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.
Terminating Copper Braid and Copper Tape Shields
– Using Glands
These shields may be terminated with either a cable gland, or if used in conduit a termination kit. Both methods are described below. Both ends of the VFD cable need to be terminated in order to allow the high frequency common mode current to flow back through the shield to the drive 1 .
– Using Termination Kits
To terminate this shield with a gland, use an EMC style cable gland which will provide a 360° connection to the shield at both the inverter and the motor. EMC cable glands are available from various manufactures. One such gland that works well with both a copper tape and a copper braid shield is the Remke Dome Cap Standard EMC Cable Gland (see attached). Follow the manufacturer’s instructions for installing the gland. If the cable is going in conduit and a gland cannot be used, you will need to create and install VFD terminations on the shield. Each termination will consist of a constant force spring sized for the overall shield diameter, a tinned copper flat grounding braid and vinyl electrical tape. All these materials are available from 3M and other electrical suppliers.
Installing the Termination
• Remove jacket to allow wire leads long enough to make the connections to the equipment.
• Remove shield back from jacket cutback 1.5 inches.
• Place a section of the tinned copper flat grounding braid long enough to be terminated to the enclosure longitudinally over the copper tape or braid shield. Secure the grounding braid with the constant force spring.
• Wrap the constant force spring over the cable shield to secure the grounding braid to cable.
• Secure the constant force spring to the cable shield by applying two half-lapped layers of vinyl electrical tape.
• Terminate the free end of the grounding braid to the metal enclosure using a nut, bolt and large flat washer. Be sure to clean the surface of the metal enclosure and broaden the grounding braid to maximize surface area.
Terminating Aluminum Armor Shields
Continuously corrugated welded aluminum armor shields are easy to terminate because the termination is performed with a gland specially made for MC cables. For the optimal physical and electrical performance, Southwire strongly recommends using American Connector metal clad cable connectors. Because of their unique screw on installation, these glands provide the maximum surface area connections possible which reduces the glands impedance. To source and size your connectors, please contact your Southwire sales representative. Follow American Connectors instructions for installing the gland. Information on American Connectors MC cable glands can be found at www.americanconnectors.com. Both ends of the VFD cable need to be terminated in order to allow the high frequency common mode current to flow back through the shield to the drive¹.
1. Most drive manufacturers recommend terminating the shield at both ends of the cable but some do not. Always follow the recom-mendations of your drive manufacturer regarding cable installation.