Data Sheet

CMC
Rogowski probe for
measuring hf common
mode currents in VSDs
PEM has developed a flexible, clip-around, current
probe to measure high frequency common mode
currents which flow around a motor drive to ground
via the bearings in large AC drive systems.
Common mode currents
- the problem:
Variable Speed Drives (VSDs) used to control AC motors can
produce large high frequency PWM voltages that can
capacitively couple to the machine shaft. The voltages on the
shaft can be sufficient to cause arcing currents to flow through
the motor bearings to ground.
The 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 and costly maintenance and down time.
x
CMC – an important tool
for engineers:
The CMC is an important tool to identify the presence and severity of
common mode currents in large motor drives. It is designed for use by
experienced personnel with knowledge of AC drive systems. Once
identified, the CMC will give an engineer a reference measurement
which can be used to evaluate the effectiveness of steps taken to
mitigate against bearing currents.
The probe is a modified version of our industry leading CWT range of
Rogowski current sensors.
The CMC can also be used for in a variety of other applications where
small, high frequency currents need to be measured.
The customised probe features:
• An electrostatically screened Rogowski
coil. The screen attenuates the effects of
unwanted interference due to capacitive
coupling from local voltage sources
• A low frequency (-3dB) bandwidth to
attenuate large fundamental power
frequency currents and magnetic
fields. This significantly improves the SNR for
measurement of high frequency bearing
currents
• A high frequency (-3dB) bandwidth of
≥10MHz for coil circumferences up to 1m
• A wide range of Rogowski coil sizes
suitable for even the largest machine shafts