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Signal in a phase on a DC motor

Discussion in 'Electronic Design' started by [email protected], Jun 4, 2007.

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  1. Guest

    Hello! How are you?

    The other day I read this:

    "There are three terminals inside most small DC motors, and it acts a
    lot like three-phase alternating current [...]. A nice side effect of
    this is that the position of the motor can be detected by taking one
    of the phases straight into the microprocessor [RA4 in a PIC16C84]."

    I would like to know how is it possible. How is the signal on the
    phases? Is it a square wave?

    Thank you!
  2. It sounds like they are referring to a brushless DC motor. Try a
    search on BLDC or brushless DC for an introduction. "most small DC
    motors" seems an exageration to me but it may be correct or it may
    depend on how you define small.

    Myself I'd quibble about them actually being DC but the terminology has

  3. redbelly

    redbelly Guest

    First, those 3 terminals are (usually? always?) rotating with the
    rotor, so I'm not sure how you could pick a signal off of them.

    I guess you could describe things this way, but it would not be a
    simple square wave, and would be at whatever frequency the rotor is
    turning at.

    For a "delta" configuration of 3 rotor coils, (AND if I am thinking
    about this correctly), the signal across any two terminals would be:

    At +V for 1/6 cycle
    At +V/2 for 1/6 cycle
    At -V/2 for 1/6 cycle
    At -V for 1/6 cycle
    At -V/2 for 1/6 cycle
    At +V/2 for 1/6 cycle

    And then repeat the cycle.


    p.s Note to Robert A: since the OP refers to "most" small DC motors, I
    think they mean the standard, brushed type.
  4. MooseFET

    MooseFET Guest

    The V/2 values won't be seen in real life. When the brush breaks
    contact with the section, there will be a largish spike. After the
    spike dies down, there will be some mixture of the V/2 value with a
    sine wave. The three windings aren't perfectly coupled to each other
    so the nonconnected winding will have some ability to do its own

    It won't be easy to get anything like an accurate angle out of the
    voltages. If you brought all 3 connections out to some sliprings, you
    could get a position good to about a third of a turn just by looking
    to see which pin is at the +V or -V voltage. You only really need two
    connections because if two of them aren't the third must be.
  5. Marra

    Marra Guest

    amounts of noise.
  6. Marra

    Marra Guest

    I did a project once to try and get tacho pulses from the commutator
    and it worked fine at a set frequency but needed lots of filtering.
  7. I suppose you could add another brush as a pickup. That does assume
    that a brushed motor has three phases wound on the rotor which seems a
    questionable assertion although more likely true on a small motor.

    Well it depends on the population they are thinking of. If your
    population is dominated by PC cooling fans and disk drive motors you'll
    certainly think most small DC motors are brushless.

    I really don't know what dominates, BLDC, stepper, PM... I wouldn't be
    surprised if the most dominant was universal.

  8. joseph2k

    joseph2k Guest

    I do not think that OP is talking about brush motors, i think 'e is talking
    about something like these: HZ/eflite/eflite_brushless/
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