Permanent Magnet Selector
Compare sintered NdFeB (neodymium) magnet grades by their demagnetisation curves, and check whether a magnet keeps its flux at your operating temperature, magnetic circuit and armature field. Set the permeance coefficient of the circuit, add any external demagnetising field from the winding or a fault current, and the selector finds each grade's operating point and its margin to the knee, the point past which flux loss becomes irreversible.
A magnet datasheet gives remanence and coercivity at room temperature, but a motor rotor runs hot. Coercivity of NdFeB drops by roughly 0.5–0.7 % per kelvin, so a high-energy grade that is comfortable at 20 °C can sit past its knee at 100 °C. Picking a grade means trading energy product (torque) against temperature margin, and this tool lays that trade-off out side by side. Every value can be shown in CGS (kGs, kOe, MGOe, as on most magnet test reports) or SI (T, kA/m, kJ/m³), and a quick unit converter sits below the tool. For the surrounding motor terms, see the electric motor glossary.
Measured vs typical data. The 52SH curves are Measured on a NIM-62000 hysteresigraph (single cylindrical sample, Ø10.04 mm, 20–150 °C, test date 2026-05-27).
N48SH, N52 and N54 are Typical values: nominal datasheet Br and HcJ at 20 °C, scaled with typical temperature coefficients. They are not measurements and not any supplier's guaranteed minimums. N52 and N54 are rated to about 80 °C, so their 100–150 °C rows are Typical · extrapolated.
Check Magnet Grades at Your Operating Point
Choose a temperature and the circuit's permeance coefficient. Click a grade, a table row or a temperature on the trend chart to focus it. The settings are kept in the page address, so you can bookmark or share a setup.
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Estimate Pc from magnet and air-gap dimensions
Simple magnetic circuit with ideal iron: Pc = (lm / g) × (Ag / Am). For a double-rotor axial flux motor with magnets facing each other across the stator, use half the magnet-to-magnet distance as g. Leakage is ignored, which gives a lower Pc than the real circuit, so the estimate errs on the safe side.
Demagnetisation Curves
Thick: intrinsic J–H curve. Thin: normal B–H curve. Dashed: load line from Pc, shifted by the extra field Ha. Filled dot: operating point. Ring: knee Hk. Hover for values.
Knee Margin at the Operating Point
Highest Usable Temperature
Highest tabulated temperature (20, 60, 80, 100, 120, 150 °C) at which each grade still holds a margin at this Pc and Ha. "Safe" means at least 1 kOe (≈ 80 kA/m) of margin to the knee.
| Grade | Safe up to | Below knee up to | Basis |
|---|
Property vs Temperature
Filled dots: measured. Hollow dots: typical. Dashed segments: beyond the grade's rated temperature (extrapolated).
All Grades
Select a row to focus that grade. Hk/HcJ is the squareness of the curve.
| Grade | BrkGs | HcBkOe | HcJkOe | (BH)maxMGOe | HkkOe | Hk/HcJ% | Basis |
|---|
Magnetic Unit Converter: Gauss, Tesla, Oersted, MGOe
Magnet test reports and many Asian datasheets use CGS units, while simulation tools and European datasheets use SI. Type a value to convert flux density, field strength or energy product.
Flux Density (B, Br)
Field Strength (H, HcJ, HcB)
Energy Product ((BH)max)
1 kG = 0.1 T · 1 kOe = 1000/4π kA/m ≈ 79.58 kA/m · 1 MGOe = 100/4π kJ/m³ ≈ 7.958 kJ/m³. In CGS, μ₀ = 1, so B in gauss and H in oersted share one scale.
How the Selector Works
Grade names. The number in an NdFeB grade is its nominal (BH)max in MGOe; the suffix sets the intrinsic coercivity class and with it the working temperature: none (about 80 °C), M (100 °C), H (120 °C), SH (150 °C), UH (180 °C), EH (200 °C). A higher suffix usually costs some remanence and adds heavy rare earths (dysprosium, terbium).
Intrinsic and normal curves. The intrinsic curve J(H) is the magnetisation of the material itself; the normal curve B = J + μ₀H is the flux density the magnet actually delivers. With h = |H| in kOe, the selector models the intrinsic curve from Br, HcJ, the knee field Hk and a recoil permeability μr = 1.05:
For the measured 52SH at 20 °C this reproduces the report's HcB to about 1.5 % (13.57 vs 13.39 kOe) and (BH)max to about 2 % (48.3 vs 47.43 MGOe). The tiles and tables always show the dataset values; the model only draws the curve shapes and finds operating points.
Load line and operating point. The magnetic circuit sets a load line B = Pc·(h − Ha), where the permeance coefficient Pc is the ratio −B/μ₀H the circuit imposes and Ha is any extra demagnetising field from the stator current. The operating point is where the load line crosses the normal curve, found by bisection. If the field exceeds HcJ there is no crossing and the magnet is fully reversed.
Knee margin. The margin is Hk minus the internal field at the operating point. At least 1 kOe is shown as Safe, 0–1 kOe as Marginal, and below zero as Past knee: the magnet loses flux that does not come back when it cools or the current drops. Coreless and large-gap designs, such as a coreless axial flux motor, run at a lower Pc, so they need the temperature margin of a higher-coercivity grade more than a slotted motor with a small air gap.
Limits. Temperatures are only evaluated at the six tabulated points; a magnet that passes at 120 °C and fails at 150 °C may fail anywhere in between. Real circuits have non-uniform Pc across the magnet, with the corners and trailing edges seeing the strongest field, so a finite element check of the final design is still needed. Rotor temperature itself depends on losses and cooling; see electric motor temperature.
This selector is provided for design exploration and does not replace supplier datasheets or detailed electromagnetic simulation. Turncircles makes no warranty as to the accuracy of results; see our Terms of Use.