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Rigid Rogowski Coils

2012-04-12 15:54:34

Rigid Rogowski Coils

Rigid Rogowski coils have a greater accuracy and stability than flexible coils and excellent rejection of interference caused by external magnetic fields. They are better than flexible coils for low-current and low- frequency measurements.   

Construction:
Rigid coils are wound on accurately-machined plastic formers. A special machine is used to apply a precision winding. The construction ensures that the calibration of the coil has a long-term stability. The coils are carefully made to ensure that they have a low pick-up of external magnetic fields. During manufacture each coil is repeatedly tested and small adjustments are made to the winding to reduce the pick-up. These precision techniques ensure that interference caused by stray magnetic fields, for example from neighbouring conductors, is very low. We can measure the mutual inductance of the coil to an accuracy better than 0.1%. All rigid coils are provided with electrostatic screening as standard to reduce noise and to minimise the effect of capacitive pick-up from the conductor voltage.
 
With a suitable electronic integrator Rigid Coils can be used to measure low currents with a resolution as low as 100KA and high currents of greater than 100kA at 50Hz. Rogowski coil sensors provide complete isolation from the circuit being measured and have no effect on the current being measured even for very low-impedance circuits.

 

               
   Coil Dimensions               Coil Type  A (mm)  B (mm)  C (mm)  Resonance   Dimensions:
The table gives the dimensions for a number of standard coil sizes. We can also make other sizes.    

Resonance:
Each coil has a self-resonant frequency caused by the self capacitance and self inductance of the winding. The frequency depends on the length of the output cable as well as the dimensions of the coil. The values in the table are for an output lead length of 2m. The resonance can be damped by using a suitable value of resistor across the coil. As a guide, with a correctly damped coil, the high-frequency -3dB point is approximately equal to the resonant frequency.
 SX-76  122  73  42   78kHz  
 SX-100  146  97  42  64kHz  
 SX-120  166  117  42  no data  
 SX-150  196  147  42  59kHz  
 SX-170  216  167  42  41kHz  
 SX-200  246  197  42  41kHz  
 SX-220  266  217 42  no data  
     

Electrical Specification
The following gives typical values. The actual specification will depend on the wire gauge used for the winding.
  • Mutual inductance: 3 - 5 microhenries: For each coil the mutual inductance is measured accurately after manufacture.
  • Current range (50Hz): < 1mA to > 100kA
  • Upper frequency see 'Resonance' above
  • Lower frequency limit: less than 0.1Hz 
   SX76 Rigid Rogowski Coil
click for a larger picture
Accuracy:
Rigid coils have a very stable output and can be calibrated to an accuracy of better than 0.1% using traceable standards. Their output is not affected significantly by the position of the conductor threading the coil.

Special Constructions:
The standard specification has been found useful for many applications. We can also make special coils to meet particular requirements for example
  • Special material formers: for example to meet high temperature requirements or with specific expansion characteristics when coils have to be encapsulated.
  • A test winding which allows the correct functioning of the coil to be monitored continuously.
  • Coils wound on formers supplied by the customer.
  Split Coils:
We also make split rigid coils which can be fitted without disconnecting the conductor. These have a similar specification to the un-split coils.
For a conventional split Current Transformer the output is extremely sensitive to small changes in the gap between the two halves. Split Rogowski coils do not suffer from this problem.

Temperature stability:
Rigid coils are affected by increasing temperature causing (1) the former to expand - which increases the mutual inductance, and (2) the resistance of the wining to become higher - which decreases the output of the integrator. With careful design these two effects can be made to cancel each other out.

 

 

 


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