Operating Points in Closed vs. Open Magnetic Circuits
In a closed magnetic circuit (e.g., a magnet sandwiched between steel yokes), the permeance coefficient (Pc) is high, typically 5-10. The magnet operates in the linear region of the demagnetization curve, far from the knee. Demagnetization risk is low even at elevated temperatures.
In an open circuit (e.g., a magnet alone in air), Pc is low (0.1-0.5). The operating point lies near the knee. Any opposing field or temperature increase can push it into irreversible loss. For this reason, magnetic separators and holding devices use steel pole pieces to increase Pc.
In a permanent magnet motor, the rotor magnets experience a varying Pc depending on rotor position and stator current. At peak load, the demagnetizing field from armature reaction can reduce local Pc below 1.0, creating a demagnetization risk at the magnet edges.
Simulating Local Demagnetization from Reverse Magnetic Fields
FEA software (e.g., Ansys Maxwell, JMAG, Motor-CAD) calculates the magnetic field vector at each finite element of the magnet. The field is then compared to the BH curve at the operating temperature. If the field magnitude exceeds the intrinsic coercivity (Hcj) at that point, the element is considered demagnetized.
We simulate three conditions:
Rated load at max ambient temperature.
Overload (2x or 3x rated current) for 10 seconds.
Stall condition (zero rotor speed, full voltage applied).
Output maps show demagnetization percentage across the magnet volume. Typically, edges and corners are most vulnerable. Acceptable threshold: <5% demagnetization after 1000 cycles; <2% for automotive or aerospace applications.
Optimizing Chamfers and Thickness Ratios via FEA Software
Geometric modifications significantly reduce demagnetization risk. Adding a 0.5-1.5mm chamfer or radius on magnet edges reduces local field concentration. FEA allows rapid iteration: one engineer can evaluate 20-30 chamfer variations in a day, whereas physical testing of each iteration would take weeks.
Thickness ratio (magnet thickness / air gap length) also affects demagnetization. For a given motor, increasing magnet thickness from 3mm to 4mm raises Pc from 1.2 to 1.6, reducing demagnetization risk by 40-50% at the same operating temperature. However, thicker magnets increase rotor inertia and material cost. FEA optimizes the thickness ratio for minimum cost while staying above the demagnetization threshold.
Cross-Verifying Prototyping Results with Computational Data
After FEA optimization, we produce physical prototypes (5-10 pieces) and perform demagnetization validation:
Measure flux at room temperature (Helmholtz coil or fluxmeter).
Heat the assembly to max operating temperature for 1 hour.
Apply a reverse field pulse or run the motor at overload.
Cool to room temperature and re-measure flux.
Irreversible loss = (flux_after - flux_before) / flux_before × 100%. Correlation with FEA should be within 10-15%. If mismatch exceeds 20%, we refine the simulation (e.g., adding the effect of material property variation, manufacturing tolerances).
For custom magnetic solutions requiring FEA demagnetization analysis – including IPM rotors, spoke-type rotor magnets, and magnetic couplings – please refer to our Finite Element Analysis technical support page on our website. We provide full simulation reports with every prototype.
To request a demagnetization risk assessment for your existing or planned motor design, contact our FEA engineering team. Send your stator current waveform, rotor dimensions, magnet grade, and max temperature. We return a simulation report within 3-5 working days.
Frequently Asked Questions
Q: How accurate is FEA in predicting demagnetization compared to real-world motor testing?
A: Within ±10% for homogenous magnet properties. Variation comes from actual magnet Hcj batch-to-batch (±5%) and temperature measurement error. We recommend a safety margin of 15% below the rated Hcj.
Q: Can you simulate demagnetization for a Halbach array assembly?
A: Yes. Halbach arrays have complex flux paths, but FEA handles them accurately. We specialize in Halbach configurations for linear motors and MRI gradient coils.
Q: What is the minimum magnet thickness to avoid demagnetization in a 150°C PMSM?
A: Depends on air gap and stator current. As a rule of thumb for a surface-mount PMSM with 1mm air gap and N35SH magnets, minimum thickness is 3.5mm. For N42SH, 4.5mm. Run FEA for your exact geometry.
