Development history of permanent magnet motor   

The development of permanent magnet motors is closely related to the development of permanent magnet materials. China is the first country in the world to discover the magnetic properties of permanent magnet materials and apply them to practice. More than two thousand years ago, China made a compass using the magnetic properties of permanent magnet materials, which played a great role in navigation, military and other fields, and became one of the four great inventions of ancient China. The first motor in the world, which appeared in the 1820 s, was a permanent magnet motor with an excitation magnetic field generated by permanent magnets. However, the permanent magnet material used at that time was natural magnetite (Fe3O4), which had a very low magnetic energy density. The motor made of it was bulky and was soon replaced by an electrically excited motor. With the rapid development of various motors and the invention of current magnetizers, people have conducted in-depth research on the mechanism, composition and manufacturing technology of permanent magnet materials, and successively discovered carbon steel, tungsten steel (maximum magnetic energy product is about 2.7 kJ/m3), cobalt steel (maximum magnetic energy product is about 7.2 kJ/m3) and other permanent magnet materials. In particular, the aluminum-nickel-cobalt permanent magnet (the maximum magnetic energy product can reach 85 kJ/m3) that appeared in the 1930 s and the ferrite permanent magnet (the maximum magnetic energy product can reach 40 kJ/m3) that appeared in the 1950 s have greatly improved their magnetic properties, and various micro and small motors have used permanent magnet excitation. The power of permanent magnet motors is as small as a few milliwatts and as large as tens of kilowatts. They are widely used in military, industrial and agricultural production and daily life, and their output has increased sharply. Accordingly, during this period, breakthroughs have been made in the design theory, calculation method, magnetizing and manufacturing technology of permanent magnet motors, and a set of analysis and research methods represented by the graphic method of permanent magnet working diagram has been formed. However, the coercivity of AlNiCo permanent magnets is low (36~160 kA/m), and the remanence density of ferrite permanent magnets is not high (0.2~0.44 T), which limits their application in motors. Until the 1960 s and 1980 s, rare earth cobalt permanent magnets and neodymium iron boron permanent magnets (collectively referred to as rare earth permanent magnets) came out one after another. Their excellent magnetic properties of high remanence density, high coercivity, high magnetic energy product and linear demagnetization curve are particularly suitable for manufacturing motors, thus making the development of permanent magnet motors enter a new historical period.

 

Characteristics and Application of Permanent Magnet Motor       

 

Compared with traditional electrically excited motors, permanent magnet motors, especially rare earth permanent magnet motors, have 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. Therefore, the scope of application is extremely wide, almost all over the aerospace, national defense, industrial and agricultural production and every field of daily life. The following describes the main features of several typical permanent magnet motors and their main applications. Compared with the traditional generator, the permanent magnet synchronous generator does not need the collector ring and brush device, the structure is simple, and the failure rate is reduced. The use of rare earth permanent magnets can also increase the air gap flux density, and increase the motor speed to the best value, improve the power quality ratio. Modern aviation and aerospace generators almost all use rare earth permanent magnet generators. Its typical products are 150 kVA 14 pole 12 000 r/min ~ 21 000 r/min and 100 kVA 60 000 r/min rare earth cobalt permanent magnet synchronous generators manufactured by General Electric Company of the United States. The first rare earth permanent magnet motor developed in China is a permanent magnet generator with a 3 kW of 20 000 r/min.

Permanent magnet generators are also used as auxiliary exciter for large steam turbine generators. In the 1980 s, China successfully developed the 40 kVA ~ 160 kVA rare earth permanent magnet auxiliary exciter with the largest capacity in the world at that time. Equipped with 200 MW ~ 600 MW steam turbine generators, the reliability of power station operation was greatly improved.
At present, independent power supply with internal combustion engine driven small generator, vehicle permanent magnet generator, wind wheel direct drive small permanent magnet wind generator is gradually promoted.

 

The important role of permanent magnet motors in various application fields

 

1 energy-saving rare earth permanent magnet motor to consumption as the main object, such as textile and chemical fiber industry with rare earth permanent magnet synchronous motor, oil, mining, rare earth permanent magnet synchronous motor in coal mine transportation machinery, rare earth permanent magnet synchronous motor drive all kinds of pumps and fans. 2 Various rare earth permanent magnet motors are used by various types of vehicles (cars, motorcycles, trains), and rare earth permanent magnet motors are a big market for zui. According to statistics, about 70% of the vehicle rare earth permanent magnet motor. For luxury cars, the motor for various applications has reached more than 70 sets. Since the requirements of various automotive motors are different, the choice of permanent magnet materials is different. Motor magnets are used in air conditioners, fans, and power windows. From a price point of view, the advantages of ferrite will continue in the future. Ignition coils, drives, sensors, still use Sm-Co sintered magnets. In addition, auto parts, also electric vehicles can not be ignored, as an environmentally friendly (EV) and hybrid electric vehicles (HEV). 3 rare earth permanent magnet motor ac servo system a set of electronic, high performance, speed control system of mechatronics machinery. The system is a self-controlled permanent magnet synchronous motor body. The system is used for the development of CNC machine tools, flexible manufacturing technology; also used for electric vehicles, rather than traditional thermal power vehicles, vehicle emissions free. Rare earth permanent magnet motor is a promising high-tech industry. 4 new areas are mainly for the support of low-power rare earth permanent magnet synchronous motor variable frequency speed control system for new air conditioners and refrigerators, wireless electric gadgets for various rare earth permanent magnet DC micromotors, rare earth permanent magnet brushless DC motors are different power instruments. The demand for such motors is also great. 5 in aerospace applications for the advantages of rare earth permanent magnet materials, making it very suitable for aircraft engine applications. Although there are some applications of rare earth permanent magnet motors in the air (such as generator voltage and short circuit protection, etc.), experts at home and abroad agree that rare earth permanent magnet motors are an important development direction of the new generation of aero engines.

