Variable-pole induction machine and multi-leg inverter system for electric vehicles

Libbos, Elie

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https://hdl.handle.net/2142/122198 Copy

Description

Title

Variable-pole induction machine and multi-leg inverter system for electric vehicles

Author(s)

Libbos, Elie

Issue Date

2023-08-28

Director of Research (if dissertation) or Advisor (if thesis)

Banerjee, Arijit

Doctoral Committee Chair(s)

Banerjee, Arijit

Committee Member(s)

• Krein, Philip
• Haran, Kiruba Sivasubramaniam
• Miljkovic, Nenad
• Singh, Brij

Department of Study

Electrical & Computer Eng

Discipline

Electrical & Computer Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Multiphase drive
• Inverter
• Motor Drive, Power Electronics
• Induction Motors
• Electric Vehicle

Abstract

Electric vehicle (EV) drivetrains require high efficiency and power density and must operate over a wide speed range. Currently, permanent magnet (PM) motors are dominating the EV industry. However, the U.S. Department of Energy (DOE) is pushing to reduce rare-earth magnet usage while at the same time reducing volume and cost. Induction machines (IMs) are an established magnet-free alternative to PM motors. Although they are rugged, reliable, and inexpensive compared to PM alternatives, conventional fixed-pole IMs cannot maintain rated power capability over a wide speed range. They can lose nearly half their power rating at the maximum speed as the machine breakdown torque fundamentally limits them. Alternatively, varying the IM pole count on the fly ensures that rated power capability is maintained up to the maximum speed. A variable-pole IM (VPIM) drive is a co-design problem of the machine stator winding and multi-leg inverter. It has been widely established that VPIMs can maintain rated power over a wide speed range, but a key question remains: can a VPIM be used to push power density and potentially meet or get close to the DOE targets? This thesis proposes a variable-pole IM (VPIM) machine and inverter co-design and control framework with a key objective of achieving high power density. The other two objectives are to increase drive cycle efficiency compared to fixed-pole IMs and PM motors and to achieve smooth and fast online pole-changing dynamics. We approach this problem from a system-level perspective by integrating machines and power electronics design and control into the same framework. An experimental GaN-based 18-leg inverter driving a toroidally-wound IM is designed, built, and tested to validate the analysis in this thesis. First, we propose a multi-leg inverter design framework that captures the impact of the number of inverter legs on pole-changing capability, torque-speed envelope, and converter size and efficiency. The framework is constrained by the torque requirement at both the base and maximum speeds at the output and the dc link voltage at the input. An 18-leg inverter that reconfigures a six-pole IM to a four- and two-pole is designed and compared to a conventional three-leg inverter, driving the same identical-sized IM. The proposed 18-leg inverter has 50% less losses and requires 62% less dc-link capacitance than a three-leg converter. In addition, the voltampere rating of the three-leg inverter leg switches must be oversized by 50% to match the 18-leg torque at maximum speed. Second, we zoom in on the machine design aspect and investigate the impact of winding selection on the machine efficiency, power density, and torque-speed envelope. When a toroidally-wound IM is designed with a low aspect ratio, defined as the ratio of stack length to rotor diameter, it can provide the largest torque-speed envelope and achieve the highest efficiency and power density compared to distributed winding designs. Third, we move to the controls piece and propose a new bumpless pole-changing approach that uses droop control. Constant shaft torque is maintained during the transition by increasing torque in the new configuration at the same rate as the decrease in the previous one. A model is derived to capture the impact of pole-transition duration and operating conditions on the inverter ratings. A fast transition of 200 ms with less than 3.9% shaft torque variation is achieved using an experimental 18-leg inverter driving a toroidal VPIM. Fourth, we introduce a new VPIM concept called continuous variable-pole induction machine (CVP). The pole count of the CVP machine is not discretely selected but instead consists of a combination of more than one configuration. CVP eliminates pole-changing dynamics and has potential power density and efficiency benefits, but it is still at a very early stage. Fifth, the co-design framework is used to design a converter-integrated VPIM with a machine power density of 40 kW/L. The space between toroidal windings is exploited to locate the inverter legs and save space. Because of the extra thermal headroom, an estimated 25% increase in current density would be enough to achieve 51.5 kW/L and exceed the DOE targets. Finally, a drive cycle comparison to a fixed-pole IM baseline designed using the same framework is performed. Results show that VPIM drives reduce drive cycle losses by up to 55%, more than interior permanent-magnet motors (IPM) have achieved in the literature. These results prove that a VPIM drive system is an excellent magnet-free candidate to meet the DOE targets.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

© 2023 Elie Libbos

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