Conductive Shield for High-Speed Rotor Loss Reduction
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Solution Overview
Problem
High-speed permanent magnet electric machines face significant challenges in reducing rotor losses due to power electronics switching harmonics, which cause eddy currents and I2R losses, especially at high rotating speeds and switching frequencies, as conventional methods like conduction and convection are inefficient, and segmentation of rotor magnets is impractical for high-speed applications.
Innovation Solution
An electrically conductive shield is placed between the stator and the rotor magnets to filter high-frequency harmonics, localizing losses at the rotor surface and reducing heat generation, allowing for easier heat dissipation, with the shield being made of high-conductivity materials like copper or copper alloys and configured as a cylindrical tube, foil, or coating covering the magnets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If rotor magnets are segmented to reduce rotor losses, then rotor losses are reduced at relatively low switching frequencies, but the magnet fill factor of the rotor is substantially reduced
Solution Approach 1:
The rotor magnets are segmented into multiple sections along the air gap surface, with electrically conductive material placed in the gaps between segments. This segmentation reduces eddy current paths in the magnets while the conductive material provides alternative current paths, thereby reducing rotor losses without significantly reducing the magnet fill factor
Solution Approach 2:
An electrically conductive material is introduced as an intermediary element between the segmented rotor magnets. This conductive material (such as copper or aluminum) provides a low-resistance path for eddy currents, reducing the I2R losses that would otherwise occur in the magnet material itself, while maintaining the magnetic field integrity
2Temperature
If conventional heat removal methods (conduction to bearings and convection to airflow) are used, then heat dissipation is achieved, but they are not effective at removing significant amounts of loss from the rotor
Solution Approach 1:
The electrically conductive material serves as a thermal intermediary that conducts heat away from the rotor magnets more effectively. By providing a dedicated conductive path parallel to the magnetic flux paths, heat generated by eddy currents can be conducted away from the magnets through the conductive material, enhancing heat removal efficiency beyond conventional methods
3Productivity
If high-speed operation and high switching frequency are used, then machine performance is improved, but rotor losses increase due to power electronics switching harmonics
Solution Approach 1:
The high-frequency switching harmonics that cause rotor losses are converted into a beneficial effect. The conductive material in the magnet gaps is designed to capture these high-frequency eddy currents, effectively using the harmful high-frequency fields to drive currents in the low-resistance conductive material rather than in the high-resistance magnet material, thereby reducing I2R losses
Solution Approach 2:
The electrical conductivity parameter of the rotor structure is changed by introducing highly conductive material (copper or aluminum) in the magnet gaps. This parameter change creates preferential paths for high-frequency eddy currents, redirecting them away from the magnet material and reducing the effective resistance encountered by these currents, thereby reducing losses at high switching frequencies
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The conductive shield effectively reduces rotor losses by localizing high-frequency harmonic fields at the rotor surface, enhancing heat dissipation and maintaining magnet fill factor, thus improving efficiency in high-speed machines.
Implementation Method 1
These very high-frequency harmonics cause eddy currents in the rotor, and thus I2R (current squared×resistance) losses
Implementation Method 2
An electrically conductive shield is provided between the permanent magnets and the rotor core, and in some instances between the magnets themselves, to filter the high-frequency harmonics
Implementation Method 3
The shield is configured to conduct heat away from the rotor surface, enhancing heat dissipation
Data Source
AI summary
An electric machine has a stator defining an interior cavity and a rotor supported to rotate in the interior cavity of the stator. The rotor has a rotor core with a plurality of permanent magnets arranged around a perimeter of the rotor core. The magnets define a cylindrical, radially facing outer surface. An electrically conductive shield is provided on and covering the cylindrical outer surface of the permanent magnets. A containment sleeve resides around the rotor core, the permanent magnets and the shield and defines an exterior cylindrical surface of the rotor. The containment sleeve is configured to retain the magnets to the rotor core during operation of the electric machine.