PWM Question Making a power feed for a mill

[xman_charl]

My testers for small motors...

 

[JohnAspinall]

PWM is Pulse Width Modulation. The controller sends out a pulse-train of direct current to the motor consisting of alternating periods of full voltage and zero voltage. The speed of the motor depends on how much of the time the motor is getting full voltage compared to how much of the time the motor is getting no voltage. All this is happening very fast. ... A potentiometer sets the duty cycle and controls the speed.

Because the motor is getting full voltage every time the pulse is high, it can put out more torque than if you just turn down the voltage using a potentiometer (actually a rheostat - almost the same). You'd have to use one with a very high current rating to control the window motor and the results wouldn't be satisfactory.

Hawkeye is right; an additional reason not to use a potentiometer directly in the power line has to do with energy loss.

Think of a regulator or switch as a device that goes from zero resistance to infinite resistance. A good switch goes fast from zero resistance to infinite resistance. Why do we care how fast? At zero resistance, no energy is being lost in the switch -- the energy is all going to the motor. At infinite resistance, no energy is being lost in the switch -- no current is flowing. But in between zero and infinite, our regulator/switch is wasting electrical energy as heat. If you do the math, the worst case is when the resistance of the regulator/switch equals the resistance of the load (motor).

Using a potentiometer/rheostat in the power flow, you are deliberately settling in this energy-losing middle ground. Conversely, by using PWM switching, you are doing the opposite -- you are getting from zero switch resistance (full on), to infinite switch resistance (full off) as fast as the digital electronics can make the edges of those pulses. PWM is a good thing.

Depending on the design, it could be turning on and off a few thousand times every second.

If the PWM frequency is in the audio range, you will hear it due to minute flexing of the motor windings turning electrical oscillations into mechanical oscillations. Some people don't care; some people find the whine annoying.
If you think you're in the second category, make sure the PWM frequency is above the audio range (say above 15-20kHz). I believe, though others should chime in, that these days faster switching is pretty easy to find, and doesn't cost a lot more.