Demagnetisation Needs Two Conditions at Once
A magnet does not weaken gradually toward failure. It performs to specification, and then at some combination of temperature and reverse field it crosses the knee and loses a permanent chunk in a fraction of a second.
Magnets and Motors · Part 4 — Limits and Supply · Chapter 9 · 18 min read
Part 4 — Limits and Supply
How rotors are actually destroyed
Not gradually, and not from heat alone. Two conditions have to arrive at the same instant.
A magnet does not weaken gradually toward failure. It performs to specification, and then at some combination of temperature and reverse field it crosses the knee and loses a permanent chunk in a fraction of a second.
2
Conditions needed simultaneously
−420 kA/m
A representative fault field
2–3×
Peak over continuous rating
180 vs 80 °C
Class H insulation vs an N42 magnet
9.1 — Crossing the knee
Important
The load line is fixed by geometry. As temperature rises the knee climbs to meet it, and a fault current that was survivable at 60 °C is destructive at 150 °C.
The magnet does not know it is about to fail, and nothing in the drive’s telemetry sees it coming. It shows up afterwards as reduced back-EMF and lost torque per amp — which is why the back-EMF test of chapter 11 is the standard health check.
9.2 — The worst case is a specific, foreseeable event
Not a hot day. The design case is a three-phase short circuit at high speed with the rotor already at maximum temperature — an inverter fault at the end of a long gradient. The short-circuit current is large, its field opposes the magnets directly, and the magnets are at their least resistant moment.
A second case is worth designing against: aggressive field weakening. The negative d-axis current that extends the speed range is a demagnetising field, applied deliberately and continuously at the top of the speed range where losses and temperatures are highest.
Why this matters
Which is why grade selection is not a magnet decision. Choosing 42SH over N42 costs energy product and money, and buys 30 °C of headroom. That trade is settled by the thermal model and the fault analysis, not by a procurement spreadsheet — and it is set at the worst combined operating point, never at the nominal one.
9.3 — Cooling, in rough order of capability
| Method | What it does | Where it falls short |
|---|---|---|
| Water jacket | A coolant spiral around the stator housing. Simple, sealed, well proven, and cools the stator well. | The rotor and magnets are only cooled indirectly across the airgap, which is a poor thermal path. |
| Direct oil spray | Dielectric oil sprayed onto end windings and rotor. Reaches the hottest copper directly and touches the rotor. Roughly 30–50 % more continuous rating from the same iron. | Needs a pump, a filter, and it adds churning loss. |
| Hollow shaft | Oil fed through the rotor shaft to cool magnets from the inside. The only method that attacks magnet temperature at its source, and increasingly standard on high-output traction motors. | Rotating seals and oil delivery into a spinning shaft. |
| Slot cooling | Coolant channels inside the stator slot itself, next to the conductors. Highest capability. | Hardest to insulate and seal reliably. |
9.4 — Continuous versus peak rating
A traction motor’s peak rating typically runs two to three times its continuous rating, and both are honest numbers. Peak is limited by how fast the thermal mass heats up — usually quoted for 30 seconds or so. Continuous is limited by steady-state heat removal.
9.4.1 — Why repeated acceleration derates
The thermal time constant of the winding is short, tens of seconds; the stator iron’s is minutes; the housing’s is longer still. A drive’s thermal model tracks all of them, which is why hard acceleration is available from cold and progressively derated on repetition.
That behaviour is the model working correctly, not a fault.
9.5 — The mismatch that shapes the design
| Class | Max winding temperature | Typical use |
|---|---|---|
| F | 155 °C | Industrial motors |
| H | 180 °C | Standard for traction |
| N | 200 °C | High-output traction |
| R | 220 °C | Specialist / aerospace |
Important
Note the mismatch that shapes the whole design: Class H insulation tolerates 180 °C, while an N42 magnet is finished at 80 °C.
In an ordinary permanent-magnet traction motor the magnets — not the copper, not the insulation — set the thermal limit. Every cooling decision is ultimately about them, which is why hollow-shaft cooling exists at all.
Quick check: test yourself
1.A rotor is validated for demagnetisation at 150 °C with no fault, and separately at full fault current at 25 °C. Is that adequate?
Show answer
2.Why does a car allow one hard launch from cold but derate on the third?
Show answer
3.Class H insulation is rated to 180 °C. Why is that not the motor’s thermal limit?
Show answer
Chapter summary
- ✓Demagnetisation needs two conditions at once — high temperature and a large reverse field — and neither alone is dangerous.
- ✓The design case is a three-phase short circuit at high speed with the rotor already hot, plus continuous field weakening at the top of the speed range.
- ✓Nothing in drive telemetry sees demagnetisation happening; it shows up afterwards as reduced back-EMF and lost torque per amp.
- ✓Cooling capability runs water jacket, direct oil spray, hollow shaft, slot cooling — and only the last two attack magnet temperature at its source.
- ✓Class H insulation tolerates 180 °C while an N42 magnet is finished at 80, so the magnets set the thermal limit in a permanent-magnet machine.
Frequently asked questions
What is the worst case a motor magnet has to survive?+
A three-phase short circuit at high speed with the rotor already at maximum temperature — typically an inverter fault at the end of a long gradient. The short-circuit current is large, its field opposes the magnets directly, and the magnets are at their least resistant moment. Demagnetisation analysis is always run at maximum temperature and maximum fault current simultaneously, never at either alone, because a hot magnet at a benign operating point is fine and a cold magnet hit with a large reverse field is fine.
Which cooling method actually cools the magnets?+
A water jacket cools the stator well but reaches the rotor and magnets only indirectly across the airgap, which is a poor thermal path. Direct oil spray reaches the hottest copper and touches the rotor, worth roughly 30 to 50 per cent more continuous rating from the same iron. A hollow shaft feeding oil through the rotor is the only method that attacks magnet temperature at its source, and it is increasingly standard on high-output traction motors.
Why is peak rating two to three times continuous rating?+
Because they are limited by different things and both numbers are honest. Peak is limited by how fast the thermal mass heats up, usually quoted for 30 seconds or so. Continuous is limited by steady-state heat removal. The winding’s thermal time constant is tens of seconds, the stator iron’s is minutes and the housing’s is longer, so a drive’s thermal model tracks all of them — which is why hard acceleration is available from cold and progressively derated on repetition. That is the model working correctly, not a fault.
What sets the thermal limit in a permanent-magnet motor?+
The magnets, not the copper or the insulation. Class H insulation tolerates 180 °C while a plain N42 magnet is finished at 80 °C. In an ordinary permanent-magnet traction motor every cooling decision is ultimately about the magnets, and choosing a higher grade such as 42SH costs energy product and money to buy roughly 30 °C of headroom — a trade settled by the thermal model and the fault analysis, not by a procurement spreadsheet.
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Magnets and Motors is an original educational series on permanent magnets and electric machines. Datasheet ranges, temperature coefficients and efficiency figures are representative standard-condition literature values, not measurements of a specific product, and sources differ on several. Always verify against the specific magnet and lamination datasheets in use before making design, procurement or certification decisions.