Seok-Hee Han

Torque Ripple Reduction in Interior Permanent Magnet Synchronous Machines Using Stators With Odd Number of Slots Per Pole Pair

This paper develops analytical principles for torque ripple reduction in interior permanent magnet (IPM) synchronous machines. The significance of slot harmonics and the benefits of stators with odd number of slots per pole pair are highlighted. Based on these valuable analytical insights, this paper proposes coordination of the selection of stators with odd number of slots per pole pair and IPM rotors with multiple layers of flux barriers in order to reduce torque ripple. The effectiveness of using stators with odd number of slots per pole pair in reducing torque ripple is validated by applying a finite-element-based Monte Carlo optimization method to four IPM machine topologies, which are combinations of two stator topologies (even or odd number of slots per pole pair) and two IPM rotor topologies (one- or two-layer). It is demonstrated that the torque ripple can be reduced to less than 5% by selecting a stator with an odd number of slots per pole pair and the IPM rotor with optimized barrier configurations, without using stator/rotor skewing or rotor pole shaping.

Analysis of Rotor Core Eddy-Current Losses in Interior Permanent Magnet Synchronous Machines

This paper presents the results of an investigation focused on the rotor core eddy-current losses of interior permanent magnet (IPM) synchronous machines. First, analytical insight into the rotor core eddy-current losses of IPM machines is developed. Next, major design parameters that have the most significant impact on the rotor core eddy-current losses of IPM machines are identified. Finite element analysis results are then presented to compare the predicted eddy-current losses in the machine core of IPM machines with one- and two-layer rotors coupled with concentrated- and distributed-winding stators. It is shown that the lowest total eddy-current losses in the machine core are achieved using a combination of distributed stator windings and two magnet layers per rotor pole, while minimizing only the rotor core eddy-current losses favors replacement of the rotor with a single-layer configuration.

Analysis of Rotor Core Eddy-Current Losses in Interior Permanent-Magnet Synchronous Machines

This paper presents the results of an investigation focused on the rotor core eddy-current losses of interior permanent-magnet (IPM) synchronous machines. First, analytical insight into the rotor core eddy-current losses of IPM machines is developed. Next, major design parameters that have the most significant impact on the rotor core eddy-current losses of IPM machines are identified. Finite-element analysis results are then presented to compare the predicted eddy-current losses in the machine core of IPM machines with one- and two-layer rotors coupled with concentrated- and distributed-winding stators. It is shown that the lowest total eddy-current losses in the machine core are achieved using a combination of distributed stator windings and two magnet layers per rotor pole, whereas minimizing only the rotor core eddy-current losses favors replacement of the rotor with a single-layer configuration.

A Magnetic Circuit Model for an IPM Synchronous Machine Incorporating Moving Airgap and Cross-Coupled Saturation Effects

A new magnetic circuit model is presented for an interior permanent magnet (IPM) synchronous machine, using a machine with three-phase distributed stator windings and three layers of flux barriers in the rotor as an example topology. The model accounts for: i) the effects of cross-coupled magnetic saturation caused by the salient rotor; ii) variation of magnetic saturation levels in the iron rotor bridges that are key elements of the unitary rotor laminations; iii) the effects of stator lamination slots on the airgap mmf distribution; and iv) the local variation of airgap permeance due to the stator slotting and the relative position of the rotor with respect to the stator. As a result of these features, the new model is capable of significantly improving the accuracy of electromagnetic performance predictions for aggressively-designed IPM machines compared to previously-available magnetic circuit models. Comparisons with finite-element analysis and measurement results are provided showing that the new model is much faster while delivering appealing accuracy compared to the FE method.

Strand-level proximity losses in PM machines designed for high-speed operation

The stator windings of surface PM synchronous machines with fractional-slot concentrated windings are vulnerable to high proximity losses when operated at high speeds due to their typically high pole numbers and resulting high electrical frequencies. The limitations of an existing 1-D flux density model are demonstrated and an improved 2-D analytical model is introduced for estimating flux densities and stand-level proximity losses inside the slots. The 2-D analytical model is shown to provide good matches with finite element analysis for the predicted flux densities and proximity losses inside a rectangular slot with a variable slot opening and electrical frequency.

