What's the The Invisible Threat and Implications from Permanent Magnet Motors

Here we expound on some engineering caveats on permanent-magnet motors.


In recent years, clients have been prioritizing and actively promoting the use of permanent magnet motors due to their excellent energy-saving features and comparable high-speed and low-speed running characteristics. CJC is a specialist in the development of permanent magnet DC motors. Now, let's delve into the significant challenges that permanent magnet motors face.

The rise of electric cars has brought renewed focus on the advancements and potential of permanent magnet DC (PMDC) motors not only in revolutionizing the way vehicles are powered, but also in providing a more convient and comfortable lifystyle to people. The increasing adoption of PMDC motors can be attributed to several factors. Firstly, PMDC motors offer a high power-to-weight ratio, making them ideal for field such as automobiles and industrial product where efficiency and performance are crucial. These motors provide enhanced torque and acceleration capabilities, allowing home appliances and robots to match or even surpass the innovative performance.


We know that faults in PMDC can bring addtional noise, harmful vibration and even euippment shutdowm. Here are the crucial faults and problems PMDC motor may have.

1.Loss of excitation is a significant challenge in permanent magnet DC motors.

The stability of the permanent magnet is crucial for generating the required magnetic field. However, the magnet's magnetic properties can become unstable during operation, resulting in insufficient magnetic field strength. This can lead to a decline in motor performance or even complete failure. Various factors contribute to this issue, including mechanical vibration, surface damage, magnet corrosion, or extreme magnetic field temperature changes. If a low-grade permanent magnet material is used, with weak magnetic properties, it may not meet the motor's requirements, necessitating re-magnetization or replacement to restore normal operation.

Overheating demagnetization is another issue that can arise.

When the magnetic properties of the permanent magnet decline, it can lead to excessive current and overheating. If the load current during motor operation exceeds the magnet's ability to withstand demagnetization, irreparable demagnetization of the magnet occurs. In other words, the decline in magnetic properties can cause overheating due to excessive current. When considering only the thermal factors and excluding the influence of magnet properties, two situations can cause overheating demagnetization. First, if the motor's internal ventilation system is inadequate and violates the natural laws of heat conduction, it can result in local heat accumulation. Second, if the winding's thermal load is excessively high and generates more heat than the motor's heat exchange system can dissipate.

Improper selection of magnet grade can also contribute to the problem.

If the motor is mistakenly designed with a lower-grade magnet, such as choosing a 155°C grade instead of the required 180°C grade, certain issues may arise. During initial testing, the motor performs well, but as it reaches thermal stability, its performance deteriorates and deviates from the design expectations. At some point, the current rapidly increases, the frequency converter quickly stops, and an overcurrent code is displayed. Subsequent testing reveals that the motor has demagnetized and requires magnet replacement.

Another problem is the excessive demagnetizing current.

When the load current exceeds the magnet's ability to withstand demagnetization, irreversible demagnetization occurs, leading to further increases in load current and aggravating the irreversible demagnetization of the magnet. This creates a vicious cycle where demagnetization accelerates until complete demagnetization occurs.

Other faults that can occur in permanent magnet motors include:

2. Deterioration of magnetic performance and overlapping demagnetization due to current.

During motor operation, if there is a degradation in magnetic performance, the motor current increases rapidly, leading to severe overheating. This further deteriorates the magnetic properties of the magnet, causing the current to increase again. The overlapping effect accelerates the deterioration, leading to motor failure in a very short period.

3. Magnet detachment. During the assembly process, adhesives are used to secure the magnets to the substrate.

The purpose of filling the gaps between the permanent magnets with adhesive is to increase the cohesion and prevent the magnets from flying off due to centrifugal forces at high speeds. However, factors such as poor adhesive performance, weak embedding, excessive temperature, water ingress, or moisture within the motor can lead to magnet detachment. This results in direct mechanical friction and loss of motor drive functionality.

Currently, there is increasing research on methods for detecting faults in permanent magnet DC motors. These methods can help companies promptly identify issues and take preventive measures. One commonly used approach is using specialized instruments to detect the air gap magnetic field of the motor and assess whether demagnetization has occurred based on the field intensity. Additionally, convenient and direct electromagnetic field analysis software can be used for rapid evaluation.

Furthermore, some literature on diagnosing faults in permanent magnet DC motors suggests extracting consecutive current signals to diagnose motor issues. This method involves analyzing the current waveform and spectral characteristics to determine if demagnetization or other faults are present (Lu & Wang, 2021).

Lu, L., & Wang, W. (2021). Fault diagnosis of permanent magnet DC motors based on multi-segment feature extraction. Sensors, 21(22), 7505. https://doi.org/10.3390/s21227505

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