Loss minimizing and saturation dependent control of induction machines in vehicle applications

Modern variable-speed induction machine drives are widely used in traction applications such as hybrid or fully-electric road vehicles despite their lower efficiency compared to interior permanent magnet synchronous machine drives. The machine terminals are fed by a voltage source inverter that implements a field-oriented current control. The rotor flux and torque producing current component can be controlled independently and should be optimized to achieve maximum efficiency in all operating points. This work presents an approach to calculate the loss distribution in the machine and the inverter explicitly. Special focus is set on the consideration of magnetic saturation with the help of a 2D magnetostatic finite-element calculation. The solutions of the optimization problems are stored in look-up-tables. The control uses them to modify the flux level for minimal total losses over the whole speed range respecting the current and voltage limits. The control structure can be implemented easily and gives optimal results for machine parameters disregarding temperature effects. To adapt to heating of the machine it is then enhanced by a new discrete search algorithm based on a simplex state machine. Simulation results for both methods are provided and compared to standard operation. The combination of model-based loss minimization and search control seems most promising to reach true loss minimal operation.