Understanding Shaft Currents: When Is Shaft Grounding Protection Needed?
For maintenance professionals, shaft currents and the resulting bearing damage are a serious concern, especially in motors controlled by variable frequency drives (VFDs). But not every motor needs protection, and not all shaft current problems are guaranteed to happen.
This post will walk you through what causes shaft currents, when they’re most likely to occur, how serious the risks are, and when shaft grounding rings or other protective measures are (or aren’t) needed.
What Are Shaft Currents?
Shaft currents are unwanted electrical currents that travel through the motor shaft and can discharge through the bearings. When this happens, the currents can pit or erode the bearing surfaces, leading to premature wear or even complete bearing failure. This is often called electrical discharge machining (EDM) damage.
How Do Shaft Currents Happen?
There are several ways these currents can be induced, but the most common causes are:
1. Variable Frequency Drives (VFDs)
If your motor is controlled by a VFD (also known as an inverter), you're more likely to experience bearing damage. ABB’s Technical Guide No. 5 notes that inverter-fed motors are especially vulnerable because the rapid voltage switching (high dV/dt) in PWM drives creates high-frequency common-mode voltages. These voltages couple capacitively between the stator windings and the rotor, inducing shaft currents. In smaller motors, this usually shows up as capacitive discharges, where voltage builds on the shaft and then arcs through a bearing, causing electrical discharge machining (EDM). In larger motors, VFDs can also drive circulating bearing currents, where current enters through one bearing and exits through the other, leading to the same pitting and fluting damage over time.2. Long Motor Lead Lengths
Long leads—especially over 50 feet (15 meters)—act like antennas and increase the common-mode voltage at the motor, leading to higher shaft currents. The longer the cable between the VFD and motor, the higher the risk.3. Improper or Missing Cable Shielding
If the VFD cable is not shielded or if the shield is not grounded properly at both ends, high-frequency noise from the VFD can escape and induce shaft currents.For motor power cables, the shield should be grounded at both the drive and motor ends. This provides a low-impedance path for high-frequency common-mode currents and helps prevent them from discharging through the bearings.
4. Electromagnetic Imbalances
Even in motors powered by line voltage (no VFD), asymmetries in the magnetic field can induce voltages in the shaft. These are usually lower in energy but can still cause long-term damage in large motors.5. Static Charges and Ground Loops
In some cases, static electricity or improper grounding in the system can also cause small shaft currents.How Shaft Currents Damage Motors and Connected Equipment
Left unchecked, shaft currents steadily degrade motor bearings. Early on, they create small defects that may show up as faint noise, excess vibration, or slightly elevated temperatures. If caught at this stage, replacing the bearings can restore reliable operation with minimal disruption. But if ignored, the damage compounds—fluting patterns form on the races, noise and vibration intensify, and heat causes lubrication to break down. Eventually, the motor suffers complete bearing failure, forcing unplanned downtime and costly repairs.Shaft currents don’t always stop at the motor. When the shaft is directly coupled to driven equipment, the current can pass into couplings, pump bearings, gearbox bearings, or fan bearings. These components can suffer the same EDM damage, pitting, and fluting seen in the motor, extending the problem into the rest of the system. The result is not just a failed motor, but also premature wear and failure throughout the connected equipment—escalating repair costs and multiplying the impact across the system.
How Often Do Shaft Currents Cause Damage?
The short answer: it depends. Not every motor on a VFD will experience shaft current damage—but many do if certain conditions are present.Factors That Increase the Likelihood of Bearing Failures:
- Motor size: Larger motors (especially above 100 HP) tend to generate and retain more shaft voltage.
- Drive configuration: High switching frequency and lack of filtering increase common-mode voltage and risk of EDM.
- Cable type and length: Long or unshielded cables raise shaft voltage. Shielded VFD cable with proper termination reduces risk.
- Grounding quality: Poor bonding and unbalanced systems force current through the bearings.
- Power quality: Harmonic distortion, voltage imbalance, and ground loops contribute to erratic shaft voltage buildup.
- Grease type and condition: Aged or contaminated grease breaks down faster, making bearings more vulnerable to EDM damage.
- Operating hours: Motors running continuously are exposed to more cumulative discharge cycles.
- Environment: Dust, moisture, vibration, or hazardous atmospheres accelerate grease degradation and insulation failure.
Real World Data
According to a technical paper from TAPPI and AEGIS, in modern AC motors equipped with VFDs, electrical damage from shaft currents has emerged as the leading cause of bearing failure—particularly in motors with sealed bearings where contamination is less of a factor.
In contrast, standard motors on across-the-line power with good grounding and no VFDs show very low rates of shaft current-related damage.
Why Does Shaft Current Damage Feel Random?
Shaft current failures often feel unpredictable because damage doesn’t follow a consistent timeline. Two identical motors, installed side by side, can have drastically different outcomes depending on subtle but critical differences.
A motor exposed to continuous voltage pulses with no shaft protection may experience bearing fluting in as little as 1–6 months, especially in harsh or continuous-duty applications. In contrast, a lightly loaded motor with decent grounding and limited runtime might never see damage.
Making things more confusing, failures due to electrical discharge often look like common mechanical wear—pitting, spalling, or overheating. Without microscopic inspection or voltage testing, these issues are frequently misattributed to misalignment, over-greasing, or contamination. As a result, electrical damage often goes unnoticed until it becomes a repeat issue, making it seem random or rare when it’s actually preventable.
Preventing Bearing Failures from Shaft Currents
When You Need Shaft Grounding Protection
If you’re using VFDs, grounding protection is almost always recommended. Here’s a quick breakdown of scenarios when you may or may not need shaft grounding protection: 
Why Grounding Is Still Advised for Dual-Insulated Motors
• Break down bearing insulation over time if insulation weakens due to heat or vibration
• Interfere with sensors or electronics mounted on the shaft or frame
Best practice: Combine insulation with a shaft grounding ring to safely divert the current and prevent unexpected failure elsewhere in the drivetrain.
Special Considerations: Belt-Drive vs Close-Coupled
Belt Drive Motors: These are at the highest risk. The belt interrupts the electrical path to the load, allowing voltages to build up more easily. Consider use shaft grounding plus an insulated bearing.
• Close-Coupled Motors: These allow current to sometimes pass through the driven equipment. You may get by with a shaft grounding ring alone, especially in clean environments.
Is Shaft Current Damage Covered by Warranty?
For example, Baldor’s warranty explicitly states: "Damage to bearings caused by electric currents, stray or circulating currents, or fluting is not covered under warranty."
That means if a motor fails due to shaft current damage, and no protection was installed, the repair cost is on the customer—not the manufacturer. This is because the failure is considered to be preventable, and it is up to the customer to decide if they want to prevent it or take their chances.
Protect Your Investment
Shaft grounding rings are one of the most effective solutions for diverting harmful shaft currents safely to ground; however, there are many more options, each suited to different applications, budgets, and scenarios.
For more details on shaft current protection, stay tuned for our next blog post on shaft grounding protection.
Rule of thumb: If you’re running a motor on a VFD, protect it with at least one shaft grounding device. And if it's belt-driven or mission-critical, go for the grounding + insulated bearing combo.
You’ll save your bearings—and your budget.
Need help finding the right protection? Contact our team at 1-800-868-3770 or sales@esrmotors.com for personalized recommendations.