Motor Thermal Protection by Continuous Monitoring of Winding Resistance

Simple practical techniques which sense ac line current afford protection of motor windings for many applications. A more complete protection system is described which continuously monitors average winding resistance and hence temperature by means of an easily applied dc injection technique. No additional motor connections are required.

[1]  R. E. Seely.  A circuit for measuring the resistance of energized A-C windings , 1955, Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics.

[2]  Gregory J. Johnson.  Determination of Temperature Rise of Energized Transformers by the Resistance Change Method , 1970 .