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Re: [Electronics_101] Thyristor Conroller

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  • lists
    In article , ... If you re anything like me you have accumalateda large collection of the things over the years! Saw a
    Message 1 of 7 , Feb 18, 2013
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      In article <006901ce0e02$5debeff0$19c3cfd0$@com>,
      Rick Sparber <rgsparber@...> wrote:
      > If your 12V DC motor doesn't draw too much power, I suggest you look at
      > buying a used "wall wart". I paid $3 for mine and use it to power the
      > motor which moves the table on my milling machine.

      If you're anything like me you have accumalateda large collection of the
      things over the years!

      Saw a lovely cartoon a while back.

      --
      Stuart Winsor

      Only plain text for emails
      http://www.asciiribbon.org
    • Jan Kok
      First of all it s a real shock hazard. The motor insulation probably isn t designed to withstand 240VAC. There could be leakage to the case. If you DO try it,
      Message 2 of 7 , Feb 18, 2013
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        First of all it's a real shock hazard. The motor insulation probably isn't
        designed to withstand 240VAC. There could be leakage to the case. If you DO
        try it, please connect a safety ground wire to the motor case.

        Second, not all thyristors are suitable for controlling motors. In
        particular, triac-based incandescent "light dimmers" may not always turn
        off between cycles of the AC line with inductive loads such as motors. The
        result is that you could end up applying full AC line voltage to your motor.

        Third, if you run the motor at the full rated torque you'll likely burn up
        the motor due to I^2R heating. Let's say the motor delivers rated torque
        when the shaft is spinning at 2/3 of the unloaded RPMs. That tells us that
        the "back EMF" produced by the motor is 2/3 of the rated voltage, i.e. back
        EMF = 8V = 2/3 of 12V. And let's say the current is 1A at full load.

        Now, assuming that 12V _DC_ is supplied: The voltage drop through the
        resistance of the windings and brushes is 12V - 8V = 4V. Thus, we can
        determine the motor resistance is R=V/I=4V/1A=4 ohms. And the power
        dissipated by the motor as heat is I^2R=1A^2*4 ohms=4W.

        But what happens when the current is supplied in narrow pulses? Torque is
        proportional to current. To get the same _average_ torque, you need a much
        higher _peak_ torque - and corresponding peak current.

        The pulses are approximately triangular (the edge of the sine wave). For
        50Hz, I calculate the triangle width as .87 ms wide and 23 amps at its
        peak. (You can verify that that gives an average current of 1A.)

        The peak power is I^2R=(23A)^2*4ohms=2.13KW. The average power _during the
        pulse_ (integrate the power) is 1/3 of that, 711W. But the duty cycle is
        .87 ms/ 10ms so the overall average power dissipated in the motor is about
        62W. Considering our hypothetical motor was designed to dissipate 4W, smoke
        seems likely.

        Bottom line, running a 12V motor with a thyristor controller from a 240V
        line does not seem like a good idea. Likely to be harmful to the motor, the
        controller - and you! :-)



        On Mon, Feb 18, 2013 at 10:37 AM, oakite2000 <stackerjack@...>wrote:

        > Hi,
        > I want to run a 12Volt D.C. motor from the mains power.
        > Has anyone got any idea what would happen if I connected the motor to my
        > half wave thyristor power controller,( which is designed for 240VAC
        > operation), and turned the output down to minimum?
        > If I connect my 240 VAC electric drill to the controller, it doesn't start
        > turning until I rotate the control pot. by about 30 degrees.
        > I don't want to try it without checking first, in case it burns out either
        > the controller, the motor, or both.
        > Thanks Guys
        > Jack
        >


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      • Dave C
        Pray tell please describe the cartoon if you can t find a link to it... Dave -=-=-=- ... [Non-text portions of this message have been removed]
        Message 3 of 7 , Feb 18, 2013
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          Pray tell please describe the cartoon if you can't find a link to it...

          Dave

          -=-=-=-

          On 18 February 2013, at 2:24 PM, lists wrote:

          > Saw a lovely cartoon a while back.
          >
          > Stuart Winsor



          [Non-text portions of this message have been removed]
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