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1. Advantages of Permanent Magnet Synchronous Motors
Since the magnetic field of the permanent magnet synchronous motor is generated by the permanent magnet, the driving loss caused by the magnetic field generated by the driving current, that is, the copper loss is avoided; the rotor works without current, which significantly reduces the temperature rise of the motor and the temperature rise is less than 20K under the same load.
A permanent magnet synchronous motor has a high power factor, independent of the number of poles of the motor. When the motor is fully loaded, the power factor of the motor is close to 1. Compared with the asynchronous motor, the motor current is lower, the copper loss of the motor stator is correspondingly lower, and the efficiency is higher. As the number of motor poles increases, the power factor of an asynchronous motor gets lower and lower. In addition, due to the high power factor of the permanent magnet synchronous motor, the capacity of the power supply (transformer) matched with the motor can theoretically be reduced, and the specifications of the supporting switch cabinet and cables can be reduced at the same time.
Compared with asynchronous motors, permanent magnet synchronous motors have a much higher efficiency value at light load and have a wide range of high-efficiency operations. The efficiency is greater than 90% in the range of 25% to 120%. The rated efficiency of the permanent magnet synchronous motor can reach 1 time the current national standard energy efficiency level, which is its biggest advantage in energy saving compared with the asynchronous motor. In actual operation, the motor rarely runs at full power when driving a load. The reason is: On the one hand, when the designer selects the motor, the motor power is generally determined according to the extreme working conditions of the load, and the possibility of extreme working conditions is very small. At the same time, in order to avoid burns under abnormal working conditions To ensure the reliability of the motor, the motor manufacturer will generally leave a certain power margin on the basis of the power required by the user to ensure the reliability of the motor. This leads to the actual operation of the motor, most of the work is below 70% of rated power, especially to drive fans or pump loads, the motor generally works in the light load area. For asynchronous motors, their light load efficiency is very low, while permanent magnet synchronous motors can still keep high efficiency in the light load area.
The permanent magnet synchronous motor also has the advantages of high starting torque, short starting time, and high overload capacity. It can reduce the installed capacity of the equipment drive motor according to the actual shaft power, save energy, and reduce investment in fixed assets. The permanent magnet synchronous motor is easy to control, has a constant speed, does not change with load fluctuations or voltage fluctuations, only depends on frequency, and runs smoothly and reliably. Because the speed is strictly synchronized, the dynamic response performance is good, and it is suitable for frequency conversion control. The installation dimensions of the permanent magnet synchronous motor meet the IEC standard, which can directly replace the three-phase asynchronous motor, and the protection level can reach IP54 and IP55.
2. The disadvantages of permanent magnet motors:
1. High cost of permanent magnet materials
2. Under the condition of high temperature, vibration, and overload current, the permanent magnet material will be irreversibly demagnetized, which will affect the performance of the motor.
3. High requirements for the control system
3. Reasons for demagnetization of permanent magnet motors:
1. Improper selection of magnetic steel-grades
If the calculation of the motor design is not accurate enough, and a lower grade is wrongly selected, such as the permanent magnet of 180°C should be selected but 155°C is wrongly selected, there may be such a situation: the initial test record index of the test process is very good, As the motor gradually tends to be thermally stable, the relevant indicators of the motor begin to deteriorate and deviate from the design expectations more and more. At a certain moment, the current increases sharply, the inverter stops quickly, and an overcurrent code is displayed. Test the no-load characteristics of the motor again, indicating that the motor has lost its magnetism, and the magnetic steel must be replaced.
2. Overheating demagnetization problem
Overheating loss of magnetism is a sensitive topic, and the decrease in the magnetic properties of magnets can also lead to overcurrent and overheating problems. If the influence of the magnetic properties of the magnetic steel is excluded and only the thermal factor is considered, it can be determined that there are two situations in which the phenomenon of overheating demagnetization will occur: first, the circulation ventilation path in the motor is unreasonable, which violates the natural law of cold and heat conduction, resulting in localized heat accumulation; second, The heat load of the winding is too high, and the heat generation exceeds the heat exchange level of the motor heat exchange system.
3. The problem of excessive demagnetization current
When the motor is running, when the load current exceeds the anti-demagnetization ability of the magnet, it will cause irreversible demagnetization of the magnet, which will further increase the load current and aggravate the irreversible demagnetization of the magnet. This reciprocation accelerates irreversible demagnetization until demagnetization.
4. How to prevent demagnetization of permanent magnet motors?
1. Correct selection of permanent magnet motor power:
Demagnetization is related to the power selection of permanent magnet motors. Correct selection of PM motor power 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 the 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.
2. Avoid heavy load starting and frequent starting:
Permanent magnet synchronous motors try to avoid direct starting or frequent starting of heavy loads. During the starting process, the starting torque is oscillating, and in the valley section of the starting torque, the stator magnetic field is demagnetizing the rotor magnetic pole. Therefore, try to avoid the heavy load and frequent starting of the permanent magnet synchronous motor.
(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 produced by the torque winding current and the magnetic flux produced by the radial force winding, the permanent magnets on the rotor surface easily cause demagnetization.
Under the condition that the air gap of the motor remains unchanged, to ensure that the permanent magnet does not demagnetize, the most effective method is to appropriately increase the thickness of the permanent magnet.
(2) There is a ventilation groove circuit inside the rotor to reduce the temperature rise of the rotor:
If the temperature of the rotor is too high, the permanent magnet will cause irreversible loss of magnetism. In the structural design, 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.
