High Frequency Bearing Current Measurement with Rogowski Current Probes

About High-Frequency Bearing Currents

High-frequency bearing currents can occur in inverter-driven motors when fast common-mode voltage transitions couple through parasitic capacitances within the motor and drive system, producing voltage on the rotor and shaft. Depending on the resulting current path and the voltage across the bearing lubricant film, the film can break down and allow a transient discharge through the bearing, contributing to electrical discharge machining (EDM), surface pitting and fluting.

The resulting bearing-current pulses can have very fast rise times and contain significant high-frequency content extending from the VFD switching-frequency region into the MHz range. Their magnitude, repetition rate and waveform depend on factors including the motor and drive design, grounding arrangement, bearing condition and parasitic capacitances within the system.

Measuring high-frequency bearing current therefore requires a current probe and measurement system with sufficient bandwidth and transient response to capture fast, short-duration pulses, together with good immunity to the high dV/dt environment around the motor and drive. Measurements are typically made with an oscilloscope to observe pulse amplitude, rise time, repetition rate and waveform shape, and can also be used to compare conditions before and after mitigation measures such as shaft-grounding devices or insulated-bearing arrangements.

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