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Switch and pot replacement for drill motor

Discussion in 'Sensors and Actuators' started by KDoran, Aug 16, 2015.

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  1. KDoran

    KDoran

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    Aug 16, 2015
    Im repurposing an AC reversible, variable speed drill for a project. I'm removing the old F/R switch and trigger and am going to replace them with a 3way toggle and a pot in a separate box.

    The original F/R switch had four wires to it. One line in, one out to each brush, and one to the motor. The way the current switch is constructed, it's difficult to discern exactly how the switch worked once I disassembled it. So my question is what kind of switch do I need (SPDT, DPDT, etc), and how should the wire from the motor interact with the switch? The line is is evident, and the lines out to the brushes seem straight forward, just not sure about the wire to the motor. Is it always hot? Is it a ground or neutral?

    I know that the pot is put into the neutral line, but I'm not sure what Ohm rating I will need. It draws 3.5 amps on 120 VAC house current.

    Thanks for the help.
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    Doesn't work that way.
    Motor is a series type with brushes to the armature in series with the field coils.
    To reverse the motor, the switch mechanism connects the brushes one way or the other with respect to to field winding.
    The switch mechanism also houses a thyristor type phase angle speed control unit (not a just a pot as you quote).
    You could add an external speed controller once the field /armature connections were sorted but I think you will find the cost would be more for the external unit than a completely new drill.
     
  3. KDoran

    KDoran

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    Aug 16, 2015
    Thank you for your reply. Starting with your last comment: I'm repurposing the drill for another project, buying a new drill doesn't solve anything, I'd likely have the same questions.

    If the trigger just changes the resistance in the ground/neutral line, why wouldn't a pot/rheostat work? It's hard to see how much more complicated it could be, considering it is just put in line with the motor ground and the neutral wire from the power cord.

    I understand that the switch sends power to one or the other brushes in order to change direction. I understand that the hot wire from the power cord is supplying the juice. What I don't know is how the wire FROM the motor interacts with the switch.
     
  4. KDoran

    KDoran

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    Aug 16, 2015
    Never mind, it dawned on me where the wire from the motor went. Now just need to find a switch that switches line and loads.
     
  5. Minder

    Minder

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    624
    Apr 24, 2015
    You need a DPDT toggle switch with centre off, IOW 3 posn.
    I can describe the hook up if needed.
    A common item.
    You do not just switch line and loads, but Field WRT the armature.
    M.
     
    Last edited: Aug 16, 2015
  6. duke37

    duke37

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    767
    Jan 9, 2011
    You will not find a pot which will take the motor current, it cannot be in the neutral line. You could use a light dimmer to reduce the voltage to the motor.
     
  7. Bluejets

    Bluejets

    4,055
    847
    Oct 5, 2014
    The "trigger" does not change the resistance as you put it.
    The trigger houses a thyristor which controls the phase angle at which the drill motor is supplied power thereby controlling the speed.
    You cannot do this with your "pot" which in most cases as stated is about 1/4W power rating.
    An external speed control unit which I mentioned earlier does the same job as the trigger phase control unit only in a remote location.Most are around 10 Amp rating at 220v with most costing around the $150 mark hence the suggestion to get a new drill.
    If that is not acceptable then perhaps a different approach to powering your device might be worth a thought.
     
  8. Minder

    Minder

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    624
    Apr 24, 2015
    You can get them all mounted in a case for $30.00 on ebay, you would have to add the reversing switch if needed.
    M.
     
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