VFD Cable Requirements and Guidelines for High Speed Applications

Variable frequency drives (VFDs) require specialized cabling to ensure reliable operation. VFDs also need specialized cabling to minimize electromagnetic interference and maintain safety for the system as a whole. Because selecting the right cables and proper installation directly impact performance and equipment longevity, understanding proper VFD cable requirements is critical.

Download the Whitepaper: Grounding and Power Cable Guidelines for VFD Applications

What are the Cable Requirements for a VFD?

VFD cable requirements include high strand count construction, XLPE insulation, and proper shielding to handle high-frequency switching and minimize EMI.

For high-speed applications, these requirements are critical due to the elevated switching frequencies and resulting electrical noise. Specialized VFD cables with XLPE insulation and symmetrical grounds help maintain system integrity and reduce noise-related issues.

The cables must also withstand rapid voltage rise times (dv/dt), resist signal distortion from harmonic content, and maintain low impedance over the drive-to-motor length. Using the wrong type of cable (such as standard THHN or building wire) can result in degraded performance, increased EMI emissions, and insulation breakdown over time.

For most industrial environments, cables rated 600V or 1000V with robust thermal and chemical resistance are preferred. This specification exceeds many competitors’ standard recommendations. Terminations should be properly grounded, and the cable’s shield must be continuous and correctly bonded at both ends to achieve the intended noise suppression and safety benefits.

Considerations for Designing Grounding and Power Cables

To achieve optimal performance, several key considerations must be taken into account when designing the power transmission components for the high-speed system. For this article, we will discuss a few highly critical components: grounding and power cables.

High-speed applications present some challenges that may not appear when running motors at standard output frequencies (60 Hz, for example). The higher frequency output required for these applications requires the VFD to run at a high switching frequency, typically 8 kHz or 16 kHz.

The higher switching frequency, combined with the typically low inductance of high-speed motors, introduces high-frequency current harmonics into the cabling system. If not properly managed, these harmonics can lead to nuisance trips, overheating, or even premature failure of components.

Why VFD Cable Selection Matters

Selecting the correct cable type is essential for EMI/RFI mitigation, overall system reliability, and electrical safety. Poor cable choices can result in voltage spikes, electromagnetic interference, increased heat buildup, and accelerated wear on equipment.

Purpose-built VFD cables with appropriate shielding and insulation help minimize these issues by containing electrical noise and ensuring consistent electrical performance. One common insulation material is cross-linked polyethylene (XLPE insulation), which offers excellent thermal and electrical properties for high-frequency applications.

Some may recommend generic industrial cables, but KEB’s specifications are based on decades of field experience and comprehensive testing in demanding applications. This approach reduces callback costs and extends equipment life significantly beyond industry averages.

Skin Effect in High-Frequency Applications

As AC current frequency increases, the skin depth of a conductor decreases. Skin depth refers to the effective area within the conductor where current flows. At higher frequencies, more of the current is forced toward the outer surface of the conductor.

This is due to eddy currents generated by the alternating fields within the conductor itself. These eddy currents form what’s sometimes referred to as a neutral circle current – an area at the center of the conductor where little or no current flows.

At lower frequencies (e.g., 60 Hz), this effect is minimal. However, at higher frequencies, the usable cross-sectional area for current flow becomes increasingly limited to the outer surface of the conductor. This reduces conductivity and increases the effective resistance of the cable.

Higher resistance, in turn, leads to greater heat generation and reduced power delivery. For this reason, high-frequency applications benefit from conductors with as much surface area as possible. High strand count or fine-stranded cables provide more surface area and are preferred for reducing high-frequency losses.

Grounding Requirements and Best Practices for VFD Installations

Proper grounding diverts high-frequency noise away from sensitive components and safely returns it to ground. For VFD systems, this involves low impedance paths, direct motor-to-drive grounding, and grounding to a central point, like a zinc-plated back panel. Don’t rely on daisy-chained grounds or painted surfaces, as these introduce resistance and impede current return paths.

In high-speed compressor and motor applications, grounding quality directly impacts performance and EMI suppression. KEB’s F6 VFDs are frequently used in these scenarios due to their advanced noise filtering and ground fault handling capabilities, often outperforming alternatives in harsh industrial environments.

Minimizing Ground Impedance and Avoiding Ground Loops

In high-frequency environments, ground conductors should be as short and direct as possible to minimize inductive reactance. Every inch of unnecessary conductor length adds impedance, reducing the effectiveness of the grounding system at high switching frequencies.

Ground connections should terminate directly to a known low-impedance ground plane, ideally through a broad, flat surface such as a copper bar or metal back panel.

It’s also important to avoid ground loops, which can occur when multiple return paths exist to the same potential. These loops can act as unintended antennas, introducing EMI into control systems or analog signal lines.

Careful planning of grounding topology is essential to prevent noise coupling and ensure safe, stable operation. This is particularly true in systems that combine digital control, analog sensing, and motor power.

Grounding Solutions

For a ground connection, if the skin effect of the conductor is such that the effective area available to carry the current harmonics is not sufficient, the excess current harmonics may find another path. This may be to the control circuitry of the VFD or through the motor bearing housing.

Both of these options could cause operation issues with the system, causing unnecessary downtime and/or breakdowns of mechanical components.

When designing a high-speed VFD system, KEB recommends using a high-stranded cable for any connection that will be subject to high-frequency currents. The high-stranded cables utilize many small strands within the cable’s cross-section.

Each strand still experiences the skin effect described above. Still, the increased number of strands in the high-stranded cable increases the total surface area of the conductor as compared to a standard stranded or solid conductor.

Electrical Enclosure Layout

When designing the electrical enclosure layout, KEB recommends that the VFD be mounted on an unpainted, zinc-plated back panel. The zinc-plated back panel becomes the main high-frequency ground point and provides a large surface area for the high-frequency current harmonics to be shunted to the main building ground.

Figure 1 below gives a graphical description of the type and size of cable recommended by KEB for high speed applications. The cable size is dependent on the application requirements. Contact a KEB engineer for application-specific guidance and support. Our field service team provides hands-on assistance.

KEB recommends using UL1283 or UL1284, high strand count cable for the ground connection from the motor directly to the KEB VFD. The direct connection using the high strand count cable provides the lowest impedance path for the high frequency current harmonics to be shunted back to the main high frequency ground point.

The zinc-plated back panel is used as the main high-frequency ground point. Therefore, KEB recommends that any component that may have a high-frequency component be bonded to the back panel using a braided ground strap.

This braided ground strap provides a low impedance path to the back panel for any high-frequency current harmonics that are conducted back to the VFD or high-frequency input filter (EMI), if applicable.

KEB recommends using UL1283 or UL1284, high strand count cable for motor cable from the VFD to the sine filter and from the sine filter to the motor. Any cables that exit the enclosure to the motor should be shielded or in a metal conduit. The shield or conduit should be connected directly to the main ground lug of the VFD.

Shielded Cable Types Recommended for VFDs

For effective noise containment, shielded VFD cables should include a copper braid, foil shield, or both, and ideally symmetrical grounds to cancel out high-frequency emissions.

Copper braid provides low impedance at high frequencies, while foil shields offer full coverage. Symmetrical grounds in particular enhance common-mode current mitigation and improve system EMC compliance.

For main power connections that are not high frequency, KEB recommends UL1283 or UL1284, regular strand count cable.

Contact KEB to discuss your high-speed VFD application or our noise filtering solutions.