Patel Bhageerath Reddy

Comparison of Interior and Surface PM Machines Equipped With Fractional-Slot Concentrated Windings for Hybrid Traction Applications

Electric drive systems, which include electric machines and power electronics, are a key enabling technology for advanced vehicle propulsion systems that reduce the petroleum dependence of the ground transportation sector. To have significant effect, electric drive technologies must be economical in terms of cost, weight, and size while meeting performance and reliability expectations. This paper will provide details of the design, analysis and testing of two permanent magnet (PM) machines that were developed to meet the FreedomCar 2020 specifications. The first machine is an Interior PM (IPM) machine and the second machine is a surface PM (SPM) machine. Both machines are equipped with fractional-slot concentrated windings (FSCW). The goal of the paper is to provide a quantitative assessment of how achievable this set of specifications is as well as a comparison with the state of the art. The paper will also quantitatively highlight the tradeoffs between IPM and SPM FSCW machines especially in the context of traction applications.

Advanced High-Power-Density Interior Permanent Magnet Motor for Traction Applications

Electric drive systems, which include electric machines and power electronics, are a key enabling technology for advanced vehicle propulsion systems that reduce the petroleum dependence of the ground transportation sector. To have significant effect, electric drive technologies must be economical in terms of cost, weight, and size while meeting performance and reliability expectations. This paper will provide details of the design, analysis and testing of an advanced interior permanent magnet (IPM) machine that was developed to meet the FreedomCar 2020 specifications. The 12 slot/10 pole machine has segmented stator structure equipped with fractional-slot concentrated-windings (FSCW). The rotor has a novel spoke structure. Several prototypes with different thermal management schemes have been built and tested. The paper will cover the test results for all these prototypes and highlight the tradeoffs between the various schemes.

Effect of Magnet Types on Performance of High-Speed Spoke Interior-Permanent-Magnet Machines Designed for Traction Applications

Interior permanent magnet (PM) machines are considered the state of the art for traction motors, particularly in light-duty hybrid and electrical vehicles. These motors usually use neodymium-iron-boron (NdFeB) PMs. These magnets include both light rare-earth materials such as neodymium (Nd) as well as heavy rare-earth materials such as dysprosium (Dy). The main purpose of Dy is to enhance the magnet coercivity to avoid demagnetization under both high temperatures as well as flux weakening. One of the key risks in terms of using these rare-earth magnets is the significant fluctuation/increase in their prices over the past few years. Applications that use large quantities of these magnets, such as traction motors and wind generators, are the most affected by these fluctuations. There has been an ongoing global effort to try to reduce or eliminate the use of rare-earth materials (particularly Dy which is the most expensive) without sacrificing too much performance. This paper will focus on advanced spoke designs targeting traction applications. The goal of this paper is to come up with new spoke designs using various grades of Dy-free magnets as well as ferrites targeting the same set of specifications. This paper will provide a detailed comparison between the various designs highlighting the key tradeoffs in terms of power density, efficiency, flux-weakening capability, and magnet susceptibility to demagnetization. Also, a prototype using ferrites has been built and tested, and the experimental results will be presented.

Transposition effects on bundle proximity losses in high-speed PM machines

Proximity effects caused by uneven distribution of current among the insulated wire strands of stator multi-strand windings can contribute significant bundle-level proximity losses in permanent magnet (PM) machines operating at high speeds. Three-dimensional finite element analysis is used to investigate the effects of transposition of the insulated strands in stator winding bundles on the copper losses in high-speed machines. The investigation confirms that the bundle proximity losses must be considered in the design of stator windings for high-speed machines, and the amplitude of these losses decreases monotonically as the level of transposition is increased from untransposed to fully-transposed (360°) wire bundles. Analytical models are introduced to estimate the currents in strands in a slot for a high-speed machine.

Effect of Number of Layers on Performance of Fractional-Slot Concentrated-Windings Interior Permanent Magnet Machines

Interior PM machines equipped with fractional-slot concentrated-windings are good candidates for high-speed traction applications. This is mainly due to the higher power density and efficiency that can be achieved. The main challenge with this type of machines is the high rotor losses at high speeds/frequencies. This paper will thoroughly investigate the effect of number of winding layers on the performance of this type of machines. It will be shown that by going to higher number of layers, there can be significant improvement in efficiency especially at high speeds mainly due to the reduction of the winding factor/magnitude of the most dominant stator mmf subharmonic component. It will also be shown that there is significant improvement in torque density. Even though there is reduction in the winding factor of the stator synchronous torque-producing mmf component, this is more than offset by increase in machine saliency and reluctance torque. The paper will provide general guidelines regarding the optimum slot/pole/phase combinations based on torque density and efficiency. Sample designs of various slot/pole combinations are used to quantify the benefit of going to higher number of layers in terms of torque density, efficiency, and torque ripple.

