Why VFD Cable Choices & Installation Matter

By Steve Wetzel, Principal Application Engineer, Southwire Company

Editor’s Note: This article is adapted from the white paper, “Testing Data Shows Why VFD Cables & Installation Matters,” from Southwire Co. Download the full paper for comprehensive information about what was tested and how; testing results; the impact of cable type and installation on system performance in variable-frequency drive-to-motor operations; and the impact of cable constructions in variable-frequency drive on motor operations.

It's a simple concept, but many are shocked by the difference the proper drive-to-motor cable can make in their facility's operational performance. And, without the appropriate test equipment, the difference is imperceptible.

Variable-frequency drives (VFDs) are unique devices that combine high voltages and currents with high-frequency waveforms. We usually don’t work with devices like this. Usually, we limit high power to 60 Hz (or up to 400 Hz in aircraft applications) and high frequencies to low-power applications. Drives combine them. Even a relatively small drive can put out many amps at hundreds of volts with frequency components in the tens of megahertz.

This combination of high voltage, current and frequency can create unexpected problems. Motors can fail, drives can trip or fail, and other equipment, including radios, sensors, alarms and programmable logic controllers (PLCs), can have problems operating and fail prematurely. Fortunately, it’s possible to mitigate these issues simply by using properly terminated VFD cables between drives and motors.

Motor Testing is Key for VFDs

To address these issues, we built a dedicated test stand (see illustration) and conducted extensive testing of the following cables:

• Five NFPA 70: National Electrical Code (NEC) Type MC (metal clad) cable constructions.
• One NEC Type TC (tray cable) cable construction with no shield.
• Two IEC (International Electrotechnical Commission) cable constructions.

In addition to testing cables, we tested different installation methods to see how system performance varies with cable termination methods. Currents and voltages were measured throughout the system while motors ran at different speeds, loads and cable lengths. We reviewed test data comparing the advantages of properly terminated VFD cables over thermoplastic high heat-resistant nylon (THHN) between the VFD and motor in factory automation environments.

Results of VFD Testing

Data analysis is ongoing. However, two interesting findings are as follows:

1. Cable type and installation have a significant influence on system performance. Cables and installations were classified into one of three groups: standard cables (unshielded THHN and XHHW), properly terminated VFD cables (using EMC (Electromagnetic Compatibility) clamps and VFD shield termination kits), and improperly terminated VFD cables (no termination of the cable shield).

System ground currents measured in each cable group significantly reduced unwanted high-frequency ground currents with properly terminated VFD cable. Radiated electromagnetic interference (EMI) also was lower when a shielded cable was installed, provided the shield was bonded to the ground at least at one end. If not, the EMI levels of the shielded cable were similar to the EMI level seen with unshielded standard wires, such as THHN and XHHW.

2. Drive speed and torque have minimal influence on system performance. It doesn’t matter if the motors are running at full speed or slow, or if the load is heavy or light; these issues exist regardless.

When test results were compared as a percentage of drive current, little difference existed between tests regardless of whether the tests ran the drives at 25 or 75 Hz and whether the motor experienced 0% or 40% load. This is because the source of these unwanted currents and voltages is due to the speedy rise times (50 to 100 ns) of the pulse-width modulated voltage waveforms the drive puts out.

Full Results

Download our white paper to get details about the testing procedures and how it revealed that the correct selection and installation of VFD cables, particularly those that are properly shielded and terminated, are crucial to minimizing high-frequency ground currents and reducing EMI — thus improving reliability of plant-floor operations.

With global headquarters in Carrollton, Georgia, Southwire is a Rockwell Automation Technology Partner. The company manufactures electrical wire and cable products, and has corporate divisions that include Industrial, OEM, energy and electrical.

Like this article? Sign up for the digital magazine (4X/year) and e-newsletter from The Journal From Rockwell Automation and Our PartnerNetwork.

^{The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Endeavor Business Media.}