Cage rotor heating at standstill

Abstract

Induction motors classified as ‘increased safety’ (Exe) must undergo a locked rotor test at full voltage, to determine the maximum cage temperature rise obtained within a specified time. The paper shows that the presence of stator MMF harmonics will cause the temperature rise observed during this test to vary from bar to bar around the rotor periphery. In a regular 3-phase machine the temperature will vary as the 6th harmonic of the fundamental pole number. In an irregular machine, such as a pole-change machine or a single-phase motor, the temperature distribution is more complex. It is shown that existing techniques of analysis can be used to give good estimates of the locked-rotor bar temperature distribution.