I just got back from EASA, and the bearing protection on display was genuinely impressive. Grounding rings with finer and finer conductive microfibers, shaft brushes, common-mode cores, conductive greases, hybrid and ceramic bearings. A lot of real engineering, all of it pointed at the same enemy: the electrical discharge that flutes a motor bearing and turns it to scrap inside a year on a variable frequency drive.
I am not here to talk anyone out of that hardware. Most of it works as advertised when it is applied on top of a sound installation. What I want to say plainly is that none of it changes the physics underneath, and when it is installed instead of fixing the cause rather than on top of a fix, it loses far more often than the brochure suggests. A grounding ring cannot outrun a bad install.
What Is Actually Happening to the Bearing
Start with why a drive does this to a motor when a line starter never did. On a motor fed straight from the line, the three phase fields in the stator are balanced, and the voltage induced on the shaft with respect to earth is essentially zero. Pulse width modulation breaks that balance. At any switching instant the motor is not seeing balanced three phase power, it is seeing a sequence of DC bus voltage pulses, and the instantaneous sum of those pulses is not zero. That imbalance is a common-mode voltage, and it oscillates at the switching frequency of the drive.
That common-mode voltage couples capacitively across the motor air gap onto the rotor and shaft, and it builds a shaft voltage. The magnitude tracks the DC bus, so a 480 volt system with a bus near 650 volts builds substantial shaft voltage, which is why these failures show up far more on 480 volt systems than on 240 volt systems, and why turning the carrier frequency up makes the problem worse, not better.
The lubricant film inside the bearing is incredibly thin, roughly 0.2 to 2 micrometers, and it behaves as a dielectric. It holds off the shaft voltage like a tiny capacitor until that voltage climbs past about 50 volts, and then it breaks down and the stored energy discharges through the bearing in a brief, intense arc. That arc is electrical discharge machining, the same process manufacturers use on purpose to cut hardened steel with sparks. Inside a bearing it is not on purpose. Each discharge is microscopic, but the switching happens thousands of times per second, and over weeks and months those micro arcs pit the races and the rolling elements into the washboard pattern of parallel grooves that we call fluting.
The Hardware Treats the Symptom
Here is the part the booths do not put on the banner. Every one of those products manages the current after the drive has already created it. They are symptom treatments, and good ones, but symptoms all the same.
A shaft grounding ring gives the shaft voltage a deliberate, low-impedance path to the frame so the current bypasses the bearing. A shaft brush does the same job with a different contact. A common-mode core reduces the amplitude of the common-mode voltage at the drive output, which lowers the shaft voltage the rest of the system has to handle. Insulated and ceramic bearings break the discharge path at the motor by removing the conductive route through the race, though they do nothing for the bearings in the driven load, the gearbox, or the coupling. Conductive grease lowers the film impedance so the energy bleeds rather than arcs. Every one of these has a real place, and every one of them is downstream of the actual cause.
The cause is upstream of all of it. The common-mode current the drive makes has to return to the drive. If you give it a clean, low-impedance way home, the shaft voltage may never reach the threshold where any of this hardware matters. If you do not, the current goes looking for a path, and the bearing is one of the easiest paths it can find. The ring becomes the last thing standing between the current and the race, and a contact device wears, loads up with grease and dust, and degrades, while a missing return path never improves on its own.
The Fix Is the Return Path
The single highest-value measure for bearing discharge is not a ring. It is proper VFD cable with symmetrical ground conductors, terminated with full 360 degree shield contact and bonded at both the drive and the motor. That construction gives the high-frequency common-mode current a low-impedance route back to the drive, so it returns through the cable instead of detouring through the shaft and bearings. ABB’s own data shows that correct cabling and 360 degree shield grounding at both ends reduces shaft and frame voltages directly. A large share of bearing current problems are cable and grounding problems wearing a costume.
The grounding and bonding around the drive and motor finish the job. Scraped, bright metal at every ground connection, a dedicated ground conductor from the motor back to the drive rather than a daisy chain through the panel, sub-plates bonded with braided straps, and low-resistance ground connections all keep the return path impedance where it belongs. For motors roughly 125 horsepower and larger that drive equipment with its own independent ground reference, a pump grounded through its piping or a gearbox grounded through oil lines, a potential equalization conductor between the two frames keeps shaft current from using the bearings as a bridge between two grounds.
Do that first, and the rings, brushes, and cores stop being the thing that saves the motor and start being what they should be, a margin of safety on a system that was already correct. The cable, shield, and grounding details that decide this are the same ones the VFD Installation Guide walks through for the rest of the system.
