In This Article
Key Takeaways
- ◆Magnet loss deposits heat in the rotor; quantify it for the actual winding, inverter and cooling design.
- ◆Segment count and direction follow the three-dimensional current paths and harmonic spectrum.
- ◆Electrical isolation must survive assembly pressure, temperature and ageing.
- ◆LOCTITE AA 326 is acrylic; its manufacturer gives 120°C maximum operating temperature. It is not a generic high-temperature epoxy recommendation.
- ◆A change in segment count does not establish a safe grade downgrade. Recheck hot demagnetization and faults.
- ◆BMW’s iX xDrive50 uses electrically excited motors, so it is not a permanent-magnet segmentation example.
Evaluate magnet eddy-current losses
Time-varying fields induce currents in conductive magnets. Their heat is deposited in the rotor and must leave through the actual thermal paths. Calculate losses over the duty cycle, including inverter and slot harmonics. A fixed percentage of motor input power or a temperature rise based only on magnet mass is not a substitute for a thermal model. Segmentation can reduce some current loops, but the loss reduction and temperature benefit require validation.
Electrical conductivity of sintered NdFeB
Sintered NdFeB is electrically conductive. Obtain resistivity data for the specified material and temperature. Simplified thin-section eddy-loss models can show a squared dependence on a relevant dimension and frequency, but field orientation, skin effects and three-dimensional return paths limit that approximation. A dimensional scaling example is not a predicted loss reduction for a complete rotor.
Choose axial segmentation
Compare candidate segment counts in the same electromagnetic model, then include the insulation and assembly geometry. The useful number of axial slices depends on stack length, harmonic field, segment contact and end effects. No universal four-to-six-slice optimum or percentage reduction is established here. Quote the chosen configuration with its machining, sorting, insulation and assembly requirements.
Circumferential segmentation and concentrated windings
Slot and inverter harmonics can produce current paths that axial cuts alone do not interrupt. Evaluate circumferential cuts where the model shows a benefit, and account for the added joints. Named vehicle and robot platforms need a verified motor configuration before serving as examples. BMW’s 2021 iX xDrive50 documentation describes electrically excited synchronous motors; that model is therefore excluded from permanent-magnet segmentation examples.
Specify inter-segment insulation
Henkel identifies LOCTITE AA 326 as acrylic, with a maximum operating temperature of 120°C in its product guidance. It is not the 150°C epoxy previously described here.
Choose an adhesive and activator or cure system qualified for the actual substrate, bond thickness, dielectric requirement and thermal duty; no substitute is specified by this article. Electrical contact between segments can defeat the intended isolation. Specify the cured joint, exposed edges and acceptance test, including voltage and conditioning. Neither an adhesive trade name nor a polymer layer by itself establishes automotive qualification.
Heating and the demagnetization knee
Magnet temperature changes coercivity and remanence. Determine the operating point on the hot demagnetization curve under normal load and the worst opposing field. Segmenting the magnet may reduce heating, but it does not independently increase the material’s coercivity. Requalify any proposed change of grade using the new loss and temperature distribution.
The reported N45H magnetic comparison and the separate historical 50H Dy-reduction example have different scopes. See the GBD evidence and limitations. Finished-part savings require a quotation and qualification of the proposed route.
Specify segmentation in the RFQ
State segment count and direction, finished dimensions, magnetization pattern, coating, insulation system and assembly datums. Define electrical-isolation and thermal-cycle acceptance criteria for this part. Agree material and assembly traceability and the required reports. Obtain a quotation for the chosen route; this guide does not establish a per-piece cost adder or a motor warranty saving.
- ●Segment layout and numbering on the drawing.
- ●Specified adhesive, preparation, cured thickness and cure.
- ●Inspection of the finished assembly after conditioning.
- ●Hot magnetic and fault-duty validation in the intended rotor.
Frequently Asked Questions
Does a low-speed motor need segmentation?
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Speed alone is insufficient. Assess pole count, slot and inverter harmonics, geometry, losses and cooling over the duty cycle.
What does segmentation cost?
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Obtain a drawing-specific quotation including cutting, insulation, sorting, fixtures and inspection. No generic cost adder is established here.
Can segments be machined after sintering?
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A route may machine sintered material into segments, then prepare, insulate and assemble them. Confirm the process, coating sequence and magnetization with the supplier.
Can segmentation justify a lower-coercivity grade?
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Magnet temperature changes coercivity and remanence. Determine the operating point on the hot demagnetization curve under normal load and the worst opposing field. Segmenting the magnet may reduce heating, but it does not independently increase the material’s coercivity. Requalify any proposed change of grade using the new loss and temperature distribution. The reported N45H magnetic comparison and the separate historical 50H Dy-reduction example have different scopes. See the GBD evidence and limitations. Finished-part savings require a quotation and qualification of the proposed route.
What adhesive should be used?
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Henkel identifies LOCTITE AA 326 as acrylic, with a maximum operating temperature of 120°C in its product guidance. It is not the 150°C epoxy previously described here. Choose an adhesive and activator or cure system qualified for the actual substrate, bond thickness, dielectric requirement and thermal duty; no substitute is specified by this article.
Does segmentation prevent demagnetization during faults?
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It may reduce the operating temperature if it reduces loss. The fault field and hot material curve still require separate evaluation; there is no fixed temperature or fault-margin benefit.
Send the segment layout, rotor duty and insulation acceptance requirements. Confirm the bond and magnetization process for the proposed assembly.
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