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The existence of interbar currents in induction motors is well known and has received serious attention in the technical literature, mainly in connection with additional losses and their impact on motor efficiency. Much less known is the fact that these currents may have critical industrial importance on the working cycle of some machines, for instance, in two-speed elevator motors, where they may cause changes in the braking torque of 100% and over when compared with motors with isolated rotor bars. However, the difficulties and uncertainties in iron current analysis together with the strong variations of the iron to bar resistance, both locally and over the motor's lifetime, seriously limit the utility of analytical studies. In view of these facts, an extended and critical review of the proposed approaches to the analytical determination of interbar currents is first presented and thereafter a deep and physical insight into the phenomenon of interbar currents is given and emphasised. The different causes of motor ageing are then described and finally the results of an experimental study into thermal ageing of two-speed elevator motors with skewed and straight rotor bars are given.