Reducing Harmonic Eddy-Current Losses in the Stator Teeth of Interior Permanent Magnet Synchronous Machines During Flux Weakening

Interior permanent magnet (IPM) synchronous machines can experience large harmonic eddy-current losses in the stator teeth under flux-weakening operation, significantly depressing the efficiency of these machines at high operating speeds. This paper presents a new analytical/finite-element hybrid design approach to reduce the harmonic eddy-current losses in IPM machine stator teeth during flux-weakening operation. The proposed technique achieves this objective by three steps: 1) developing an analytical index for the harmonic eddy-current losses in IPM machine stator teeth; 2) designing the spatial distribution of the rotor MMF to minimize the analytical index; and 3) synthesizing the rotor geometry to implement the desired rotor MMF function while maintaining the basic machine characteristics unchanged. It will be shown that two-layer rotors, if properly optimized, are significantly more effective than one-layer rotors for the purpose of reducing the harmonic eddy-current losses in IPM machine stator teeth during flux-weakening operation at high speeds.

Design and Experimental Verification of a 50 kW Interior Permanent Magnet Synchronous Machine

This paper presents the design details for an IPM machine designed to deliver 50 kW constant power over a 5:1 speed range extending from 850 rpm to 4250 rpm, with a gradual reduction in the required output power up to 8000 rpm (25 kW). Electromagnetic, thermal, and structural considerations have been included in the design optimization process. The resulting machine is designed with two magnet layers per pole and a distributed stator winding. Special features of the machine include its deep stator slots and four-layer winding, made necessary by the desire to minimize the machines moment of inertia. Test results available to date demonstrate that the machine is capable of delivering the required output torque and power, and the agreement between the predicted and measured machine parameters is generally quite good. Calculated iron losses for high-speed flux-weakening operation are presented in the final section of the paper, illustrating the challenges associated with minimizing the impact of high-frequency harmonic flux density components

Design Tradeoffs Between Stator Core Loss and Torque Ripple in IPM Machines

High stator core losses can pose a significant problem in interior permanent-magnet (IPM) machines operating over wide constant-power speed ranges. At lower speeds, the torque ripple can be undesirably large in some IPM machine designs, contributing to acoustic noise and vibration. While previous work has addressed these two problems independently, this paper shows that the conditions for reducing stator core losses during flux-weakening operation, dominated by harmonic eddy-current losses in the stator teeth, can conflict with the conditions for reducing the torque ripple of IPM machines. It is also shown that the resulting design tradeoffs depend on the details of the IPM machine topologies that are under consideration. The appropriate IPM machine topologies that offer more favorable tradeoffs are identified to achieve the best possible compromise of high-speed stator core losses and torque ripple characteristics.

Design Tradeoffs between Stator Core Loss and Torque Ripple in IPM Machines

High stator core losses can pose a significant problem in interior permanent-magnet (IPM) machines operating over wide constant-power speed ranges. At lower speeds, the torque ripple can be undesirably large in some IPM machine designs, contributing to acoustic noise and vibration. While previous work has addressed these two problems independently, this paper shows that the conditions for reducing stator core losses during flux-weakening operation, dominated by harmonic eddy-current losses in the stator teeth, can conflict with the conditions for reducing the torque ripple of IPM machines. It is also shown that the resulting design tradeoffs depend on the details of the IPM machine topologies that are under consideration. The appropriate IPM machine topologies that offer more favorable tradeoffs are identified to achieve the best possible compromise of high-speed stator core losses and torque ripple characteristics.

Impact of Maximum Back-EMF Limits on the Performance Characteristics of Interior Permanent Magnet Synchronous Machines

Interior permanent magnet (IPM) synchronous machines are vulnerable to uncontrolled generator (UCG) faults at high speed that can damage the inverter. One approach to reducing this risk is to impose limits on the maximum machine back-EMF voltage at top speed. This paper presents the results of a comparative design study that clarifies the nature and extent of the penalties imposed on the IPM machine metrics and performance characteristics as a result of imposing progressively tighter values of back-EMF voltage limits. As an alternative to limiting back-EMF and penalizing machine designs, this paper also investigates the effectiveness of the system-side protection approach to the same UCG fault problem