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generate 3 phase delta?

Discussion in 'Electronic Basics' started by BobG, Dec 16, 2005.

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  1. BobG

    BobG Guest

    I have a small ac motor.. I looked up the part number... globe
    75a120-2.. 3 wires... datasheet says its a delta connection. I'd like
    to understand how to drive this thing... seems like it would be simpler
    if it had 4 wires and was connected wye.... but its not, so here I am
    asking for ideas. I guess I need 3 'power amps' that will put out 3
    phase sin waves. I guess the clever part is generating 4 or 5 bit sin
    waves using pwm? I guess this is just like a synchro... will work at
    10hz or 1hz or .1hz? The magnet just follows the field around?
     
  2. You will need some source of 3 phase power (3 sine waves, each shifted
    120 degrees with respect to the other two) at the appropriate volts
    per hertz (higher frequency requires higher voltage) capable of the
    required current. Each winding runs on the voltage difference between
    two of the phases.
    With the correct voltage to frequency ratio, it can run over a wide
    range of frequency. I don't know if that model is a permanent magnet
    synchronous type or an induction type.
     
  3. Bob Eldred

    Bob Eldred Guest


    This is a four pole hysteresis synchronous mortor, B-2700 series. It runs at
    1800 RPM on 60Hz single or three phase, 115VAC. It gives one ounce-inch of
    torque at 1800 RPM, 12 watts.

    To run single phase connect the AC line to the Blue and Green leads. Connect
    a 2uF, 200VAC capacitor between the White and Green leads. To reverse
    direction, connect the capacitor between the White and Blue leads.

    A hysteresis synchronous motor does not have permnanet magnets nor is it an
    induction motor. The rotor is iron and assumes a permanent magnetic flux
    when energized. There are no fixed poles on the rotor. The rotor will lock
    onto sychronous speed of the rotating field, in this case 1800 RPM. These
    motors are used where accurate rotational speed is required but they don't
    give much torque.
    Bob
     
  4. BobG

    BobG Guest

    BE:
    This is a four pole hysteresis synchronous mortor, B-2700 series. It
    runs at
    1800 RPM on 60Hz single or three phase, 115VAC. It gives one ounce-inch
    of
    torque at 1800 RPM, 12 watts.
    To run single phase connect the AC line to the Blue and Green leads.
    Connect
    a 2uF, 200VAC capacitor between the White and Green leads. To reverse
    direction, connect the capacitor between the White and Blue leads.
    ===================================
    Hi Bob. I saw where one could run it from single phase with a cap....
    but I wanted to see if I could drive it with a microcontroller
    somehow...wondering how to generate 3 phase. I noticed it put out a sin
    wave between any pair of wires.... that's why I thought it had a perm
    magnet in it. Sounds like a lot of work. There's a dozen or so of em at
    Skycraft.
     
  5. BobG

    BobG Guest

    BE:
    This is a four pole hysteresis synchronous mortor, B-2700 series. It
    runs at
    1800 RPM on 60Hz single or three phase, 115VAC. It gives one ounce-inch
    of
    torque at 1800 RPM, 12 watts.
    To run single phase connect the AC line to the Blue and Green leads.
    Connect
    a 2uF, 200VAC capacitor between the White and Green leads. To reverse
    direction, connect the capacitor between the White and Blue leads.
    ===================================
    Hi Bob. I saw where one could run it from single phase with a cap....
    but I wanted to see if I could drive it with a microcontroller
    somehow...wondering how to generate 3 phase. I noticed it put out a sin
    wave between any pair of wires.... that's why I thought it had a perm
    magnet in it. Sounds like a lot of work. There's a dozen or so of em at
    Skycraft.
     
  6. Bob Eldred

    Bob Eldred Guest

    Yes you could generate three phase signals in a microprocessor. You'd have
    to run the motor on higher voltage, however, 115 volts for 60Hz. You could
    run at higher speed (frequencies) with higher voltages and lower speeds with
    lower voltages. The volt-time product per cycle is held constant for
    variable frequency drive. The frequency varies the speed and the constant
    volt-time product keeps the torque controlled without allowing the motor to
    overheat at low speeds. Usually such drives use pulse width modulation to
    generate sinusoidal currents for the windings. Again a microprocessor could
    easily develop the required three PWM switching signals for a three phase
    system. The PWM signals would also incorporate the necessary constant
    volt-time product characteristic. The drive elements for the motor could be
    FETs or IGBT's or power transistors all driven from the processor. It sounds
    like a big deal though just for a one ounce-inch motor. Such variable
    frequency inverter drives are used on all kind of motors from fractional HP
    on up.
    Bob
     
  7. Rich Grise

    Rich Grise Guest

    Generating 3-phase in a uC isn't hard at all - just have three look-up
    tables of 3 sine waves 120 degrees out of phase from each other, and
    run 3 DACS to give you the three phases. Or, you could use one LUT and
    three scrolling indices 1/3 of the way apart from each other, but that'd
    be nasty ugly because your LUT would have to be 3N entries long. ;-) You
    probably wouldn't need very big look-up tables; it's surprising how
    quickly harmonic content drops off with just a few samples.

    That also gives you absolute speed control, without relying on a
    cap, but mind what John Popelish said about making the drive voltage
    inversely proportional to the frequency. (at least, I _think_ that's
    what he said - it's been a couple of posts ago. ;-) )

    I don't know how to turn three, ground-referenced outputs into a
    delta, however. I wonder what would happen if you just used three
    half-H's, which, I guess, amounts to 3x totem-pole outputs? My intuition
    says, "of course", but I haven't done the grunt work yet.

    Good Luck!
    Rich
     
  8. Jasen Betts

    Jasen Betts Guest

    possibly there's no magnet but a rotor that picks up a current from the
    magnetic field of the windings and behaves a bit like a magnet - google for
    induction motor,

    Bye.
    Jasen
     
  9. Jasen Betts

    Jasen Betts Guest

    it may be possible to drive only two of the inputs (and ground the
    third). the two driven inputs should have a phase difference of 60 degrees.

    bear in mind that with this setup the driven inputs have nearly twice the
    voltage from the undriven input than they would have from ground if being
    driven from a three phase supply... so check the winding to frame insulation
    in that datasheet before trying that with the undriven input grounded.

    Bye.
    Jasen
     
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