Speed control for woodworking - how does it work?

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Robert Scott

Posted: Fri May 02, 2008 3:24 pm

Guest

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:

http://www.rockler.com/product.cfm?Offerings_ID=10760&TabSelect=Details

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

Rheilly Phoull

Posted: Fri May 02, 2008 4:13 pm

Guest

"Robert Scott" <---@---> wrote in message
news:481b7700.5839046@newsgroups.comcast.net...

Quote:

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:

http://www.rockler.com/product.cfm?Offerings_ID=10760&TabSelect=Details

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

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

--
Rheilly P

Jim Yanik

Posted: Fri May 02, 2008 9:43 pm

Guest

"Rheilly Phoull" <rheilly@bigpong.com.au> wrote in
news:481b83e9$0$14266$5a62ac22@per-qv1-newsreader-01.iinet.net.au:

Quote:

"Robert Scott" <---@---> wrote in message
news:481b7700.5839046@newsgroups.comcast.net...
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:

http://www.rockler.com/product.cfm?Offerings_ID=10760&TabSelect=Detai
ls

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

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

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.

--
Jim Yanik
jyanik
at
kua.net

Bob Eld

Posted: Fri May 02, 2008 10:21 pm

Guest

"Robert Scott" <---@---> wrote in message
news:481b7700.5839046@newsgroups.comcast.net...

Quote:

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:

http://www.rockler.com/product.cfm?Offerings_ID=10760&TabSelect=Details

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

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.

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