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dc motor speed control

Discussion in 'Electronic Repair' started by Rick, Jan 2, 2005.

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  1. Rick

    Rick Guest

    I have a dremel scroll saw with a dc motor and a broken speed control. I
    assume it is some kind of pot. There are 3 wires going to the terminals on
    the pot. If i connect all 3 the motor sometimes makes 1/2 turn but usually
    just hums. I traced the 3 wires back to a connector on the pcb power supply
    board. Do I need a minimum resistance to make this work. I'd be happy to
    just bypass the switch and run at full speed.

    Thanks
     
  2. Ross Herbert

    Ross Herbert Guest

    It sounds like the electronic speed controller is faulty. This unit is
    not a simple resistive speed controller where the potentiometer varies
    the current through the motor directly. It will most likely be a Pulse
    Width Modulated (PWM) or phase triggering speed controller where the
    potentiometer varies the duty cycle of a SCR or TRIAC device which
    carries the heavy motor current. Whatever the actual circuit, it will
    be a semiconductor speed controller of some sort. Simply shorting out
    the potentiometer will not allow the motor to run at full speed
    because it is the semiconductor device which is in series with the
    motor and you would need to pull the controller apart in order to
    bypass this element.

    I would recommend against bypassing the controller in any case and
    since it will be necessary to pull it apart anyway it would be best to
    repair it if possible. If you are not able to do this yourself then
    perhaps you should enquire as to a local Dremel service agent and get
    them to do it. Of course the cost of parts and labour would probably
    be half the cost of a new scroll saw (unless it is still under
    warranty.
     
  3. Kim  Sleep

    Kim Sleep Guest

    From my past experience with Dremel repairs, they are quite helpful with
    replacement parts, and their parts are also quite inexpensive. A controller
    pot from them wont be much more expensive than a aftermarket pot.
     
  4. Jerry G.

    Jerry G. Guest

    You most go to the origional manufacture for the parts. It is most
    likely not a simple pot. It is also possible that the motor may have a
    fault as well.

    Jerry G.
    =======
     
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