Why Proper VFD Cable Termination is Crucial

By Tony Holderby, Product Specialist and Regional Sales Manager, Service Wire Company

The ability to optimize energy consumption through the precise control of motor speed has led to the increased usage of variable-frequency drives (VFDs) across many industries. These drives require specifically engineered cable and termination solutions to function properly. When VFD issues arise, a common reason is the improper termination of VFD cable, which allows uncontrolled electromagnetic interference (EMI) to wreak havoc throughout the system.

The EMI emitted from the power cable can induce voltages, common-mode stray currents or crosstalk onto adjacent cables damaging the drive, motor, motor bearings and surrounding equipment. The source of this damage can be difficult to diagnose and cost millions in downtime. Implementing proper cable termination in a VFD system is an easy way to avoid these costs.

Consider the Cable

When considering termination, start with the cable itself. A properly designed cable is an essential part of any VFD system. To ensure full electrical containment, VFD cables should have thermoset insulation (such as XHHW-2 or RHW-2), three conductors, three symmetrical balanced grounds and a 5-mil copper tape shield with 50% overlap.

However, without proper termination of the copper tape shield, the benefits these cables offer can be lost. For the most reliable termination, we recommend that a VFD system include self-terminating connectors and termination kits for each end of the cable.

To control EMI, VFD cable should be terminated at both ends — at the VFD cabinet and at the motor. When bonded properly at both ends with connectors and termination kits, the copper tape shield becomes the path of least resistance and can return stray currents back to the source — reducing EMI emissions from the cable.    

Copper Tape Shielding

As noted, copper tape shielded cable is essential for proper VFD termination. In fact, properly designed and grounded shielding is considered by many to be the most important consideration for cable system performance.

Overlapping the tape eliminates gaps, ensuring 100% electrical containment even when the cable is bent. Additionally, the tape’s large surface area, combined with the low impedance of copper, provides an ideal return path for stray current.

Self-Terminating Connectors

Field testing has found that, in addition to properly engineered cables and copper tape shielding,  creating the lowest possible impedance path for current requires a 360-degree termination to earth ground at both the motor and drive end of the cable. The recommended way to achieve such a termination is through self-terminating connectors.

VFD self-terminating connectors are a specialized type of cable gland that make 360-degree contact with a cable shield when tightened. This connection creates a low-impedance path for stray currents to follow, keeping them contained in the VFD cable and away from surrounding equipment.

The connectors should be used on both ends of a cable to prevent stray current from damaging the VFD or motor. Connectors are available for direct wiring, conduit and liquid-tight applications to accommodate various installation requirements.

Termination Kits

VFD termination kits should be used in conjunction with connectors to reinforce the low-impedance return path. This helps extend the life of the motor and prevent motor bearing failure.

Termination kits typically consist of flat-woven tinned copper braids, a stainless-steel spring clamp and adhesive-backed copper tape. The braids are installed on opposite sides of the cable against the cable’s copper tape shield. Adhesive-backed copper tape is wrapped around the braids to hold them in place against the shield, and the clamp is placed over the tape to further secure them.

This configuration is wrapped with more copper tape for additional support. The other ends of the braid are joined together and then terminated to earth-grounded bare metal — of an enclosure wall or motor junction box — with a brass bolt or flat plate with bolt. This creates a clear path for any common-mode stray current to follow.