CMC Common Mode Current Probe
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.
- Peak Current: ±37.5A .. 150A
- Coil Circumference: 500 .. 4000mm (Standard)
- Coil Cross Section: 8.5mm
- Bandwidth: 1.9kHz to 14MHz (-3dB)
- Robust, flexible, clip-around coils
- High voltage isolation and common mode rejection
- Not influenced by high dc magnetic fields
- Both battery (4xAA) and external DC power supply
- CE marked
- Special long coil versions
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.
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.
The low noise integrator design allows better measurement accuracy of high frequency currents and enables a wide dynamic measurement range.
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.
- Accurate Power Conversion Measurements on High Power Motor Drives
- Case Study I – HF Circulating Currents in a Propulsion Drive
- Case Study II – HF Common Mode Currents
- “High Performance Rogowski Current Transducers”, W. F. Ray and C. R. Hewson
- “Next-Generation Current Probes for High-Speed and High Power-Density Applications”, Dr Chris Hewson, Power Electronic Measurements Ltd
- “Power Integrity” by Steve Sandler. See Chapters 5: Probes and 7: Measuring Impedance for applications using PEM Rogowski Coils (purchase of book required).
Select product options from drop-down menus to quote standard products not normally in stock. For fully custom products, fill in the message box below.
Order Code for CMC: Type / Battery / Cable Length / Coil Circumference
For example: CMC015/B/2.5/1000
For descriptions of the possible selections, please see the Specifications above.
CMC Common Mode Current Probes
|Part No.||Description||Price||In Stock|
CMC Common Mode Current Probe 200mV/A, 37.5A, 19kHz – 11MHz, Alkalai battery 4xAA, 4m cable length from Coil to Integrator, 500mm coil circumference length, 8.5mm cross section. Includes US13S power supply.
CMC Common Mode Current Probe 50mV/A, 150A, 1.9kHz – 14MHz, Alkalai battery 4xAA, 4m cable length from Coil to Integrator, 1000mm coil circumference length, 8.5mm cross section. Includes US13S power supply.
|Part No.||Description||Price||In Stock|
CWT UM Regulated Power Supply. 120VAC, 60Hz, 12V ±10%, 300mA, Barrel type: 1.3 x 3.5 x 10mm. Power disconnects battery when plugged into the integrator.
CWT UM Regulated Power Supply. 100V-220AC, 50/60Hz, 12V ±10%, 500mA, Barrel type: 1.3 x 3.5 x 10mm. Power disconnects battery when plugged into the integrator. European plug type.
CWT UM Regulated Power Supply. 100V-220AC, 50/60Hz, 12V ±10%, 500mA, Barrel type: 1.3 x 3.5 x 10mm. Power disconnects battery when plugged into the integrator. UK plug type.
Yokogawa compatible power cord, 0.5m, supports all PEM units fitted with a 1.3mm DC power socket.
Terms and Conditions
Quantity Discount Schedule:
|Quantity||1 – 9||10 – 24|
Terms and Conditions:
Prices above are for domestic deliveries to Canada, USA and Mexico only.
Prices are expressed in U.S. Dollars, FCA San Carlos, California.
Prices are subject to change without notice.
Higher quantity and OEM discounts are available.
Visa, MasterCard, Amex, Prepayment before shipment, or Net 30 days (subject to credit
Items in stock shipped next business day. All other items up to 6 weeks.