A variable frequency drive (VFD) is a type of controller that varies the frequency of the signal and voltage to control motor speed. Other common names for VFD include variable speed drive, adjustable speed drive, constant torque drive, inverter drive, and pulse-width modulation (PWM) drive. But what is VFD cable, and why is it important? VFD cable is a specially designed power cable engineered to connect a variable frequency drive to a motor while withstanding the unique electrical stresses created by VFD systems. Unlike standard cable, VFD cable is designed to reduce electrical noise, manage voltage spikes and reflected waves, and help protect motors, drives, and connected equipment from premature failure.

When working with a VFD system, you need VFD cable that can handle that harsh electrical environment. VFD systems characteristically create a significant amount of electrical noise in the form of voltage spikes, reflected waves, charging currents, common-mode voltage, and RF/EM interference. This leads to voltages that are two to three times the source voltage.

In the case of a non-VFD cable, this could result in insulation breakdown and cable failure. Non-VFD cable also allows common-mode (or stray) voltage to find its way through the VFD motor and equipment grounds, which causes motor and equipment failure. VFD cable significantly reduces the risk associated with these issues.

"VFDs are becoming more and more popular," says Eddie Seibolt, Product Manager at Amphenol TPC Wire and Cable (ATPC). "To put it simply, VFDs provide better process control, precise motor speed, and improved efficiency throughout the plant, whether it's a steel plant, food and beverage manufacturing, or other industrial applications."

As VFD technology becomes increasingly common across the manufacturing world, selecting the right cable has become more important than ever.

Diving deeper: So, what is VFD Cable? Why is it special?

Explaining something as complex as variable frequency drive cable can be complicated to those outside of electrical engineering, but simply put, VFD cable is a specialized motor supply cable designed to operate between a variable frequency drive and an AC motor. Because VFDs use high-frequency switching to control motor speed, the cable must withstand conditions that standard power cable was never designed to handle. And what makes this type of cable special, you ask? The answer comes down to engineering.

When a VFD controls motor speed, it uses high-frequency switching to create a pulse-width-modulated (PWM) output signal. While this improves process control and energy efficiency, it also generates electrical phenomena that can damage motors, drives, and cable systems if not properly managed.

These conditions include:

• Voltage spikes

• Reflected wave voltage

• Common-mode (stray) voltage

• Electromagnetic interference (EMI)

• Radio frequency interference (RFI)

• Charging currents

• Corona discharge

A properly engineered VFD cable incorporates specialized design features to mitigate these effects and improve long-term system reliability.

Enhanced Shielding Design

One of the most important components of a VFD cable is its shielding system, Seibolt says. "At ATPC, we're using a braided shield that's going to provide robustness, longevity, durability, and overall performance."

Unlike many commodity cables that rely on tape shields, high-performance VFD cables engineered at ATPC often use a braided copper shield that provides a low-impedance path to ground. This helps contain electrical noise, reduce EMI/RFI emissions, and safely return common-mode currents to the drive rather than allowing them to flow through motor bearings or other equipment.

Corona-Resistant Insulation Systems

Voltage spikes generated by VFD systems can exceed the motor circuit's nominal operating voltage. These spikes create electrical stress within the cable insulation that can lead to corona discharge, insulation degradation, and premature cable failure.

VFD cable is engineered with insulation systems capable of withstanding these repetitive voltage stresses, helping extend cable life and protect critical equipment.

"The ATPC version withstands corona voltages up to 2,000 volts. Commodity cable is usually lower rated and may be insufficient even with a 480-volt system," Seibolt adds.

Low-Capacitance Construction

Engineered VFD cables are often designed with lower capacitance to reduce charging currents generated between the drive and motor. Excessive charging currents can place additional stress on drive electronics, particularly in longer cable runs.

By controlling capacitance, VFD cable helps improve overall system performance and reduce the likelihood of drive-related issues.

Symmetrical Grounding for Bearing Protection

"Stray voltage can travel through the motor bearings, causing pitting, frosting, and fluting over time, which leads to vibration, noise, and eventually, motor failure," says Seibolt.

Many VFD cable designs incorporate symmetrical ground conductors positioned around the power conductors to help equalize electrical fields and provide a predictable return path for common-mode currents. Without proper grounding design, stray voltage may seek alternate paths through motor bearings, resulting in the failure mechanism Seibolt outlined.

Industrial-Grade Materials

Electrical performance is only part of the equation. VFD cables are frequently installed in harsh manufacturing environments where they are exposed to abrasion, chemicals, impacts, oil, and extreme temperatures.

