Outer-Rotor Brushless Motor Cogging Torque Reduction
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Solution Overview
Problem
Outer-rotor permanent magnet brushless motors face challenges with cogging torque and trapezoidal-shaped back electromotive force (EMF) waveform, leading to vibration and noise, particularly at lower speeds, and existing solutions are costly and difficult to implement due to the arc-shaped rotor magnets.
Innovation Solution
The motor is designed with a 16-pole outer-rotor and 15-slot inner-stator configuration, featuring a boot part with a specific arc-design and correction angle ratio (0.2 < a1/a < 0.6) to reduce cogging torque and modify the EMF waveform, approaching a sine wave for improved performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If arc-shaped rotor magnets are used in outer-rotor permanent magnet brushless motor, then the motor can be constructed with surface-mount attachment, but cogging torque increases and causes vibration and noise
Solution Approach 1:
The patent applies local quality by modifying only the stator tooth tip geometry (creating a boot part with specific arc radius and correction angle) rather than changing the entire rotor magnet structure. This localized modification reduces cogging torque while preserving the surface-mount attachment advantage of arc-shaped rotor magnets.
Solution Approach 2:
The patent changes geometric parameters of the stator tooth tip by introducing a boot part with specific arc radius (R1) and correction angle (α1). These parameter modifications alter the magnetic flux distribution at the tooth tip, thereby reducing cogging torque while maintaining the overall motor structure.
2Ease of manufacture
If conventional rotor magnet arrangements are used, then manufacturing is simpler, but modifying back-EMF and reducing cogging torque becomes costly and difficult
Solution Approach 1:
Instead of modifying the rotor magnets to change back-EMF waveform and reduce cogging torque, the patent inverts the approach by modifying the stator tooth tip geometry. This reverse engineering approach achieves the same goals (reduced cogging torque and improved back-EMF) while keeping the rotor magnet arrangement simple and easy to manufacture.
3Object-generated harmful factors
If stator tooth tip is modified with boot part and correction angle, then cogging torque is reduced and back-EMF waveform is improved, but manufacturing complexity increases
Solution Approach 1:
The patent introduces a curved boot part at the stator tooth tip with a specific arc radius (R1). This curvature modification smooths the magnetic flux distribution at the tooth tip, effectively reducing cogging torque. The curved geometry is relatively simple to implement compared to other complex structural modifications.
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 configuration effectively minimizes cogging torque, enhances motor smoothness, reduces manufacturing costs, and minimizes iron loss, resulting in improved performance and efficiency while maintaining a trapezoidal EMF waveform.
Implementation Method 1
the permanent magnet motor has the following advantages: it is comparatively simple in structure, reliable, smaller in size, lower operation loss, high operation efficiency
Implementation Method 2
Cogging torque of electrical motors is the torque due to the interaction between the permanent magnets of the rotor and the stator slots
Data Source
AI summary
An outer-rotor permanent magnet brushless motor is disclosed, which comprises an outer-rotor and an inner-stator. The outer-rotor configured with P poles, P is a natural number and denotes a multiple of 4. The inner-stator has a stator axle, and S teeth which configured on outer surface of stator, S=P−1, S is a natural number. A circle is formed about a center of the stator axle from the center of the outer end surface. A correction angle a1 and an original angle a are defined from the center of the outer end surface. T The ratio of a1/a is defined by the following formula: 0.2<(a1/a)<0.6.