Hybrid Current Control of Rare Earth Free Biaxial Excitation Synchronous Machines

Rare-earth free biaxial excited synchronous machines (BESM) are best suited for achieving unity power factor at their base speed which enables the characteristic of high-power density and lower cost for a traction motor drive system. Feedforward and feedback current control techniques are widely employed control techniques for any electric machine due to their features of low cost and dynamic performance respectively. Due to the nature of dual excitation of the machine, a hybrid current control technique is explored in this paper. Feedforward current control is being employed for stator currents in synchronous reference frame and feedback current control is employed for rotor field current. Typically, feedforward current control requires accurate knowledge of motor parameters. However, the parameters vary nonlinearly based on the operation of BESM. Hence, it is important to analyze and understand the impact of machine parameter estimation errors on the steady state as well as the dynamic performance of the system. This paper presents a detailed implementation of the hybrid current control technique along with the parameter sensitivity analysis. Analytical expression for the actual currents in the hybrid current controlled BESM is developed under parameter estimation errors and the analysis is verified extensively under various circumstances.