Brushless Wiper Motor Magnet Geometry for Demagnetization Control
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Solution Overview
Problem
In motors with a 2:3 ratio of magnetic poles to teeth, salient poles can cause demagnetization of permanent magnets, leading to rotor core stopping at unfavorable rotation angles, requiring excessive power for restart.
Innovation Solution
A motor design with parallel-oriented permanent magnets and inclined surfaces on the salient poles, reducing demagnetizing fields and increasing magnetic flux density, while using ferrite magnets to control costs and prevent sudden torque fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If salient poles are provided on the rotor core to generate reluctance torque, then the rotation force of the rotor is improved, but the interlinking magnetic flux easily flows toward the salient poles causing demagnetization of the permanent magnets
Solution Approach 1:
The patent applies local quality by providing inclined surfaces at specific locations on the permanent magnets where they contact the salient poles. These inclined surfaces are localized to the circumferential side surfaces of the permanent magnets, creating a regional modification that directs magnetic flux away from vulnerable areas while preserving the overall magnet structure and reluctance torque generation capability.
Solution Approach 2:
The patent introduces asymmetry through the inclined surfaces on the permanent magnets, which are angled relative to the radial direction. This asymmetric geometry creates a preferred path for magnetic flux that avoids the circumferential side surfaces, thereby preventing demagnetization while maintaining the symmetric salient pole structure needed for reluctance torque.
2Force
If salient poles are provided on the rotor core, then reluctance torque is generated contributing to rotation force, but the order of cogging torque becomes twice the least common multiple causing the rotor core to stop at unfavorable rotation angles
Solution Approach 1:
The inclined surfaces are applied locally to the permanent magnets to modify the magnetic flux distribution in specific regions. This localized modification changes the magnetic characteristics at the magnet-salient pole interface, thereby altering the cogging torque pattern and preventing the rotor from stopping at unfavorable positions while preserving reluctance torque generation.
3Device complexity
If the ratio of magnetic poles to teeth is 2:3, then the motor structure is simplified, but the demagnetizing field is intensified when salient poles are present
Solution Approach 1:
The patent maintains the simplified 2:3 ratio structure while applying local quality modifications through inclined surfaces on the permanent magnets. These inclined surfaces are specifically positioned at the contact regions with salient poles, creating a localized solution that mitigates demagnetization effects without requiring changes to the overall motor structure or pole-to-tooth ratio.
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
Prevents rotor core from stopping at demagnetization angles, suppresses cogging torque, and achieves high reluctance torque with reduced inductance values, enhancing motor efficiency and reliability.
Implementation Method 1
The rotor has a shaft, a substantially cylindrical rotor core that is externally fitted and affixed to the shaft, and permanent magnets provided on the rotor core. For example, ferrite magnets are used as the permanent magnets.
Implementation Method 2
magnetic attractive and repulsive forces are generated between the interlinking magnetic flux formed in the stator and the permanent magnets provided in the rotor core, whereby the rotor continuously rotates
Implementation Method 3
By providing the salient poles, in the rotor core, a direction in which an interlinking magnetic flux (q axis magnetic flux) formed by coils of a stator easily flows and a direction in which the interlinking magnetic flux does not easily flow (d axis direction) are formed. As a result, a reluctance torque is generated in the rotor core
Data Source
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AI summary
Provided are a motor and a brushless wiper motor capable of preventing a rotor core from stopping at a rotation angle where permanent magnets are most easily demagnetized when the ratio of the number of magnetic poles of the permanent magnets to the number of teeth is 2:3. The motor is provided with: a rotor core 32; a plurality of permanent magnets 33 disposed on the outer circumferential surface 32b of the rotor core 32 and magnetized in parallel orientation; and a salient pole 35 saliently formed between the permanent magnets 33 adjacent in the circumferential direction of the outer circumferential surface 32b of the rotor core 32. The ratio of the number of magnetic poles of the permanent magnets 33 to the number of teeth is 2:3. Sloped surfaces 33e are formed on the side surfaces of the permanent magnets 33 in the circumferential direction. The angle θ2 between lines L3 and lines L2 is an electrical angle of 13°or more, said lines L3 connecting outer circumferential corner portions 33f where the sloped surfaces 33e and the outer circumferential surface 33a are connected to each other and the shaft center C1, said lines L2 connecting the radial direction outermost side of the circumferential direction side surface of the salient pole 35 and the shaft center C1.