Takashi Fukushige

Efficiency contours and loss minimization over a driving cycle of a variable-flux flux-intensifying interior permanent magnet machine

In this paper, experimental evaluations for the efficiency of a novel interior permanent magnet (IPM) machine with variable-flux characteristics using low coercive force magnets is presented. The variable-flux characteristics allow improving the efficiency of machine and also reducing the usage of rare-earth material in the high-coercive magnets, which are currently used for the IPM machines in electrified vehicles. A flux-intensifying interior permanent magnet (FI-IPM) type having positive saliency is employed for a positive d-axis current to mitigate a demagnetizing field in the magnet due to a q-axis current. A proof-of-principle machine is designed, fabricated and evaluated. A series of experiments are conducted to capture the efficiency contours with different magnetization states of the low coercive force magnets. The designed machine shows benefits in improving efficiency when the magnetization state is optimally operated. With these results, the loss over a driving cycle is then simulated and the benefits of changing the magnetization state are quantified.

Efficiency Contours and Loss Minimization Over a Driving Cycle of a Variable Flux-Intensifying Machine

In this paper, experimental evaluations for the efficiency of a novel interior permanent-magnet (IPM) machine with variable-flux characteristics using low-coercive-force magnets are presented. The variable-flux characteristics allow improving the efficiency of a machine. A flux-intensifying IPM type having positive saliency is employed for a positive d-axis current to mitigate a demagnetizing field in the magnet due to a q-axis current. A proof-of-principle machine is designed, fabricated, and evaluated. A series of experiments are conducted to capture the magnetization and demagnetization properties of the low-coercive-force magnets by applying control d-axis current pulse. The efficiency contours with different magnetization states (MSs) of the magnets are then experimentally obtained. The designed machine shows benefits in improving efficiency when the MS is optimally operated. With these results, the loss over a driving cycle is then simulated, and the benefits of changing the MS are quantified.

Variable-Flux Machine Torque Estimation and Pulsating Torque Mitigation During Magnetization State Manipulation

This paper focuses on dynamic control, under loaded conditions, of the magnetization state of suitably designed variable-flux (VF) permanent-magnet (PM) machines. Such VF-PM machines have been shown to achieve low loss operation over a wide range of load and speed. For this type of machine, the PM flux linkage varies during the magnetization manipulation process. Published magnetization techniques have occurred at zero load conditions and thus did not generate torque pulsations. However, under loaded conditions, the existing methods would produce unwanted torque pulsation. This paper proposes a parameter insensitive method to solve this issue. This method generates a decoupling current command that is calculated from accurately estimated stator flux linkage. Accurate flux estimation, i.e., insensitive to inductance saturation and PM flux linkage variation (e.g., temperature or magnetization level) is achieved by using the voltage disturbance estimated by a closed-loop stator current vector observer. In both simulations and experiments, it is shown that even during magnetization processes under loaded conditions, the flux can be estimated correctly, and smooth torque output can be achieved.

Operating within dynamic voltage limits during magnetization state increases in variable flux PM synchronous machines

This paper examines the issue of staying within the voltage capacity of a fixed bus voltage inverter during increasing magnetization state (MS) of variable flux permanent magnet synchronous machines (VF-PMSMs). MS manipulation using a fast stator id pulse is shown to induce a large voltage even at low speeds. In order to utilize the power conversion capability and improved efficiency regions of VF machines, MS should be manipulated over a wide speed range. Therefore, reaching the voltage capacity of the inverter when increasing MS is a significant dynamic voltage limitation for the existing methods. A reverse rotating current vector trajectory (RRCVT) method is proposed in this paper to mitigate the dynamic voltage limitation. The RRCVT method is a partial inverse model solution, where the current vector trajectory causes the contributing voltage components to partially cancel each other. The RRCVT method allows for MS to be increased at significantly higher speeds before reaching the voltage capability of the inverter.

Design methodology for variable-flux, flux-intensifying interior permanent magnet machines for an electric-vehicle-class inverter rating

This paper presents a design methodology for variable-flux, flux-intensifying interior permanent magnet (VFI-IPM) machines for a 30 kW electric vehicle class inverter rating requirement. Feasible VFI-IPM design considering current in operating range limits of 1 pu is a design objective. Positive Id vector control with positive reluctance torque, resulted from having Ld > Lq, under loaded conditions and magnetization state changes within 1 pu current (defined at continuous current rating) are primary constraints. Analytical relationships between magnet size, rotor and stator design parameters are used to facilitate a feasible design. Impact of design parameters on machine torque capability and saliency characteristics is evaluated.

