CMC

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

Variable Frequency/Speed Drives (VFD / VSD) used to control AC motors can produce high frequency voltages that may ultimately appear on the machine shaft. These voltages are the result of capacitive coupling of the VSD PWM voltage to the motor shaft. The voltages on the shaft can be sufficient to cause arcing currents to flow through the motor bearings to ground. 

These discharging currents can cause heating and even melting of the surface of the bearing raceways. The damage caused by bearing currents can lead to premature failure of the motor drive as well as costly maintenance and down time. 

PEM have a range of probes for engineers to monitor these currents, directly measuring the bearing current, which ultimately can be used to evaluate the effectiveness of steps taken to mitigate against bearing currents.

Rogowski current probes do not measure the DC component of a current. However unlike a current transformer or a fluxgate device they contain no magnetic materials so are unaffected by the DC current. Therefore it is possible to use a small, flexible, Rogowski coil to measure a small AC current in the presence of a large DC current, whereas a sensor based on a magnetic principle would be expensive and bulky to prevent saturation effects. One such common application is measuring the ripple current in capacitors where the main current is DC or slow time varying.