Low switching frequency stator flux linkage observer for interior permanent magnet synchronous machines

Deadbeat-direct torque and flux control (DB-DTFC) is a direct torque control (DTC) method that provides fastest possible torque response with smooth and continuous operation even at the voltage limits. DB-DTFC achieves the smoothest possible torque and flux trajectory tracking performance by using the machine inverse model to select Volt-sec solutions that will produce the desired torque and flux by the end of each PWM interval. Since the DB-DTFC inverse model requires accurate steady state and dynamic flux estimates, discrete time stator current and flux linkage observers are needed to properly implement DB-DTFC. The existing high switching frequency stator current and flux linkage observers are formed based on approximate discrete time models for IPMSMs by assuming high switching frequency (10 kHz) operation. This paper presents a more exact discrete time model for IPMSMs, such that even at low switching frequencies, proper stator current and flux linkage observers can be formed to achieve accurate current and flux linkage estimation. The proposed low switching frequency stator current and flux observers are developed using discrete time modeling with an inverter-sourced Volt-sec. input model. The proposed observers are shown to be very feasible for realtime implementation and their experimental evaluation documents the improvement achieved compared to high switching frequency observers even under machine parameter variations.