Twelve Failures, One Pattern
I reviewed twelve VFD bearing failures last year and looked at every one. All twelve were discharge failures, the frosting and fluting signature, not lubrication and not load. All twelve traced back to a grounding deficiency. Not one of them was a worn-out ring or a failed brush. Every one was a return path that was never built in the first place.
That is the pattern under almost all of these failures, and it is why the fix is not where the money usually goes. When a VFD-driven motor with no special bearing protection runs for years without trouble, it is rarely luck. It is almost always because the cable and the grounding give the common-mode current an adequate way home and keep the shaft voltage under the breakdown threshold. Bearing discharge is a grounding problem first and a bearing problem second.
Why It Stays Hidden Until It Is Too Late
The reason these failures keep happening is that they are silent while they happen. The motor runs normally, the drive shows no fault, and vibration readings sit inside tolerance through the early frosting and pitting stages. Only when the races have organized into full fluting does the vibration signature finally announce a problem that has been running for a year or more.
That matters for how you watch these systems. By the time vibration analysis or electrical signature analysis flags it, the metal has already moved, which makes those tools lagging indicators for this specific failure. If you want to catch the risk before damage forms, measure the cause. A shaft voltage probe on the running motor tells you whether shaft voltage is reaching the discharge threshold, and a clamp measurement of common-mode current on the motor cable tells you whether the current has a clean return path or is hunting for one through the bearings. Those are the early measurements. Vibration confirms the damage after it exists. Building those measurements into a routine is part of the broader program covered in the VFD Maintenance and Reliability Guide.
The Takeaway
A drive does not just spin a motor. It changes the electrical environment the motor lives in, and a motor that was perfectly healthy on line power can lose its bearings on drive power if the common-mode current is never given a way home. The bearing is where the damage shows up. The grounding is where the failure was decided. All the clever hardware at EASA is worth specifying, but specify it on top of a correct return path, not in place of one. Bolted onto a good ground it is a margin. Bolted onto a bad one it is a prayer, and a grounding ring cannot outrun a bad install. For where this fits in a full reliability program, see the VFD Maintenance and Reliability Guide.
Dr. Carl Lee Tolbert, PhD, CMRP, is the author of Before the First Fault: A Field Guide to VFD Installation and Reliability. The book covers the bearing current mechanism, the full mitigation hierarchy, and the grounding and bonding practices that prevent these failures in depth. To build the same discipline into your team, see theVFD training and courses.
Author: Dr. Carl Lee Tolbert, PhD, CMRP, Wayward Leaders LLC, waywardleaders.com
Frequently Asked Questions
Does a shaft grounding ring prevent VFD bearing damage?
It helps, but it treats the symptom rather than the cause. A grounding ring gives shaft voltage a low-impedance path to the frame so current bypasses the bearing, which works well when it is installed on top of a sound cable and grounding system. On an installation with a poor common- mode return path, the ring is the last line of defense, and contact devices wear and degrade over time. The more durable fix is a proper return path through VFD cable and grounding.
Why do grounding rings and brushes still fail to protect a motor?
Usually because they were installed to compensate for a grounding or cable deficiency rather than alongside a correct one. If the common-mode current has no clean way back to the drive, it loads the ring or brush beyond what a single contact device can carry, and it continues to find the bearing path. The hardware is a margin, not a substitute for the return path.
What actually causes EDM bearing failure on a VFD?
The pulse width modulated output creates a common-mode voltage that couples onto the motor shaft and builds a shaft voltage. When that voltage exceeds roughly 50 volts, it discharges through the thin lubricant film as an electrical discharge machining arc, thousands of times per second, until the bearing races pit and flute. The underlying cause is almost always an installation that gives the common-mode current no low-impedance return path.
What is the most effective way to prevent bearing currents?
Give the common-mode current a clean way home. Use VFD-rated cable with symmetrical ground conductors, bond the shield 360 degrees at both ends, and ground and bond the drive and motor correctly, including potential equalization on larger motors whose driven equipment has its own ground reference. Bearing hardware such as rings, brushes, cores, and insulated bearings then adds margin on top of a system that is already correct.
Will vibration analysis catch bearing currents in time?
Not early enough to prevent the damage. Vibration analysis and electrical signature analysis detect the mechanical damage only after fluting has formed, which makes them lagging indicators for this failure mode. To catch the risk before metal moves, measure the cause directly with a shaft voltage probe and a common-mode current measurement on the motor cable.