Why Cable Specifications Matter ？

The increasing popularity of variable frequency drives (VFDs) over the past decade has increased the need for specially designed wire and cable solutions to ensure long-term performance. To avoid costly downtime, diagnostics and repairs in these critical operations, it is critical to specify specially designed cables and termination systems that are effective against motor bearing failures, cable failures, stray currents and electromagnetic interference (EMI).

However, there is some confusion in the market when it comes to specifying the right cable system and termination methods that will optimize performance and increase the life of motors using VFDs.

Importance of Drive Cable Construction

The higher the switching speed of a VFD, the greater the electromagnetic interference (EMI) generated, which is inevitably radiated into the environment through the inverter-to-motor power lines. EMI emitted from the power lines can cause induced voltages, crosstalk or common-mode stray currents in adjacent cables and equipment, which can damage control systems and mechanical components.

To avoid these issues, be sure to select XLPE insulated, electrically balanced, shielded VFD cables that meet NFPA 79 standards when specifying cable systems for VFD applications. For optimal performance, RHW-2 2kV rated cables are designed with superior dielectric properties to resist voltage spikes generated under operating conditions. Increasing insulation thickness reduces impedance mismatch with the system and reduces issues with reflected wave phenomena.

Commonly Used Inefficient Cable Constructions for VFD Applications

Single and Multi-Conductor Thermoplastic Insulated Cables

Problem

Unshielded tray cables or single-conductor cables with thermoplastic insulation (such as THHN/THWN or TW90) are commonly used in VFD applications. However, thermoplastic insulation cannot withstand the voltage spikes introduced by VFDs. These spikes can cause hot spots and pinholes in the insulation and arcing on wires or grounded surfaces.

Solution

High-quality thermoset insulated shielded VFD cables are critical for VFD applications due to their higher dielectric strength, impedance characteristics, and physical properties. The copper tape shield has a smooth surface that provides a low impedance path for high frequency noise or common mode stray currents.

Continuously Welded Aluminum Armor

Problem

Shielded VFD cables are critical because AC current tends to flow on the outer surface of the conductor (known as the “skin effect”), increasing the effective resistance of the conductor and generating stray currents if not properly terminated. Continuously Welded Armored Cables rely on the aluminum armor as a shield path.

Solution

However, due to its higher conductivity and lower resistance, a 5mm flat copper tape shield with 50% overlap and proper termination can better direct high frequencies away from the ground plane and provide the lowest impedance path to avoid damage to motor bearings and sensitive electronics.

Braided Tray Cable

Problem

Braided tray cables have other issues that can adversely affect performance in VFD applications. Because the braid only provides a certain percentage of total coverage, and because the braid strands can move when the cable is installed with bends, these gaps can cause electromagnetic interference (EMI) radiation, increasing the risk to the motor and surrounding equipment.

Solution

In contrast, a copper tape shield with a 50% overlap can be routed around corners during installation and maintain complete electrical enclosure. Flat copper tape shields have lower path resistance than braided shields.