Bearing currents are one of the most common failure modes in industrial motors. Yet, surprisingly few reliability engineers truly understand how to measure them—or how to choose the right mitigation strategy.
If you’re only measuring shaft voltage, you’re missing half the story.
In this post, we’ll explain why a Rogowski coil is superior to shaft voltage measurements and how common mode current analysis can save your motors from premature failure.
***
First, Understand the Types of Bearing Currents
Not all bearing currents are the same. Separating them is the first step toward choosing the right fix.
The real damage comes from the magnitude of the common mode current (peak to peak). Shaft voltage sees only a fraction of that picture.
Common mode current is simply the sum of the three phase currents. Ideally, it should net to zero—a flat line on your oscilloscope. When it doesn’t, you have a problem.
But here’s the critical part: frequency matters. And shaft voltage doesn’t measure frequency.
***
Low Frequency Trouble: Magnetic Symmetry Breakdown
If the dominant frequency matches the inverter’s line frequency (Hz output), you’re looking at a break in magnetic symmetry—typically a current imbalance.
This is known as a circulatory current (see IEEE 1415 standard). It’s induced by the rotor and flows through the motor frame via the bearings.
Worse, no shaft brush can stop it. Why? Because the current path is inside the motor.
Motors with high impedance imbalance are especially vulnerable. That’s why 3Phi Reliability recommends acceptance testing below 8% impedance imbalance.
Mitigation?
- Buy better quality motors
- Fit insulated bearings or end shields
The catch: Many motors exceed 8% imbalance—some brands, dramatically so.
On an oscilloscope, this imbalance shows up as a repeating, out-of-balance common mode current trace.
***
High Frequency Trouble: The Inverter’s Dark Side
The second damaging common mode current is high frequency, tied to:
- The inverter’s switching frequency
- Cable resonance
- Grounding and termination characteristics
These currents often resonate in the MHz range, so your oscilloscope and Rogowski coil need sufficient bandwidth.
My recommended specs:
- Oscilloscope: 250 MHz bandwidth, 1 GHz sample rate
- Rogowski coil: at least 10 MHz bandwidth
Lower specs = compromised results.
At these specs, you can capture individual switching events. Most traces show a mix of circulatory and high frequency dV/dt current.
But there’s a third type—even more destructive.
***
The Real Threat: Reflective Voltage
Reflective voltage happens when high frequency dV/dt hits resonance in the cable, amplifying the damage.
High frequency current sees very high inductive reactance (XL = 2πfL), blocking most current from entering deep into the motor. The first few turns take the hit, reflecting the energy back.
Think of a skipping rope: the wave travels down, reflects at the other end, and bounces back. This can more than double the voltage about 30 cm before the motor terminals.
A small portion of this high frequency current does enter the motor via winding and insulation capacitance—causing bearing fluting.
On the common mode trace, reflective voltage appears as non-repeating spikes of energy, driven by cable resonance.
This damages motor insulation. Why nano-crystalline cores beat shaft brushes: They attenuate reflective voltages before they reach the motor. Shaft brushes do nothing for insulation protection and are not recommended.
***
Stop Guessing. Measure Common Mode Current.
Your mitigation strategy should be based on what type of bearing current is actually present in the common mode current.
Without that data—without a Rogowski coil and oscilloscope—you’re guessing.
That’s exactly why so many shaft grounding devices fail. The common mode current was never measured in the first place.
***
Final Takeaway If you’re serious about bearing current failure prevention, retire the shaft voltage-only approach. Invest in a Rogowski coil and proper oscilloscope setup. Your motors—and your maintenance budget—will thank you. Note: A good quality Oscilloscope and Two Rogowski coils are only 650 euro (2026 prices).
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}