Cost issues

Ferrite permanent magnet motors, especially miniature permanent magnet DC motors, have been widely used because of their simple structure, reduced mass, and generally lower total cost than electrically excited motors. Because the price of rare earth permanent magnet is still relatively expensive, the cost of rare earth permanent magnet motor is generally higher than that of electric excitation motor, which needs to be compensated by its high performance and operating cost savings.

In some cases, such as the voice coil motor of computer disk drive, the performance is improved after using NdFeB permanent magnet, the volume and mass are significantly reduced, and the total cost is reduced. In the design, it is necessary to compare the performance and price according to the specific application and requirements, and to make the choice, but also to innovate the structural process and optimize the design to reduce the cost.

Types and characteristics of permanent magnet motors

Compared with traditional electrically excited 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 flexible motor shapes and sizes. Therefore, it is more and more widely used. Permanent magnet motors use permanent magnets as a magnetic field and do not require external energy to maintain their magnetic field, while ordinary motors require current to have a magnetic field.

regular permanent magnet motors are usually divided into the following five categories:Permanent magnet DC motor, asynchronous start permanent magnet synchronous motor, permanent magnet brushless DC motor, speed control permanent magnet synchronous motor and permanent magnet synchronous generator.

Permanent magnet motor mainly has the following characteristics:

       1. Permanent magnet DC motor

The difference between the permanent magnet DC motor and the ordinary DC motor is that the former cancels the excitation winding and the pole core, and substitutes the permanent magnet pole. The characteristics of permanent magnet DC motors are similar to those of separately excited DC motors. The difference between the two is that the main magnetic field is generated in a different way. The former magnetic field is not controllable, and the latter magnetic field is controllable. 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 starting 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 stator rotating magnetic field and the cage rotor to achieve starting. During normal operation, the rotor runs at a synchronous speed, and the cage rotor no longer works. Its working principle is basically the same as that of an electrically excited synchronous motor.

Compared with induction motors, asynchronous start permanent magnet synchronous motors have the following characteristics:

(1) Constant speed, synchronous speed.

(2) The power factor is high, even the leading power factor, thus reducing the stator current and stator resistance loss, and there is no rotor copper loss during stable operation, thus reducing the fan (small-capacity motor can even remove the fan) and the corresponding wind friction loss, and the efficiency can be increased by 2% ~ 8% compared with the induction motor of the same specification.

(3) has a wide range of economic operation. Not only the rated load has a higher power factor and efficiency, but also in the 25% ~ 120% rated load range has a higher power factor and efficiency, so that the light load operation energy saving effect is more significant. This type of motor is generally provided with a starting winding on the rotor and 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 the permanent magnet motor is only 92kg, which is equivalent to 45.8% of the mass of asynchronous motor.

(5) the impact on the power grid is small. The power factor of the induction motor is low, and the motor has to absorb a large amount of reactive current from the power grid, resulting in a decline in the quality factor of the power grid, increasing the burden of power transformation and distribution equipment and power loss. However, there is no induced 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, so that the power grid does not need to install reactive compensation devices.

(6) Because neodymium iron boron permanent magnet materials are usually used, the price is high; when the motor is not designed or used properly, irreversible demagnetization may occur.

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

(8) Motor performance is greatly affected by factors such as ambient temperature and power supply voltage.

      3. Permanent magnet brushless DC motor

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

      4. Speed regulating permanent magnet synchronous motor

The speed-adjustable permanent magnet synchronous motor and the permanent magnet brushless DC motor are basically the same in structure. The stator is multi-phase windings and the rotor has permanent magnets. 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 a set of 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. Unlike ordinary synchronous generators, it uses permanent magnets to establish a magnetic field, eliminating the excitation winding, excitation power supply, collector ring and brushes, etc., and has a simple structure. Reliable operation, high efficiency and maintenance-free. When using rare earth permanent magnets, the air gap has high magnetic density, high power density, small size and light weight. However, due to the use of permanent magnets to establish a magnetic field, it is difficult to adjust the output voltage and reactive power by adjusting the excitation method. In addition, the permanent magnet synchronous generator usually uses neodymium iron boron or ferrite permanent magnet. The temperature coefficient of the permanent magnet is relatively high, and the output voltage changes with the change of the ambient temperature, which causes the output voltage to deviate from the rated voltage and is difficult to adjust.

Why are permanent magnet motors more efficient?

