Detection of broken rotor bars. Part I - New theory

Conceptually, a broken rotor bar in a squirrel cage induction motor would imply an open circuit in one of the bars. On the contrary, research [1] has shown that in most copper cage motors, the contact impedance between bar and core is small, allowing current to continue to flow into the broken bar. This current enters the broken bar, and flows through the core to the adjacent healthy bars, as interbar currents. The paper shows that these interbar currents produce an axial force, and by monitoring the associated frequencies - interbar current and consequently broken rotor bars are detected at an early stage of development such as a high resistance joint (bar-end ring joints) or one broken bar. Experimental measurements in part II of the paper verify the theory presented in this paper.