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dc motor speed control

R

Rick

Jan 1, 1970
0
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
 
R

Ross Herbert

Jan 1, 1970
0
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
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.
 
K

Kim Sleep

Jan 1, 1970
0
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.
 
J

Jerry G.

Jan 1, 1970
0
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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