motor speed control

[goodpickles]

Hello!
What I need to do is control the speed of a dc motor. As I need to control the speed fairly accurately and have a decent amount of torque at low speed I think I need to use PWM. I will be running it off of and ac line, so I could use that for the trigger signal? Right?

Seems I saw a circuit a while back that would use the low side of the wave (when no power is being sent to the motor) to provide feedback from the motor to determine required power, so the circuit could vary power going to the motor to maintain a constant speed even if the load varies.

Can anyone provide a circuit like this?

 

[Styx]

PWM really is the simples and best way to control a machine

A simple DC machine has the advantage that speed is proportional to the voltage applied to the motor windings.

However, this is only true for a constant, unchanging load. IE if the machine is to drive a load that will never change then the machine constant can be calculated faily acurrately.

IF as you say the load could change, then you will need some sort of speed-feedback to ensure the speed is held at the demanded level.

and also from what it soulds like you need a current-loop as well to maintan the load