Protect Light Industrial Motors with Specialized VFD Cables

Variable frequency drives (VFDs) are a popular choice for light industrial applications due to their excellent efficiency and precise control. However, many compact VFDs produce noisy outputs with major implications for motor supply cable selection. Choosing the wrong cable can result in complications, including hard-to-diagnose downtime, premature motor failure, and safety hazards.

Many of these issues relate to the fast-switching technology underpinning VFDs. High-frequency switching introduces significant voltage spikes, electromagnetic interference (EMI), and stray currents that can harm the motor, nearby systems, and the cable itself.

This article discusses the cabling challenges designers face when using VFDs. It then introduces Belden VFD cables and shows how they can be used to address these challenges.

Challenges of VFDs for light industrial motors

VFDs pose difficult design tradeoffs in light industrial applications, such as fans, pumps, conveyors, extruders, mixers, presses, and mills. These applications often involve motors deployed by the dozens or even hundreds, and the scale of these installations creates pressure to use the lowest-cost solutions.

As a result, these applications often call for entry-level, general-purpose compact VFDs based on silicon (Si) insulated-gate bipolar transistors (IGBTs). Typical characteristics of these drives include:

• Integral horsepower (HP) in the range of 1 to 30 HP (0.74 kW to 22 kW)
• Three-phase, low-voltage output ranging from 200 V to 575 V
• Switching frequencies in the 2 kHz to 16 kHz range

Given the moderate power levels and the pressure to keep costs down, it can be tempting to pair these drives with whatever cables are used elsewhere in the facility. Depending on the application, this may include TECK90 armored cable, continuously corrugated welded (CCW) cable, or even standard building wire, such as thermoplastic high-heat-resistant nylon-coated (THHN) cable. However, none of these options properly addresses the unique characteristics of VFDs.

Why VFDs require specialized cables

Entry-level VFDs use two-level pulse width modulation (PWM), in which the output rapidly toggles between two states (Figure 1). The square-wave outputs of these drives have steep voltage rise times (dV/dt), which can cause several problems.

Figure 1: Entry-level VFDs use two-level PWM outputs to approximate a sine wave. (Image source: Wikipedia)

First, the square waves emanating from a VFD introduce high-frequency harmonic noise into the motor supply cable. These waves can reflect when they encounter an impedance mismatch, such as at the cable termination. The constructive interference of these waves can generate voltage spikes two or three times the nominal voltage. For example, a VFD supplying a 460 V motor can readily see spikes well above 1 kV.

These high voltages can exceed the corona inception voltage (CIV), producing corona discharge that can damage standard cable insulation materials such as polyvinyl chloride (PVC). This can lead to equipment failure and shock hazards.

Second, rapid voltage changes can introduce excessive EMI, leading to hard-to-trace operational errors. For example, this noise can impact the operation of programmable logic controllers (PLCs) and other control systems in the vicinity. This is particularly problematic when motor supply cables must be laid near other cables, as interference can affect network and sensor signals.

Third, rapid switching causes imbalances in the three-phase voltages, creating stray voltages that require a return path outside the phase wires. Without proper containment, the resulting common-mode current (CMC) often discharges through motor bearings, causing microscopic melting and pitting that can significantly shorten motor life.

In some cases, CMC may be returned via the VFD enclosure or other structures. This can cause electrical noise to radiate into surrounding areas, posing a shock hazard. These are risks associated with using TECK90 or CCW cable, for example.

Finally, supply cables can present high capacitive reactance to high-frequency noise. As a result, VFDs are vulnerable to capacitive charging currents that can lead to significant energy loss. This is particularly problematic for long cable runs and small motors, where the cable’s capacitive impedance can represent a sizable percentage of the overall system load.

It is worth noting that high-frequency voltages can be mitigated by pairing a VFD with an output filter or by choosing multi-level VFDs with inherently smoother outputs. However, neither approach fully eliminates the hazards, and both options may be out of reach for cost-sensitive applications.

Benefits of specialized VFD cables

Belden Basics 2 kV VFD cables (Figure 2) are engineered to address hazards associated with PWM-driven VFDs. The core design feature is cross-linked polyethylene (XLPE) insulation rated at 2 kV. Even when exposed to voltages exceeding the CIV, XLPE remains highly resistant to corona discharge because of its tighter molecular structure. This provides sufficient resilience against voltage spikes and accommodates most compact VFDs.

