Induction Machine Characterization for Short-Term or Momentary Stall Torque

The conventional wisdom on peak-duty drive design leads to vastly oversized machines, following design strategies that are based on continuous ratings. This paper investigates induction machine time ratings that allow peak electrical capabilities to be fully attained. The machine transient thermal response is used to identify four heating regimes based on the dominant heat-transfer mode: subtransient, transition, transient, and temperature creep. A strong connection between the subtransient mode and i²t ratings is presented. The impacts of stator and rotor losses on the end-winding temperature are independently identified. This information is combined into a transient thermal impedance characteristic, which provides time ratings as a function of stator and rotor losses with less than 10% error. This method is also adapted for unbalanced supply conditions and rotor-related excess heating conditions with similar error. An implementation of time rating information on a drive system is shown to exploit machine peak ratings. The presented characterizations inform the short-term peak energy density of induction machines.