Importance of Specifying the Right Cable System for VFD Applications
Written March 2020 | Steven Stanford, National Sales Manager - Industrial at Service Wire Company
The increased adoption of variable frequency drives (VFDs) over the past decade has increased the need for specifically engineered wire and cable solutions that ensure long-term performance. To avoid costly downtime, diagnosis, and repair of these critical operations, it is important to specify cable systems designed to address motor bearing failure, cable failure, stray current, and electromagnetic interference (EMI).
Importance of Cable Selection for VFDs
The higher switching speeds of VFDs inherently generate more high frequency currents that inevitably get radiated through the power cable damaging control systems and mechanical parts. To avoid these issues, an electrically balanced, copper tape shielded cable with NFPA 79 compliant crosslinked polyethylene (XLPE) insulation is the best solution to maximize drive performance.
Independent lab and field-testing by prominent manufacturing companies have proven that the ServiceDrive® VFD Cable System is the best solution available. However, there are several less effective wiring methods commonly used for VFD applications that cannot provide the complete protection for commercial or industrial applications.
Cable Constructions Commonly Used for VFD Applications
|Single and Multiconductor Thermoplastic Insulated Cables
||Cables insulated with thermoplastic insulation (like THHN/THWN) cannot withstand the voltage spikes associated with VFDs. These spikes result in hotspots and pinholes in the insulation, allowing arcing across wires or grounded surfaces. High quality thermoset insulated shielded cables are critical for VFD applications because of the higher dielectric strength, impedance characteristics, and physical properties. Copper tape shielding provides a smooth surface with a low impedance path for the high frequency noise or common mode stray currents.
|Continuously Welded Aluminum Armor
Continuously welded armor cable relies on aluminum as armor and the shield. However, a copper shield with proper termination provides a superior low impedance, low resistance path, allowing the current to flow back out of the system on a controlled path and avoiding damage to motor bearings and sensitive electronics. A flat copper tape shield with 50% overlap has been proven to be the best solution, providing better flexibility and ease of installation.
|Braided Tray Cable
Braided tray cable presents additional issues that adversely impact performance in VFD applications. These cable designs will typically use an aluminum-backed mylar wrap, which has increased electrical resistance when compared to bare copper tape. Because the braid can only offer a percentage of total coverage and braid strands move when the cable is installed with bends, these gaps allow EMI to radiate and provide a less than desirable return path for high frequency currents, compounding the risk to the motor and surrounding equipment. By contrast, a flat bare copper tape shield with 50% overlap can be bent around corners during installation and maintain full electrical containment.
The ServiceDrive® VFD Cable System is the only complete solution for issues commonly associate with VFD applications. The system consists of insulated cable with three NFPA 79 compliant XLPE conductors and three balanced grounds, 5-mil copper tape shield, and matching self-terminating connectors and termination kits.
- The heavy-duty flat copper tape shield is helically applied over the cable core with 50% overlap, ensuring full electrical containment of EMI emissions at all points even when the cable is bent.
- The conductor and ground configuration and overall copper tape shield provide the best low impedance return path for stray currents.
- The thermoset insulation is engineered to withstand voltage spikes, hotspots, and pinholes, preventing premature cable failure.
When specifying cable systems for VFD applications, it is important to select an electrically balanced, copper tape shielded cable with XLPE insulation. This design will:
- Reduce EMI
- Provide a low impedance return path
- Reliably withstand high voltage spikes
However, lesser cable constructions are used that can’t provide the protection required for VFDs.
Unshielded single and multiconductor cable does not address any of the issues associated with VFDs. Continuously welded armor relies on aluminum as a shield, instead of copper. Braided tray cable only offers a percentage of total coverage, allowing stray currents to occur.
The ServiceDrive® VFD Cable System is the only off-the-shelf complete solution. The cable is made of thermoset insulation, and the conductor and ground configuration with overall copper tape shield provides the best low impedance return path.
The ServiceDrive® VFD Cable System now includes self-terminating tray cable connectors for direct wiring, liquid tight, NPT conduit and interlocked armor installations. These patented connectors are key to unlocking the full benefits of the complete ServiceDrive® System, protecting drives, motors, bearings, and sensitive electronic equipment.
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