PMSM Motor Failure Analysis-Rotor Demagnetization

1. Structure of PMSM motor

Stator of PMSM motor:

As with ordinary AC induction motors, power is provided through stator windings. PMSM stator windings are usually distributed over multiple slots to approach a sinusoidal distribution, thereby generating a sinusoidal back electromotive force waveform.

Rotor of PMSM motor:

The structure of permanent magnet synchronous motor is similar to that of basic synchronous motor, the only difference is the rotor. The rotor does not have any field windings, but permanent magnets are used to generate field poles. The permanent magnets used in PMSM are composed of samarium cobalt and dielectric, iron and boron because of their high magnetic permeability.

The most widely used permanent magnet is neodymium iron boron because of its low cost and easy availability. In this type, the permanent magnets are mounted on the rotor. The structure of permanent magnet synchronous motor can be divided into two types according to the way the permanent magnets are mounted on the rotor.

If the magnets are mounted on the surface of the motor rotor, the PMSM motor is called a surface-mounted permanent magnet motor (SPM).

If the magnets are mounted inside the rotor, the PMSM motor is called an interior permanent magnet synchronous motor (IPM). Motors with interior permanent magnet (IPM) rotors can provide extremely high efficiency.

2 What are the possible causes of rotor demagnetization?

2.1. Temperature factors

High temperature environment: Permanent magnet materials such as neodymium iron boron are very sensitive to temperature. When the motor runs at high temperature for a long time, the Curie temperature of the magnet will drop, resulting in weakening or loss of magnetism.

Overheating: When the motor is overloaded for a long time or has poor heat dissipation, the internal temperature will rise, which may cause the demagnetization of the magnet.

2.2. Magnetic field factors

Reverse magnetic field: When the motor is running, the current in the stator winding will generate an alternating magnetic field. If the strength of this magnetic field exceeds the anti-demagnetization ability of the magnet, it may cause the magnet to be partially or completely demagnetized.

External strong magnetic field interference: If the motor is disturbed by a strong magnetic field (such as a nearby strong magnetic device) in the operating environment, it may cause the magnet to demagnetize.

2.3. Mechanical stress

Mechanical stress during assembly and operation: During the installation of the magnet or the operation of the motor, the magnet may be subjected to greater mechanical stress (such as compression, bending, impact, etc.), which may cause cracks or microstructural damage inside the magnet, thereby causing demagnetization.

2.4. Material quality issues

Stability of magnetic materials: If the quality of magnetic materials (such as NdFeB) is not up to standard, such as impurities or improper composition ratio, the anti-demagnetization ability of the magnet will be reduced, and it is more likely to demagnetize under high temperature or strong magnetic field.

Poor surface treatment of magnets: If the surface of the magnet is not effectively protected (such as incomplete or unqualified coating), it may be corroded in a humid or corrosive environment, resulting in demagnetization.

2.5. Environmental factors

Humid environment: In a high humidity environment, the surface of the magnet is prone to oxidation or corrosion, especially if the coating is poorly protected, the risk of demagnetization is higher.

Corrosive gas: In an environment containing corrosive gas, the surface of the magnet may be chemically corroded, resulting in weakened magnetism.

2.6. Current factors

Overload current: When the motor is running, if the current is too large, it will cause the coil to heat up, thereby increasing the temperature of the magnet and increasing the risk of demagnetization.

Harmonic current: High-frequency harmonic current will also produce additional thermal effects on the magnets and form additional magnetic fields, which may cause partial demagnetization of the magnets.

2.7. Insufficient magnetization process

Incomplete initial magnetization: If the magnets are not fully magnetized during the production process, the magnets themselves are weak and are prone to demagnetization in subsequent use.

2.8. Vibration and shock

Vibration and mechanical shock during operation: If the motor is frequently subjected to vibration or external shock during operation, it may cause changes in the microstructure inside the magnets, thereby causing demagnetization.

3 The two most likely factors in practical applications

Among the many factors that lead to demagnetization of the permanent magnet synchronous motor (PMSM) rotor, the two most likely main reasons are excessive temperature and reverse magnetic field interference.

3.1 Excessive temperature

The effect of temperature on permanent magnet materials is very significant. Excessive temperature is the most common and most likely cause of rotor demagnetization.

Thermal demagnetization: Permanent magnet materials (such as NdFeB) are highly sensitive to temperature. When the temperature rises close to or exceeds the Curie temperature of the magnet, the magnetic properties of the magnet will decrease significantly. Especially in high temperature environment, the coercive force of the magnet is greatly reduced, resulting in partial or complete loss of magnetism, that is, irreversible demagnetization.

Overload and poor heat dissipation:

When the motor is overloaded for a long time or has poor heat dissipation, the internal temperature will continue to rise, further increasing the risk of demagnetization. If there are no adequate heat dissipation measures in the motor design, or the operating environment temperature is too high, it may cause magnet demagnetization.

Long-term temperature fluctuations:

Even if the temperature does not reach an extremely high value, long-term temperature fluctuations and cumulative effects will cause the magnetic properties to gradually decline, eventually leading to partial demagnetization.

3.2 Reverse strong magnetic field interference

Reverse magnetic field interference is another important factor leading to rotor demagnetization, especially in the working environment of the motor, which is easy to be ignored.

Reverse action of stator magnetic field:

During the operation of the motor, the current in the stator winding will generate an alternating magnetic field. If the magnetic field is strong enough and the direction is opposite to the original magnetization direction of the magnet, it may partially demagnetize the magnet, especially when the coercive force of the magnet has been reduced due to factors such as high temperature.

External strong magnetic field:

If there are other strong magnetic field sources (such as other motors, transformers, etc.) in the motor installation or operation environment, these external magnetic fields may interfere with the rotor magnets, especially when the magnets have low anti-demagnetization ability or are close to the saturation magnetization state. This reverse magnetic field interference may cause some magnetic domains to flip, thereby causing demagnetization.

4 Summary

Excessive temperature and reverse magnetic field interference are the two main causes of permanent magnet synchronous motor rotor demagnetization. Excessive temperature will directly weaken the coercive force of the magnet, making it more susceptible to the reverse magnetic field, while the reverse magnetic field may accelerate demagnetization when the coercive force of the magnet decreases. These two factors often complement each other to form a composite demagnetization effect. Special attention should be paid to these two factors in motor design, operation and environmental control.

In addition to paying attention to the factors that may appear in the application link, we should also pay attention to the rotor production link, especially the impact of the rotor magnetization process on demagnetization.

In short, factors such as excessive current, uneven magnetization, temperature rise and residual magnetic field during the rotor magnetization process will increase the risk of magnet demagnetization. Therefore, during the magnetization process, it is necessary to accurately control the current and temperature and eliminate the residual magnetic field to ensure the magnetic stability of the magnet.