Switched brushless doubly-fed reluctance machine drive

Agrawal, Shivang

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https://hdl.handle.net/2142/121955 Copy

Description

Title

Switched brushless doubly-fed reluctance machine drive

Author(s)

Agrawal, Shivang

Issue Date

2023-10-24

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

Banerjee, Arijit

Doctoral Committee Chair(s)

Banerjee, Arijit

Committee Member(s)

• Haran, Kiruba
• Liberzon, Daniel
• Stillwell, Andrew
• Englebretson, Steven

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)

• Motor
• power electronics
• turbo-electric propulsion
• brushless
• reluctance

Abstract

Turbo-electric distributed propulsion systems are considered to be a critical enabler for low-carbon emission in the aircraft industry. A standard approach for constructing the megawatt-scale electromechanical drivetrains in these systems has been to use single electrical port motors and generators. A power-electronic converter is required between the motor and the generator to decouple the generator and motor speeds. The converters along with the associated filters process the full-rated machine power, leading to a bulky, expensive and less efficient system. A brushless doubly-fed reluctance machine offers several advantages over a single-port machine since it requires a partially-rated power converter, provides reduced maintenance, and operates without permanent magnets. However, BDFRMs have inherently poor torque density and high torque ripple. Since the flux modulation in a BDFRM is carried out by the rotor, the rotor structure plays an important role in torque production. The complexity of the air-gap magnetic fields due to differing winding-pole numbers interacting with a reluctance rotor have precluded the use of an analytical framework to optimize the rotor. Instead, researchers have relied upon time-consuming finite element analysis (FEA). This thesis presents an analytical method to compute closed-form solutions for the mean torque, captured by the coupling coefficient, using a practical circular ducted rotor. The method also accounts for the potential drop across the flux-barriers, leading to a better estimation of mean torque. After accounting for the primary and secondary stator harmonics, an analytical approach to model the effect of the rotor flux-barriers on the mean and ripple torque is also formulated in this work. The method shows that the instantaneous torque is highly sensitive to the location of these flux-barriers. The analytical torque profile is validated using both the finite element analysis (FEA) and experimental tests. A rotor with two flux-barriers per pole is used to illustrate the effectiveness of the proposed approach. The modeling framework is used to optimize the rotor geometry, stator geometry and excitations to maximize mean torque and minimize ripple. One of the candidate designs obtained from the pareto front is manufactured. The candidate design proves that while the designs with equal electrical loadings on both stators, and a torque angle of 90o between the two stators, may seem a good design approach, they are far from optimal. The optimal design has unequal electrical loadings and a torque angle of 117 degrees and is capable of producing 84% more torque than the conventional design with the same motor geometry. Experimental results are conducted to verify this claim. BDFRM utilizes a partially rated power converter but is restricted to applications which have limited operating speed range. The switched-drive architecture is presented in this thesis to extend the use of BDFRM to wide-speed range applications, such as propulsion systems, while preserving the benefit of fractionally rated power converter. The switched-drive architecture has already been implemented for a doubly-fed induction motor. In this proposed architecture, a controlled back-to-back converter is always connected to the secondary stator, while the primary stator is connected to different sources on-the-fly depending on the operating speed. The primary stator is connected to a low voltage dc supply at low-speeds and to a fixed voltage fixed frequency ac grid at high-speeds. The prototype BDFRM is chosen as an example to show the design process of power converter connected to the secondary stator. The switched-drive architecture can reduce the converter voltage rating by 47% as compared to the traditional drive while still operating from zero to 78% of maximum rotor speed. A unique controller is needed to fit the different demands of a switched BDFRM drive, specifically where both ac and dc supplies are available. This thesis also presents a control architecture for a switched-BDFRM drive. The proposed architecture adapts to the on-the-fly changes in the primary stator configuration and seamlessly controls the speed and torque during these variations. Experimental results are presented to validate the proposed approach. The proposed switched-BDFRM drive architecture opens up opportunities to create a compact, efficient, cost-effective, and sustainable solutions for constructing the megawatt-scale electromechanical drivetrains in turbo-electric propulsion systems as well as other high power electromechanical energy conversion systems.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Shivang Agrawal

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