The Dark Side of VFDs: Understanding and Mitigating Harmonics and Shaft Currents

09/17/2025

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The Variable Frequency Drive (VFD) is one of the greatest energy-saving devices in the modern industrial playbook. Its ability to precisely control the speed of a motor on a pump, fan, or compressor to match fluctuating demand has saved facilities countless megawatt-hours and dramatically reduced operational costs. However, as with any powerful technology, VFDs are not without their complexities. While they solve many mechanical and efficiency problems, they can introduce a host of subtle, yet potentially damaging, electrical phenomena into your system. Two of the most significant of these are harmonics and shaft currents. For engineers, ignoring these issues can lead to mysterious equipment failures, power quality problems, and a shortened lifespan for your most critical motors. At Blackhawk Equipment, we integrate VFDs into many of our high-efficiency solutions, and we believe a deep understanding of the entire system—both mechanical and electrical—is key to long-term reliability.

Electrical Noise: What Are Harmonics and Why Do VFDs Create Them?

Imagine the electrical power coming from your utility as a perfect, smooth sine wave at 60 Hz. This is the clean power that standard AC motors are designed to use. A VFD, in its process of converting AC power to DC and then back to a simulated AC waveform at a variable frequency, disrupts this smooth wave.

The Source of the Distortion:

The front end of a VFD, the rectifier, rapidly converts the incoming AC voltage to DC voltage. It does this by drawing current in short, non-linear pulses instead of a smooth, continuous wave. This pulsed current draw distorts the original sine wave.
This distortion can be mathematically broken down into the fundamental 60 Hz frequency plus a series of additional, unwanted frequencies at whole number multiples of the fundamental frequency—these are the harmonics (e.g., the 5th harmonic is 300 Hz, the 7th is 420 Hz, and so on). This "dirty" power then flows back from the VFD into your facility's electrical distribution system.

The Consequences of Harmonic Distortion:

This electrical "noise" can cause a variety of problems throughout your plant, often in places you wouldn't expect:

Overheating Transformers and Conductors: Harmonic currents don't contribute to useful work, but they do generate heat. This can cause transformers, bus bars, and neutral conductors to overheat, even when they appear to be operating below their rated load.
Nuisance Tripping: The distorted waveforms can confuse sensitive circuit breakers and protective relays, causing them to trip unexpectedly and shut down processes.
Interference with Electronics: Harmonics can interfere with sensitive electronic equipment, including PLCs, computers, and even some monitoring sensors, causing erratic behavior or data corruption.
Reduced Power Factor and Utility Penalties: High levels of harmonic distortion can lead to a poor power factor, which may result in financial penalties from your electric utility.

The Silent Killer: What Are VFD-Induced Shaft Currents?

While harmonics affect the electrical system feeding the VFD, shaft currents affect the motor driven by the VFD. This is a far more direct and mechanically destructive issue.

The Cause: Common-Mode Voltage:

The output of a VFD is not a perfect AC sine wave; it's a very fast-switching, pulsed DC voltage known as Pulse Width Modulation (PWM). Due to imperfections in this switching, a "stray" voltage, known as common-mode voltage, is created.
This voltage can't find a clean path back to the drive, so it uses the motor itself as a path to ground. A voltage potential builds up on the motor's rotor and shaft. The motor's bearings, filled with conductive grease, are all that separates the energized shaft from the grounded motor frame.

The Damage Mechanism: Electrical Discharge Machining (EDM):

The lubricant in the bearings is a dielectric, but it can only withstand so much voltage. When the shaft voltage builds high enough, it arcs through the bearing lubricant, discharging to the grounded motor frame in a tiny, high-current lightning strike.
This happens thousands of times per second. Each arc is a miniature Electrical Discharge Machining (EDM) event, blasting a tiny pit or fusion crater onto the mirror-smooth surface of the bearing race. Over millions of cycles, these pits multiply and merge, a phenomenon known as "fluting," which looks like washboard-like ridges on the bearing race.

