Techno Isel Lathe Refit Using Centroid Acorn and Teknic Clearpath Drives

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Re: Techno Isel Lathe Refit Using Centroid Acorn and Teknic Clearpath Drives

Post by BodeRacing »

I'm just reading this post now. Let me make some clarifications on proper VFD cabling.

There are 3 sources of what I refer to as stray currents caused by VFDs. Stray currents cause bearing failure in motors and equipment, and can cause nearby electronic equipment to misbehave or even fail. Been there done that many times (actually I fix it not create it). Normally stray currents don't hurt bearings in equipment this size, but in testing I catastrophically failed both bearings in a 120 vac fed VFD 1 hp motor in 70 hours.

The stray currents are caused by common mode voltage generated by the drive. Common mode voltage means that the voltage between the neutral and ground is not zero. This causes common mode current flowing in unwanted paths.

First is circulating current. Not usually an issue in motors this size and no action is required.. Solution is an insulated opposite drive end bearing in the motor.

Second is stator to rotor capacitive currents. Usually not much of an issue in equipment of this size. Solution is a ground brush on the drive end of the motor.

Last is the most important. Stator to frame capacitive currents. This is what usually gets you in trouble in equipment of this size. The common mode currents are in the motor frame and WILL get back to the drive (back plane of the drive). You need to control this current path. This is where the shielded cable comes into play. A proper shielded cable will be the preferred low impedance path from the motor frame to the drive. The shield must be well high frequency bonded at the motor and drive. High frequencies don't want to travel in regular wire, even short pieces. The shield needs to be directly bonded to the motor frame and the drive. High frequency strap can be used for the connections if kept short.

Not practical for most folks, but the way to verify proper installation is to put a Rogowski coil around the motor leads and measure the cable summation current. You want to see zero high frequency current flow.

In addition, to protect against cross talk (what you refer to as EMI), even with the shielded cable, keep the motor leads at least 3 inches from all other cables and 6 inches from very sensitive cables.

This is proper VFD installation. I have been trying to get a VFD installation paper done for an IEEE conference for a few years now, but just haven't gotten to finishing it.
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