Technology / Technical guide
Permanent-Magnet Temperature & Demagnetisation
Temperature can change a permanent magnet’s field and its resistance to demagnetising conditions. A winding’s high-temperature wire does not describe those magnetic properties. Distinguish reversible changes from a change that remains after cooling.
Temperature affects more than winding insulation.
A magnet’s material, operating point and thermal history determine which magnetic change a temperature excursion can produce.
01 / Magnetic quantities
Separate remanence from resistance to demagnetisation.
Remanence describes retained magnetic induction under its defined material condition.
Remanence describes retained magnetic induction under its defined material condition. Coercivity describes resistance to a reverse magnetising field, using the specified curve definition. A larger remanence does not automatically mean a larger coercivity.
Both quantities can vary with temperature, and their changes need not follow the same coefficient. A label naming the broad magnet family leaves material grade and the actual magnetic circuit unspecified.
02 / Reversible change
Recognise a field change that follows temperature.
A reversible change returns toward the original magnetic output when the temperature returns, within the applicable conditions.
A reversible change returns toward the original magnetic output when the temperature returns, within the applicable conditions. Arnold’s temperature note distinguishes this from irreversible magnetic loss. Reversible does not mean the output stayed constant while the temperature differed.
A local linear description might write Br(T) ≈ Br(Tref)[1 + γ(T − Tref)], with a material-specific coefficient γ. This is an approximation around a stated reference, not a universal coefficient or a product operating-temperature limit.
03 / Lasting loss
Distinguish cooling from restoring magnetisation.
An irreversible loss remains after the magnet returns to the reference temperature.
An irreversible loss remains after the magnet returns to the reference temperature. Some losses may be recoverable through remagnetisation of an otherwise intact material; others involve structural degradation. These are different physical categories.
Arnold’s magnetisation-loss paper describes thermal and reverse-field effects on magnetic state. Explaining recoverability is not a field-service instruction. Simply cooling a partly demagnetised assembly does not establish restoration of its previous field.
04 / Operating point
Connect the material curve to the magnetic circuit.
The magnet operates as part of a magnetic circuit, not at an arbitrary point on a material chart.
The magnet operates as part of a magnetic circuit, not at an arbitrary point on a material chart. Geometry and surrounding magnetic paths affect its operating point. The winding’s field can also contribute to the magnetic conditions experienced by the magnet.
Temperature-dependent curve changes can move the operating point into a region producing lasting loss. This is why one temperature number cannot describe every combination of material, geometry and electromagnetic loading.
05 / Different limits
Do not substitute Curie temperature for useful operation.
Curie temperature concerns a material’s magnetic ordering.
Curie temperature concerns a material’s magnetic ordering. It is different from a practical assembly temperature limit or the onset of unacceptable magnetic change under a particular operating condition. Important changes can occur without reaching Curie temperature.
Likewise, insulation classification concerns an insulation system, and copper resistance concerns a conductor. Those quantities interact thermally with a machine but do not define the rotor magnet’s demagnetisation margin.
06 / Machine scope
Explain thermal interaction without assigning a rating.
Reduced magnetic linkage can change the induced-EMF relationship described in the EMF guide .
Reduced magnetic linkage can change the induced-EMF relationship described in the EMF guide. A changed terminal voltage alone, however, cannot identify demagnetisation uniquely because speed, loading and other conditions also matter.
High-temperature copper wire is confirmed across the current five MARSTON models, while its exact grade remains unconfirmed. Only FD14-50KW’s permanent-magnet architecture is confirmed here; no magnet grade, coercivity curve or demagnetisation temperature is assigned.
Continue the review with PMSG Magnetic Flux, Flux Linkage & Induced EMF.
Continue the review with Generator Magnetic Saturation & Flux Density.
Continue the review with Copper Winding Resistance & Temperature Approximation.
For an additional magnet-component review, see magnet operating curves and load lines.
General background: Arnold Magnetic Technologies: temperature effects on magnetic output · Arnold: reversible, irreversible and thermal magnetisation losses. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Remanence | Br under defined material conditions | Describes retained induction. |
| Coercivity | Reverse-field property with a stated curve definition | Different from magnetic output strength. |
| Reversible change | Field changes with temperature and returns | Output need not be constant during heating. |
| Irreversible loss | Change remaining after return to reference conditions | Cooling alone does not establish recovery. |
| Operating point | Magnetic circuit and thermal/electromagnetic state | Makes a material chart assembly-dependent. |
Engineering questions.
Does high-temperature winding wire prove magnet heat resistance?
No. Winding and magnet properties describe different parts of the machine.
Is reversible change the same as no field change?
No. The field may change temporarily with temperature.
Does cooling always restore lost magnetisation?
No. Irreversible loss can remain after cooling.
Is Curie temperature a safe operating rating?
No. It is a material transition, not a complete assembly limit.
Can lower terminal voltage prove demagnetisation?
Not alone. Speed, load and other electrical conditions also affect the result.
Is a MARSTON magnet grade or thermal threshold given?
No. Those product-specific magnetic properties are not established here.
Technical enquiry
Discuss the technical question.
Temperature can change a permanent magnet’s field and its resistance to demagnetising conditions. Share the model and applicable operating or design information for an engineering discussion.