Himavarsha Dhulipati

Design considerations for permanent magnet machine drives for direct-drive electric vehicles

Understanding the need for improvement in efficiency of an electric vehicle drivetrain system, this paper exclusively discusses various design aspects of a permanent magnet machine drive for direct-drive electric vehicles (EV). Firstly, the motivation to employ a direct-drive configuration in EV is discussed. Thereafter, initial electric machine rating design considerations for a typical Supermini or B-segment EV employing a direct-drive configuration is discussed. Furthermore, employing an existing stator, investigations are performed through analytical equations and designed machines to understand different permanent magnet machine design aspects with regards to selection of: number of poles, type of permanent magnet rotor, stator winding configuration and number of phases. The study performed here will assist in providing decision points on various structural design indices of the machine before venturing into the FEA based permanent magnet machine design and assessment for the direct-drive EV application.

Investigation of 6-phase surface PM machines with concentrated windings for reduction in space harmonics, leakage inductance and magnet loss in direct-drive EV

Surface Permanent Magnet (SPM) synchronous machines with fractional slot concentrated windings (FSCW) have been found to provide high torque density with low torque ripple and cogging torque, making them suitable for direct-drive electric vehicle (EV) application. This paper initially analyses a 3-phase 36/30 FSCW SPM machine for direct-drive application in terms of its space harmonics, steady-state characteristics and losses over a wide speed range using winding function theory and MTPA control in conjunction with finite element analysis (FEA). It was found that the 3-phase machine produces high space harmonics in the flux density which resulted in increased magnet eddy current loss and high stator leakage inductance which leads to extended constant power speed range (CPSR) as well. Since, the CPSR requirement for a direct-drive EV motor is lesser than that of a high-speed EV motor, there is scope for reducing the stator leakage inductance. Hence, a 6-phase 36/30 FSCW SPM machine employing the same stator, rotor and current rating as that of the 3-phase machine is investigated in an effort to reduce space harmonics, stator leakage inductance and magnet eddy current losses while delivering the desired output characteristics. Also, an analytical method to calculate the 6-phase machine d- and q-axis inductances from winding and slot permeance functions are proposed. Thereafter, a comparative performance analysis is conducted on both the 3-phase and 6-phase machines designed and results are discussed.

3-D Sub-Domain Analytical Model to Calculate Magnetic Flux Density in Induction Machines With Semiclosed Slots Under No-Load Condition

In this paper, a novel 3-D sub-domain analytical model is developed to determine magnetic flux distribution in single-cage induction machines (IMs) with skewed rotor bars under no-load condition in an effort to more detailed analysis of spatial harmonics. The proposed model, along with an optimization algorithm, is as an alternative solution to finite-element analysis (FEA) in optimizing the geometry of IMs. The analytical method is based on the resolution of 3-D Laplace and Poisson’s equations in cylindrical coordinates using the separation of variables method to calculate the magnetic vector potential for corresponding sub-domain. The proposed model includes the effect of slotting and tooth tips for the stator and rotor slots, which is usually neglected in a 2-D analysis due to the complexity of differential equations. Also, the proposed 3-D model can be used for any slot-pole combination in addition to considering the asymmetrical effect in the axial direction, which is a source of noise, vibration, and excessive losses in IMs. To evaluate the performance of the proposed 3-D analytical model, calculated magnetic-field distribution is compared with the results obtained from the 3-D FEA.

Design approach incorporating MTPA and winding function theories for on-board direct-drive surface PM machines with concentrated windings in EVs

Removal of the gear-box from a conventional all-electric vehicle (EV) power-train and incorporating direct-drive topology is expected to improve motor-to-wheel efficiency. Firstly, this paper discusses the merits and challenges of a novel direct-drive scheme employing a single on-board motor in an EV. Thereafter, motor design targets established for such an application in a typical super-mini EV are discussed. A novel bottom-up approach based on maximum-torque-per-ampere (MTPA) control and winding function theories of PM machines is proposed to design an on-board direct-drive surface permanent magnet (PM) machine with fractional-slot concentrated-windings in the stator. A typical direct-drive motor is designed employing the proposed approach and its performance is analyzed using its electromagnetic model in conjunction with finite element analysis and MTPA control scheme over the entire speed range of the motor. Comparative analysis of results obtained from analytical calculations and finite element analysis is performed. It is also shown that the proposed direct-drive scheme in EV is worth considering for advancement of state-of-the-art EV drive-train systems technology.

Response surface methodology based optimization of surface PM machine incorporating stator slotting and PM sizing effects to extend the operating limits for direct-drive EV application

Direct-drive electric vehicle motors have requirements such as high-torque, low-speed and a constant power speed range (CPSR) between 3 and 5 depending on the tire size. Furthermore, these motors must deliver lower cogging torque and torque ripple when compared to conventional electric vehicle high-speed motors due to absence of any damping mechanism. Surface permanent magnet synchronous machines (SPMSMs) with distributed winding configuration are found to favor the aforementioned characteristics for the above application. However, in spite of the lower CPSR requirements for direct-drive application, SPMSMs suffer from poor flux weakening operation. Various rotor structural modifications as well as optimal PM sizing solutions are proposed in literature. However, they fail to take into account the stator slotting effect which significantly affects the flux weakening operation of the machine. Thus, in order to alleviate the challenges involved in realizing a SPMSM as a direct-drive motor, Response Surface Methodology (RSM) is implemented with magnet size and stator slot dimensions as design variables in an effort to optimize the characteristic current and further enhance the CPSR of the machine. Finite element models of the optimal machine are used to verify the output power- and torque-speed characteristics over entire operating range calculated from analytical equations.

3-D sub-domain analytical model to calculate magnetic flux density in induction machines with semi-closed slots under no-load condition

In this paper, a novel 3-D sub-domain analytical model is developed to determine magnetic flux distribution in a squirrel-cage induction machine with skewed rotor bars under no-load condition. The analytical method is based on the resolution of 3-D Laplace and Poissons equations in cylindrical coordinates using separation of variables method to calculate the magnetic vector potential for corresponding sub-domain. The proposed model is sufficiently general to be used for any slot/ pole combination including slotting and tooth-tips for both stator and rotor, which were usually neglected in previous 2-D solutions due to complexity of the differential equations. To evaluate the performance of the proposed 3-D analytical model, calculated magnetic field distribution is compared with those obtained from the 3-D finite element analysis (FEA) and experimental results.