Current probes measure HF motor-drive common-mode currents

April 05, 2016 // By Graham Prophet
Motor installers and design engineers can deal more effectively with high frequency common-mode currents caused by variable-frequency drives, using the latest non-contact current probes from Power Electronic Measurements (PEM): accurate non-contact current measurement helps counter threat of damage to motor bearings and to the drive.

CMC current probes, using Rogowski principles, allow engineers to assess the magnitude of the threat posed by HF common-mode currents, that can damage motor bearings and interfere with nearby electrical equipment. The current measurements can help determine suitable corrective action, such as fitting brushes, insulated bearings, or choke coils, and with further measurements assess the effectiveness of the chosen remedy.


The screened coils have high immunity to voltage disturbances and a low frequency (-3 dB) bandwidth that attenuates unwanted 50/60Hz magnetic fields while maximising the SNR of the measurement of the high frequency common mode currents. Availability in coil sizes from 300 to 1000 mm, and longer if required, suits them for use with a wide range of motor shafts. Even with a coil circumference of 1000 mm the CMC probes can achieve a high frequency (-3dB point) bandwidth of over 10 MHz.


The probes are available in three sensitivity grades for measuring bearing currents up to 37.5A, 75A or 150A peak. The peak detected current corresponds to a maximum output voltage of ±7.5V when connected to an oscilloscope input via the BNC output provided.