Reduced Rare-Earth Flux-Switching Machines for Traction Applications

There has been growing interest in electrical machines that reduce or eliminate rare-earth material content. Traction applications are among the key applications where reducing cost and, hence, reduction of rare-earth materials are key requirements. This paper will assess the potential of different variants of flux-switching machines (FSMs) that either reduce or eliminate rare-earth materials in the context of traction applications. Two designs use different grades of dysprosium-free permanent magnets (PMs), and the third design is a wound-field variant that does not include PMs at all. A detailed analysis of all three designs in comparison to the required set of specifications will be presented. The key opportunities and challenges will be highlighted. The impact of the high pole-count/frequency of the FSMs will also be evaluated. Experimental results for one of the designs with dysprosium-free PMs will also be presented.

Modeling and analysis of proximity losses in high-speed surface permanent magnet machines with concentrated windings

Surface permanent magnet (SPM) machines can be designed with fractional slot-concentrated windings (FSCW) to achieve extended speed ranges. High-speed operation can lead to significant levels of proximity losses in the stator windings due to substantial spatial harmonic magnetic fields in the air-gap as well as the high-frequency currents themselves. An integrated analysis tool is presented in this paper to calculate the strand- and bundle-level proximity losses in slotted stator conductors without requiring finite element analysis. A combination of analysis techniques, including Laplace equation solutions and conformal mapping, is used to estimate the magnetic field in each stator slot, providing the basis for proximity loss estimation. The match between losses estimated using the analytical model and finite element analysis is very promising.

Generalized Approach of Stator Shifting in Interior Permanent-Magnet Machines Equipped With Fractional-Slot Concentrated Windings

Electrical drive systems, which include electrical machines and power electronics, are a key enabling technology for advanced vehicle propulsion systems that reduce the petroleum dependence of the ground transportation sector. To have significant effect, electric drive technologies must be economical in terms of cost, weight, and size while meeting performance and reliability expectations. Interior permanent magnet machines with fractional-slot concentrated windings have been shown to be good candidates for hybrid traction applications. One of the key challenges is the additional stator magnetomotive force sub- and superharmonic components that lead to higher losses in the rotor as well as saturation effects. This paper tries to address this issue by looking into the concept of stator shifting. The generalized concept of stator shifting in the context of the harmonic components that are targeted for cancellation is presented; the focus is on single-layer and double-layer windings that have stator space subharmonics. It is shown that the stator shifting can reduce the loss-producing harmonics on the rotor as well as help the flux weakening performance of the fractional-slot concentrated winding designs. The cancellation of the loss harmonics is introduced as a method in which a particular harmonic can be targeted as well as reduce the phase inductance of the machine allowing for more room in terms of the operating voltage at higher speed. The concept of stator shifting will be explained, and the effect of varying the shift angle on the various harmonic components and winding factors will be investigated. Various designs, arising out of single-layer and double winding layer 10-pole, 12-slot configuration (targeting the FreedomCAR specifications) with varied shift angles are evaluated. The comparison between these designs in terms of their power density, efficiency, and torque ripple is presented.

Analysis of bundle losses in high speed machines

Elevated frequencies in high-speed ac machines increase the skin effect in stator windings, making it necessary in many cases to divide each phase winding into high numbers of small-diameter strands connected in parallel. Unfortunately, circulating currents among the strands in a single bundle can significantly increase the total copper losses depending on several factors including the twisting (i.e., transposition) of the strands in the bundles. An analytical model for these bundle proximity losses is presented for individual bundles as well as slot-bound bundles configured for high-speed machines. Loss predictions provided by this model match well with finite element results for high-speed operation of a 55 kW (peak) PM machine with concentrated windings. Stators with litz and non-transposed windings are compared to highlight the large differences in their bundle proximity losses. Analytical and finite element results are shown to match well with experimental results at frequencies of 700 and 800 Hz.

Design, analysis and fabrication of a high-performance fractional-slot concentrated winding surface PM machine

A high-performance 55 kW (peak) fractional-slot concentrated winding (FSCW) surface permanent magnet (SPM) machine has been designed to meet demanding performance requirements based on the FreedomCar advanced traction motor requirements prepared by the US Department of Energy. This paper describes key steps in the design, analysis, fabrication, and testing of this machine. Design challenges arising from the high-speed, high-efficiency requirements are presented followed by a discussion of design features that have been introduced to address them. A prototype version of the machine has been fabricated using a segmented stator configuration and segmented neodymium-iron rotor magnets. Test results gathered to date demonstrate promising torque production and efficiency characteristics that are consistent with the machine performance predictions.