For this reason, premium VFD cables utilize specially compounded jacket materials designed to maintain flexibility and durability long after commodity PVC jackets begin to degrade.

Questions to Ask Yourself (If You’re Not Seeing a Problem With the Cable You’re Using on Your VFD System)

Images courtesy of CJ Cowie

• Have you ever made adjustments to your VFD controller to reduce or eliminate nuisance faults? If so, this may indicate that the standard cable you’re currently using is inadequate for the VFD system.
• Are you experiencing VFD motor failures sooner than you expected? What about other equipment failures? Cable failures? All three types of failure are symptoms indicating you may be using ill-suited cable.
• Have you noticed unusual motor vibration, bearing noise, or repeated bearing replacements? These may be signs that stray voltage is affecting your motor.
• Have you observed white, powdery residue on cable insulation? This could indicate corona damage caused by voltage stress within the system.

Benefits of Working with a VFD System

• Improved control: VFD motor speed controlled within 0.1% of tolerance, facilitating complex processes
• Higher efficiencies: Properly designed systems can reduce energy costs
• Smooth/soft motor start: Reduces motor fatigue and early failure
• Quiet(er) operation
  

Challenges Often Associated with a VFD System

• Excessive electrical noise: Voltage spikes, electromagnetic (EM) and radio frequency (RF) noise, reflected waves potentially doubling/tripling voltage, common-mode or stray voltage, charging currents
• Failure of motor (bearings) and controller
• Failure of cable due to corona damage
  
• Motor bearing damage caused by common-mode voltage
• Premature equipment downtime resulting from electrical stress

The Importance of Selecting a High-Quality VFD Cable Over Standard Cable

Standard cable poses a risk of failure due to high-voltage spikes (1,500 volts or greater) and corona discharge, which can damage the jacket and/or insulation. VFD cable provides the proper shielding to reduce EM/RF noise and provide a low-impedance signal return path, preventing common-mode or stray voltage that can lead to premature cable and/or equipment failure.

"You can get away with not using a VFD cable at first," Seibolt says. "But you'll likely see some potential issues down the road that could lead to future downtime in the plant if they're not using a VFD-specific cable. With VFD systems in general, high-frequency switching creates electrical noise. That's just the nature of the VFD motor. If you're using a commodity cable instead of a VFD cable, that can result in motor, controller, and cable damage."

Smaller-sized VFD controllers are even more susceptible to charging currents. In this case, the charging current associated with a higher cable capacitance may damage the drive output. Again, installing a lower capacitance VFD cable reduces this risk.

TPC offers several VFD cables engineered to withstand the harsh environment generated by VFD systems:

Super-Trex® VFD Shielded Power Cable: 

• Constructed with both foil and braid shields to withstand corona voltages up to 2,000 V 
• Features symmetrical ground wires to reduce the impact of common-mode voltage noise 
• Ideal for applications where a longer-lasting cable is desired 

Trex-Onics® Low Capacitance VFD Shielded Power Cable

Trex-Onics® VFD Shielded Power Cable (with Option for Brake and Signal Pairs): 

• Constructed with a heavy-duty, tinned copper braid shield to protect equipment from damage caused by electrical noise and stray voltage and to provide a shield against EMI/RFI noise. 
• Features a semi-conductive layer designed to withstand corona voltages up to 2,000 V

Thermo-Trex® High Temperature Shielded VFD Reeling Cable

Thermo-Trex® High Temp VFD Cable

Top 3 Reasons to Choose TPC’s Solution Over the Commodity Product

1. Braid shields: TPC’s tinned copper braid shields greatly improve flexibility and ease-of-use when compared with the copper tape shield found in commodity cable, which is stiff and difficult to bend in tight spaces.
2. Voltage/Corona ratings: TPC’s products are designed to withstand corona voltages up to 2,000 V as compared to commodity cable, which offers lower ratings that may be insufficient even with a 480 V system.
3. Jacket life: TPC’s products feature a specially compounded jacket that is designed to hold up in harsh industrial environments for extended periods of time, as compared with commodity cable, which provides a simple PVC jacket.

While standard cable may appear to work in a VFD system, it often lacks the shielding, voltage handling capability, and construction needed for long-term reliability. By choosing a VFD cable specifically engineered for the application, facilities can reduce electrical noise, protect motor bearings, minimize downtime and extend equipment life. Whether you're installing a new VFD system or troubleshooting recurring failures, selecting the right VFD cable is one of the most important steps in protecting your investment.

Want to discuss your specific VFD application and issues you may be experiencing? Contact us today.