[ralph@nes]

Not really a servo drive but very, very similar, I have an AC Tech / Lenze motor drive issue.

We are using this drive/VFD to control the RPM of a large AC motor on a test rig which we use to test bearings.

I have several sensors at work on this rig, some of which measure pressures, temperatures, and flow. Most of these are 0-5v or 4-20ma output and are working fine.

I have three sensors on the machine which are proximity or magnetic sensors. These are for monitoring RPM on the machine in three location. They also are a 0-5v signal but are a TTL pulse train (basically on-off very fast). Every time we try to use the VFD on this rig, my readings from these magnetic sensors go haywire!

When using a straight-up motor starter on the system, we have no issues. Apparently when the VFD converts the voltage to a frequency to control the RPM of the motor, it is creating some sort of RF or EM interference. We are using shielded power wire from the VFD to the motor and the sensor leads are also shielded up til the last little bit where it has to be exposed in order for the signal from the magnetic pickups to work.

Does anyone have any experience with motor drives causing interference, and if so, what did you do? I'm tearing my hair out on this project. Any input is greatly appreciated.

[Al_The_Man]

VFD's can put out alot of interference, there are a few measures I would check into, There is special cable made by Beldon for VFD's, but is very expensive, first , I would make sure the three phase wiring to the motor is tightly wound together, do not include the ground wire, the motor and VFD should be tied to the same ground conductor directly back to the Machine service ground supply.
There are also three phase inductors or chokes, that can be fitted on the motor as well as feed conductors.
I would try and establish that all the sensor power supply commons and the shields are taken to a common ground plate. The machine should already have this in the electrical enclosure.
A while back I retro-fitted a CNC machine that has three 5hp VFD's and using these precautions and have not seen a problem.
Al.