Measuring High-Frequency Bearing Currents with PEM’s CMC Rogowski Probe

About High-Frequency Bearing Currents

High-frequency bearing currents represent a significant threat to modern motor systems driven by variable frequency drives (VFDs). These transient electrical currents, generated by rapid semiconductor switching in power electronic converters, cause premature bearing failure through electrical discharge machining (EDM) effects. Unlike conventional mechanical wear, EDM damage progresses rapidly, potentially reducing bearing life from years to mere months.

These currents are characterised by extremely fast rise times (typically nanoseconds), high peak amplitudes (0.5-40A) and frequency components ranging from kilohertz to megahertz. When the voltage across a bearing exceeds the breakdown threshold of the lubricant film (typically 5-30V), electrical discharge occurs, creating microscopic craters on bearing surfaces that eventually develop into characteristic fluting patterns.

The technical challenge lies in accurately capturing these fast, transient phenomena in operational environments. Conventional current measurement tools lack the necessary bandwidth, risking millions in equipment damage and unplanned downtime across marine propulsion systems, water treatment facilities and critical industrial applications.

PEM’s CMC Rogowski Technology

PEM’s CMC Rogowski probe addresses this precise challenge with purpose-designed measurement capability for high-frequency bearing currents. Drawing on decades of expertise in power electronic measurement, our technology provides:

• Optimised high-frequency response
  2kHz  into the MHz range bandwidth specifically engineered to capture fast transients associated with VFD switching edges
• Superior transient performance
  Accurately measures current pulses with rise times as fast as  20ns
• Non-invasive measurement
  Flexible, clip-around design requires no electrical connection or circuit breaking
• Air-core design
  Eliminates magnetic saturation and hysteresis effects that compromise measurement accuracy
• Electrostatic screening
  Reduces capacitive coupling interference in electrically noisy environments

Key Features

• Frequency range: 2kHz  to >10 MHz
• Fast transient response: Captures rise times as fast as 20ns
• Peak current measurement: Up to 150A peak
• Flexible sizing: Available in various diameters to accommodate different cable and shaft dimensions
• Rugged construction: Designed for harsh industrial environments

Applications

• Bearing protection verification
  Confirm effectiveness of insulated bearings, shaft grounding rings and other mitigation measures
• Condition monitoring 
  Detect developing issues before catastrophic failure occurs
• Root cause analysis
  Distinguish between different bearing current types (EDM, circulating, rotor ground) through waveform analysis
• Design validation
  Verify motor system designs against bearing current risk factors