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2023

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07

Permanent magnet motor magnetic circuit structure and design

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Permanent magnet motor magnetic circuit structure and design

1. Permanent magnet motor

1.1 Magnetic circuit structure and design calculation

The biggest difference between a permanent magnet generator and an excitation generator is that its excitation magnetic field is generated by permanent magnets. Permanent magnets are both a magnetic source and an integral part of a magnetic circuit in a motor. The magnetic properties of permanent magnets are not only related to the manufacturing process of the manufacturer, but also related to the shape and size of the permanent magnets, the capacity of the magnetizer and the magnetization method. The specific performance data are highly discrete. Moreover, the magnetic flux and magnetomotive force that the permanent magnet can provide in the motor also vary with the material properties, size and operating status of the rest of the magnetic circuit. In addition, the magnetic circuit structure of the permanent magnet generator is various, the leakage magnetic circuit is very complex and the proportion of the leakage flux is relatively large, the ferromagnetic material part is relatively easy to saturate, and the magnetic permeability is nonlinear. All of these increase the complexity of the electromagnetic calculation of the permanent magnet generator, making the accuracy of the calculation result lower than that of the electric excitation generator. Therefore, it is necessary to establish a new design concept, re-analyze and improve the magnetic circuit structure and control system; must apply modern design methods, study new analysis and calculation methods to improve the accuracy of design calculations; must study the use of advanced testing methods and manufacturing craft.

1.2 Control issues

After the permanent magnet generator is made, it can maintain its magnetic field without external energy, but it also makes it extremely difficult to adjust and control its magnetic field from the outside. These limit the application range of permanent magnet generators. However, with the rapid development of control technologies for power electronic devices such as MOSFETs and IGBTTs, permanent magnet generators do not require magnetic field control but only motor output control in applications. The design requires the combination of three new technologies of NdFeB material, power electronic devices and microcomputer control, so that the permanent magnet generator can operate under brand-new working conditions.

1.3 Irreversible demagnetization problem

If the design and use are improper, the permanent magnet generator will be under the action of the armature reaction generated by the impact current when the temperature is too high (NdFeB permanent magnet) or too low (ferrite permanent magnet), or under severe mechanical vibration Sometimes irreversible demagnetization, or demagnetization, may occur, which will reduce the performance of the motor and even make it unusable. Therefore, it is necessary to research and develop methods and devices for checking the thermal stability of permanent magnet materials suitable for motor manufacturers, and to analyze the anti-demagnetization ability of various structural forms, so that corresponding measures can be taken during design and manufacture to ensure permanent magnetism. Magnetic generators do not lose their magnetism.

1.4 Cost issues

Since the current price of rare earth permanent magnet materials is relatively expensive, the cost of rare earth permanent magnet generators is generally higher than that of electric excitation generators, but this cost will be better compensated in the high performance and operation of the motor. In the future design, performance and price comparison will be carried out according to specific usage occasions and requirements, and structural innovation and design optimization will be carried out to reduce manufacturing costs. It is undeniable that the cost price of the product under development is slightly higher than that of the current general-purpose generator, but we believe that with the further perfection of the product, the cost problem will be well resolved. The person in charge of the technical department of DELPHI (Delphi) company in the United States believes: "Customers pay attention to the cost per kilowatt." His statement fully demonstrates that the market prospect of AC permanent magnet generators will not be troubled by cost issues.

1.5 permanent magnet rotor features:

Structure 1:

Parallel magnetic field structure; the rotor is made of casting and pressed, with permanent magnets embedded in it, with high energy, light weight, small size, solid and reliable overall structure, and the maximum working speed is greater than 15,000 rpm.

Patent No.: ZL96 2 47776.1

Structure 2:

Series magnetic field structure; the rotor adopts steel structure, and the surface is embedded with permanent magnets in sequence. The surface of the rotor has strong magnetic flux, light weight, small size, solid and reliable overall structure, and the maximum working speed is greater than 15,000 rpm.

Patent No.: ZL98 2 33864.3 Features of voltage stabilization system for the whole machine:

A half-controlled bridge rectifier circuit is composed of silicon controlled rectifiers and diodes. The voltage stabilizing system is a chopper modulating voltage stabilizing device, and its voltage stabilizing accuracy is plus or minus 0.1v, so the generator has the characteristics of being able to withstand large current instantaneously, reliable operation and durability, and because it can directly use the generator The reverse voltage of the generated alternating current makes the thyristor turn off by itself, so there is no need to add a turn-off circuit, which makes the circuit structure simple and reliable.