Figure 2: Belden Basics 2 kV VFD cables feature copper tape shields, symmetric grounding, and robust insulation and jacketing. (Image source: Belden)

XLPE also exhibits a lower dielectric constant than PVC, reducing cable capacitance and associated charging currents.

Standard electrical cables typically use a single, moderately sized grounding conductor. In contrast, the Basics 2 kV VFD cables feature three large grounds, symmetrically spaced among the phase conductors. This geometry ensures that CMC has a low-impedance, balanced return path.

In addition, the Belden cables incorporate a dual-layer, helical copper tape shield. This provides a continuous, low-impedance barrier against radiated and conducted EMI. The ground conductors are bare and make direct contact with the shield, providing a shared low-impedance return path for all undesired currents.

The Basics 2 kV VFD cables also include mechanical features designed to withstand the demanding environments typical of light industrial installations. The oil-resistant PVC outer jacket withstands exposure to lubricants and industrial fluids. The cables are direct burial-rated and carry a TC-ER listing, allowing installation without conduit for exposed runs. FT4/IEEE 1202 flame retardance provides an additional safety margin in facilities with strict fire codes.

How to select VFD cables based on motor power

Among other considerations, cables should be selected based on their current-carrying capacity. This capacity is primarily determined by cable gauge, but depending on the application, derating factors must be considered. Belden has created an illustrative selection guide (Figure 3) based on typical full-load current (FLC) ratings of three-phase AC motors as published in NEC Table 430.250, multiplied by 125% per NEC Article 430-22(A).

Figure 3: The Belden Basics 2 kV VFD cable selection chart indicates which cable to use based on motor voltage and power. Asterisks indicate that upsizing may be required for longer runs. (Image source: Belden, modified by Kenton Williston)

For example, for a 10 HP motor operating at 230 V, the 29723C 0102500 is a good choice. This cable features 10 AWG conductors and 14 AWG grounds.

For a 10 HP motor operating at 460 V, Belden instead recommends the lighter 29721C 0102500 with 14 AWG conductors and 18 AWG grounds. This is a two-gauge step down, reflecting the lower current draw at higher voltage.

Note that ampacity is not the only design consideration. Another key issue is cable length. For runs longer than 50 feet (ft.), the voltage drop may require a larger gauge. Returning to the example of a 10 HP/460 V motor, Belden recommends upsizing to the 29722C 0102500 (12 AWG conductors, 16 AWG ground conductors) for long runs.

Benefits of custom-length VFD cables

It is important to obtain a cable cut to the precise length needed to optimize performance and cost. Ordering cable on spools and cutting it to length in the field is one option. However, this approach often results in waste, and in-field cuts may be poorly executed. Similarly, ordering cable in standard lengths and cutting it down can be cost-inefficient.

To address these constraints, the Digi-Spool custom cable length program allows ordering by the foot with no minimum length, no additional cut fees, and precise cuts with no splices. The program is UL certified, and all the Belden cables highlighted in this article are available in custom lengths.

How to terminate VFD cables

Selecting the right cable is critical, but so is proper termination. Standard methods bond the shield (and, in some cases, the ground conductors) to the VFD enclosure. However, this approach can inject noise currents near the drive, PLC, and other sensitive equipment.

Belden’s recommended solution is to use an isolating (pass-through) gland, which allows the cable to enter the enclosure without terminating the shield or grounds at that point (Figure 4). This allows the shield and grounds to be terminated to protective earth (PE) at the VFD and motor, providing robust isolation.

Figure 4: VFD cable shield and grounds should be connected to PE at the drive and motor. (Image source: Belden)

Belden offers a line of BTC cable glands designed for this purpose, including the BTC075A0791RA (Figure 5). Its pass-through design keeps the shield and ground conductors isolated at the enclosure entry, and it is compatible with all cables covered in this article. The gland features a 3/4 inch (in.) National Pipe Tapered (NPT) fitting suited to standard industrial enclosures.

Figure 5: The BTC075A0791RA cable gland is designed specifically to allow pass-through for VFD cables. (Image source: Belden)

Conclusion

VFDs offer compelling efficiency and control, but their fast-switching outputs present challenges that require careful cable selection. The Belden Basics 2 kV VFD cables address these challenges with features optimized for compact VFDs. When paired with DigiKey’s Digi-Spool custom cable-length program, these cables provide a practical solution for a wide range of light industrial applications.