VFD/DRO problem

EMC normally means electromagnetic compatibility and refers to the ability of the device to withstand outside electromagnetic interference without compromising the operation of the device.
EMI means electromagnetic interference and refers to suppression of electromagnetic interference generated by the device to avoid issues with other devices.
If the VFD manual is accurate, their EMC filter is concerned with preventing outside signal from interfering with the operation of the VFD and not with the VFD interfering with other devices.
Interference issues are usually best dealt with at the source of the interference. In the case of a VFD, one possible solution might be adding an EMI filter to the power cord. You may also have high frequency signals running on the 3 phase lines to the motor. The line would act as an antenna. radiating the high frequency to the surrounding space which is then picked up by the DRO. The simplest solution might be installing snap-on ferrite chokes on the line cord and the output cord. Low pass filters on the line and output leads is a more aggressive solution. Having a good ground to the VFD is a good start. You may have to install additional filtering on the on the DRO as well.
 
Thanks. But to be clear the ferrite suppressors go on the power cords, not on the DRO data cables?
 
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I use SY cable on the outputs of my vfd's and have the screen grounded at the supply end only(the earth terminal on the vfd).

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Is supposed to help with not radiating noise so much. Not that expensive for a few meters of it so might be worth a go. (mileage may vary and all that :) )

Stu
 
tstr said:
Thanks. But to be clear the ferrite suppressors go on the power cords, not on the DRO data cables?
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I don't have a 3 phase VFD but a brushed DC motor using PWM at around 25 khz. I have ferrites on both the line cable and the output to the motor. I had previously had interference when the OEM induction motor was switched off with my early version TouchDRO but to date, have only had one instance of interference with DRO during an excessively high current draw in a crash induced stall situation.

Belts and suspenders. Extra ferrites won't hurt.
 
The answer to post #12 is both, possibly. But try the power cords first.
 
You might try verifying the metal shield on the DRO data cables are grounded. Disconnect power, put an ohm meter with one lead to the shield, one to the ground pin on the DRO head. Then make sure that pin is connected in the power cord to a safety ground.

Make sure the VFD is grounded properly as well.

Make sure any cables for the VFD and DRO (power in/out or data) do not run parallel. They should cross at 90 degree angles if separation is not possible.
 
A bit of an update. I got ferrite supressors and some shielded cable sleeves and used them on most of the cables. I also tried relocating the DRO as far as possible from the lathe (about 1,5 m from the tailstock end) and moved the VFD as far as the cables alloved from the electrical box (about 0,5 m from the lathe). After all of this the reward was, wait for it... no change at all.
 
Another update. After getting shielded cable and an EMI-supressor for the inlet line, I changed the cable from the VFD to lathes electronic box and the motor power cable from the electronic box to the motor and added EMC cable glands to both cables going through the lathes electronic box. Also grounded the cable shield and wrapped the motor wires leaving from the VFD a few times around ferrite supressor rings.

The running numbers issue on the DRO got a lot better but did not completely stop (now the numbers on Y didn't seem change but on the X they did change from time to time), so any ideas besides getting an EMC cable gland for the motors connection box too? Maybe that will fix the issue completely, but as this has taken about forever (feels like it anyway) I might as well prepare for the worst.

I could also try to move the VFD into a metal enclosure a bit away from the lathe or change the cable from the wall outlet to a shielded one but don't currently have enough shielded cable to do both as the cable I got was expensive as hell and I only got 5 meters of it.
 
Yeah, now I'm feeling stupid. I went to add a missing screw to the other DRO cable that I had ordered a few months ago. While changing it I spotted a knob with letter PE on it and after some investigation decided to try to add a cable between the knob and the VFD ground. No more DRO problems. Thanks for everyone for the advice.
 
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