How much do you know about the advantages and disadvantages of permanent magnet motors
Permanent magnet motors use permanent magnets to generate the magnetic field of the motor, without the need for excitation coils and excitation current; with high efficiency and simple structure, it is a very good energy-saving motor, with the introduction of high-performance permanent magnet materials and the rapid development of control technology. With the introduction of high-performance permanent magnet materials and the rapid development of control technology, the application of permanent magnet motors will become more widespread.
Characteristics of permanent magnet motor
Compared with the traditional electrically excited motor, a permanent magnet motor has a simple structure, reliable operation, small size, lightweight, low loss, high efficiency, the shape and size of the motor can be flexible, and other significant advantages. Therefore, the scope of application is wide, almost all over aerospace, national defense, industrial and agricultural production, and daily life in various fields.
1. Permanent magnet DC motor
Permanent magnet DC motor and ordinary DC motor structure are different; the former canceled the excitation winding and pole core and replaced it with a permanent magnet pole. The characteristics of a permanent magnet DC motor are similar to other excitation DC motors; the difference is that the main magnetic field is generated in different ways. The former magnetic field is not controllable; the latter magnetic field can be controlled. In addition to the good characteristics of his excitation DC motor, the permanent magnet DC motor also has a simple structure, reliable operation, high efficiency, small size, lightweight, and so on.
2. Asynchronous starting permanent magnet synchronous motor
An asynchronous starting permanent magnet synchronous motor is a permanent magnet synchronous motor with self-starting ability, which has the characteristics of both an induction motor and an electro-excitation synchronous motor. It relies on the asynchronous torque generated by the interaction between the rotating magnetic field of the stator and the cage rotor to realize starting. During normal operation, the rotor runs at synchronous speed. The cage rotor no longer plays a role, and its working principle is the same as that of the electro-excited synchronous motor. Asynchronous starting permanent magnet synchronous motor has the following characteristics compared with an induction motor:
- (1) Constant speed, synchronous speed.
- (2) High power factor, even for the superpower factor, thus reducing the stator current and stator resistance loss, and stable operation without rotor copper consumption, which can reduce the fan (small-capacity motors can even remove the fan) and the corresponding wind friction loss, efficiency than the same specification of the induction motor can be increased by 2%-8%.
- (3) Wide economic operation range. The rated load has a high-power factor and efficiency, and the 25%-120% rated load range has a higher efficiency, making the energy-saving effect more significant for the light load operation. These motors are generally set up on the rotor starting winding, which can start directly at a certain frequency and voltage.
- (4) The volume and mass of permanent magnet motors are greatly reduced compared with induction motors. For example, the mass of the 11kW asynchronous motor is 220kg, while the permanent magnet motor is only 92kg, equivalent to 45.8% of the mass of the asynchronous motor.
- (5) Small impact on the power grid. The power factor of the induction motor is low, and the motor has to absorb a large amount of reactive current from the grid, which causes the quality factor of the grid to drop and increases the burden of the grid substation and distribution equipment and power loss. While permanent magnet motor rotor without induction current excitation, the motor power factor is high, improving the quality factor of the grid so that the grid no longer needs to install a reactive power compensation device.
- (6) As NdFeB permanent magnet material is usually used, the price is high; irreversible demagnetization may occur when the motor is not designed or used properly.
- (7) Complicated machining process and poor mechanical strength.
- (8) The performance of the motor is greatly affected by the ambient temperature, supply voltage, and other factors.
3. Permanent magnet brushless DC motor
A permanent magnet brushless DC motor uses an electronic commutator to replace the commutator of the DC motor, which retains the excellent characteristics of the DC motor. It has the advantages of simple structure, reliable operation, and convenient maintenance of AC motor. Also, it has the advantages of large starting torque and good speed regulation of DC motors. Due to the abolition of the brush commutator, the reliability is high; the stator mainly generates a loss, has good heat dissipation conditions, small in size, and is lightweight.
