A linear combination of current and voltage based flux observers for direct torque controlled IPM drives

Flux observers in direct torque controlled (DTC) interior mounted permanent magnet (IPM) machine drives are of paramount importance as the drives rely on observed flux for control of the machine flux and torque. It is well known that a current based (CB) observer performs better at low speeds, whereas a voltage based (VB) observer is more accurate at high speeds. Hence, a large number of state-of-the-art DTC drives utilize the combination of CB and VB observers, known as hybrid observer (HO), for a wide speed operating range. However, because the VB observer accuracy becomes extremely poor when the speed approaches to zero, the performance and current waveforms of the DTC drive with the conventional HO significantly deteriorate. To overcome the performance deterioration at low speeds, an alternative combination of the CB and VB observers are proposed with a simple approach. In the proposed scheme, the performance deterioration and inaccurate estimations at low and high speeds due to the cross-interference between the CB and VB observers are avoided without an extra hardware or computational burden. Experimental results on a 10kW IPM machine designed for traction applications validate the superiority of the proposed approach.