When considering an air-core current sensor design, several issues need to be taken into account.
The measurement of DC current requires the use of a field sensing element; due to the absence of a field focused area, a highly sensitive field sensing device must be considered ideally in an array around the conductor.
When available, a magnetic circuit can be used as a shield to external magnetic field disturbances; with air-core technologies the sensitivity to external disturbances must be managed in a different way, for example an array of field sensors instead of a single sensor or, when coils are considered, special design execution such as the Rogowski method of routing the return wire; the ability to accurately measure the desired current while also rejecting external fields is a significant challenge for air-core technologies.
Three efficient air-core sensors technologies are introduced below: the Rogowski, PRiME™ and Split core current transformer technologies.
Rogowski technology is an Air-core technology (without magnetic circuit).
A pick-up coil is magnetically coupled with the flux created by the current to be measured IP. A voltage VOUT is induced on the pick-up coil proportional to the derivative of flux and thus proportional to the derivative of the current to be measured IP. Because the derivative of DC is zero this technology is only useful for the measurement of AC or pulsed currents.
The waveform of the measured current requires the integration of the induced voltage VOUT. Therefore, the current transducer may includes an integration function in the processing electronics (option).
Rogowski RT type current sensors has a lightweight measuring head combined with remote electronics (distance between head and electronics can be as great as 4 meters, or 12 feet).
This, along with all of the previous described attributes, lead to a device suitable for use in a wide range of applications.
PRiME operates on the basic Rogowski principle.
Instead of a traditional wound coil, the measuring head is made of a number of sensor printed circuit boards (PCBs, each made of two separate air cored coils) mounted on a base-PCB.
Each sensor PCB is connected in series to form two concentric loops. The induced voltage at their outputs is then integrated in order to obtain both amplitude and phase information for the current being measured.
Typical applications include:
A transformer is a static electrical device transferring energy by inductive coupling between the windings making part of it. It is made with a primary coil (WP) with NP turns and a secondary coil (WS) with NS turns, wound around the same magnetic core (C).
A varying current IP in the primary winding (assimilated here to the primary conductor crossing the aperture: NP = 1) creates a varying magnetic flux in the transformer’s core crossing the secondary winding. This varying magnetic flux induces a varying electromotive force or voltage Vind in the secondary winding. Connecting a load to the secondary winding causes a current IS to flow. This compensating secondary current IS is substantially proportional to the primary current IP to be measured so that NP.IP = NS.IS
DC currents are not measured and not suitable because they represent a risk of magnetic saturation. The relationship here above is respected only within the bandwidth of the current transformer.
Warning!: Never let the output unloaded because there is a risk of safety for users.
Split Core AT & TT Type current transformers are used in Smart Grid and Automation environments.
Typical applications include: