Why do Permanent Magnet Motors Demagnetize?

Q: Which magnet grade is best for air compressors?

35SH, 38SH, or 40SH are the industry standards due to their balance of magnetic strength and thermal stability. For heavy-duty models, UH grades provide even higher safety margins.

Q: How long does it take for demagnetization to become apparent?

It depends on the severity:Extreme Cases: Under severe overheating, demagnetization can occur within a few hours.Progressive Demagnetization: This typically takes months or even 1 to 2 years to evolve from a “slight increase in current” to “frequent inverter overload alarms.” This inherent stealthiness is precisely why demagnetization is so often overlooked. Therefore, it is highly recommended to perform a Back EMF spot check every quarter.

Q: What is the appropriate increase for magnet thickness?

For standard PM motors, increasing the magnet thickness by 10% to 20% is ideal. This enhancement improves anti-demagnetization capability and output torque without causing air-gap imbalances or magnetic saturation overload.For high-load or high-temperature applications, an increase of 20% to 25% is suggested; however, this must be coupled with a redesign of the stator-rotor air gap for proper matching.

Ethan
Maggio 14, 2026
Base di conoscenze

A magnete permanente VSD (Variable Speed Drive) air compressor runs for less than two years. Then, it suddenly has unexplained high current and frequent inverter overload alarms. When checked, the magnets’ densità di flusso magnetico has dropped by nearly 30%.

This problem is common. Demagnetization is the most hidden failure mode for PM motors. Unlike a burnt coil that stops the machine right away, demagnetization lets the equipment keep running in a “sub-healthy” state. By the time problems become obvious, the damage is often irreversible. Knowing why demagnetization happens helps you fix issues and prevent risks during design and selection.

Punti di forza

Focus on preventing irreversible demagnetization of permanent magnet motors; conduct back EMF sampling quarterly.
Five core causes of demagnetization: high temperature, reverse magnetic field, overload/start-stop, mechanical impact, design & manufacturing defects.
Select magnetic steel matching working conditions.
Judge demagnetization by symptoms and back EMF/gaussmeter testing.
Prevention keys: reserve 20% load margin, control temperature, standardize parameters, thicken magnetic steel.

What is Demagnetization?

A permanent magnet gets its magnetism from the orderly alignment of tiny domini magnetici inside. When external pressures like high heat, reverse magnetic fields, or mechanical shocks cross a critical point, these domains shift. This weakens the external magnetic field. This process is called demagnetization.

Demagnetization is split into two types, based on whether the loss can be reversed:

Reversible Demagnetization:As the name suggests, the magnet can recover on its own. Once the external problem is removed (like when temperature returns to normal), the flux density goes back to normal. No permanent damage occurs.
Irreversible Demagnetization:This is true failure. Once the magnetic domains stay permanently disordered, magnetism cannot come back even after the problem is fixed. You need to remagnetize the magnet, or more often, replace the whole motor.

You must focus on preventing irreversible demagnetization. It develops slowly and quietly. Checking back EMF every three months is strongly recommended to catch it early.

Core Causes of Demagnetization

Demagnetization rarely comes from one single factor. It can happen from misuse, design flaws, or a single major event.

Excessively High Temperature

Heat is the most common cause. Every magnetic material has a Temperatura massima di esercizio. If the temperature stays above this limit, the orderly magnetic domains become disturbed. The material’s coercive force (resistance to demagnetization) drops sharply. If the temperature reaches the Curie Temperature, magnetism is lost forever.

Reverse Magnetic Fields

In normal operation, the magnetic fields inside a motor work together. However, during a short circuit, controller error, or due to poor design, the stator coils may suddenly create a powerful opposing magnetic field. If this “demagnetizing field” exceeds the magnet’s natural resistance, demagnetization happens right away.

Deep Field Weakening control can also cause this. To run faster than the rated RPM, un inverter sends a reverse current to counteract the permanent magnet field. If the control algorithm parameters are not set properly, this strong demagnetizing force can easily damage the magnets.

