In the joints of precision robots, under the rotors of drones, and even in the subtle operations of medical equipment, a key component is hidden—the frameless torque motor. Among these, the arc coefficient of the rotor magnet is the mysterious force influencing motor performance.
In modern technology, frameless torque motors have become core components for robotic joints, medical robots, and drone propulsion systems. Unlike traditional motors, frameless torque motors adopt a frameless design, characterized by small size, light weight, low inertia, and a compact structure.
Among the many factors affecting motor performance, the arc coefficient of the rotor magnet plays a crucial role in magnetic field distribution and overall motor performance. This article will provide an in-depth understanding of this seemingly small yet vitally important parameter.
01 Introduction to Frameless Torque Motors
A frameless torque motor is a new type of motor designed specifically for special application scenarios. It removes the frame structure of traditional motors and integrates the stator and rotor directly into the customer's equipment.
This design gives the motor a higher power density and a more compact structure, making it very suitable for applications where space is limited.
02 Definition and Importance of the Arc Coefficient
The arc coefficient (or pole arc coefficient) refers to the ratio of the permanent magnet pole arc length to the pole pitch. It is an important parameter describing the coverage range of the magnetic poles. In motor design, the arc coefficient directly affects the distribution and waveform of the air gap magnetic field, thereby influencing the motor's torque output performance and operational smoothness.
An appropriate arc coefficient can make the air gap magnetic field distribution closer to a sinusoidal wave, reduce harmonic content, lower torque ripple, and thus improve the motor's control accuracy and operational efficiency.
Research shows that using a pole arc coefficient of 0.85 can achieve relatively ideal output characteristics.
03 Influence of Arc Coefficient on Magnetic Field Distribution
The arc coefficient influences the motor's magnetic field distribution in several ways:
Magnetic Flux Magnitude:
A larger arc coefficient usually means a larger cross-sectional area of the magnet, enabling it to generate more magnetic flux, thereby increasing the motor's output torque.
Magnetic Field Waveform:
A suitable arc coefficient can make the air gap magnetic field distribution more sinusoidal, reduce harmonic content, and consequently lower the motor's torque ripple and operational noise.
Cogging Torque:
Optimization of the arc coefficient can effectively reduce cogging torque (a periodic torque ripple caused by the interaction between the stator slots and the permanent magnets).
Iron Core Saturation:
The arc coefficient, together with the stator tooth width, affects the degree of iron core saturation in the motor. Excessive saturation increases the non-linearity of the motor's torque characteristic curve and increases torque fluctuation.
04 Interactive Effects of Arc Coefficient with Other Parameters
The arc coefficient does not act independently; it has complex interactions with other motor parameters:
Table: Interactive Effects of Arc Coefficient with Other Parameters
Parameter Name | Interaction Manifestation | Optimization Suggestion |
Number of Poles | Increasing the number of poles leads to a decrease in the arc of individual magnetic poles, potentially reducing magnetic flux. | Find the optimal balance between pole number and arc coefficient. |
Stator Tooth Width | Stator tooth width is the main parameter affecting iron core saturation, jointly influencing magnetic field distribution with the arc coefficient. | Optimize stator tooth width and arc coefficient simultaneously. |
Air Gap Length | Air gap length affects magnetic reluctance, thereby influencing magnetic flux and field distribution. | Consider the combined effect of air gap length and arc coefficient on the magnetic field. |
PM Material | Different permanent magnet materials (e.g., N38EH, N48UH) have different magnetic properties, requiring different arc coefficient optimization. | Adjust the arc coefficient according to the PM material properties. |
05 Optimization Methods for the Arc Coefficient
Optimizing the arc coefficient is an important part of motor design. Main methods include:
Finite Element Analysis (FEA):
Use FEA software for precise simulation of the motor's magnetic field, finding the optimal arc coefficient through parametric scanning.
Skewing (Slots or Poles):
Using skewing techniques can effectively weaken cogging torque. Combined with arc coefficient optimization, it can further improve motor performance.
Auxiliary Slot Design:
Adding auxiliary slots on the stator tooth tips can alter the magnetic field distribution and reduce torque ripple. Studies show that adding 0.5mm auxiliary slots can reduce torque ripple by 0.25 percentage points.
Multi-objective Optimization:
Comprehensively consider the impact of the arc coefficient on torque output, torque ripple, iron loss, and copper loss to find the best compromise that meets multiple performance requirements.
06 Practical Applications and Case Studies
In practical applications, optimization of the arc coefficient has brought significant performance improvements:
Robotic Torque Motors:
Research indicates that measures such as optimizing the pole arc coefficient can reduce the impact of iron core saturation on torque characteristics, improving the linearity of the motor's torque characteristic curve and reducing torque fluctuations.
Frameless Motor Design:
A frameless motor for a collaborative robot, using a 24-slot 28-pole design and parameter optimization (including the arc coefficient), achieved a rated torque of 0.52Nm, a peak torque of 1.2Nm, while the cogging torque was only 0.0047Nm, keeping the torque ripple ratio below 1%.
Halbach Array Application:
Using a rotor structure with a Halbach array compared to traditional surface-mounted magnets can increase the torque constant by 7.6% under rated conditions and by 21.6% under overload conditions.
07 Future Development Trends
With technological advancements, the optimization of the rotor magnet arc coefficient in frameless torque motors continues to progress:
Multi-physics Coupling Analysis:
Future optimization will not only consider electromagnetic performance but also integrate the effects of multiple physical fields such as thermal performance and mechanical stress.
New Material Applications:
The development and application of new permanent magnet materials will provide more possibilities for arc coefficient design, such as magnet materials with better anti-demagnetization performance at high temperatures.
Intelligent Design:
Use artificial intelligence algorithms to accelerate the optimization process of the arc coefficient, achieving automation and optimization of motor design.
Customized Solutions:
Develop customized arc coefficient optimization solutions tailored to the characteristics of different application scenarios (e.g., robotic joints, medical equipment, drones).
The optimization of the arc coefficient is just one part of frameless torque motor design, but its synergistic effect with parameters like the number of poles, stator tooth width, and air gap length can create a more powerful and precise power source.
In the future, with the application of new materials and intelligent design technologies, the optimization of the arc coefficient will become more precise, opening up new possibilities for high-precision motor applications.
