Exploring the Limits: Maximum Operating Temperature

Ι What Is Maximum Operating Temperature?

The magnetic properties of permanent magnets can be influenced by various external factors, such as environmental temperature, mechanical shock, corrosion, and radiation. Among these, temperature is the most critical, as fluctuations in ambient temperature can significantly and severely affect magnetism. The impact of temperature is particularly widespread and profound, making it a key concern for many users, especially when it comes to understanding the maximum operating temperature of permanent magnets.

The maximum operating temperature (T_w or maximum working temperature) of permanent magnets refers to the highest temperature at which a magnet can maintain its magnetic properties with a specific irreversible flux loss.

More specifically, for a saturated cylindrical magnet with an L/D ratio of 0.7, this temperature is determined by heating the magnet from room temperature to a constant temperature in an open-circuit state, holding it at that temperature for two hours, and then cooling it back to room temperature. The temperature at which the irreversible flux loss does not exceed 5% is considered the maximum operating temperature.

Maximum Operating Temperature vs. Curie Temperature

Many people mistakenly confuse the Curie temperature with the maximum operating temperature, but these terms are not interchangeable. It’s important to understand the distinction between them when evaluating the thermal limits of permanent magnets.

The Curie temperature (T_c), also known as the Curie point, is a key property of ferromagnetic materials. It is the temperature above which a ferromagnetic material loses its permanent magnetism and becomes paramagnetic. In simpler terms, at or above this temperature, permanent magnets can no longer retain their magnetization and lose their magnetic properties entirely.

The Curie temperature reflects the material’s resistance to thermal disruption of its magnetic ordering. A higher Curie temperature indicates greater stability against heat, which is why permanent magnets are preferred to have as high a Curie temperature as possible. The Curie temperature is inherent to the material itself and is typically much higher than the maximum operating temperature.

Since different magnetic materials have different Curie temperatures, they also exhibit varying thermal performance characteristics. Understanding these differences is crucial for selecting the right material for specific applications.

What is the Maximum Operating Temperature of a Magnet?

The maximum operating temperature of a magnet varies depending on its material type and grade. Each type of magnet is designed to operate within a specific temperature range to maintain its magnetic properties. Understanding these limits is crucial for selecting the right magnet for different applications.

Permanent magnets come in various types, each with its own temperature tolerance:

Neodymium Magnets: Known for their high strength, but with lower temperature tolerance compared to other types.
Samarium Cobalt Magnets: Notable for their ability to withstand higher temperatures, with some grades capable of handling temperatures up to 550°C.
Alnico Magnets: These magnets offer excellent high-temperature stability and are commonly used in high-heat environments.
Ferrite Magnets: While not as strong as other types, they provide good heat resistance and are widely used in industrial applications.

Below, we outline the specific temperature limits for each of these materials.

Magnet type		Maximum Operating Temperature T_w (degrees celsius)	Curie Temperature T_c (degrees celsius)
Neodymium magnets	N grade	80	310-350
	M grade	100
	H grade	120
	SH grade	150
	UH grade	180
	EH grade	200
	AH grade	230
Samarium Cobalt magnets	1:5 type	200	700-750
Samarium Cobalt magnets	2:17 type	250-550	800-850
AlNiCo magnets		550	750-850
Ferrite magnets		250	450
Notes: The data are sourced from the National Standards of the People’s Republic of China. The listed maximum operating temperatures are based on an appropriate L/D ratio or a properly designed magnetic circuit.

Factors Affecting Maximum Operating Temperature

Besides the aforementioned magnet type and grade, the maximum operating temperature of a permanent magnet is also affected by its shape and size, specifically the permeance coefficient (Pc) value. A higher Pc value means the magnet is less susceptible to demagnetization, helping it maintain its magnetic properties better under various conditions. In contrast, magnets with lower Pc values, such as thinner or shorter magnets, are more prone to demagnetization and can lose magnetization more easily when exposed to external magnetic fields or temperature fluctuations.

If we compare magnets of the same material and diameter at the same temperature but vary the thickness, we observe that thinner magnets are more susceptible to demagnetization. For example, a magnet with dimensions 30×30 mm has a Pc value of 3.46, meaning its operating point is above the knee point of the B-H curve, within the linear portion. In contrast, a magnet with dimensions 30×3 mm has a Pc value of 0.22, placing its operating point below the knee point, within the non-linear portion of the B-H curve, resulting in irreversible magnetization loss.