The Symptoms and Failure:

Initially, the damage is microscopic and silent. Eventually, the bearing becomes noisy and starts to vibrate. By the time you can hear it, the damage is severe. This process can destroy a brand-new motor's bearings in as little as three to six months, leading to a costly and unexpected failure.

Mitigating the Dark Side: Proven Engineering Solutions

Fortunately, both harmonics and shaft currents are well-understood problems with proven solutions.

Solutions for Harmonic Distortion:

Line Reactors (AC or DC): These are essentially inductors installed on the input side of the VFD (AC line reactor) or within its DC link (DC link choke). They "smooth out" the pulsed current draw, offering a simple and cost-effective way to reduce harmonics for smaller drives.
Passive Harmonic Filters: For more significant harmonic reduction, a passive filter—a specially designed network of inductors and capacitors—can be installed. It's tuned to "trap" the most problematic harmonic frequencies.
Active Harmonic Filters: This is a more advanced, electronically controlled solution. An active filter measures the harmonic distortion in real-time and injects an equal and opposite "anti-harmonic" current to effectively cancel out the noise. They are highly effective but come at a higher cost.
Low-Harmonic or 18-Pulse VFDs: These drives are designed with more complex rectifiers (e.g., 18-pulse instead of the standard 6-pulse) that inherently produce much lower levels of harmonic distortion.

Solutions for Shaft Currents:

Shaft Grounding Rings: This is one of the most common and effective solutions. A conductive microfiber grounding ring (like those made by AEGIS®) is installed around the motor shaft. The fibers ride on the shaft, providing a highly reliable, low-impedance path for the shaft currents to safely travel to ground, bypassing the bearings completely.
Insulated Bearings: One of the motor's bearings (typically the non-drive end) can be replaced with a bearing that has an insulated outer race (e.g., ceramic-coated). This breaks the electrical circuit, preventing current from flowing through the bearings. This is an effective but more costly solution, often specified on new, large motors.
VFD Shielded Cable: Using properly grounded, shielded cable between the VFD and the motor is a crucial best practice. It helps to contain some of the electrical noise and provides a better path to ground for common-mode currents.

Blackhawk Equipment's System-Wide Approach

At Blackhawk Equipment, our expertise lies in the reliable performance of the mechanical systems we provide. We understand that a VFD is a powerful tool, and its proper application is key to the longevity of the pumps, compressors, and blowers it controls. When we design and supply a system with a VFD, we:

Partner with Electrical Experts: We collaborate with qualified electrical engineers and power quality specialists to ensure that issues like harmonics are considered in the overall project design.
Specify Robust Equipment: We recommend and supply motors with features like "inverter-duty" windings that are designed to withstand the stresses of VFD operation.
Advise on Best Practices: We strongly advise our customers to implement proven shaft current mitigation strategies, like shaft grounding rings, to protect their investment and ensure the long-term reliability of the motor bearings.
Focus on the Complete System: We ensure that the entire mechanical system—the pump, compressor, couplings, and base—is robust and correctly aligned to operate smoothly across the entire speed range controlled by the VFD.

Conclusion: Harnessing the Power of VFDs Safely and Reliably

Variable Frequency Drives are indispensable for modern industrial efficiency, but their benefits come with a responsibility to manage their electrical side effects. Harmonics and shaft currents are not minor issues; they are serious electrical phenomena that can compromise your entire facility's power quality and destroy your motors. By understanding their causes and proactively implementing proven engineering solutions—from harmonic filters to shaft grounding rings—you can harness the full power and efficiency of VFDs without falling victim to their dark side. A holistic, system-wide approach is key to ensuring a long and reliable life for your VFD-driven equipment.

Contact Blackhawk Equipment to discuss your next VFD application. Let our team ensure your system is designed not just for efficiency, but for total operational reliability.