Shunt wound or permanent magnet DC motors are inherently constant torque
devices because the magnetic field is constant at or below base speed. Any way
to get variable armature voltage will produce variable speed at constant
torque. Most AC supplied DC motor controllers are phase controlled SCR bridges
with freewheeling diodes. PWM supplies, variac (autotransformer) with diode
bridge, or power resistor will all work.

Brush type AC/DC motors (universal motors) as found in most hand held power
tools are series wound meaning the armature current also provides the field
excitation. That is why they will run (sort of) on AC -- the magnetic field
and the armature current both reverse at the same time. The speed of a series
motor can be controlled by controlling terminal voltage. That is what simple
triac controllers and light dimmers do -- but simply reducing the terminal
voltage will also reduce the available torque. Slightly more sophisticated
controllers have a feedback circuit that senses when the motor starts to slow
below the intended speed and increases the voltage to maintain the intended
speed. This feedback allows the motor to produce constant torque. Many
variable speed power drills depend upon the operator's finger to provide this
feedback -- pushing the trigger down to maintain something like constant speed
as the tool is loaded. The better controllers, and probably the more expensive
router speed control that started this string, have an electronic feedback
circuit of some kind. The cheap HF controller that did not work very well
probably does not have an electronic feedback feature.

Mill


"william_b_noble" wrote in message
...

ok, you asked. these are triac based controls that duty cycle modulate
the input AC power - basically they delay the turn on of the AC to the
motor until a desired phase angle is reached - the later the turn on
(each half cycle) the less power is delivered to the motor.

That sounds like PWM. Is that what these controllers are?

It is my understanding that a PWM drive will offer max torque over the speed
control range because the motor always sees peak voltage (but that the
voltage is chopped so that the average current determines the speed).

I am trying to build a power table feed for a mini mill. I've got a
high-torque 24VDC gear motor, but I'm still looking for an electronic
circuit to drive it. I know some people have used a light dimmer with the
output routed through a step-down transformer; perhaps a more version would
use one of these universal motor speed controllers in place of the light
dimmer.

Anyone have suggestions for either an economical off-the-shelf solution, or
a schematic for a basic PWM circuit?

Thanks!