Frequent Overload or Abnormal Starts

Every time a motor starts from rest, the magnets face “magnetic resistance” until the speed stabilizes. This puts pressure on the magnets. One start is not a problem. But starting and stopping dozens of times a day for years causes serious cumulative damage.

Similarly, long-term overload keeps current high. This creates heat and extra demagnetizing pressure at the same time — a “double whammy” for magnet life.

Mechanical Impact and Vibration

Sintered NdFeB (Neodymium) magnets are hard but very brittle. They cannot absorb energy through bending. Instead, impacts create tiny micro-cracks. At high speeds, centrifugal force can also cause micro-damage, making magnetic properties fade faster. Rough handling during shipping or hammering during installation often leads to future failure.

Design and Manufacturing Defects

Low-quality magnetic materials, poor assembly that creates uneven air gaps (causing high local magnetic density), and weakrotor field-weakening designs are “built-in” problems. These flaws make the motor’s operating point too close to the “knee” of the demagnetization curve. Even small changes can trigger failure.

Magnet Grade Temperature Reference Table

Choosing the right grado di magnete is the first defense against thermal demagnetization. Below are key temperature details for common materials:

Material Series	Grade Examples	Temperatura massima di esercizio	Curie Temperature	Application Scenarios
NdFeB — N Series	N35, N42, N52	60~80°C	~310°C	Room temperature equipment, consumer electronics
NdFeB — M Series	35M, 42M	100°C	~340°C	General industrial motors
NdFeB — H Series	35H, 42H	120°C	~340°C	Variable frequency air compressors, servo motors
NdFeB — SH Series	30SH, 35SH	150°C	~350°C	High-temperature industrial equipment
NdFeB — UH/EH Series	28UH, 28EH	180~200°C	~350°C	New energy vehicles, aviation
SmCo — Samarium Cobalt	SmCo5, Sm2Co17	250~350°C	~750°C	High temperature, military industry, aerospace
Ferrite	Y30, Y35	250°C+	~450°C	Low-cost general occasions

Selection Advice: For industrial VFD air compressors, use H series as a minimum (120°C). For continuous full-load or high-ambient conditions, upgrade to SH series.

How to Diagnose a Demagnetized Motor

To diagnose demagnetization, check symptoms and use instrument measurements together.

Typical Symptoms

Early signs of demagnetization are often confused with other faults. Check these at the same time:

Current runs 10%~30% higher than rated during normal operation.
Energy use per unit output rises sharply, and efficienza drops.
The frequency converter has overload alarms more often.
Torque fluctuation increases, along with abnormal vibration and noise.

Note: Occasional overload only at low or certain high-speed ranges is usually not related to demagnetization. It is more likely a load matching o parameter setting issue. Do not jump to the conclusion that demagnetization has occurred.

Back EMF Detection Method

This is the easiest, most cost-effective field test:

Disconnect: Completely decouple the motor from the load (air end).
Run: Use the inverter to run the motor at its rated speed (no-load).
Measure: Record the output line voltage.
Standard: If the measured voltage is $\ge$ 50V lower than the Back EMF value on the nameplate, the motor is demagnetized.

Direct Measurement via Gaussmeter/Tesla Meter： This method is intended for scheduled maintenance involving disassembly. By sampling magnetic flux density across the rotor surface, demagnetization is confirmed if there is an overall drop of >15% or a localized deviation of >20%.

Preventive Measures Against Demagnetization

Once demagnetization happens, it is almost impossible to fix cheaply. Replacing the motor or rotor is expensive. But most risks can be avoided with good practices when buying, installing, and using the motor.

Correct Sizing and Selection

Many demagnetization problems start during the selection phase.

Motor power margin: If you choose a motor with too little margin, it will run in overload when fully loaded. Long-term heat buildup makes demagnetization inevitable. In practice, pick a motor with ~20% margin above your expected load — do not size based on ideal conditions.
Magnet temperature rating: Choose based on the real environment, not just the cheapest option. If the machine room is hot year-round or the equipment runs continuously, low-temperature-rated magnets will fail early.

