Measuring Low-Frequency Bearing Currents with PEM’s RCTi & LFR Rogowski Probe

About Low-Frequency Bearing Currents

Low-frequency bearing currents pose a significant yet often overlooked threat to essential rotating machinery. These currents, occurring at power frequency (50/60 Hz) and harmonics, silently damage bearings through continuous electrical stress. Unlike their high-frequency counterparts from variable frequency drives, low-frequency bearing currents typically remain undetected until significant damage has already occurred.

When these currents flow through bearings in hydroelectric generators, turbogenerators and large industrial motors, they progressively degrade lubrication, create heating effects and cause premature failure. This leads to unexpected downtime, costly emergency repairs and substantially reduced asset lifespans—often in the most critical equipment where failure impacts are severe.

The technical challenge lies in detecting and measuring these low-frequency phenomena. Conventional measurement tools lack the necessary sensitivity and frequency response, leaving operators blind to developing issues until catastrophic failure occurs.

PEM’s RCTi/25 & LFR/015/1.5

PEM has developed a specialised sensor to address this exact challenge. Drawing on decades of Rogowski current measurement expertise, our RCTi/25 (permanent monitoring) and LFR/015/1.5 (diagnostics) Rogowski current sensors are specifically engineered to detect and measure low-frequency bearing currents.

These Rogowski sensors address the unique requirements of this application:

• Customised frequency response from 4 Hz to 40 kHz, optimised to maximise SNR at power frequencies to >50^th harmonic.
• High sensitivity (200 mV/A) for detecting even low-magnitude damaging currents.
• Air-core design that eliminates the magnetic saturation issues.
• Customisable dimensions adaptable to any shaft size.
• Flexible installation options with cable lengths from 2.5m to 30m for complex environments.

Key Features

• Optimised bandwidth for best SNR:    
  • 0.6Hz to 1MHz (RCTi/25) (coil length dependent)
  • 4 Hz – 40 kHz (LFR/015/1.5)
• High sensitivity and low noise:   
  • 200 mV/A
• Current range:
  • up to 25A rms (RCTi/25)
  • 30A pk (LFR/015/1.5)
• Flexible coil circumference (500–7000 mm) for large shafts. Longer coils possible.
• Long cable lengths available (2.5–30 m) for installation flexibility.
• Electronics to suit your needs: The RCTi in either a DIN rail case or robust, EMI immune panel mount enclosure for permanent installation, or the LFR battery powered electronics for troubleshooting applications and simple connection to a scope or datacorder.