Enhancing System Performance With Proper VFD Cable Installation

Variable frequency drives (VFDs) are unique devices that combine high voltages and currents with high-frequency waveforms. Plant engineers typically do not work with these devices. 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 on the factory floor. Motors can fail, drives can trip or fail and other equipment, including radios, sensors, alarms and programmable logic controllers can have problems operating and prematurely fail. Fortunately, it is possible to mitigate these issues and the solution is simple: use properly terminated VFD cables between drives and motors.

The History of VFDs

VFD cable constructions became available in the late 1990s after incorporating the insulated gate bipolar transistor (IGBT) into drives. These new power transistors had much faster rise times than their predecessors. This evolution allowed drives to become smaller, less expensive, more energy efficient, and more precise in control, but they also had the undesirable side effects mentioned above. Cable manufacturers quickly offered new cable constructions, causing many customers to believe that VFD cable was the magic cure-all for all factory automation issues caused by these new drives.

Customers’ results often did not live up to the manufacturer’s claims. One primary reason for the disconnect was that proper installation practices were not communicated or followed. Additionally, sales representatives unknowingly overpromised what the cable could do (for instance, VFD cable does not help with harmonics or reduce reflected wave amplitude). It has also been more than 25 years since the first VFD cable hit the market, and up until 2024 there was no standard in the U.S. for this specific cable construction. As a result of these obstacles and misconceptions, some VFD cable constructions have done more harm than good.

Motor Testing Is Key for VFDs

It was determined that testing should be performed to help educate the industry on this topic. However, the only existing testing found on drive to motor cables was a 1997 IEEE paper titled “Evaluation of Motor Power Cables for PWM AC Drives” by John Bentley and Patrick Link.(1) While the paper did evaluate eight different cable constructions, many of these constructions are not used in North America and none of the cables chosen included standard building wire—the most common cable used for application in the U.S. or Canada.

The cables tested in this paper were as follows:

• Five NFPA 70: National Electrical Code (NEC) (2) Type MC (metal clad) cable constructions.
• One NEC Type TC (tray cable) cable construction with no shield.
• Two IEC (International Electrotechnical Commission) cable constructions.

Two currents (total ground and standard mode current) and two voltages (motor frame and crosstalk voltage) were measured. The results clearly showed that shielded cable outperformed unshielded cable. The best-performing cable was a continuously corrugated and welded aluminum armored cable with three conductors and three grounds. This cable is seldom used on factory floors because of its installation difficulty. There was no comparison between the performance of the most common cable used in these installations (unshielded building wire) and other constructions. Also, shield termination methods were briefly addressed but were not a significant part of the study. Finally, as the paper is more than 25 years old, most of the cables tested are no longer installed in factories.

Over the past year and a half, Southwire engineers have been conducting tests on various cable constructions currently used in this application on the factory floor. In addition to testing cables, we tested different installation methods to see how system performance varies with cable termination methods.

Extensive testing helped to determine how the cable connecting the drive to the motor and its installation method could affect factory floor performance. Currents and voltages were measured throughout the system while motors ran at different speeds, loads and cable lengths. The selection of the cable and proper shield termination significantly impacted system performance. Tests were run on seven different cable constructions, three building wire constructions (4-conductor thermoplastic high heat-resistant nylon (THHN) tray cable, four-conductor XHHW (Cross Linked High-Heat Water-Resistant) tray cable and four single conductor THHN wires) and four VFD cable constructions (two four-conductor nonsymmetrical cables and two three-conductor three-ground symmetrical cables). Tests were run on cables of 30 and 300 feet in length, with up to seven different shield termination methods. Testing was also performed at two fundamental motor speeds (25 and 75 Hz) and two loads (0% and 40% torque).

Results of VFD Testing

Data analysis is ongoing; three interesting findings include:

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, like THHN and XHHW.
   
   Radiated EMI can adversely affect nearby electrical equipment, causing intermittent operation, false alarms and premature failure. The effect of system ground currents drives create (a form of conducted EMI) can have a similar impact as radiated EMI. However, while radiated EMI decreases the farther from the source, conducted EMI does not.
   
   These conducted EMI ground currents consist of high-frequency current spikes and can be several times the drive's output current. Properly terminated VFD cable creates a low impedance path at high frequency via the cable shield. It pushes most of that unwanted, noisy current along this controlled path. It isolates it from the building ground, thus minimizing the ground current spikes from flowing past factory equipment where it can cause damage.
   
2. Drive speed and torque have minimal influence on system performance.
   It does not 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, there was little difference between tests irrespective 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 the speedy rise times (50 to 100 ns) of the pulse width modulated voltage waveforms that the drive puts out.
   
3. Some cable constructions are much easier to install correctly than others.
   One unexpected but significant finding was that some VFD cables allow for a more “foolproof” installation than others. Southwire’s three-conductor VFD cable with three bare grounds (3 + 3) performed well regardless of the shield termination method. This is because the bare ground, which is in contact with the cable shield, bonds the shield to the ground at both ends. The classic four-conductor design incorporates an insulated ground and performance varies depending on the shield termination methods. Testing confirms most drive manufacturers’ recommendations that the cable shield be bonded to the ground at both the motor and the drive. Properly terminated VFD cable is the best way to control unwanted current spikes on the ground. It also provides the best protection against EMI.

In conclusion, while VFDs offer significant advantages in energy efficiency and motor control, their unique combination of high voltage, current, and frequency introduces complex challenges on the factory floor. From equipment malfunctions to electromagnetic interference, these challenges can be mitigated through the use of properly constructed and installed VFD cables. The latest testing confirms that cable type and shield termination methods play a critical role in system performance, with properly terminated VFD cables significantly reducing harmful ground currents and EMI. Understanding and applying best practices in VFD cable selection and installation will be essential to ensuring reliable, safe and efficient operations.

Sources Referenced:
1. https://ieeexplore.ieee.org/document/567793
2.https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70