Curved Permanent Magnet Geometry for Electric Power Steering Torque Ripple
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Solution Overview
Problem
Permanent magnet rotating electric machines experience cogging torque and irreversible demagnetization issues, particularly at high temperatures, leading to torque reduction and increased vibration and noise, which affect the performance of electric power steering devices.
Innovation Solution
The design incorporates a specific geometry for the permanent magnets with a curved surface like a circular arc, optimized armature winding configurations, and a demagnetization evaluation parameter to minimize cogging torque and irreversible demagnetization, including the use of magnetic projection portions and careful consideration of the air gap length and magnetic flux density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If the curved surface of permanent magnet is made like a circular arc to decrease cogging torque, then the cogging torque is reduced, but the height of the end portion of permanent magnet in the peripheral direction becomes smaller, making irreversible demagnetization more likely to occur
Solution Approach 1:
The invention applies different geometric characteristics to different portions of the permanent magnet. The curved surface opposite the stator is designed with a specific radius of curvature to reduce cogging torque, while the end portions are designed with increased height relative to the central portion. This local differentiation allows each region to fulfill its specific function: the curved surface minimizes cogging torque while the taller end portions prevent irreversible demagnetization.
2Temperature
If the permanent magnet rotating electric machine operates at high temperature, then the coercive force of permanent magnet is decreased, making irreversible demagnetization more likely to occur, but increasing the magnet's susceptibility to demagnetization
Solution Approach 1:
The invention performs preliminary action by designing the permanent magnet geometry in advance to prevent irreversible demagnetization before it occurs. The end portions are designed with greater height than the central portion, creating a geometric configuration that inherently protects against demagnetization under high-temperature conditions. This preventive design ensures that even when coercive force decreases at elevated temperatures, the magnet maintains its magnetic properties.
3Power
If irreversible demagnetization occurs, then the torque is decreased and the cogging torque and torque ripple are increased, but the magnetic flux produced by the permanent magnet becomes different from the design value
Solution Approach 1:
The invention provides beforehand cushioning by designing the permanent magnet with taller end portions that act as a protective buffer against irreversible demagnetization. This geometric configuration creates a margin of safety that prevents demagnetization under extreme operating conditions, thereby cushioning against the harmful effects of torque reduction, cogging torque increase, and magnetic flux deviation from design values.
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
This approach effectively reduces cogging torque and irreversible demagnetization, enhancing the performance and reliability of permanent magnet rotating electric machines, especially in electric power steering devices, by maintaining low torque ripple and noise levels even at high temperatures.
Implementation Method 1
a plurality of permanent magnets provided in order in a peripheral direction around the rotor core
Implementation Method 2
an interaction between a stator core and a permanent magnet
Implementation Method 3
an armature winding wound around each of the plurality of teeth to configure the multiple phases
Data Source
Figure 1
Figure 2~3
Figure 4~5
AI summary
A permanent magnet rotating electric machine includes: a stator including: a stator core having teeth, and an armature winding wound around each of the teeth to configure the multiple phases; and a rotor including a rotor core, and permanent magnets provided in order around the rotor core, The rotor is arranged to be spaced apart from the stator with an air gap therebetween. Each of the permanent magnets has a curved surface opposed to the stator and is configured to satisfy the following relationship: 0.65≤Rm×h1Wh1+g≤1.37 where Rm denotes a radius of curvature of the curved surface, h1 denotes a thickness of a central portion of the permanent magnet in the peripheral direction, W denotes a width of the permanent magnet in the peripheral direction, and g denotes an air gap length of the air gap.