4. Speed regulation permanent magnet synchronous motor
Speed-regulated permanent magnet synchronous motor and permanent magnet brushless DC motor structure is the same, the stator for the multi-phase winding; the rotor has permanent magnets, the two advantages are similar. Their main difference is that the permanent magnet brushless DC motor is synchronized according to the rotor position information. In contrast, the speed-controlled permanent magnet synchronous motor needs a set of electronic control systems to realize synchronization and speed control.
5. Permanent magnet synchronous generator
A permanent magnet synchronous generator is a kind of synchronous generator with a special structure, different from the ordinary synchronous generator; it adopts a permanent magnet to establish a magnetic field, canceling the excitation winding, excitation power supply, collector ring, and brushes, etc., which has simple structure, reliable operation, high efficiency, and maintenance-free. When rare earth permanent magnets are used, the air gap magnetism is high, the power density is high, the volume is small, and the quality is light. However, since permanent magnets are used to establish the magnetic field, it is difficult to adjust the output voltage and reactive power by adjusting the excitation. In addition, permanent magnet synchronous generators usually use neodymium-iron-boron or ferrite permanent magnets, and the temperature coefficient of permanent magnets is high, and the output voltage changes with the ambient temperature, resulting in the output voltage deviating from the rated voltage and difficult to regulate.
Disadvantages of permanent magnet motors
Permanent magnet motors (PMM) generate torque through the interaction of stator current with permanent magnets on or in the rotor. Small, low-power motors are used in IT equipment, commercial machines, and automotive auxiliary equipment where surface rotor magnets are common. Internal magnets (IPM) are common in larger machines such as electric vehicles and industrial motors.
In PM motors, centralized (short-pitch) windings may be used for the stator if torque pulsations are not considered, but distributed windings are common in larger PM motors. The Inverter Is Critical For Controlling The Winding Current since PM motors do not have a mechanical commutator. PM motors do not require current to support their magnetic field, unlike other brushless motors. As a result, PM motors can provide the most torque and may be the best choice if they are small or lightweight. The lack of magnetizing current also means higher efficiency at the “sweet spot” load – where the motor performs best.
In addition, while permanent magnets offer performance benefits at low speeds, they are also the technology’s “Achilles heel”. For example, as the speed of a PM motor increases, the reverse electromotive force approaches the inverter supply voltage, making it impossible to control the winding current. This defines the base speed of a general-purpose PM motor and usually represents the maximum possible speed for a given supply voltage in a surface magnet design.
At speeds greater than the fundamental speed, IPMs use active magnetic field weakening, in which stator currents are manipulated to depress magnetic flux intentionally. The range of speeds that can be reliably implemented is limited to about 4:1. As before, this limit can be achieved by reducing the number of windings turns and accepting greater cost and power losses in the inverter.
The need for magnetic field weakening is speed dependent and has associated losses regardless of torque. This can reduce efficiency at high speeds, especially at light loads.
Other disadvantages include that it is difficult to manage under fault conditions due to its inherent reverse electromotive force. Even if the inverter is disconnected, as long as the motor is rotating, current will continue to flow through winding faults, resulting in cogging torque and overheating, which can be dangerous. For example, the weakening of the magnetic field at high speeds due to inverter shutdown can lead to uncontrolled power generation, and the DC bus voltage of the inverter can rise to dangerous levels. Operating temperature is another important limitation except for those permanent magnet motors equipped with samarium cobalt magnets. And high motor currents due to inverter failure can lead to demagnetization. The holding force of the mechanical magnets limits the maximum speed. If a permanent magnet motor is damaged, repairing it usually requires returning it to the factory because safely extracting and handling the rotor is difficult. Finally, recycling at end-of-life is problematic, although the current high value of rare earth materials may make this more economically viable.
Despite these drawbacks, permanent magnet motors remain unrivaled in terms of low speed and efficiency, and they are useful in situations where size and weight are critical.