A dc bias may be applied to the core in such a manner as to offset the dc component in the conductor signal. Bias may also be used to increase the available flux swing and consequently improve I-t capability.
The dc component of a signal increases the flux within the core of the transformer and is capable of reducing its effective permeability. It can reach a level where the reduction in permeability of the core is sufficient to degrade its low-frequency characteristics. It can cause signal distortion, an increase in the low-frequency cut-off point and a reduction in I-t capability and droop performance. The monitor recovers once the dc current is removed.
An offset dc bias is applied by adding one or more turns through the aperture of the transformer and setting the current to the appropriate value. It should be noted that the ac component of the signal to be measured will induce a voltage in this auxiliary winding. The impedance of the current source should be great enough to prevent a significant level of induced current flow in the conductor. An inductor or resistor of greater than 2 ohms impedance should prove adequate for all models. Bias current may be reduced by increasing the number of turns on the auxiliary winding. This will necessitate a proportionate increase in the impedance of the bias supply.
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