Strict Temperature Control

VFD parameter settings affect magnet health more than most people realize. Focus on two key areas:

Current limit parameters: Setting these too high heats the magnets and adds extra demagnetizing stress. Engineers familiar with these motors should set parameters — do not change factory defaults casually.
Flux-weakening control: For speeds above the rated value, poorly set d-axis current parameters can create strong demagnetizing force. Be extra careful when tuning these parameters.
Starting method: Use soft starting whenever possible. Reduce the number of start-stop cycles to lower cumulative damage from starting impacts.

Increasing Magnet Thickness During Design

Thinner magnets cost less but are less resistant to external problems. Even small temperature rises or current fluctuations can make them demagnetize easily. Slightly thicker magnets give the motor more safety margin in imperfect operating conditions. Minor issues are less likely to cause failure.

When buying a motor, ask the supplier directly about the magnet design margin. If they give vague answers, that is a warning sign.

Conclusione

Demagnetization in permanent magnet motors comes down to a balance of temperature, current, mechanical stress, e design margin.

Demagnetization itself is not scary. The real risk is not noticing it early. For industrial users, the key to prevention is:

Build safety margins during selection.
Control temperature during operation.

When buying, do not blindly chase the lowest cost by using cheaper magnetic steel grades or thinner magnets. When using the motor, add temperature monitoring and regular back EMF testing to your maintenance plan.

With scientific management and maintenance, permanent magnet motors can deliver their alta efficienza e high power density benefits. They will become long-lasting core power sources on your production lines.

Alcune domande frequenti

Can a demagnetized motor be re-magnetized?

Theoretically yes, but the labor of disassembly and the inconsistency of results usually make it more expensive than simply replacing the motor or rotor.

How long does demagnetization take to show up?

In extreme overheat scenarios, it can happen in hours. In gradual cases, it usually takes 6 months to 2 years to progress from a slight current increase to frequent system failures.

Which magnet grade is best for air compressors?

35SH, 38SH, or 40SH are the industry standards due to their balance of magnetic strength and thermal stability. For heavy-duty models, UH grades provide even higher safety margins.

How long does it take for demagnetization to become apparent?

It depends on the severity:

Extreme Cases: Under severe overheating, demagnetization can occur within a few hours.
Progressive Demagnetization: This typically takes months or even 1 to 2 years to evolve from a “slight increase in current” to “frequent inverter overload alarms.” This inherent stealthiness is precisely why demagnetization is so often overlooked. Therefore, it is highly recommended to perform a Back EMF spot check every quarter.

What is the appropriate increase for magnet thickness?

For standard PM motors, increasing the magnet thickness by 10% to 20% is ideal. This enhancement improves anti-demagnetization capability and output torque without causing air-gap imbalances or magnetic saturation overload.

Per high-load or high-temperature applications, an increase of 20% to 25% is suggested; however, this must be coupled with a redesign of the stator-rotor air gap for proper matching.

Per ulteriori approfondimenti, consultate questi blog correlati:

Guida completa alla magnetizzazione dei magneti

Una guida completa ai metodi di degaussing

Come i gruppi statorici alimentano i sistemi motore

Injection Molded Magnets – All You Need to Know

2026 Magnet Report: Rare Earths & Supply Chain Truths Prices

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Ethan Huang

Mi dedico alla scrittura scientifica divulgativa sui magneti. I miei articoli si concentrano principalmente sui principi, le applicazioni e gli aneddoti del settore. Il nostro obiettivo è fornire ai lettori informazioni preziose, aiutando tutti a comprendere meglio il fascino e il significato dei magneti. Allo stesso tempo, siamo desiderosi di ascoltare le vostre opinioni sulle esigenze legate ai magneti. Sentitevi liberi di seguirci e di impegnarvi con noi per esplorare insieme le infinite possibilità dei magneti!

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