CMC Probe

Rogowski probe for measuring hf common mode currents in VSDs. Variable Frequency Drives (VSDs) used to control AC motors can produce large high frequency voltages that may appear on the machine shaft. These voltages are the result of capacitive coupling of the PWM voltage. 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 as costly maintenance and down time. PEM has developed a flexible, clip-around, current probe to measure these high frequency common mode currents which flow through a motor to ground via the bearings in large AC drive systems. The probe is a modified version of our highly successful, industry leading, CWT range of Rogowski current sensors. The CMC is an important tool for identifying 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.