Effects of Conducted Emissions on Closed-Loop Current Sensor Alignment

Qualification testing of variable frequency drives (VFDs) involves the use of precise current sensors to measure the drive output currents. In some cases, these current sensors are crucial even for controlling and safeguarding machinery during normal operations. Many such current sensors are designed based on closed-loop Hall effect and fluxgate technologies. This paper explores the impact of switching noise from VFDs on the scale and offset in the outputs of these closed-loop current sensors. We review the basic principles of closed-loop current-sensing techniques and their construction. A formula for calculating the total flux in the core is then developed. When the relaxation time of the sensor exceeds the dwell time between feedback pulses, the control loop’s ability to null the core is strained. The paper estimates the remanence or residual flux density of the core using the tracking error in the compensation circuit.