Most current transducer technologies can be modified into a voltage transducer by ensuring the small measurement current (proportional to primary voltage) is applied to a large number of primary turns to create the Ampere-turns necessary for measurement. This is commonly accomplished with Fluxgate technologies, but less typically with open loop and Eta Hall based transducers.
The Fluxgate technologies discussed here cover several types of isolated current and voltage transducers based on the same basic measurement principle - the magnetic field created by the primary current to be measured is detected by a specific sensing element. The latter is driven through its B-H loop by dedicated electronics and the resulting magnetic effects are used for primary current detection. There are a wide variety of methods for concentrating the field, driving the magnetic core, and sensing the field intensity, but in all cases the underlying working principle is the same.
Working principle of Fluxgate technologies
The "standard" Fluxgate design will be described as this has a construction similar to the closed loop Hall effect current transducer. This example will provide the basic principles of Fluxgate technologies, after which other topologies can be understood.
Standard Fluxgate - working principle
An isolated Fluxgate current transducer can be designed in the same manner as a Hall effect based closed loop transducer, using the same magnetic circuit including a gap and secondary winding. The secondary winding is driven to provide zero flux in the gap as sensed by the Fluxgate element, rather than a Hall generator.
The main difference between the closed loop Hall technology and the Fluxgate is on the way the airgap field is detected - by a Hall cell in the first case, and by a so-called "saturable inductor“ in the second. This implies a drastic change of the transducer electronics used to drive the sensing element and to process the resulting signal.
The Fluxgate sensing element (Figure 1) is a "saturable inductor“ made of a small, thin magnetic core with a coil wound around it. It is generally made of discrete pieces of material (lamination sheet & copper wires), but various designs can be considered, including advanced concepts based on MEMS technologies.
Figure 1. Fluxgate magnetic sensing head (saturable inductor)
As with any inductor, the value of inductance of the "saturable inductor“ depends on the magnetic permeability of the core. When the flux density is high, the core is saturated, its permeability low, and the inductance high. At low field density, the inductance is high.
The "saturable inductor“ is purposely designed so that any change in the external field, Bext, affects its saturation level, changing its core’s permeability and consequently its inductance. Thus, the presence of an external field changes the inductance value of the field sensing element. This change can be very pronounced if the saturable inductor is adequately designed. The second factor affecting this inductance is the current, Isi, injected into the coil of the saturable inductor. This current produces a flux, channeled into the magnetic core, resulting in an additional magnetic field component, Bsi. The saturable inductor is generally designed in a way that Bext and Bsi have the same order of magnitude, both affecting the inductance value.
With the "standard Fluxgate“ design (Fig. 2), the primary current creates an airgap flux, ΦP (corresponding to Bext), which adds to the flux Φsi created by the saturable inductor current I
Figure 2. Standard Fluxgate - gap flux distribution – additive flux
In conclusion, changes in the magnetic saturation of the Fluxgate sensing head leads to inductance variations to be detected by the processing electronics (Figure 3). The closed loop principle is then used, where variations in inductance created by the primary current, IP, can be detected and compensated using the closed loop principle, feeding a current, IS, into the transducer’s secondary coil to return the gap field to zero and thus inductance back to a reference case. The relation between primary and secondary current is then simply given by the primary to secondary turns ratio.
Figure 3. Principle of the "standard Fluxgate"
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