CURRENT SENSORS

Zlan has developed new technology for implementing the arc fault circuit interrupter (AFCI) which reduces the manufacturing complexity and, at the same time, greatly simplifies testing of the final product.  A key element is a differential transformer for sensing the line current.  Normally, toroidal transformer cores small enough for use in molded case circuit breakers cannot handle the entire load current without saturating.  After the transformer core saturates, the output is no longer proportional to the current.  There are several ways to reduce the magnetic field in the transformer so that it doesn't saturate: Figure 4 -  Divert a small fraction of the line current into a wire that passes through the transformer core.  For example, if the load current is 10 amperes, the small wire passing through the toroidal transformer might be carrying only 1 ampere.  This works very well for breakers without the ground fault sensing requirement, when only one transformer is needed for sensing line current. Figure 4 Figure 5 -   Use a core with an airgap.  This reduces the magnetic field in the core but it also raises the low frequency cutoff point of the transformer.  This causes it to become a differentiator at low frequencies.  Analysis of signals from the transformer may be more complex when the roll-off in low frequency response has to be taken into account.  The assembly process may be affected because the heavy neutral wire has to pass around the outside of the current sensing core. This configuration also raises an economic issue since the breaker now requires different types of transformers for current sensing and for ground fault sensing.  Furthermore, it introduces a logistic problem in stocking two different components. Figure 5 Figure 6 -  Run both of the current conductors through the cores, which are aligned on the same axis, and shunt a small part of the current around the outside of one core.  This allows the current and the ground fault cores to be of the same construction, which reduces the cost of parts and simplifies assembly.  Also, the external current shunt is a physically small conductor, so the size of the assembly is not increased.  Figure 6 Figure 7 - Part of the line current is diverted around the current sensing transformer so its output remains linear. Figures 6 and 7 show how the two transformers for sensing load current and ground fault leakage current can be conveniently mounted side by side and the two power conductors run through both of them.  This configuration is compact and relatively easy to assemble.  By using a small conductor to bypass part of the main current around one of the tranformers its response will be proportional to the load current.  The flux in the transformer can be made a small fraction of what it would be if only one power wire passed through it because most of the flux due to the current in the hot wire is canceled by the equal but opposite current in the neutral wire.  In this way, core saturation is prevented and the output of the transformer remains linear even when the load current is several times the handle rating of the breaker. The fraction of load current that is diverted through the shunt of the AFCI/GFCI breaker depends on several design factors, but it may typically be on the order of 5 to 10 percent.  As the breaker's maximum design current increases, it's an advantage to make the shunt fraction smaller in order to keep the flux in the core material low and, as we will soon see, reduce the power required for testing.  In the following discussion, we will assume the current fraction is 10 percent.  Therefore, if 10 amperes are flowing in the load, 1 ampere will flow around the current sensing tranfomer via the shunt conductor.  This produces a net flux in the transformer corresponding to 1 ampere.  The output of the current sensing transformer provides the signal to the arc fault processing hardware and software.  The ground fault sensing transformer is an independent circuit and will be neglected for now.  Normally the analog output of the current transformer is digitized by an analog to digital converter (ADC) and the digital data goes to a microprocessor.  In some applications there may be various stages of analog filtering and amplification prior to the digitizing process but the output of the transformer is the starting point for arc detection. When the load current is 10 amperes, the transformer responds as if only 1 ampere were flowing through its center ( 9 amperes minus 10 amperes).  This current sets up a magnetic field in the transformer core and it is this field that determines the output of the transformer's secondary Since the transformer doesn't make direct contact with either of the power conductors, the line voltage has no effect on the magnetic pickup. This independence of line voltage is a very significant feature and can be exploited to greatly reduce the power needed to generate the arcing waveforms. Note that if we let one ampere flow through the neutral wire, which doesn't have a shunt, and no current flows in the hot wire to cancel it, the resulting magnetic flux in the transformer core corresponds to one ampere.  Hence, the output of the transformer will be the same as it would be for a full 10 ampere load because the transformer doesn't know the source of the magnetic field.  By using this smaller current, we have reduced the power level by a factor of 10, which is definitely in the direction to simplify the test equipment. In addition, this current of one ampere only has to flow through the neutral circuit which connects the AFCI/GFCI breaker's input and output terminals.  Since the resistance of this path is typically a small fraction of an ohm, the voltage drop between the input and output terminals will be small.  This low voltage at the output of the current source results in another reduction in power, on the order of 10X to 20X.  Thus we see that the total power required may be less than one percent of the power required if a brute force approach were used, yet the response of the current sensor is exactly the same.  This means that comprehensive testing of the breaker can be done with relatively low power equipment.  [Prev]   1    2    3    [Next]   Home