Segmented Rotor Laminations with Fir-Tree Core for Stress Distribution
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Solution Overview
Problem
Spoke-type permanent magnet rotors with segmented laminations face stress concentration issues due to radial forces, and existing attachment methods are inefficient in distributing these stresses while maintaining magnetic flux.
Innovation Solution
A rotor design featuring a stack of ferrous laminations with fir-tree shaped openings for a non-ferrous metal rotor core, which maximizes stress distribution by evenly applying centrifugal loads and minimizing interference with magnetic flux, using a cast non-ferrous metal or plastic material for the rotor core.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the steel laminations are made in one-piece, then the manufacturing process is simpler, but the magnetic circuit is negatively affected
Solution Approach 1:
The rotor is divided into multiple steel lamination segments in the circumferential direction, with permanent magnets arranged between adjacent segments. This segmentation improves the magnetic circuit by creating proper magnetic flux paths while allowing each segment to be independently manufactured and assembled, resolving the contradiction between manufacturing simplicity and magnetic performance.
2Reliability
If the steel lamination segments are separated in the circumferential direction, then the magnetic circuit is improved, but each segment must be fixed to the shaft to withstand radial forces
Solution Approach 1:
The attachment structure is merged with the rotor core itself. The rotor core is formed by casting non-ferrous material into the openings of the lamination segments, creating an integrated structure that both connects the segments and provides the necessary mechanical strength to withstand radial forces, eliminating the need for separate attachment components.
Solution Approach 2:
A rotor core made of non-ferrous material (such as aluminum) is introduced as an intermediary element between the steel lamination segments. This intermediary material fills the openings in each segment and connects them together, providing both mechanical support and electrical insulation, thereby simplifying the overall structure while maintaining magnetic circuit performance.
3Strength
If traditional attachment methods are used to fix lamination segments, then the segments are secured to the shaft, but stress concentrations occur in the region between lamination segments and the cast rotor core
Solution Approach 1:
The openings in the lamination segments are given a fir-tree shaped profile with curved surfaces instead of sharp corners or straight edges. This curved geometry distributes stresses more evenly across the interface between the lamination segments and the cast rotor core, eliminating stress concentration points while maintaining strong mechanical attachment.
Solution Approach 2:
The geometric parameters of the attachment interface are optimized by using a fir-tree profile with specific branch angles and thickness variations. The radially outer branch has a smaller angle with the radial direction compared to the radially inner branch, creating a gradient in stress distribution that reduces peak stresses while maintaining attachment strength.
4Strength
If a large amount of casting material is used to reduce stress concentrations, then the load bearing capability is improved, but the weight and complexity of the rotor increases
Solution Approach 1:
The amount and distribution of casting material is optimized by using a fir-tree profile with varying branch thicknesses. The material is concentrated in regions of high stress (thicker branches) and reduced in regions of lower stress (thinner branches), achieving the required load bearing capability with minimal material usage, thereby reducing rotor weight.
Solution Approach 2:
The casting material is distributed non-uniformly within the rotor core, with higher material density in regions requiring greater load bearing capability (such as the outer branches of the fir-tree profile) and lower density in regions with lower stress requirements. This local optimization reduces overall material usage while maintaining structural integrity.
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 fir-tree shaped profile reduces stress concentrations and minimizes the amount of casting material required, enhancing the durability and efficiency of the motor by evenly distributing loads and maintaining magnetic flux efficiency.
Implementation Method 1
whereby the rotor core is formed by casting or molding a non-ferrous material, in particular a non-ferrous metal, in a space radially inwards of the lamination segments and into the radially outwards extending openings in each of the lamination segments
Implementation Method 2
Surfaces of the opening section profile which have a component facing away from the axis of rotation of the rotor are subject to centrifugal loading and the fir-tree profile maximizes the area of these surfaces, therefore minimizing stress concentrations
Data Source
Figure 1~2
AI summary
A rotor for an electric machine comprising a stack of ferrous laminations, the stack is divided into segments in the circumferential direction, whereby at least one permanent magnet is arranged between two adjacent segments, each segment comprising an opening extending in a radial direction outwards from a radially inner surface. A rotor core is provided for connecting the adjacent segments, whereby the rotor core is formed by casting a non-ferrous material in a space radially inwards of the segments and into the radially outwards extending openings in each of the segments, whereby the openings have a generally fir-tree shaped section profile.