Permanent magnets are all good, but demagnetization is too distressing?-Xinda Motor- electric motors and EV accessories supplier|INDEX--

Permanent magnets are all good, but demagnetization is too distressing?

Date：2023-04-17　　　Author：XINDA MOTOR

Once the permanent magnet motor loses its magnetism, basically you can only choose to replace the motor, and the maintenance cost is a lot. How to judge the loss of magnetism of the permanent magnet motor, let's look down.

1. The current of the machine is normal when it starts to run. After a period of time, the current becomes larger. After a long time, it will report that the inverter is overloaded.

First of all, it is necessary to confirm that the air compressor manufacturer’s frequency converter is correct, and then confirm whether the parameters in the frequency converter have been changed.

If there is no problem with both, you need to judge by the back electromotive force, disengage the machine head from the motor, carry out no-load identification, and run it to the rated frequency without load. At this time, the output voltage is the back electromotive force. If it is lower than the motor nameplate If the counter electromotive force is above 50V, it can be determined that the motor is demagnetized.

2. After the permanent magnet motor is demagnetized, the operating current will generally exceed the rated value by a lot

Those situations where overload is only reported at low or high speed or occasionally reported are generally not caused by demagnetization.

3. It takes a certain amount of time to demagnetize the permanent magnet motor, some months or even a year or two

If the manufacturer chooses the wrong type and causes the current overload to be reported, it does not belong to the demagnetization of the motor.

Reasons for motor demagnetization

The cooling fan of the motor is abnormal, resulting in high temperature of the motor
The motor is not equipped with a temperature protection device
Ambient temperature is too high
Motor design is unreasonable

How to prevent demagnetization of permanent magnet motor? ? ?

Correct selection of permanent magnet motor power
Demagnetization is related to the power selection of the permanent magnet motor.

Correct selection of the power of the permanent magnet motor can prevent or delay demagnetization.

The main reason for the demagnetization of the permanent magnet synchronous motor is that the temperature is too high, and overload is the main reason for the high temperature.

Therefore, a certain margin should be left when selecting the power of the permanent magnet motor. According to the actual situation of the load, generally about 20% is more appropriate.
Avoid heavy starting and frequent starting

The cage-type asynchronous start synchronous permanent magnet motor should try to avoid heavy load direct start or frequent start.

In the process of asynchronous starting, the starting torque is oscillating, and in the trough section of the starting torque, the stator magnetic field demagnetizes the rotor magnetic poles.

Therefore try to avoid heavy load and frequent starting of asynchronous permanent magnet synchronous motor.

Improved Design

1. Appropriately increase the thickness of the permanent magnet

From the perspective of permanent magnet synchronous motor design and manufacture, the relationship between armature reaction, electromagnetic torque and permanent magnet demagnetization should be considered.

Under the combined action of the magnetic flux generated by the torque winding current and the magnetic flux generated by the radial force winding, the permanent magnets on the rotor surface are likely to cause demagnetization.

When the air gap of the motor remains unchanged, the most effective way to ensure that the permanent magnet does not demagnetize is to increase the thickness of the permanent magnet appropriately.

2. There is a ventilation groove circuit inside the rotor to reduce the temperature rise of the rotor

An important factor affecting the reliability of permanent magnet motors is the demagnetization of permanent magnets.

If the temperature of the rotor rises too high, the permanent magnet will lose its magnetism irreversibly.

When designing the structure, the internal ventilation circuit of the rotor can be designed to directly cool the magnetic steel. Not only reduces the temperature of the magnetic steel, but also improves the efficiency.

Compared with ordinary three-phase asynchronous motors, permanent magnet synchronous motors have the advantages of high starting torque, short starting time, and high overload capacity, and can reduce the installed capacity of equipment-driven motors according to the actual shaft power, saving energy and reducing fixed assets investment.

Relatively speaking, the permanent magnet synchronous motor is easy to control, the speed is only determined by the frequency, the operation is stable and reliable, and does not change with the fluctuation of the load and voltage. In view of the characteristics of strict synchronization of the speed of the permanent magnet synchronous motor, it determines the advantage of good dynamic response performance of the motor, which is more suitable for frequency conversion control.

The advantage of permanent magnet synchronous motor lies in its two lows and two highs, that is, low loss and temperature rise, high power factor and high efficiency. This is exactly what people pursue for motor performance, and it also determines the market application status of permanent magnet motors.

Why does the permanent magnet motor have low loss and low temperature rise?

Since the magnetic field of the permanent magnet synchronous motor is generated by the permanent magnet, the excitation loss caused by the magnetic field generated by the excitation current is avoided, which is what we call copper loss; when the motor is running, the rotor runs without current, which significantly reduces the temperature rise of the motor. According to incomplete statistics, under the same load conditions, the temperature rise will be about 20K lower.

High power factor and high efficiency of permanent magnet synchronous motor

Compared with asynchronous motors, permanent magnet synchronous motors have much higher efficiency values at light loads, and have a wide range of high-efficiency operation. The efficiency is greater than 90% in the range of 25% to 120% load rate, and the rated efficiency of permanent magnet synchronous motors can reach the current national standard. This is the biggest advantage compared with asynchronous motors in terms of energy saving.

In actual operation, motors rarely run at full power when driving a load. The reason is: on the one hand, when the designer selects the motor, the power of the motor is generally determined according to the limit working condition of the load, and the chance of the limit working condition is very small. If the motor is damaged, the power margin of the motor will be further reserved during design; on the other hand, in order to ensure the reliability of the motor, the motor manufacturer usually leaves a certain power margin on the basis of the power required by the user. This leads to the fact that most of the motors in actual operation work below 70% of the rated power, especially for driving fans or pump loads, and the motors usually work in the light load area. For the asynchronous motor, its light load efficiency is very low, while the permanent magnet synchronous motor can still maintain a high efficiency in the light load area.

The power factor of the permanent magnet synchronous motor is high, and it has nothing to do with the number of motor stages. When the motor is fully loaded, the power factor is close to 1. Compared with the asynchronous motor, the motor current is smaller, and the stator copper consumption of the motor is smaller, and the efficiency is higher. . The power factor of asynchronous motor is getting lower and lower as the number of motor stages increases. Moreover, because the power factor of the permanent magnet synchronous motor is high, the capacity of the power supply (transformer) supporting the motor can be reduced theoretically, and the specifications of the supporting switchgear and cables can be reduced at the same time.

Disadvantages of permanent magnet synchronous motor

The permanent magnet synchronous motor also has its disadvantages. For example, the starting current multiple of the permanent magnet synchronous motor is about 9 times, which is larger than the starting current of the asynchronous motor; the permanent magnet synchronous motor cannot use the step-down starting method. Because under the condition of step-down power supply, the asynchronous starting torque drop is larger than that of the asynchronous motor, which will cause difficulty in starting. For the self-starting characteristics of the permanent magnet synchronous motor and the feedback current when the system is short-circuited, the parameters of different equipment manufacturers are quite different, and because it is difficult to obtain relevant data, the application of the permanent magnet synchronous motor will affect the short-circuit level of the power system. And start calculation verification brings some uncertain factors.

Permanent magnets are all good, but demagnetization is too distressing?

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