Rotor assembly method for permanent magnet motors to reduce cogging torque slot harmonic components

Assembly Method for Reducing Cogging Torque in Permanent Magnet Motors Using Magnet Placement to Offset Strength Variations

This technology is a rotor assembly method for permanent magnet motors that involves measuring the strength of each magnet, reselecting their placement positions until the sum of the squares of the strengths of magnets with the same torque phase and the total strength of N-pole and S-pole magnets are balanced within an allowable tolerance, and then assembling them into their final positions.

While permanent magnet motors offer high power density and efficiency, their inherent torque ripple makes them problematic for applications sensitive to noise and vibration, such as HVAC systems and precision machining equipment. Conventional cogging torque reduction designs assume ideal geometric and magnetic conditions, failing to account for unexpected cogging torque components caused by variations in magnet strength and manufacturing tolerances during actual production.

This technology calculates pole positions with identical torque phases based on the number of poles and slots, and uses iterative re-placement to balance the sum of the squares of magnet strengths and the total strength per polarity, allowing variations between magnets to offset one another. By optimizing the assembly sequence rather than requiring additional structural changes or high-precision magnet sorting, it reduces slot harmonic components of cogging torque and improves torque ripple characteristics. It can be applied to HVAC compressor motors, precision servo motors, and EV drive motor production lines, enabling stable mass production of low-noise, low-vibration motors even when magnet quality varies.

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Key Features:

• Measuring the strength of each magnet and randomly selecting initial positions for each magnet within the rotor
• Reselecting magnet positions until the sums of the squares of the strengths of magnets with the same torque phase are equal within an allowable tolerance
• Comparing whether the total strength of magnets at the N-pole and S-pole positions is equal within an allowable tolerance
• Assembling the magnets into the rotor at the final selected positions once both balance conditions are met

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This invention was developed with support from the Ministry of Trade, Industry and Energy for the development of core technologies for 5kW-class high-power-density motors for electric vehicles using a hybrid approach of non-rare-earth and rare-earth magnets.

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