Cost Issue

   Ferrite permanent magnet motors, especially miniature permanent magnet DC motors, are widely used because of their simple structure and process, light weight, and generally lower total cost than electric excitation motors. Since rare earth permanent magnets are still relatively expensive at present, the cost of rare earth permanent magnet motors is generally higher than that of electric excitation motors, which needs to be compensated by its high performance and operating cost savings.

 In some occasions, such as the voice coil motor of the computer disk drive, the performance of NdFeB permanent magnets is improved, the volume and mass are reduced, and the total cost is reduced. In the design, it is necessary to compare the performance and price according to the specific use occasions and requirements to decide the choice, and to innovate the structure and process and optimize the design to reduce the cost.

Types and characteristics of permanent magnet motor

   Compared with traditional electric excitation motors, permanent magnet motors, especially rare earth permanent magnet motors, have significant advantages such as simple structure, reliable operation, small size, light weight, low loss, high efficiency, and the shape and size of the motor can be flexible and diverse.  so, it is more and more widely used. Permanent magnet motors use permanent magnets as the magnetic field, which can maintain their magnetic field without external energy, while ordinary motors require current to have a magnetic field.

Conventional permanent magnet motors are usually divided into the following five categories: permanent magnet DC motors, asynchronous start permanent magnet synchronous motors, permanent magnet brushless DC motors, variable speed permanent magnet synchronous motors and permanent magnet synchronous generators.

Permanent magnet motors mainly have the following characteristics:

1. Permanent magnet DC motor the structural difference between a permanent magnet DC motor and an ordinary DC motor is that the former cancels the field winding and the pole core, and replaces it with a permanent magnet pole. The characteristics of the permanent magnet DC motor are similar to those of the separately excited DC motor, the difference between the two is the way the main magnetic field is generated. The former has an uncontrollable magnetic field, while the latter has a controllable magnetic field. In addition to the good characteristics of separately excited DC motors, permanent magnet DC motors also have the characteristics of simple structure, reliable operation, high efficiency, small size, and light weight.

2.  Asynchronous start permanent magnet synchronous motor Asynchronous start permanent magnet synchronous motor is a permanent magnet synchronous motor with self-starting ability, which has the characteristics of induction motor and electric 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 achieve starting. During normal operation, the rotor runs at synchronous speed, and the cage rotor no longer works, and its working principle is basically the same as that of the electric excitation synchronous motor.  

Compared with the induction motor, the asynchronous start permanent magnet synchronous motor has the following characteristics:

(1) The speed is constant, which is synchronous speed.

(2) High power factor, even leading power factor, thereby reducing stator current and stator resistance loss, and there is no rotor copper loss during stable operation, thereby reducing fans (small-capacity motors can even remove fans) and corresponding wind friction Loss and efficiency can be increased by 2% to 8% compared with induction motors of the same specification.

(3) It has a wide range of economic operations. It not only has high power factor and efficiency at rated load, but also has high power factor and efficiency in the range of 25% to 120% rated load, which makes the energy saving effect more significant when running at light load. This type of motor generally has a starting winding on the rotor, which has the ability to start directly at a certain frequency and voltage.

(4) The volume and quality of permanent magnet motors are greatly reduced compared with induction motors. For example, the mass of 11kW asynchronous motor is 220kg, while that of permanent magnet motor is only 92kg, equivalent to 45.8% of the mass of asynchronous motor.

(5) Little 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, resulting in a decrease in the quality factor of the grid, increasing the burden on the grid transformation and distribution equipment and power loss. However, there is no induction current excitation in the rotor of the permanent magnet motor, and the power factor of the motor is high, which improves the quality factor of the power grid and makes it no longer necessary to install a reactive power compensation device in the power grid.

(6) Because NdFeB permanent magnet materials are usually used, the price is high; when the motor is improperly designed or used, irreversible demagnetization may occur.

(7) The processing technology is complex and the mechanical strength is poor.

(8) The performance of the motor is greatly affected by factors such as ambient temperature and power supply voltage.

3. Permanent magnet brushless DC motor

The permanent magnet brushless DC motor replaces the commutator of the DC motor with an electronic commutation device, which retains the excellent characteristics of the DC motor. It not only has the advantages of simple structure, reliable operation, and convenient maintenance of AC motors, but also has the advantages of large starting torque and good speed regulation performance of DC motors. Due to the cancellation of the brush commutator, the reliability is high; the loss is mainly generated by the station, and the heat dissipation condition is good; the volume is small and the weight is light.

4. Speed-regulating permanent magnet synchronous motor

The variable speed permanent magnet synchronous motor and the permanent magnet brushless DC motor are basically the same in structure, with multi-phase windings on the stator and permanent magnets on the rotor. The advantages of the two are similar. The main difference between them is that the permanent magnet brushless DC motor realizes synchronization according to the rotor position information, while the speed regulating permanent magnet synchronous motor needs an electronic control system to realize synchronization and speed regulation.

5. Permanent magnet synchronous generator

The permanent magnet synchronous generator is a synchronous generator with a special structure. Different from the ordinary synchronous generator, it uses permanent magnets to establish a magnetic field, and cancels the excitation winding, excitation power supply, collector ring and brush, etc., and has a simple structure. Reliable operation, high efficiency, maintenance-free. When rare earth permanent magnets are used, the air gap magnetic density is high, the power density is high, the volume is small, and the weight is light. However, since a permanent magnet is used to establish a magnetic field, it is difficult to adjust the output voltage and reactive power by adjusting the excitation method. In addition, permanent magnet synchronous generators usually use NdFeB or ferrite permanent magnets. The temperature coefficient of permanent magnets is high, and the output voltage changes with the change of ambient temperature, resulting in output voltage deviation from the rated voltage, and it is difficult to adjust.




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