Brushless Motor Stator Rotor Air Gap Design for Torque Ripple Reduction
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Solution Overview
Problem
Fractional-slot winding motors experience increased harmonics leading to motor iron loss, magnetic saturation, vibration, and noise due to asymmetry in the magnetic circuit, resulting in cogging torque and torque ripple.
Innovation Solution
A brushless motor design with a stator and rotor configuration featuring evenly distributed teeth and magnets, where the distance between the rotor and stator varies to reduce cogging torque and torque ripple, and a sine wave back electromotive force is achieved, minimizing harmonic current and iron loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If fractional-slot winding is used to reduce slot area occupied by insulation bracket and improve slot fill factor, then winding resistance decreases and motor efficiency improves, but armature reaction increases higher harmonics leading to increased motor iron loss
Solution Approach 1:
The patent applies local quality by designing different air gap lengths in different regions: shorter air gap at tooth tips and longer air gap at tooth roots. This local variation optimizes the magnetic circuit characteristics specifically at the tooth regions where harmonics are generated, reducing iron loss without compromising the overall slot fill factor achieved through fractional-slot winding
Solution Approach 2:
The patent changes the air gap parameter from a uniform value to a variable value that depends on the radial position. By making the air gap length a function of position (shorter at tooth tips, longer at tooth roots), the magnetic circuit parameters are optimized to reduce harmonic content and iron loss while maintaining the benefits of fractional-slot winding
2Object-affected harmful factors
If fractional-slot winding is used to improve slot fill factor, then winding resistance decreases, but asymmetry of magnetic circuit causes partial magnetic saturation leading to motor vibration and increased noise
Solution Approach 1:
The patent addresses magnetic saturation and its harmful effects by applying local quality differentiation in the air gap design. The shorter air gap at tooth tips prevents magnetic saturation in high-flux regions, thereby reducing vibration and noise sources, while the overall fractional-slot winding configuration maintains high slot fill factor
Solution Approach 2:
The patent converts the potential harm of magnetic saturation into a benefit by strategically placing shorter air gaps at tooth tips where flux concentration occurs. This prevents saturation and its harmful effects (vibration and noise) while the fractional-slot winding continues to provide its efficiency benefits
3Object-affected harmful factors
If air gap between stator and rotor is reduced to smooth parasitic slot torque, then torque ripple decreases, but manufacturing precision requirements increase
Solution Approach 1:
The patent applies local quality by setting different air gap lengths at different locations: shorter air gap at tooth tips and longer air gap at tooth roots. This local differentiation smooths parasitic slot torque through magnetic cam action while the variations are built into the design, reducing dependency on ultra-high manufacturing precision for uniformity
Solution Approach 2:
The patent introduces dynamic variation in the air gap length along the radial direction, creating a non-uniform but controlled air gap profile. This dynamic design (varying air gap) provides magnetic cam action to smooth torque ripple while being manufacturable with standard precision tolerances
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 design effectively reduces motor vibration, noise, and iron loss while improving efficiency by controlling flux saturation and maintaining a sine wave back electromotive force, enhancing manufacturing simplicity.
Implementation Method 1
A brushless motor design with a stator and rotor configuration featuring evenly distributed teeth and magnets, where the distance between the rotor and stator varies to reduce cogging torque and torque ripple, and a sine wave back electromotive force is achieved
Implementation Method 2
the distance between the rotor and stator varies to reduce cogging torque and torque ripple
Data Source
Figure 1
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AI summary
A brushless motor includes a stator having stator core and winding teeth evenly distributed on the stator core; and a rotor rotatably disposed within the stator with the winding teeth facing the rotor, where the rotor has a rotor core and magnets evenly distributed around the rotor core. A first symmetry axis is defined passing through a center of a one of the magnets to a center of the rotor, and a second symmetry axis is defined passing between adjacent magnets to the center of the rotor. A first distance between the outer surface of the rotor to a surface of a winding tooth when the first axis is aligned with the winding tooth is smaller than a second distance between the outer surface of the rotor to the surface of the winding tooth when the second axis is aligned with the winding tooth.