Subject: Re: phase converters was Re: 3 phase etc

From: [email-masked] (Pete Lancashire)
Date: Thu, 1 Jul 93 16:54:19 GMT

[email-masked] writes:

>Actually, 3 phase is far from my home; I live in the center of residences.
>But anyway...I don't really understand this "transformer" or "starter"
>stuff you all have been talking about. What I gather is that one can take
>a 3 phase motor and somehow rig it up to run another 3 phase motor using
>only single phase power supply. But how is this done???? What I have is a

If you know the basics of what a transformer does.

Here is how I 'try' to explain it.

First a 'transformer';

One winding with AC going in, iron to couple the magnetic field, and
another winding to take that mag. field and convert it back to
electricity. 

Then translating that to a simple motor with two windings.

The windings of the motor are the same as the windings of the
transformer. The rotor/stator is the iron.

Then try to picture a motor in sync with the AC, with the input winding
on one side and the output winding on the other side. The motor is
rotating a a fixed speed, and it takes 1/1800 of a minute or 1/3600 for
the output field to 'get the magnetic field' from the input. I know this
is not exactly 'correct' but it helps visulize how it works.

Now a 3 phase motor has its windings 120 deg or 1/3 of a revolution
apart. You energize two of the winding, and the third then output
voltage 120 degrees from the others. (not exactly true but...)

Now if someone better at 'teaching' can help this up PLEASE do !!
And I'm no good at ASCII drawings :(..
 
>big 3 phase motor with six wires hanging out one end and single phase power
>coming out of the wall of my workshop. Could someone please advise?

The six wires and sometimes up to twelve !! allow for multiple voltages
and for rotational direction.

-pete

          --------------------------------------------
          ----- Pete Lancashire [email-masked] ----
          ----- Kid in spirit. Big Brother to be! ----
          ---                or now                ---
          - Adopt ? ME ? HELP !!, Am I going crazy ? -
          --------------------------------------------

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From: [email-masked] (Jim Kirkpatrick)
Date: 1 Jul 93 14:06:35 MDT

> [email-masked] writes:

(etcetera) about running 3-phase stuff on 1 phase.

There appear to be two ways to run a 3-phase motor on single phase power.
I don't have any direct experience with either of these methods, but am
anxious to try, since I have some 3-phase motors and equipment.

This applies to induction motors, not "universal" or other types.

1.  Run single-phase power into one of the 3 motor windings.  If you
    simply apply power, the motor will stall because it has no preference
    as to which direction to turn.  So, it's better to manually get the
    motor spinning in the desired direction, then apply power.  This method
    has the disadvantage of requiring an external starting force, and that
    since the motor is running on 1 of 3 windings, the available power
    needs to be derated (by how much?  probably a lot).  It may also have
    a stronger tendency to stall under load.

    A variation is to use a starting capacitor and a relay or two, to give
    a second winding some out-of-phase power to help it get started.  Once
    the motor is up to some decent speed, switch out the capacitor.  The
    arrangement should cause the motor to completely drop power in case of
    a power failure, rather than possibly restarting (safety issue).

2.  Wire up a 3-phase motor as above, but then steal the power generated
    by the other two windings (along with the 1-phase input power).  This
    is sometimes called a "spinner" and is sort of like a 1-phase motor
    and 3-phase generator all in one package.  The output power won't have
    ideal phase angles, nor will it have perfectly balanced voltages, but
    you can run most 3-phase stuff nicely.  Two rules of thumb:  the spinner
    must be rated about 2 times the hp of the combined load, and the starting
    capacitor should be about 100 microfarad per spinner hp (non-polarized
    if you hadn't guessed).  In large units, I've heard of people using a
    small 1-phase motor just to get the spinner up to speed and going in
    the right direction.

The above has been gleaned from various sources, most notably several
articles in the magazine Home Shop Machinist over the past few years.
Corrections, alternatives, etcetera, welcome.

Always keep safety in mind.  We're not only talking about healthy
voltages (220, 208 typically), but rotating machinery as well (e.g.
shield the shaft of the spinner so it won't scramble Rover).

Jim

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From: [email-masked] (Mark A Fleming)
Date: 2 Jul 1993 16:48:47 GMT

I haven't been following this thread closely, so maybe this has already been
pointed out.  I'm running a 3 Hp 3 phase motor for my Southbend from 220 using
a phase converter box (I have the manual at home if more detailed info is
needed).  Basically, two windings are powered as 120V, 180 degrees out of
phase.  The "box" is simply a switch and capacitor, which shifts the phase of
one line and feeds the third winding.  The third winding is energised for only
about 20 seconds to start the motor.  Power was "stolen" from an electric
rangetop flush to a wall between the kitchen and garage.  The box was around
$90US.

maf