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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.

Technical Specification

Peak current

10A peak to 150A peak


±7.5V peak

High Frequency Bandwidth (-3dB)

upto 14MHz (CMC06 | 1000mm coil)

Low Frequency Bandwidth (-3dB)

Varies with model type (refer to datasheet)

Accuracy (typ.)

Calibrated to ±0.5% with conductor central in the Rogowski loop


Typical variation with conductor position ±3% of reading

DC Offset

±3mV maximum at 25℃

di/dt ratings

Absolute max. 70kA/μs (peak); 1.5kA/μs (rms)

Operating temperature range

0℃ to +40℃ (Integrator electronics)


–20℃ to +90℃ (Coil and cable)

Coil Lengths

500, 700 or 1000mm (custom lengths available)

Coil thickness

8.5mm max.

Peak coil insulation

10kV peak

Cable Length (coil to integrator)

2.5 or 4m (custom lengths available)

Power Supply

Option ‘B’ Battery 4 x AA (1.5V standard alkali batteries)
Plus - 2.1mm socket for 12V (±10%) DC input


Option ‘R’ Rechargeable Battery 4 x AA (1.2V NiMH batteries) with on-board trickle charge circuitry.
Plus - 2.1mm socket for 12V (±10%) DC input

Output load

≥ 100.0 kohm (for rated accuracy)


Low Frequency Characteristics

The low frequency bandwidth is set to attenuate any large fundamental frequency currents and magnetic fields. The CMC06 integrator has a gain of typically -90dB at 50Hz, this means that if there is a 100Arms, 50Hz current passing through the coil the output of the CMC will be <0.2mVrms.

Typical low frequency amplitude response

High Frequency Characteristics

The high frequency bandwidth of the CMC is determinded by the coil length, the cable length and the integrator design. The high frequency bandwidth for each model is quoted for a 2.5m cable and a 1000mm coil in the specification table.

Typical High Frequency Response (Model CMC06 -- 50mV/A)

Showing the variation of HF performance with coil length, 500mm coil up to 5000mm coil

Noise Characteristics

The low noise integrator design allows better measurement accuracy of high frequency currents and enables a wide dynamic measurement range.

Typical noise – Model CMC03

Ch1 - CMC03/B/2.5/1000
(Peak current 75A, Sensitivity 100mV/A)
Timebase 2ms/div

Delay Characteristics

The trace shows the CMC03 measuring a 2MHz sinusoidal current source compared with a coaxial shunt measurement of the same current. There is a delay between the actual current and the output of the CMC which is predictable and is determined by the coil and cable length as well as the integrator design. The predicted delay for the CMC03B/2.5/1000 is 35ns.

2MHz damped sinusoidal current 16Apk

- CMC03/B/2.5/1000 (Peak current 75A, Sens. 100mV/A)
- Co-ax shunt 2GHz Timebase 200ns/div

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