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Speed control for woodworking - how does it work?

Does anyone know how these aftermarket speed controls for woodworking tools like routers work? At first I thought they were just open-loop duty-cycle controllers. If that were the case, then the router speed would sag a lot when the work is applied to load it down. I'm sure the built-in speed controls use some sort of RPM sensor for feedback, and so can maintain speed under varying loads. But from the reports from woodworkers in one forum I have been reading, these controls really do work. For example, this one:

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But there is no obvious speed feedback to the external controller. Is it possible that the controller can sense the RPM based on detecting small variations in the current?

Robert Scott Ypsilanti, Michigan

Reply to
Robert Scott
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Most work on the back emf produced by the motor. Note the warning about 'soft start' tools.

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Rheilly P
Reply to
Rheilly Phoull

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A brushed motor will go at a constant speed (within reason) if you apply a constant voltage to it. A small speed drop occurs on load due to the armature resistance but this can be compensated for by measureing the current and applying a proportional increase in voltage. Around 1% variation is possible over the full load range is possible by this method.

Reply to
cbarn24050

"Rheilly Phoull" wrote in news:481b83e9$0$14266$ snipped-for-privacy@per-qv1-newsreader-01.iinet.net.au:

I use a lamp dimmer to control my Dremel MotoTool,an older model 270. It's just a common triac dimmer. I installed the dimmer and a duplex outlet in a plastic junction box,with a power cord to the wall outlet.

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Jim Yanik
jyanik
at
kua.net
Reply to
Jim Yanik

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This thing is a triac phase angle controller like a lamp dimmer. The pot controls the firing angle of an SCR or Triac thus controlling the effective voltage the motor sees.

It may have back emf feedback to sense the motor speed but probably not. The way to tell is to load the motor at low speed and see if the torque picks up to "fight" the load. If it has feedback it will do that, if not, the motor will just slow down. Run at low speed and grab the shaft with your hand and try to slow or stall it. (Obviously do this without any bits in place). If it has feedback it will be obvious.

When used, back emf is sensed with a bridge circuit that subtracts the applied voltage leaving the back voltage (emf). This goes to a controller and represents the speed.

Reply to
Bob Eld

The comomon back-EMF controller uses an SCR, but unlike an SCR lamp dimmer, the motor is in the cathode side, not the anode side - so the back EMF affects the gate- cathode potential.

This type of SCR circuit can maintain almost full torque down to almost zero speed on suitable motors.

Clifford Heath.

Reply to
Clifford Heath

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