I spent some time the week before last investigating the noise issue that I’ve been having on my Bridgeport BOSS mill. The system architecture involves a single phase 240V input, which powers a VFD for the 2HP spindle motor, a 120V step down transformer to power some devices on the machine (spray mister, accessory lights, cabinet ventilation fans), and a 72V/24V/12V DC power supply to power the USC board and the three Gecko 203V stepper drivers. First, I disconnected every device from the DC power supply by disconnecting the positive terminal and disconnected one of the 240V AC wires to the VFD. I measured the noise from the ground bolt to the ground pin of the 12V output of the power supply. Noise was nominal, perhaps 20-40mV peak to peak at 50ms/div (as I recall). Reconnecting the VFD gave approximately 600mV of noise peak to peak. This seemed rather surprising given the fact that I’m essentially measuring the same wire across about 18".
I installed a Rasmi VFD filter and I re-wired the grounds. Instead of 2 grounds which were connected via the machine, I ran all grounds back to the ground bolt in the power cabinet. Previously, the 72V ground to the drives was only wired directly from the power supply to the drives. It was isolated from the other power and logic grounds, but is now common. The only ground that was not run to the power cabinet ground was the stepper’s step and direction signal ground. This ground is connected at the USC board to a ground which is common with the 12V power input ground that powers the USC board (and is therefore run back to the main ground bolt). Upon powering up everything I discovered that the spindle index encoder noise had increased from around 1.4V peak to peak to more like 7V peak to peak. I’m shocked that a ground wiring change could impact this measurement so dramatically. The VFD only noise is now measuring approximately 400mV p-p at the power supply 12V ground, which I believe is acceptable. However, I’m sure I could find higher spikes if I reduced the time scale based on my previous test results. I got some conflicting results when trying to isolate the cause, so I rechecked my measurements today. Rechecking my measurements at the 12V output ground on the power supply, I found: VFD only: ~300mV USC only: ~40mV USC and 24V devices (mostly SSRs for pneumatic valves): ~40-50mV USC, 24V devices, and Geckos with NO 72V input (i.e. only step & direction inputs) ~40-50mV USC, 24V devices, and Geckos with 72V input (i.e. ready to move motors) ~1-1.2V Now I tested the 72V wire at the gecko’s power input and found noise of approximately 5V. Then I removed the wire at one of the two 72V supply outputs (it has dual 72V outputs) and measured at the + terminal with two other geckos powered from the other 72V output. Noise was minimal (though difficult to measure as my scope wouldn’t let me scale to see the ripple as clearly — at most it’s probably 1-2V). This leads me to the conclusion that either the supply somehow struggles under load or the Geckos are introducing inordinate noise. I’m doubtful the supply is the culprit because the noise appears to be a much higher frequency than would be expected from a simple rectified power supply with filtering capacitors. The geckos are in the control cabinet with the USC. Would it help to move them closer? Thanks, Matt _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
