Connect with us

2 phase to 3 phase for motors

Discussion in 'Electronic Basics' started by Bob Masta, Dec 4, 2003.

Scroll to continue with content
  1. Bob Masta

    Bob Masta Guest

    I recently saw a small machine shop that
    ran 3-phase machines from a homebrew
    converter. According to the owner-builder,
    he used a big 3-phase motor as a sort of
    "flywheel". This motor ran off the incoming
    2-phase (standard 220V from utility company)
    through some banks of capacitors to the
    flywheel motor. Somehow, once it was up
    and running, this setup was able to supply
    3-phase to the actual working motors in the rest of the
    shop. Anyone heard of this before, or have
    any links to Web pages?


    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  2. You have described it fairly well.
    They look like this:

    The motor is run like a standard (2 phase) capacitor run motor, and
    the magnetized rotor shifts the phase a bit to generate 3 phase power
    as it rotates.
  3. Eric R Snow

    Eric R Snow Guest

    Greetings Bob,
    If you do a google search for rotary phase converter you will find all
    the info you need to build your own. Or do a deja news of the
    rec.crafts.metalworking newsgroup. And, look at In
    the dropbox are pictures and descriptions of home made rotary phase
    converters. I'm not an expert by any means on these things but I
    understand a little bit about how they work. After the three phase
    idler motor is started on single phase it generates the third phase
    which is used to power all the three phase equipment. You can use
    capacitors to shift the phase to start the idler motor, or use a
    single phase motor to spin it up and then apply the single phase or
    even wrap a rope around the shaft, spin it up, and then apply single
    phase power to it. Once spinning it will generate the third phase
    because all motors, when running, are also generators. Capacitors are
    used on the output phases of the idler motor to balance the voltages
    under actual running conditions. The capacitors change the phase angle
    so the voltage and current are in phase with each other.
  4. als

    als Guest

    Not long ago, I built one for a friend that seems to be
    working out well (10 HP rotary). There are some pictures:
    I did some extensive testing on phase relationships
    while I was learning, and this was the simplest version
    that seemed to work well. I did not publish the results
    of the tests of various other designs (other rotaries
    and statics), and of the various components tested (some
    to destruction), but I can.
    <als> alsAThal-pcDOTorg (don't bother to reply to the spam address).
  5. Very neat. But I have two small concerns. At the moment you hit the
    start, there is an unknown voltage on the pair of 250 uf caps and a
    possibly quite different unknown voltage on the bank of 4 running caps
    that get connected directly in parallel with a contact. I think I
    would have added a small inductance or resistance in series with the
    start caps to limit that equalization current, and also a bleed
    resistor across the start cap bank.
  6. Kim Clay

    Kim Clay Guest

    Hmmm... How does it start? Shouldn't K1 be powered from the main breaker
    instead of its own contacts (using the 0.5A fuse)?

    If the idler (or a load motor) is not spinning there will be no voltage
    across the pair of 250V start caps (discharged through the deactivated
    start relay), or the run caps. Both are discharged through the idler

    A bleed resistor across the start caps is always a nice feature just in
    case the start relay contacts go open.

    rec.crafts.metalworking is where they do lots of this.

    Check out (at the top) for

    Also, by the two 24V transformers, there seems to be an option to use
    the !safety ground! rather than the neutral as return for the control

    Nice looking install!
  7. als

    als Guest

    I apologize for the errors in the schematic. Drew it from memory,
    which seems to be failing more often lately. :)
    I will correct it, and also add some more drawings of some other
    converters (static and rotary) as soon as I get a round tuit.
    Explanatory defense follows :)

    There is a 33k, 2W resistor across the start caps (not shown in the
    drawing). It is actually not necessary, since the potential relay
    recloses when the converter is shut down, which discharges the start
    caps into the idler. I had added it so I would not be startled when
    You are correct. The fuse IS connected to one of the LINE IN terminals.
    My error in the drawing. Sorry.

    The option to use safety ground is there since the converter is mounted
    in close proximity to the main panel. I incorporated it in order to
    use the ground as a reference to insure that the shop ground was in order.
    The original design used 3 transformers (one on each leg), and a couple
    more relays, but it started to get complicated (!).
    I made a decision to use only parts that were readily available, and
    to make it simple enough that any decent electrician could understand
    and repair it if necessary.
    Thanks. It would have looked nicer, but in went thru several iterations,
    since I was mainly building it out of parts on hand. Woulda' done it a
    little different if 1) the parts had to be purchased, and 2) the application
    or environment was different.

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day