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Optimization of Small Induction Machine Rotors for Enhanced Performance with Inverters

This research explores the design and optimization of small induction motors, specifically rotors tailored for inverter use. With a focus on rotor slot geometry, we employ finite-element method (FEM) simulations and a Monte Carlo optimization approach to identify the ideal rotor shapes. Our findings reveal potential for significant performance improvements in efficiency and torque, especially when using innovative slot designs. Future work includes refining rotor assembly methods and developing better testing procedures to further compare different rotor designs under standardized conditions.

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Optimization of Small Induction Machine Rotors for Enhanced Performance with Inverters

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  1. POWER AFFILIATES PROGRAM Rotor Designs for Small Inverter-Dedicated Induction Machines M. Amrhein and P. T. Krein University of Illinois at Urbana-Champaign

  2. Outline • Overview of induction motors and design • Design approach • Optimization process • Simulation results • Rotor assembling • Future work

  3. Induction Machines, Key Facts • Exist about 100 years • Efficient, rigid, easy to use • Low cost in manufacturing and service compared to other types of machines • Designs are well established for general purpose machines → Standardized designs, design classes (NEMA) • Designs have trade-offs to satisfy certain performance criteria

  4. Induction Machines, Key Facts • Rotor slot shapes are a key issue in the machine design • Slot shapes are optimized and well-known for standard machines • Innovative and new designs are rare • Electronic drives introduce new performance aspects • Drive technology improved (e.g., control algorithms, power electronics components)

  5. Why a New Design? • The machine designs have not been altered to take advantage of the new technology • Same motors are used for line-start applications as well as for drive applications • Machines can be optimized for operation with power electronics drives • Different performance criteria: • No line-starts • Motor operates only in low-slip regime

  6. Performance Criteria

  7. Project Idea • Idea: New designs of small induction motors optimized for use exclusively with inverters • Numerous parameters for optimization (stator-design, rotor-design, materials,...) ÞFocus on rotor slot design • Power range: ~ 3HP and less • Goal: Maximize performance with respect to • Steady-state operation efficiency • Maximum torque

  8. Design Approach • Use finite-element-method (FEM) solver to calculate machine performance, given a slot-geometry • Alter geometries, compare results using a performance function approach • Performance function should reflect efficiency

  9. Performance Evaluation

  10. Optimization Process, Generic Slot Model • Rotor slot is modeled with 14 geometric parameters • Most reasonable slot shapes can be modeled (e.g., deep bar slots and double cage geometries are possible) • Effects of closed / open slots can be simulated

  11. Optimization Process • Challenging optimization problem (14-dimensional) • A “brute force” approach with 10 values for each parameter would require 1014 test cases! • A sophisticated nonlinear approach still requires a large number of test cases • Convergence and analysis of converged solution would be an issue

  12. Optimization Process, Monte Carlo • “Solution”: Monte Carlo approach • Randomly chosen numbers are assigned to the 14 parameters within a given parameter space defined by geometric constraints • Advantages: • Broad exploration of different slot geometries • Likely to find good designs with enough test cases • Unexpected designs far from conventional ones are possible • Convergence is achieved with about 1000 test cases

  13. Simulation Results, Design Comparison • 4 possible rotor geometries • Design-01 is the yields highest performance index • Slot area of Design-01 ≈ 4 x slot area of conv. design

  14. Simulation Results, Design Comparison

  15. Simulation Results, Design Comparison

  16. Simulation Results, Area Sensitivity • Trend towards a consistent cross-sectional slot area was observed • Designs with equal area have a similar performance value q • Different bar materials yield an optimum slot shape similar to Design-01

  17. Simulation Results, Shape Sensitivity • Equal area criterion: Changes of up to 15% in parameter values do not have an effect on performance, as long as the total slot area does not change. • Conclusion: A certain amount of conductor material can be placed within a rotor to increase efficiency. Distribution of material is relatively insensitive.

  18. Rotor Assembling

  19. Rotor Assembling

  20. Rotor Assembling

  21. Experimental Setup

  22. Experimental Setup

  23. Future Work • Currently rotors are in production • Different issues need to be solved • Better way (easier) for rotor assembling • Casting process • Test procedures (how to test different rotors with a given stator for reasonable comparison) • Machine model more suitable for optimization • Full integration of the optimization process

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