This paper presents a novel algorithm for online rotor flux linkage estimation for a variable flux interior permanent magnet synchronous machine drive system at different flux density levels. A modified adaptive nonlinear filter is used to instantaneously estimate the amplitude, phase angle, and frequency of the major back EMF harmonic components, from which the total air-gap flux linkage is estimated. The algorithm avoids the averaging method that depends only on the fundamental back EMF component in estimating the air-gap flux linkage. The d-axis inductance versus current measurement test is performed at variable magnetization states to account for the d -axis inductance variation when estimating the rotor flux linkage. Since the stator winding resistance is temperature dependent, a thermocouple is used to obtain the actual stator winding temperature for accurate stator winding resistance measurement. The core loss resistance has been neglected for simplicity. The method was experimentally evaluated for different magnetization states and showed a good performance in tracking the rotor flux linkage variations. The method was tested for field-oriented control (FOC) schemes (i _d = 0) and for FOC with the negative d-axis current operation as well.

This paper presents the effect of different magnetization pulse widths and methods on the back-EMF, no-load losses, and minimum inverter rating at different operating conditions of a variable flux permanent magnet synchronous machine (VF-PMSM/VFM). Precise information of the back-EMF helps to estimate the cogging torque and torque ripple. An accurate determination of the core loss leads to a better efficiency estimation and machine design. An experimentally determined minimum inverter rating reduces the cost of the motor drive system. An adaptive nonlinear filter is used to estimate the VFM back-EMF during the motoring mode. The VFM is often demagnetized and remagnetized by injecting a current pulse to achieve the high speed and high torque requirements. The sudden injection of a d-axis current pulse will change the magnet flux linkage and back-EMF harmonics. In this paper, harmonics present in the machine back-EMF due to different magnetization and demagnetization current pulse widths and magnetization methods are analyzed. Besides, the quality of the back-EMF for different speeds and machine no-load losses are presented for different magnetization levels. The minimum inverter sizing is determined for magnetizing current and magnetizing current at full load considering the minimum pulse amplitude and width to magnetize the VF-PMSM.

Variable-flux interior permanent magnet synchronous motors (VFIPMSMs) find growing attention for electrified transportation applications, especially in the area of electric vehicles. While a prior focus was on optimizing the magnetization requirement to not oversize the inverter, and improving the machine power over a wide operating range, this paper aims to investigate and compare the different possible operating envelopes of the VFIPMSM from the drive point of view. Demagnetizing the low-coercive magnets via only a short d-axis current pulse eliminates the need of continuously applying a negative d-axis current in the flux-weakening region; hence lower copper loss and improved motor efficiency is expected. In this paper, this has been investigated and compared with the utilization of continuous negative d-axis current in the flux-weakening region considering the non-linear demagnetization characteristics of the low-coercive magnets. The latter scheme has been seen to improve the high-speed output characteristics and to extend the speed-range. Although, a constant-power-speed range with VFIPMSMs is not feasible due to the irreversible demagnetization of lowcoercivity magnets, an improvement of high-speed output power is shown to be feasible via saliency manipulation with the latter scheme. A VFIPMSM with a positive-reluctance torque (inverted saliency Lq < Ld) is used for experimental validation.

The paper shows the reduction of core losses by using a three-level inverter over a two-level inverter for the same dc-bus voltage and switching frequency. A synchronous reluctance machine stator-core toroid was used in the analysis. Hence, the analysis is realistic as it accounts for mechanical effects through the stator core, and the distorted supply of the line-to-line inverter voltage supplies. The paper also shows the limitations of using finite element-derived excitation; hence, the toroid was directly supplied by the inverter. The reduction of core losses by use of a three-level inverter is significant at very high flux densities and frequencies; approximately 60% lower core losses. Therefore, it can reduce the cooling burden especially in the hard to cool teeth, increase the service life, and allow increased output. The latter is because the output of the three-level fundamental voltage is higher for the same dc-bus and switching frequency at lower machine losses.

Variable-flux motors (VFMs) with hybrid permanent magnets (PMs) can limit the utilization of rare-earth PMs and reduce the high-speed losses of traction motors. These motors are often characterized by two main magnetization states (MS), i.e., the maximum and minimum. The flux variation is achieved by applying current pulses. This can be multiples of the rated current and changes the rare-earth PM operating point so that a minimum flux is produced by the low-coercive force PM (LCFPM). This paper presents a detailed study of the modeling and analysis of rare-earth PM demagnetization in hybrid PM VFMs. An iterative simulation procedure is proposed for predicting rare-earth PM demagnetization while driving the LCFPM to minimum MS. Two plausible causes of demagnetized rare-earth PM operations are investigated. Then, a series-hybrid PM VFM with partially demagnetized rare-earth PMs is tested experimentally to validate the proposed simulation procedure, and a reasonable match is found between the simulation and experimental results. Finally, some design remarks are presented, and two remedial design modifications are proposed, allowing minimal changes in rotor geometrical constraints. Results reveal that for hybrid PM VFMs, it is crucial to model the irreversible demagnetization behavior of rare-earth PM while predicting the re/demagnetization performance.

This paper uses an adaptive nonlinear filter to estimate the variable flux permanent magnet synchronous machine (VF-PMSM) back EMF. The VF-PMSM is demagnetized at high speed to limit the machine terminal voltage and magnetized at low speed to maintain the torque by injecting a short current pulse. In this paper, harmonics present in the machine back EMF due to different magnetization/demagnetization current pulse widths and magnetization methods are analyzed. In addition, the effect on the harmonic spectrum present in VF-PMSM back EMF when demagnetized with pulse current and continuous current is discussed. Usually, a d-axis current required to magnetize the VF-PMSM is larger than the machine rated current. Thus, the inverter rating corresponding to the machine rating cannot achieve 100% magnetization. The minimum pulse amplitude and width to magnetize the VF-PMSM is experimentally determined to calculate the inverter oversize considering the available DC bus.