This is an interesting and complex situation with several potential contributing factors. Let's break down the possibilities of why driving a ground rod near the VFD might appear to alleviate LED flickering, and what other areas you should investigate.

Potential Reasons Why a Ground Rod Might Seem to Help (and the associated caveats)



RF Noise Mitigation

VFDs generate significant Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI). This noise can couple into the building's wiring, causing various issues, including LED flickering. A dedicated ground rod, directly bonded to the VFD chassis, could provide a lower impedance path for this high-frequency noise to dissipate into the earth. It's acting like a very crude RF ground.

Caveats

This is likely not a proper grounding solution, and depends heavily on soil conductivity. The improvement might be coincidental or masking a more fundamental problem. The noise can still couple into the wiring through other paths. It's more of a band-aid than a cure. This doesn't address VD.

Equipotential Bonding (Potentially Misinterpreted)

The VFD's chassis should already be bonded to the electrical system's grounding electrode system via the equipment grounding conductor (EGC). If this bonding is poor or missing, the VFD chassis might be floating at a slightly different potential than other grounded equipment. Driving a rod and bonding to it might inadvertently improve this equipotential bonding, but it's not the intended method.

Caveats

NEC (National Electrical Code) requires all grounding electrodes to be bonded together to form a single grounding electrode system. An isolated rod violates this principle. This is dangerous as it could potentially create a path for fault current outside of the intended EGC. If the house is wired with older 2 wire and no EGC then this may be the issue.

Improved Common Mode Noise Filtering (Indirectly)

Common-mode noise is noise present equally on all conductors relative to ground. The VFD's input filter (if present) typically relies on a good ground connection to effectively attenuate common-mode noise. A better connection to earth could indirectly improve the filter's performance, reducing noise that couples into the lighting circuits.

Caveats

Again, the quality of the primary EGC to the panel is crucial here. A supplemental ground rod is not a substitute for a proper EGC.

Reduced Voltage Transients (Less Likely)

It's possible that the ground rod is providing some damping effect on voltage transients caused by the VFD switching. However, this is less likely and would depend on the specific characteristics of the installation and the soil. Voltage transients are more commonly addressed with proper surge protection at the panel.

Caveats

Voltage transients usually cause a wider range of problems than just LED flickering.

Important Considerations and Areas to Investigate


Grounding and Bonding Integrity



Verify the EGC

Is there a proper equipment grounding conductor (EGC, the bare or green wire) running from the VFD back to the main electrical panel? Is it securely connected at both ends? This is the MOST important thing to check. If the home is older and doesn't have an EGC this may be the issue. Then the rod may be the path to ground.

Bonding to the Service Entrance

Confirm that the main electrical panel's grounding electrode system (ground rod, water pipe bond, etc.) is intact and properly connected.

Ground Resistance Testing

Consider using a ground resistance meter to test the resistance of the existing grounding electrode system. High resistance can indicate a problem.2.

VFD Installation and Configuration



Correct Wiring

Ensure the VFD is wired correctly, including the input power, motor leads, and any control signals.

VFD Parameters

Verify that the VFD's parameters are properly configured for the specific motor and application. Incorrect settings can lead to increased noise and harmonics.

Input Filter

Does the VFD have a built-in input filter? If not, consider adding one (though Franklin's quoted cost is high - there might be alternatives).

Output Filter

If the motor cable is long, an output filter (dV/dt filter or sine wave filter) might be necessary to reduce voltage reflections and stress on the motor insulation.3.

LED Compatibility



Dimmable vs. Non-Dimmable

Are the LEDs dimmable? Non-dimmable LEDs are often more susceptible to flickering when exposed to voltage fluctuations.

LED Driver Quality

The quality of the LED driver significantly impacts its susceptibility to noise and voltage variations. Lower-quality drivers are more prone to flickering.

LED Compatibility with VFDs

Some LEDs may simply be incompatible with the electrical noise generated by VFDs.4.

Voltage Drop



Measure Voltage Drop

Use a multimeter to measure the voltage drop at the LED fixtures while the pump is running. Excessive voltage drop can cause flickering.

Circuit Loading

Are the lighting circuits heavily loaded? Overloaded circuits are more susceptible to voltage drop.

Wire Gauge

Verify that the wire gauge is adequate for the circuit length and load.5.

Harmonics



Harmonic Analysis

A power quality analyzer can be used to measure the harmonic content of the electrical system. High harmonic distortion can cause various problems, including LED flickering. VFDs are notorious for injecting harmonics into the power system.6.

Ferrite Rings



Placement

Ferrite rings can be helpful in reducing conducted EMI. Place them as close as possible to the noise source (VFD) and the affected equipment (LEDs).

Coverage

Use enough ferrite rings to effectively suppress the noise. Experiment with different locations and configurations.

Troubleshooting Steps


Start with the Basics

Verify proper grounding and bonding throughout the electrical system. This is the foundation for a stable and safe installation.2.

Isolate the Problem

Try turning off other loads in the house to see if the flickering is reduced. This can help identify the source of the noise.3.

Observe the LEDs

Are all the LEDs flickering, or just specific ones? This can help pinpoint the affected circuits.4.

Measure Voltage

Measure the voltage at the LED fixtures while the pump is running and while it's off.5.

Document Everything

Keep detailed records of your observations, measurements, and any changes you make.

Why Some Homes Aren't Affected

The susceptibility to VFD-induced noise depends on a variety of factors, including:

Distance from the VFD

Homes closer to the VFD are more likely to be affected.

Wiring Configuration

The layout of the wiring can influence how noise is coupled into the lighting circuits.

Grounding Quality

Homes with better grounding systems are less susceptible to noise problems.

LED Sensitivity

Some LEDs are simply more tolerant of noise than others.

In summary

The ground rod fix is highly suspect as a proper solution. It's more likely masking a deeper issue. Focus on verifying and improving the grounding and bonding system, addressing voltage drop, and mitigating EMI. Investigating the VFD's installation and parameters, as well as the LED compatibility, is also crucial. Bring a multimeter and a clamp meter to the site visit and check the ground. You may have to pull the VFD and test the ground with a known good source like a generator. You want to ensure a good, low-impedance ground path exists.

Before any of that, I would call the VFD manufacturer's tech support. They may have seen this before, may have a filter recommendation, or may have parameter changes that would reduce the issue. They are the experts on their product, and they may have a quick fix. Flag for review