I have run into a tricky problem. I have had a homemade CNC mill, made from a drill press, for several years. I use a PC with TurboCNC software driving a much modified Chinese Stepper controller with the Toshiba 6560 driver chip.
I recently modified the Mill to use a commercial column and spindle head. I retained the AC drill motor but got rid of the belt and pulleys by using a direct gear drive. To control the speed I built a variable frequency drive based on a paper on the microchip website that synthesizes two sine waves 90 degrees apart using PWM at 12kHz.
My problem is that when the motor is running it is generating noise that causes the Y and Z steppers to creep. The co-ordinates in the software are not changing so the software isn't causing the creep. If I disconnect the LPT cable and run the motor I do not get the creep so I don't think it is coming from the Chinese controller. I suspect that I am either getting induced noise in the LPT cable although I believe it is shielded, or I am getting noise on the AC line that is getting back into the computer. I looked for noise on the AC line with a scope but didn't see anything obvious. I tried moving the cable away from the motor and control without any noticeable difference. If I replace VF control with the original start capacitor there is no problem so the noise is definitely coming from the VF controller. Does anyone have an idea how to fix this or isolate what is causing it?