice to have. You can even get
ahold of
them used for free/next-to-nothing *because* Nobody Has 3 Phase Power!
can buy some very expensive electronic converters, but why bother?
You can run a 3 phase motor from standard 220 single phase power.
Really. It
is an old technique, known mostly only to old tinkerers and the like.
It works like this:
First, you get the 3 phase motor turning it (manually, or better, with
a small
110 v motor), and THEN turn on the 220 (connected to two legs) it will
run. It
will not run at rated power, or smoothly, but it will run (at speed).
This is
OK for some machines. Now for a magic trick.
        Take a second (free, same size or smaller) 3 phase motor, and connect
it to the first (three leg switch, and zing!, the second motor turns on
instantly, and both motors run smoothly! And with more HP than off of
The first motor is acting like a "generator" to provide the third leg.
This is
not "full three phase power", but it works quite nicely. The 220
power to both motors, with one more wire to connect them. You can can
additional motors also.
        This works best if the first motor is a larger HP (2 vs 1), or higher
speed rating (3400 vs 1750) than the slave. Older, "beefy" motors are
preferable. A 3 HP, 3400 RPM motor works nicly. The reason for this is
if you try to start a bigger load than your master motor, you may
reverse the
rotation direction of the master (3 phase can be wired to run either
way). A
large/fast motor will have enough momentum to resist changing
The more motors you get running, the more stable the system becomes. The
limit may be the amount of power drawn through the third leg of any
motor, or your switch, etc.

        A real world, functioning system:
master motor: 3 HP, 3 Ph., 3400 RPM
Starter "pony" motor- 110, 1/2 HP, 1 Ph washing machine motor
Slave Motor- 3 Ph, 1750 RPM punch press or lathe. (Insert your machine

        The Washing machine motor and the master are mounted to a board/bench,
and their shafts are connected with a flexible coupling.
Power to them is switched with a Double Throw, Double Position, CENTER
switch. The 3 legs of the master also go to a 3 pole switch, To the

   (DP/DT/C off Switch, shown (Three phase
    in 110 V motor on position) Shown off)
           SWITCH 1 SWITCH 2 /
220 leg1 o--------------------------------+-----------/ o---->
>--------o\ o |
             \o |
                                               | /
220 leg2 o---------------------------+----------------/ o---->
>--------o\ o | | To
             \o--------| | |
                       | | | /
Neutral | | | |--------/ o---->
>--------o------| | | | |
                 | | |--------------|
              |------------| | 3 HP, 3 Ph. |
              |(110V Motor) |==[]====| Motor |
              |------------| | MASTER |
        1) Start with Switch 1 OFF.
        2) Make sure that Switch 2 is OFF.
        3) Turn on the 110 V "start up" motor with switch 1. (as drawn) This
           will drive the master motor.
        4) Once running, flip Switch 1 so that power now goes to the Master
           Motor. It will now run at it's speed, and drive the 110 motor.
           (note that power is NEVER connected to both at once)
        5) You may now turn on Switch 2, which will start the machine motor.
        6) Turn off BOTH switches when done.

Additional motors must have their own switch (of course), wired to the
and should only be started one at a time.

CAUTION: Always observe proper safety proceedures when working with
electricity! If you don't know what you're doing, DON'T!

copyright 1989 Roger Garnett
  Roger Garnett (
         "The South Lansing Centre For Wayward Sports Cars"
       "All donations of stray, orphaned, odd, neglected, etc.
        sports cars and bits in need of a good home accepted."
         "The drop off bin is right there- behind the barn..."

On Tuesday, Mar 30, 2004, at 05:56 US/Pacific, Jules Richardson wrote:

> Rats, I just found out the museum's dead Sperry drum store uses a 3
> phase motor - is there any way I can run this from a single phase
> supply
> (UK mains, ~240V, 50Hz) without things blowing up? Or am I resigned to
> replacing the motor with a single phase equivalent?
> I just want to get the thing spinning so that people can hear it
> running
> - it's way beyond actually being able to restore it to working
> condition
> again :-(
> cheers
> Jules
Received on Tue Mar 30 2004 - 08:20:49 BST

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