Suggested design space in a PMSM parameter plane for variable flux machines

In this paper, design space for variable flux (VF) machines is examined in a permanent magnet synchronous machine (PMSM) parameter plane based on constant parameter, lossless power conversion properties, and estimated total loss distributions (i.e. copper and iron losses) while applying a d-axis current constraint based on an assumed demagnetization characteristic. Medium-to-high speed, partial torque operation is typical for duty cycle loads, such as in the case of the electric vehicle. VF machines can reduce losses in these operating conditions by reducing the internal magnetization state of the magnet, thereby reducing flux linkage associated with flux produced by the magnet. In this paper, VF machines with large normalized permanent magnet flux linkage which are in the highly salient, flux intensified (FI) machine design space (VFI-IPMs) are shown to have large maximum torque, rated power, inverter utilization, and constant power speed ratio (CPSR) as well as low losses in the low torque region over a wide speed range when compared to conventional fixed magnet flux PMSMs.

Analysis of Magnetizing Trajectories for Variable Flux PM Synchronous Machines Considering Voltage, High-Speed Capability, Torque Ripple, and Time Duration

This paper discusses various transient trajectories for increasing the magnetization state (MS) of variable flux permanent magnet synchronous machines (PMSMs), a class of PMSM where the magnets are demagnetized and remagnetized using the drive inverter during drive cycle operation. To manipulate MS, a magnetizing current pulse can be used, which may drive a large amount of flux linkage in the machine. As a result, above low-speed operation, a large voltage may be required from the inverter. This paper extends several existing methods for improved voltage properties, and proposes and experimentally evaluates a new method—a straight line stationary frame flux linkage trajectory—for higher speed capability. The presented trajectories, along with existing trajectories, are organized into families and compared regarding their required voltage, high-speed capability, torque ripple, and time duration.

Zero/low speed magnet magnetization state estimation using high frequency injection for a fractional slot variable flux-intensifying interior permanent magnet synchronous machine

This paper focuses on zero/low speed magnetization state (MS) estimation using high frequency injection for a fractional slot variable flux-intensifying interior permanent magnet synchronous machine (VFI-IPMSM). For VFI-IPMSMs, the knowledge of the MS is necessary to achieve loss minimizing control, since loss properties vary with MS. The MS can be estimated by measuring EMF, however, voltage sensors are not commonly used in standard drives. If a flux observer is used, accurate estimation is difficult at zero/low speed due to the diminishing EMF signal. To solve this issue, a superimposed high frequency (HF) injection method for MS estimation is proposed. Physically, higher MS implies a higher saturation condition which results in lower differential inductance. With a constant HF voltage injection, lower inductance (higher MS) results in a larger HF current response and vice versa. As a result, by imposing a HF voltage signal, the MS can be estimated through the HF current response. The proposed MS estimation methodology is evaluated experimentally with a fabricated fractional slot VFI-IPMSM and demonstrates effective MS estimation within 5 % error.

Variable flux machine torque estimation and pulsating torque mitigation during magnetization state manipulation

This paper focuses on dynamic control, under loaded conditions, of the magnetization state of suitably designed variable flux (VF) permanent magnet (PM) machines. Such VF-PM machines have been shown to achieve low loss operation over a wide range of load and speed. For this type of machine, the PM flux linkage varies during the magnetization manipulation process. Published magnetization techniques have occurred at zero load conditions and thus did not generate torque pulsations. However, under loaded conditions, the existing methods would produce unwanted torque pulsation. This paper proposes a parameter insensitive method to solve this issue. This method generates a decoupling current command which is calculated from accurately estimated stator flux linkage. Accurate flux estimation, i.e. insensitive to inductance saturation and PM flux linkage variation (e.g. temperature or magnetization level) is achieved by using the voltage disturbance estimated by a closed-loop stator current vector observer. In both simulations and experiments, it is shown that even during magnetization processes under loaded conditions, the flux can be estimated correctly and smooth torque output can be achieved.

Variable Leakage Flux IPMSMs for Reduced Losses Over a Driving Cycle While Maintaining Suitable Attributes for High-Frequency Injection-Based Rotor Position Self-Sensing

Variable leakage flux (VLF) interior permanent-magnet synchronous machines (IPMSMs), which use intentional magnet leakage flux paths cross-coupled with the q-axis flux paths to reduce losses over driving cycles, have been introduced recently, but self-sensing has not been discussed for these machines. This paper focuses on the saliency angular offset attributes that are the primary metric for the well-established injection-based self-sensing methods. This paper begins by evaluating the effect of the intentionally designed cross-coupling in the VLF-IPMSM with rotor surface bridges on the saliency angular offset. This paper then proposes alternate rotor designs with side-loop structure that cause relatively linear and well-behaved cross-coupling that improves the load-dependent saliency angular offset characteristics. The effect of VLF properties on power conversion and driving cycle loss reduction capability is evaluated for the proposed designs. This paper also applies the concept of VLF design to conventional flux-weakening interior permanent-magnet machine rotor design and presents the loss reduction capability of such designs.