2. Advantages of permanent magnet generator

2.1 Simple structure and high reliability

The permanent magnet generator eliminates the excitation winding, carbon brush and slip ring structure of the excitation generator, and the structure of the whole machine is simple, avoiding the easy burning and disconnection of the excitation winding, carbon brush and slip ring structure, and the structure of the whole machine is simple, It avoids failures such as easy burnout and disconnection of the excitation winding of the excitation generator, easy wear and tear of carbon brushes and slip rings, etc., and the reliability is greatly improved.

2.2 Small size, light weight, high specific power

The adoption of the permanent magnet rotor structure makes the internal structure design of the generator very compact, and the volume and weight are greatly reduced. The simplification of the structure of the permanent magnet rotor also reduces the moment of inertia of the rotor, increases the practical speed, and achieves a very high specific power (that is, the ratio of power to volume).

2.3 Good power generation performance at medium and low speeds

In the case of the same power level, at idle speed, the output power of the permanent magnet generator is twice that of the excitation generator, that is to say, the excitation generator of the actual power level of the permanent magnet generator.

2.4 Can significantly prolong battery life and reduce battery maintenance work

The main reason is that the permanent magnet generator adopts a switch-type rectification and voltage stabilization method, which has high voltage regulation accuracy and good charging effect. It avoids the shortening of battery life caused by over-current charging. The open-type rectification output of the permanent magnet generator uses small current pulses to charge the battery, and the charging effect is better with the same charging current, thereby prolonging the service life of the battery.

2.5 High efficiency

The permanent magnet generator is an energy-saving product. The permanent magnet rotor structure eliminates the excitation power required to generate the rotor magnetic field and the mechanical loss of friction between carbon brushes and slip rings, which greatly improves the efficiency of the permanent magnet generator. The average efficiency of ordinary excitation generators is only 45% to 55% in the speed range between 1500 rpm and 6000 rpm, while that of permanent magnet generators can be as high as 75% to 80%.

2.6 Using a self-starting voltage regulator

No need for external excitation power supply. The generator generates electricity as long as it spins. When the battery is damaged, the car charging system can still work normally as long as the engine is running. If the car does not have a storage battery, the ignition operation can also be realized as long as the handle is turned or the car is slipped.

2.7 Especially suitable for working in humid or dusty harsh environments 2.8 No radio interference

The permanent magnet generator has no carbon brush and no slip ring structure, which eliminates the radio interference caused by the friction between the carbon brush and the slip ring; Requirements for ambient temperature.

3. Comparison of structural characteristics

3.1 Permanent magnet generator

The permanent magnet generator has a simple structure, a large rotor magnetic field, no excitation winding, no carbon brush, no slip ring, large air gap, no contact, and the only worn part of the whole machine is the bearing. The reliability of the product is improved, no external regulator is needed, and the distance between the magnetizer materials is optimized to reduce magnetic flux leakage, so that the generator has good idle speed performance and sufficient output circuit.

Excitation generator:

The excitation generator has poor power generation performance at idling speed, and the battery is easily discharged; the efficiency of the generator is only 40%-50% in the range of 1500-6000 rpm, and the efficiency is low. Because the rotor has an excitation coil, the temperature of the whole machine rises; Excitation generators are also prone to problems such as carbon brushes and slip ring damage.

3.2 Performance comparison (take 28V, 36A, 1000W generator as an example)

3.3 Volume temperature rise comparison

Based on the comparative analysis of the above performance, structure, and volume, the permanent magnet type is simpler in structure, smaller in size, lighter in weight, and superior in performance than the excitation type, and it can generate sufficient power at low speeds.

Volume and weight comparison table of JFYJ permanent magnet generator (3000W) and common excitation generator (3000W)

4. The charm of rare earth NdFeB

"The Crown of Rare Earths" - NdFeB, a unique rare material, will become the darling of the industrial revolution in the 21st century because of its powerful magnetic carrying function. And the motor industrial revolution triggered by it has become the high attention of motor experts and automobile experts all over the world. According to the prediction of industry experts, the motor in the 21st century will be a world of permanent magnets.

As a country of rare earths, China's reserves account for 80% of the world's total, but the utilization and development of rare earths is not ideal. On the contrary, some developed countries have fully realized the potential value of rare earths and are doing in-depth research. China should develop unique rare earth resources like South Africans mine diamonds and Middle Easterners develop oil, and give full play to this resource advantage. At present, we have found a breakthrough in the deep development and utilization of rare earths in wind power projects.