Brent S. Gagas

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.

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.

Variable flux permanent magnet synchronous machine (VF-PMSM) design to meet electric vehicle traction requirements with reduced losses

Variable flux permanent magnet synchronous machines (VF-PMSMs) in which the magnetization state (MS) of low coercive force (low-Hc) permanent magnets can be actively controlled to reduce losses in applications that require wide-speed operation have been proposed recently. While prior focus has been on achieving MS manipulation without over-sizing the inverter and obtaining higher torque capability, this paper extends the design objectives to include the power requirements of an electric vehicle traction motor over its entire speed range. Finite element methods are used to study the effect of combinations of low-Hc and high-Hc permanent magnets arranged in either series or parallel on the performance of VF-PMSMs. It is shown that while both configurations help improve the torque density, only the series configuration can help improve the high speed power capability. Experimental results showing the variable MS property, torque-speed capability and loss reduction capability of a series magnet configuration VF-PMSM test machine are presented.

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.

Magnet Temperature Effects on the Useful Properties of Variable Flux PM Synchronous Machines and a Mitigating Method for Magnetization Changes

Variable flux permanent magnet synchronous machines (VF-PMSMs) use permanent magnet magnetization as an additional degree-of-freedom to reduce losses based on operating conditions (e.g., at medium to high speeds, losses are reduced by using a lower magnetization). Magnet properties are known to be dependent on temperature; therefore, the magnet temperature effects on magnetization manipulation and maximum torque properties in VF-PMSMs are investigated in this paper with FEA simulations and experiments. Increased magnet temperature changes the available range of attainable magnetization levels and makes demagnetization occur more easily; therefore, a different current angle and magnetization are needed for maximum torque operation. The temperature effects on high speed magnetization manipulation methods, which are needed for driving cycle loss reduction and full power capability, are evaluated with simulation and experiments on a prototype 80 kW traction machine. A closed loop method for magnetization manipulation that mitigates the effect of temperature is proposed.

Variable Flux Permanent Magnet Synchronous Machine (VF-PMSM) Design Methodologies to Meet Electric Vehicle Traction Requirements with Reduced Losses

Variable flux permanent magnet synchronous machines (VF-PMSMs) in which the magnetization state of low coercive force permanent magnets can be actively controlled to reduce losses in applications that require wide-speed operation have been proposed recently. While prior focus has been on achieving magnetization state manipulation without oversizing the inverter and obtaining higher torque capability, this paper extends the design objectives to include the power requirements of an electric vehicle traction motor over its entire speed range. Finite-element methods are used to study the effect of combinations of low coercive-force and high coercive-force permanent magnets arranged in either series or parallel on the performance of VF-PMSMs. While both configurations help improve the torque density, only the series configuration can help improve the high speed power capability. Experimental results showing the variable magnetization state property, torque-speed capability, and loss reduction capability of a series magnet configuration VF-PMSM test machine are presented.

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.

Magnet temperature effects on the useful properties of variable flux PM synchronous machines and a mitigating method for magnetization changes

Variable flux permanent magnet synchronous machines (VF-PMSMs) use permanent magnet magnetization as an additional degree-of-freedom to reduce losses based on operating conditions (e.g., at medium to high speeds, losses are reduced by using a lower magnetization). Magnet properties are known to be dependent on temperature; therefore, the magnet temperature effects on magnetization manipulation and maximum torque properties in VF-PMSMs are investigated in this paper with FEA simulations and experiments. Increased magnet temperature changes the available range of attainable magnetization levels and makes demagnetization occur more easily; therefore, a different current angle and magnetization are needed for maximum torque operation. The temperature effects on high speed magnetization manipulation methods (which are needed for driving cycle loss reduction and full power capability) are evaluated with simulation and experiments on a prototype 80 kW traction machine. A closed loop method for magnetization manipulation that mitigates the effect of temperature is proposed.

Machine design for self-sensing

This paper provides a review of key concepts for concurrent design for different machine types that are suitable for self-sensing at both zero to low speed and high speed operation. The impact of modifying the machine design to be suitable for self-sensing on power conversion properties (Average, cogging and ripple torque) is presented for several classical and advanced machine types. General recommendations to yield both self-sensing performance and good power conversion properties are provided for induction machines, surface permanent magnet machines, flux-intensifying interior permanent magnet machines, and variable flux machines with variable magnetization state and variable leakage flux.