Motor controller warning.

[tonyfoale]

Many forum members use tread mill motors and similar commutated DC motors for driving various tools.
I have just been browsing the net looking at motor controllers and noticed many inaccurate descriptions. Many adverts claim PWM motor control and SCR control. In fact these are just SCR controls. Real PWM controllers tend to be more expensive. I have also seen claims of PWM only for what is plainly an SCR controller.

So what is the problem? Well it depends on what you use the motor for. An SCR chops away part of the normal rectified sinewave shaped input to reduce the average voltage and hence reduce motor speed. A PWM system uses a much higher frequency than the 50/60 Hz mains and varies the density of a square wave pulse train to vary average voltage and speed.

The SCR gives an output with sharp spikes when not at full voltage. This does not matter too much for applications such as a saw or even a milling machine or lathe spindle but the motor speed will be pulsing to some degree due to both the low drive frequency (double the mains value) and aliasing with the commutator segments. Mechanical inertia and motor inductance will tend to smooth these effects.

Due to its much higher frequency, 1-10 kHz being typical, PWM control is inherently much smoother and so makes it infinitely preferable for applications such as tool post and other grinders. You would probably get better finish on small lathes also. Use PWM when you want the smoothest drive, motors will last longer too.

Anyway this warning is not to say don't get an SCR controller but to alert forum members to misleading for sale notices on the net. Whatever type you go for make sure that you get one with sufficiently high voltage and current rating for your motor.