Category archives: VFD Cable

VFD cable related application notes

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Did you know that if you run cables that connect your variable frequency drives (VFDs) to your motors you could have a significant safety risk in your plant or factory? Fear not, there is a simple solution to this potential problem. It’s a fact. Non-shielded cables emit noise. In many cases, this is not a significant prob- lem. Most of us have heard that 60 Hz hum that happens when a phone line is run too close to a standard 600 Volt power cable. It’s really nothing more than a nuisance with standard power. But the same physics behind that hum may be creating a safety issue in your facility. VFDs change standard 60 Hz power in to variable frequency power that allow us to ex- perience significant energy savings, better control of our equipment, and reduced main- tenance costs. However, like most things in life, there are trade-offs. The down-side of a drive system is that it generates lots of high frequency voltage components that can cause problems with motors, drives, and other plant equipment. These same high frequency waveform components can also cause safety issues. Let’s look at how.

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Overview While it is acceptable to terminate the phase conductors of a VFD cable as you would any other industrial power cable, special attention needs to be paid to the termination of the cable shield. The shield is an important part of the VFD cable but if it is not terminated properly, most of the benefits that this shield provides are negated. If you don’t properly terminate a VFD cable’s shield you may as well have not spent the extra money on VFD cable to begin with! Proper shield termination allows the shield to become a low impedance path for high frequency common mode current to flow from the motor back to the inverter. Without this controlled path, these currents can travel through motor bearings and building infrastructure and cause problems with other sensitive equipment like PLCs, control, and communication systems located throughout your facility. There are three main types of shield found in VFD cables and Southwire makes VFD cables with each of these shield types. The shield types are: copper braid shield with aluminum foil (Copper Braid); A helically applied copper tape (Copper Tape); and a continuously corrugated welded aluminum used in MC cables (Aluminum Welded Armor). This application note will detail how to terminate each of these shield types.

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A variable-frequency drive (VFD) cable is a special cable construction for the inverter-to-motor cable that has some or all of the following attributes:
• An overall shield that keeps bad stuff such as electrical magnetic interference (EMI) from escaping.
• A robust insulation system that keeps good stuff such as voltage and current from escaping.
• A symmetrical design that reduces the amount of bad stuff in the cable, such as common mode current and EMI.

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