A laboratory system to produce a highly accurate and uniform magnetic field for sensor calibration

Abstract

This paper addresses one particular issue that the authors came across when developing a DC Motor Duty Meter - a comprehensive condition monitoring tool for large DC motors based on flux density measurement inside the motor air gap. This particular issue crucial for the project was how to calibrate, with high accuracy, a large number of flux density sensors to be used to measure the flux density distribution along the surface of the motor poles. From available literature and IEEE standards, the recommended and most common way to generate a calibration magnetic field is based on Helmholtz coil principle, which in practice is limited to small flux density magnitudes. A method used by the authors and described in this paper utilizes a step test and simple calculations based on Faraday's Law. The novelty of the described method is in particularities of the system, measurements and calculations, which make the method highly immune to typical sources of errors. The method can be used to produce a very accurate and uniform magnetic field with densities ranging from fractions of Tesla to above one Tesla to calibrate flux sensors in this flux range. Its accuracy is only limited by that of a current source, a current probe and a digital oscilloscope. Uniformity of the generated field is discussed and experimentally confirmed. Experimental results obtained within the DC Motor Duty Meter project are included to illustrate correctness of the sensor calibration. Conditions that guarantee the maximum efficiency of the method are discussed. The paper is concluded by a step by step calibration procedure that other researchers may find useful.