Apoorva Athavale

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.

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.

Variable leakage flux (VLF) IPMSMs for reduced losses over a driving cycle while maintaining the feasibility of high frequency injection-based rotor position self-sensing

This paper presents variable leakage flux (VLF) IPMSM rotor designs which use intentionally created leakage flux paths in order to reduce losses over a wide torque-speed operating range and produce a high peak torque. Above base speed, the high leakage flux within the rotor reduces the stator flux linkage, thus reducing both iron and copper losses. Below base speed, as the load current iq increases, the magnet leakage flux decreases, and the magnet flux crossing the air gap (oriented in the d-axis) increases such that the machine can have a high peak torque. The effect of this intentional cross-coupling of d- and q-axis flux paths on high-frequency injection based position self-sensing capability, which is used at low speed conditions (including zero speed) is examined. A proof-of-principle machine is tested to experimentally evaluate the capability of obtaining iq dependent variable leakage flux characteristics. This machines saliency angular offset for use in high frequency injection based position self-sensing is also analyzed with FEA and evaluated with experiments. A new leakage flux path structure with side-loops in the rotor is then proposed. This paper presents a suitable selection of the dimensions of this side-loop structure to balance the tradeoff between the capabilities to obtain a large variation in leakage flux along with good position self-sensing characteristics. The effect of the VLF properties on total losses over a driving cycle is then evaluated. The mechanical integrity of the side-loop rotor structure at the maximum speed under consideration is examined. Flux weakening machine designs with VLF properties (FW-VLF-IPMSM) are also analyzed to assess the loss reduction capability of VLF machines compared to conventional FW-IPM machines.

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.

Magnetization state estimation in variable-flux PMSMs

Variable flux permanent magnet synchronous machine (VF-PMSM) traction drives are capable of significant loss reduction over an electrical vehicle drive cycle using dynamic magnetization state (MS) manipulation. A continuous, online MS estimation method is desirable for VF-PMSM drives to optimally utilize the loss minimization capability with precise MS control. This paper presents a MS estimation method that uses flux observers and a structured neural network (SNN) based machine model to achieve high accuracy with low sensitivity to load current and a fast dynamic response. The steady state performance of the proposed method in the presence of load current is evaluated experimentally in the current plane as well as in the torque-speed plane. The effect of magnet temperature on MS estimation accuracy is also evaluated experimentally. The transient performance of the proposed method during MS manipulation transients is evaluated using FE methods.

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.