500kW VFD Motor Vibration and Noise: How to Fix It

Introduction

Hello everyone! Today, we’re diving into a puzzling real-world case involving a 500kW VFD system. This motor had been operating flawlessly for over a year. However, in the past six months, two issues emerged: increased electromagnetic noise (Motor Noise) and excessive bearing vibration (Motor Vibration)—despite motor temperature and current readings remaining normal.

Servo motor experiencing early-stage Motor Vibration due to VFD-induced harmonics and resulting Motor Noise

When It’s Not a Hard Fault, but a Systemic Feedback Loop

The fact that the system ran smoothly for over a year indicates that both the VFD parameters and motor configuration were correctly set up from the start. The sudden onset of vibration and noise points to a gradual degradation process rather than a one-time failure—likely due to VFD harmonics and induced shaft voltage/currents accumulating over time.

Root Cause: Understanding the Electric Erosion Behind Motor Vibration

In VFD-powered motors, harmonic spikes from the inverter can induce shaft voltage, which results in shaft current. This current flows through bearing oil film capacitors, forming transient discharges when the oil film is breached during operation.

These discharges cause electrical arcing, generating heat over 10,000°C, which creates pitting on the bearing raceways and balls/rollers. Over time, this forms the typical washboard pattern seen in electrical erosion damage.

This leads to:

  • Rotor imbalance and micro-oscillations
  • Magnetic air-gap variation
  • Magnetic flux instability
  • Ultimately triggering more Motor Vibration and Motor Noise


This becomes a vicious feedback loop—shaft current leads to damage, which causes more vibration, which in turn generates more shaft current.

Diagnostic Tip: Use Carrier Frequency to Identify the Source

Try increasing the carrier frequency in the VFD settings:

  • If noise is reduced: It’s likely caused by harmonic-induced electromagnetic noise, and the bearings are still intact.
  • If noise increases or stays the same: It indicates serious electrical erosion and bearing surface damage.

Three Proven Solutions for Motor Vibration from Electrical Erosion

1.Three Proven Solutions for Motor Vibration from Electrical Erosion

Prevents shaft current from flowing through the bearings, eliminating the root cause of EDM damage.

2.Install Shaft Grounding(e.g., carbon brushes)

Provides a low-impedance path to ground, releasing shaft voltage and protecting the bearings.

3.Add VFD Output Reactors

Suppresses harmonic spikes and reduces voltage reflections, stabilizing motor operation.

Additional Measures to Control Motor Vibration

  • Adjust carrier frequency (for harmonic suppression)
  • Use vibration analysis tools to monitor bearing health
  • Use VFD-rated cables with proper grounding
  • Limit long motor lead lengths to prevent overvoltage
  • Consider shaft voltage monitoring systems for predictive maintenance

Final Thoughts: Motor Vibration Isn’t Always Mechanical—It’s Often Electromagnetic

This case highlights that Motor Vibration and Motor Noise aren’t always caused by mechanical failures. In VFD-controlled motors, they often stem from shaft voltage, bearing currents, and electrical erosion.

To solve the problem, engineers must look beyond mechanics and adopt a system-wide perspective that includes electromagnetic coupling and resonance feedback. Only then can truly effective and sustainable solutions be implemented.