Choosing the Right Cable for VFD to Motor Applications
Variable Frequency Drives (VFDs) have become increasingly popular over the past decade for their ability to enhance energy efficiency in motor-driven systems. However, with these advancements come new challenges, particularly in ensuring proper cable selection between the VFD and motor. This article will address the risks of improper cabling, how to mitigate these issues, and key features to look for in high-quality VFD cables.
The Impact of Proper vs. Improper Cabling
Using improper cabling for your VFD to motor connection can lead to significant problems, such as electromagnetic interference (EMI). The high-voltage spikes generated by VFDs during normal operation can induce unwanted currents and voltages into nearby circuits, causing radio frequency interference (RFI) and potentially damaging sensitive equipment. Additionally, inadequate cabling can result in common-mode voltages, which can harm motor bearings over time.
On the other hand, proper VFD-to-motor cabling can mitigate these issues by minimizing ground currents, reducing crosstalk between cables, and preventing common-mode currents from affecting motor bearings. By choosing the right cable, you can enhance both the reliability of your electronics and the safety of your system.
Shielding: A Crucial Layer of Protection
Effective shielding is a key component of a high-quality VFD cable. Proper shielding provides a low-impedance path to ground, which helps prevent the noise generated by VFDs from escaping the cable and keeps external noise from entering. There are three main types of shielding available:
- Foil Shielding: Made from a thin layer of aluminum laminated to a substrate for added strength, foil shielding provides 100% coverage of conductors. However, it can be difficult to work with when attaching to connectors.
- Woven Braid Shielding: Woven braid shielding offers a low-resistance path to ground and is easier to connect than foil shielding. It typically covers 70-95% of conductors. The copper used in woven braids is more conductive than aluminum, making it very effective for noise conduction, though it does add size and cost to the cable.
- Combination Foil/Braid Shielding: This option combines the strengths of both foil and woven braid shielding, providing effective noise control by returning excess noise back to the drive, which helps keep both the motor and surrounding equipment running smoothly.
Cable Shape: Go for Round and Symmetrical
When it comes to VFD to motor cabling, round, symmetrical cables perform best. This shape ensures more uniform electrical characteristics within the cable, reducing potential performance issues caused by irregularities.
Insulation: Durability and Flexibility
The insulation of your VFD cable should be resistant to harsh industrial environments and possess strong dielectric properties. This will protect the cable from breakdown due to high voltage, current spikes, and other stressors like corona discharges. Additionally, the insulation should be able to withstand various environmental factors, such as grit, humidity, sunlight, and oil, while still being flexible enough for easy installation and movement.
Cross-linked polyethylene (XLPE) insulation is the top choice when compared to standard PVC/nylon insulation. XLPE insulation helps reduce the effects of harmonics and corona discharge, provides stable electrical performance, and can extend the lifespan of your cable. XLPE can also handle voltages up to three times higher than PVC, allowing for longer cable runs, better motor protection, and more efficient power transfer between the drive and motor.
Grounding: More Copper Means Better Performance
Effective grounding is essential for minimizing the impact of EMI and ensuring safe, reliable operation. The best VFD cables feature three phase conductors, an overall shield, and three symmetrical ground wires. While some cables on the market only include a single ground wire, these are not recommended for use in 2-level drives under 690V and are only considered marginally acceptable for higher-level drives. In contrast, cables with three ground wires are rated as “good” for all types of drives.
A proper VFD cable cross-section should look like this:
[Insert image of VFD cable cross-section with three symmetrical grounding wires]
Conclusion
Choosing the right VFD-to-motor cable can make a significant difference in preventing EMI, voltage spikes, and other common issues that arise in VFD and motor applications. By investing in proper cabling—featuring effective shielding, round symmetrical design, durable insulation, and robust grounding—you can protect both your motor and your surrounding equipment, ensuring smoother, more reliable operation.
Make sure to consult with a professional when selecting your VFD cable to ensure you get the best performance and longevity from your motor system.
