Thermo-Mechanical Stresses of the Squirrel Cage Rotors in Adverse Load Conditions

As life extension of applications becomes increasingly important, the motors life span must also be increased. This result can be expected only if motor's design and use is properly matched to the driven machine. A rotor designed for a specific application is always a sensitive compromise between performances and costs. Thermal and mechanical strength of the rotor are often limiting factors due to stresses imposed by load characteristics. Die cast aluminum rotor designs performing satisfactory in standard conditions become unreliable when exposed to adverse load conditions. Load patterns like multiple direct-on-line stop/starts, re-closures or prolonged overload/stall conditions may induce high rate failures of the bearings and windings - as the most repaired components. Further investigations confirmed that main root cause of such failures is the extra-heat transferred from the rotors as a result of their performances degradations. Key transmitting torque from the rotor to the shaft Bottom bar, part of the running cage Top filament (tf), part of the starting cage "Leakage slot" (ls) Rotor's performance degradations are due to internal thermo- mechanical stresses of their electrical circuit initiated as a result of the transient rotor-bar currents generated in adverse load conditions. Resembling in form the wave propagation on transmission lines, the rotor bar current density variation along the slot generate "hot spots" in specific points where the temperature can reach values beyond safety limits. By approximating transitory phenomena as stationary, the currently used methods neglect the "hot spots" temperatures by averaging them to the entire rotor. Frequent occurrences of thermo-mechanical stresses could bring the conductive material close to its fatigue conditions initiating degradation process of the rotor's performances. As a result, the rotor lifetime and neighbour components are drastically reduced, increasing ownership expenses to unacceptable values. Analyzing specific industrial applications and using mathematical models, the paper proposes a practical method for designers and maintenance/application engineers in evaluating rotor capability to withstand thermo-mechanical stresses. Proposed thermal characteristics (� T i = f (t)) are useful in setting motor protection and control devices and assessing the rotor life span. This exercise offers practical guidance about the nature of design trade-offs that may be invoked to suit unusual loads - the option in boosting performances of electric motors without reducing their life span.