CMC series - Measurement of bearing currents
CMC series - Measurement of bearing currents

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Manufacturer: PEM

CMC series - Measurement of bearing currents

CMC Series - 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.

 

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 dla cewek o obwodach do 1m.
  • A wide range of Rogowski coil sizessuitable for even the largest machine shafts

Seria CMC – pomiar prądów łożyskowych



Model Sensitivity
[mV/A]		 Peak
current
[A]		 Noise
max
[mVP-P]		 LF (-3dB)
bandwidth
[kHz]		 Typical LF LF
(<1%)
bandwidth
[kHz]		 Peak
di/dt
[kA/µs]		 HF (-3dB) bandwidth
(-3dB)
[MHz]
(Coil length 1000mm)

CMC015

 200.0 37.5 4.0 19.0 50.0 4.0 11.0

CMC03

 100.0 75.0 4.0 6.0 15.0 8.0 13.0

CMC06

 50.0 150.00 4.0 1.9 5.0 16.0 14.0



Output ±7.5V peak corresponding to ‘Peak Current’ into >100kΩ (e.g. DC 1MΩ oscilloscope)
Accuracy Variation with conductor position in the coil typically ±3% of reading (for a 5cm2 conductor) Linearity (with current magnitude) 0.05% of reading
Calibration Calibrated to ±0.5% reading with conductor central in the coil loop
Offset DC ±3mV maximum at 250C
Temperature Coil and cable -200C to +900C. Integrator 0 to +400C
di/dt ratings These are ‘absolute maximum di/dt ratings’ and values must not be exceeded:
Absolute max. peak di/dt: 70kA/μs
Absolute max. rms di/dt: 1.5kA/μs
Coil voltage 10kV peak
Safe peak working voltage to earth. Rating established by a 15kVrms, 50Hz, 60sec flash test. Information
about continuous use of the coils at high voltage can be obtained from PEM.

Key features

  • Coil length (circumference)
    500mm, 700mm, 1000mm - longer coils available on request
  • Coil cross-section (thickness)
    8.5mm max (14mm with removeable silicone sleeve – only for mechanical protection)
  • Cable length
    2.5m and 4m as standard (connecting cable coil to integrator) - longer cables available on request.
  • Battery options
    B-Standard: 4 x AA 1.5V alkali batteries. Lifetime typically 25 hours
    R-Rechargeable: 4 x AA 1.2V NiMH batteries. Lifetime
    typically 10 hours. External adaptor recharges
    batteries and powers unit.
  • Socket for external power adaptor (1.3mm diameter) (adaptor available from PEM as an option)
  • Electronics enclosure
    Dimensions H=183mm, W=93mm, D=32mm
  • Output BNC socket
    Supplied with 0.5m BNC:BNC cable.
Seria CMC – pomiar prądów łożyskowych - dane kluczowe
 


Seria CMC – pomiar prądów łożyskowych - charakterystyki - niskie częstotliwości

Low Frequency
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

 
Seria CMC – pomiar prądów łożyskowych - charakterystyki - wysokie częstotliwości

High Frequency
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

 
Seria CMC – pomiar prądów łożyskowych - charakterystyki - szumy

Noise
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

 


Seria CMC – pomiar prądów łożyskowych - charakterystyki - opóźnienie

Delay
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.
Typical noise – Model CMC03



 

Generating the order code
Type / Power supply / Cable length (m) / Coil circumference (mm)
e.g. CMC06 – 50mV/A battery supply, 2.5m cable from coil to integrator, 1000mm circumference coil
CMC06 / B / 2.5 / 1000
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CMC Series - 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.

 

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 dla cewek o obwodach do 1m.
  • A wide range of Rogowski coil sizessuitable for even the largest machine shafts

Seria CMC – pomiar prądów łożyskowych



Model Sensitivity
[mV/A]		 Peak
current
[A]		 Noise
max
[mVP-P]		 LF (-3dB)
bandwidth
[kHz]		 Typical LF LF
(<1%)
bandwidth
[kHz]		 Peak
di/dt
[kA/µs]		 HF (-3dB) bandwidth
(-3dB)
[MHz]
(Coil length 1000mm)

CMC015

 200.0 37.5 4.0 19.0 50.0 4.0 11.0

CMC03

 100.0 75.0 4.0 6.0 15.0 8.0 13.0

CMC06

 50.0 150.00 4.0 1.9 5.0 16.0 14.0



Output ±7.5V peak corresponding to ‘Peak Current’ into >100kΩ (e.g. DC 1MΩ oscilloscope)
Accuracy Variation with conductor position in the coil typically ±3% of reading (for a 5cm2 conductor) Linearity (with current magnitude) 0.05% of reading
Calibration Calibrated to ±0.5% reading with conductor central in the coil loop
Offset DC ±3mV maximum at 250C
Temperature Coil and cable -200C to +900C. Integrator 0 to +400C
di/dt ratings These are ‘absolute maximum di/dt ratings’ and values must not be exceeded:
Absolute max. peak di/dt: 70kA/μs
Absolute max. rms di/dt: 1.5kA/μs
Coil voltage 10kV peak
Safe peak working voltage to earth. Rating established by a 15kVrms, 50Hz, 60sec flash test. Information
about continuous use of the coils at high voltage can be obtained from PEM.

Key features

  • Coil length (circumference)
    500mm, 700mm, 1000mm - longer coils available on request
  • Coil cross-section (thickness)
    8.5mm max (14mm with removeable silicone sleeve – only for mechanical protection)
  • Cable length
    2.5m and 4m as standard (connecting cable coil to integrator) - longer cables available on request.
  • Battery options
    B-Standard: 4 x AA 1.5V alkali batteries. Lifetime typically 25 hours
    R-Rechargeable: 4 x AA 1.2V NiMH batteries. Lifetime
    typically 10 hours. External adaptor recharges
    batteries and powers unit.
  • Socket for external power adaptor (1.3mm diameter) (adaptor available from PEM as an option)
  • Electronics enclosure
    Dimensions H=183mm, W=93mm, D=32mm
  • Output BNC socket
    Supplied with 0.5m BNC:BNC cable.
Seria CMC – pomiar prądów łożyskowych - dane kluczowe
 


Seria CMC – pomiar prądów łożyskowych - charakterystyki - niskie częstotliwości

Low Frequency
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

 
Seria CMC – pomiar prądów łożyskowych - charakterystyki - wysokie częstotliwości

High Frequency
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

 
Seria CMC – pomiar prądów łożyskowych - charakterystyki - szumy

Noise
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

 


Seria CMC – pomiar prądów łożyskowych - charakterystyki - opóźnienie

Delay
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.
Typical noise – Model CMC03



 

Generating the order code
Type / Power supply / Cable length (m) / Coil circumference (mm)
e.g. CMC06 – 50mV/A battery supply, 2.5m cable from coil to integrator, 1000mm circumference coil
CMC06 / B / 2.5 / 1000
Less
Read more
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