Also, terminating VFD cable at both ends allows common mode current to return to the drive through the cable shield.
But you might think if you terminate the shield at both ends, that’s going to create a ground loop. That’s not true. The ground loop already exists. Drives create common mode current. That current gets pushed to the motor from the drive through the cable’s phase conductors. It has to return to the drive. We want that current to return through a path we control. The best return path we have is through the cable shield.
If the current doesn’t return through the cable shield, it will return through a different path.
One possible path is for the current to return through the motor frame to motor bearings to motor shaft to connected factory equipment to building steel, and, finally, back to the inverter. If this path is taken, the current will flow in packets when it discharges across the bearings, causing bearing fluting — repetitive ridges in the bearing race — that damages motors.
Rule #2: How to Handle Drain Wires
A common VFD cable construction is a cable with:
· Three-phase conductors.
· One insulated full-sized ground.
· A full-sized bare drain wire.
· An overall copper braid shield covered with a jacket.
Drain wires are most commonly used in foil-shielded communication cables to provide an easy and reliable method to bond the shield. Because it’s difficult to reliably bond a thin length of aluminum foil, the drain wire, which is in contact with the foil, can be easily grounded.
The problem with drain wires in VFD cables is many electricians think they know exactly what to do with them: cut them off at one end and use the drain to terminate the other end. This is exactly what we don’t want to happen.
We want the cable to be terminated at both ends with low impedance, 360°, large surface area terminations. Drain wires don’t accomplish this. It’s not necessary to connect the drain wire to ground if you terminate the shield with a proper termination kit or EMC cable gland. If this is done, you can simply cut the drain wire off.
Southwire Company is a participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™ program. Based in Carrollton, Georgia, the company manufactures electrical wire and cable products.
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.