Direct torque control for interior permanent magnet synchronous motors with respect to optimal efficiency

Due to their high torque and power per volume ratio, interior permanent magnet synchronous motors (IPMSM) are widespread electrical machines for traction drive applications, e.g. in the drive train of hybrid electric vehicles (HEV). IPMSM exhibit a significant degree of saliency along the rotor circumference. Thus, in order to exploit the full potential of an IPMSM drive, it is mandatory to utilize the reluctance torque. As a result, using a rotor flux oriented torque control, the clear separation between flux and torque generating currents is no longer possible, i.e. that the determination of the reference currents for the subordinated current control is rather difficult. Furthermore, drive applications demand for a wide constant power range with flux weakening ratios of up to 1:5 or even higher. However, flux weakening operation demands for additional measures to ensure that the current controllers dispose of a minimum amount of voltage reserve, which is necessary to govern transients and to compensate for disturbances. In this contribution a Direct Torque Control (DTC) structure is proposed to cope with the afore mentioned problems. In a DTC structure, torque and flux are controlled directly. Hence, the approach is very well suited for operation at saturated voltage. Furthermore, with torque and flux as control variables, it is not necessary to determine any reference currents. However, the proposed structure utilizes two Look Up Tables (LUT) for the determination of efficiency-optimal set values for flux and torque. The LUT are generated based on measurement results. So, they do already account for saturation effects. The proposed DTC structure has been implemented and verified on a test bed.