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How good a "sine" wave to drive a selsyn/synchro?

Discussion in 'Electronic Design' started by jtaylor, Sep 29, 2005.

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  1. jtaylor

    jtaylor Guest

    The simplest way to get this pair of devices working in a non-400hz 3-phase
    environment seems to be to program a pic to generate three-phase and amplify
    the output. If I have three pins per phase and run each through a resistor
    of some calculated value I can make a psuedo-sinewave. Is this likely to
    have any issues because it's not a true sinewave? This is a hobby-level
    application, so good enough will be good enough....
  2. What speaks against PWM ? It is simpler to
    amplify to any level rather lossless.

  3. Joerg

    Joerg Guest

    Hello Rene,
    Some uC don't have enough timer links for three-phase. I think most of
    the MSP430 can do that and then PWM clearly looks like the solution
    here. No idea about PICs but it would be worth to scour the app notes
    for any three-phase tasks.

    The issue with not-so-perfect sine waves is often audible noise and
    sometimes faster wear due to resonances that travel to the bearings.

    Regards, Joerg
  4. Genome

    Genome Guest

    If you have three pins per phase then you have nine pins. If you make a
    johnson counter for 6 of the pins and take a weighted sum through the
    resistors then use the seventh bit for polarity then you get something or
    another with two bits to spare. To get your three phases you offset the
    summing resistors for each phase.

  5. jtaylor

    jtaylor Guest

    This sounds good but it's a bit over my head - how will I get the three
    phases to vary at 120 degrees?
  6. Rich Grise

    Rich Grise Guest

    It sounds like he's saying you could use a 9-stage Johnson Counter and
    interleave your summing resistors for a resulting 3-phase.

    Good Luck!
  7. Mook Johnson

    Mook Johnson Guest

    Just look at some of the PICs designed for motor control. The have three
    phases for PWM.
  8. Genome

    Genome Guest

    Have a look at the bit about LFO generation using a ring counter at....

    There is also a circuit in H&H, the art of electronics. I think you scale
    the resistors as some Sin(X)/X function but I can't remember.

    Use six of your pins on the pic to implement the walking ring counter and
    then add the summing resistors like this (arbitrary values chosen)

    Phase 1) Phase 2) Phase 3)
    10K 15K 20K
    15K 10K 20K
    20K 10K 15K
    20K 15K 10K
    15K 20K 10K
    10K 20K 15K

    So each set of resistors is shifted two bits compared to the previous one.

  9. Genome

    Genome Guest

    Oh Crap...... that doesn't seem to work.

  10. Jon

    Jon Guest

    Your post seems to be a continuation of another thread. As I
    understand your problem, you need to provide drive for a synchro or
    selsyn for some kind of motion control application. There is a common
    misconseption that a selsyn/synchro is a 3-phase device. For the usual
    motion control application, this is not true. Time-wise, the voltages
    are either in-phase or 180 degrees out of phase with respect to a
    reference. As the synchro shaft rotates at a constant rotational
    velocity, the AMPLITUDES (forgive my shouting) vary sinusoidaly and are
    120 degrees apart with respect to the MECHANICAL (I did it again)
    position. To generate these voltages, you only need to generate a
    single phase (electrically) sine (or pseudo-sine) wave of an
    appropriate frequency, i.e., 400Hz. You then vary the amplitudes of
    the other two voltages so that they vary sinusoidally 120 degrees apart
    with respect to the desired mechanical position of the shaft.
    There is one application that does require drive voltages that are 120
    degrees apart electrically. If you want to generate a sine wave whose
    electrical phase varies sinusoidally with respect to a reference, you
    can drive the 3 stator windings with sine waves that are electrically
    120 degrees apart. Now, as the shaft is rotated, the rotor voltage
    electrical phase will vary linearly from 0-360 degrees as the shaft
    position varies from 0-360 degrees.
    I hope this helps.
  11. Bob Stephens

    Bob Stephens Guest

    Yes, that's right. And one more thing - I didn't see the OP either - as
    regards the quality of the sine, fuhgeddabowdit, I've seen some truly
    hideous signals driving aircraft synchros just fine.

  12. Robert Baer

    Robert Baer Guest

    Why not be simple, and use a phase-retard oscillator that uses 3
    stages to work, and pick off each phase?
  13. jtaylor

    jtaylor Guest

    Um, yes, but...

    First off, it looks like you know what yer talkin' about.

    (I don't.)

    I have a synchro sender for a fuel tank and an indicator that matches it. I
    wish to provide them with the electrical necessaries to get the indicator to
    show what the sender sees.

    They have each three wires (but a clever fellow like you would know that).

    What do I need to make this happen? I thought it was three phases at 400
    Hz, but...
  14. jtaylor

    jtaylor Guest

    Oh good.

    Would square be too too hideous?
  15. budgie

    budgie Guest

    On Fri, 30 Sep 2005 20:56:35 -0300, "jtaylor"

    (now the problem/question becomes MUCH clearer ...)

    UIAVMM (and it has happened) you are looking at these devices the wrong way.

    A Selsyn has a "single phase" rotor and a "three phase" stator. The intended
    application for remote (shaft) position indication used excitation of the rotor
    to induce voltages in each stator leg which represented the vector components
    seen at the three 120deg displaced positions.

    The stator windings of one Selsyn were connected to those of the second unit,
    and when the rotor of the sender was excited the receiving unit would adopt the
    same rotational position if its rotor was excited in phase with the sender.

    So your requirement is to excite both rotors with a common SINGLE PHASE
    alternating drive. Ideally a sinusoid, but square waves work as well. Do note
    though that you are driving an inductive load.

    As always, I am ready to be corrected, as I haven't played with them for 30
  16. Robert Baer

    Robert Baer Guest

    Ah....Been 40 years or so.
    A selsyn / syncro sender/receiver has 5 leads: 2 for the AC drive
    (nominally 120VAC 400Hz), and the three from one goes to the 3 of the other.
    Technically, they are transformers, and the movement of the shaft of
    one "picks off" various phases of the exciting field; those voltages
    drive the other and set up the same field.
    The rotor of the second then moves to minimum energy (via magnetic
    attraction and repulsion), which is the same position as the first.
    If connected improperly, the "receiver" will turn in the opposite
    There are a few rare 60Hz versions; look at the label / plate for
    info; small ones are always 400Hz.
    They can be driven with 60Hz, but at vastly reduced voltage (being
    400Hz transformers, they have a much lower inductance at 60Hz).
    Naturally, the torque will also be a lot lower.
  17. Robert Baer

    Robert Baer Guest

    A square wave will not do too well, and one could zap the insulation
    if one uses a 120V 400Hz square wave to drive the 2-lead drive winding.
  18. jtaylor

    jtaylor Guest

    This particular pair of units want 12v (so the labels say).

    But square is too bad, oh well...
  19. Robert Baer

    Robert Baer Guest

    If i remember correctly, (been about 40 years), 120V units rated for
    400Hz run nicely on 24V 60Hz.
    Never heard of 12V units, rather hard to believe; typically selsyns /
    synchros are run from the line and are made for military applications -
    hence 120VAC 600Hz.
  20. Glen Walpert

    Glen Walpert Guest

    IIRC 12 and 26 volt rated synchros were fairly common a couple decades
    ago when I was using them. I see no reason why a square wave would
    not work for a simple indicator application, perhaps with the voltage
    slightly reduced for the same RMS voltage or with the sharp edges
    removed with a low pass filter. Square wave excitation of 12 volt
    reslovers is quite common and works well.
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