Measurements of magnetic flux density in rotating machines : development of a calibrated and portable measurement system

Abstract

Diagnostics of generator poles has traditionally been done by measuring the impedancein each pole and assume that the magnetic ux density is proportional to the impedance.With Hall-sensors it is possible to measure the actual magnetic ux density in each poledirectly. It is also possible to measure the ux density while the machine is runningwhich is not possible with impedance tests. In this project a complete system for makingmeasurements with Hall-sensors has been developed and tested on three generators. Twoof the generators were hydropower generators of 50 and 90 MVA. The measurementsystem is comprised of hall-sensors, power supply and a connection units as well as acalibration unit. 9 V lithium-batteries are powering the system making it easy to setupin any environment. Output of the system is a BNC-connector enabling connectionsto a wide variety of data acquisition interface units possible. The calibration systemhas been tested and veri_ed with a professional teslameter. Results from the tests showvirtually no deviation in the measurements made with the hall-sensors compared to theteslameter.Hall-sensors used in this project has an output voltage of about +/- 0.2 V with the uxdensitiesmeasured. In power stations there might be electromagnetic noise interferingwith the signal and this problem has been observed. It would therefore be favourable toget a higher amplitude on the signal from the Hall-sensors making them less exposed tonoise and unwanted voltages. An amplifier was added to the same circuit board as theactual Hall-sensors and placed in the air-gap. The resulting measurements made withthe ampli_ers had less noise than the measurements without the ampli_er. However, theampli_ers seems to be a_ected by the magnetic _eld in the air-gap leading to malfunctionof the ampli_ers.