The permanent magnet synchronous motor is mainly composed of stator, rotor and housing parts. Like ordinary AC motors, the stator core is a laminated structure to reduce the iron loss due to eddy current and hysteresis effects during motor operation; the winding is usually a three-phase symmetrical structure, but the parameter selection is quite different. The rotor part has various forms, including a permanent magnet rotor with a starting squirrel cage, and a built-in or surface-mounted pure permanent magnet rotor. The rotor core can be made of solid structure, can also be laminated. The rotor is equipped with permanent magnet material, which is customarily called magnetic steel.

Under the normal operation of the permanent magnet motor, the rotor and the stator magnetic field are in a synchronous state, the rotor part has no induced current, no rotor copper loss, hysteresis, and eddy current loss, and no need to consider the problem of rotor loss and heating. General permanent magnet motor for dedicated inverter power supply, natural with soft start function. In addition, the permanent magnet motor is a synchronous motor, which has the characteristics of adjusting the power factor of the synchronous motor through the excitation strength, so the power factor can be designed to the specified value.

 

 

From the starting point of view, due to the fact that the permanent magnet motor is started by the variable frequency power supply or the matching frequency converter, the starting process of the permanent magnet motor is easy to realize; similar to the starting of the variable frequency motor, it avoids the starting defect of the ordinary cage asynchronous motor.

In short, the efficiency and power factor of permanent magnet motors can reach very high, and the structure is very simple. The market has been very hot in the past ten years. However, demagnetization fault is an unavoidable problem for permanent magnet motors. When the current is too large or the temperature is too high, it will cause the temperature of the motor winding to rise instantaneously, the current will increase sharply, and the permanent magnet will lose its magnetism rapidly. In the control of the permanent magnet motor, the over-current protection device is set to avoid the problem of the motor stator winding being burned, but the resulting loss of excitation and equipment outage are inevitable.

Compared with other motors, the application of permanent magnet motors in the market is not very popular. For motor manufacturers and users, there are some unknown technical blind spots, especially when it comes to matching with frequency converters, which often leads to serious discrepancies between design values and test data and must be verified repeatedly.

The motor design process involves some basic considerations, such as the starter, the requirements of the application environment, when and what torque and speed are needed, how often? What is the working cycle? What are the environmental conditions such as temperature and pressure? Even the most efficient motor, if the motor is used in the wrong area, it will not exert its maximum efficiency. Many motors are used in the combination of gear motors, gear reducers and motors. The gear motor provides high torque at low speed. In short, the gear motor will absorb the motor power and reduce the speed while amplifying the torque. The duty cycle of the gear motor will affect the performance rating of the motor, such as the continuous duty cycle.

Best Cooling Design Enclosure

A well-cooled motor runs more efficiently. In order to obtain the best airflow, the design of the cooling fan and fan cover is optimized to ensure that the tight combination between the stator and the motor housing provides the best cooling performance. The electrical efficiency of the motor has improved a lot, but the power of the cooling fan accounts for a larger proportion of the total loss. The optimization of the cooling fan size includes using the minimum power of the fan while providing sufficient cooling. The optimized fan design can reduce fan power requirements by 65%. An important design feature is the gap between the blades and the housing. The space between the casing and the fan blades should be as small as possible to prevent turbulence and reduce backflow.

Choosing low-friction bearings for the working speed

Ball or roller bearings are used in high-efficiency motors. They consist of an inner and outer ring and a cage containing steel or ceramic rollers or balls. The outer ring is connected to the stator, and the inner ring is connected to the rotor. When the shaft rotates, the element also rotates, and the friction of the shaft rotation is minimized. They have a long service life and low maintenance costs. High-precision applications allow for minimal air gaps. Thermal shrinkage and thermal expansion will affect the fit of the shaft and bearing housing and the internal bearing clearance. Power output controls shaft size and bearing bore. Load magnitude and direction determine bearing size and type. Consider additional forces, such as asymmetric air gaps that cause magnetic pull, out-of-balance forces, pitch errors of gears, and thrust loads. For bearing load calculations, the shaft is treated as a beam supported on a rigid moment-free support. Ball bearings are more suitable for high-speed applications than roller bearings. High speed factors include cage design, lubricant, running accuracy, clearance, resonant frequency, and balance.

The bearing requires minimal load, so the rolling elements rotate to form a lubricating film instead of sliding, which increases the operating temperature and degrades the lubricating oil. The minimum allowable load is equal to 0.01 times the dynamic radial load rating of the ball bearing. This is especially important when the bearing is close to 70% of the recommend rating. Knowledge of the ambient temperature range and the normal operating temperature range will help determine the most effective lubrication method for bearings: oil or grease. In general, the normal operating temperature range of the gear motor considered is -25 to 40°C. Synthetic greases have good performance in various temperature ranges. Greases can simplify maintenance, cleaning, leakage reduction and pollution protection.

Use high-quality balancing machine, high standard and balance under motor running speed

When the shaft center and the rotating shaft do not coexist, noise and vibration will be generated. The balance has a limited impact on efficiency, but it will affect operating noise and life expectancy, which is also important to maximize the use of resources. Bearing vibration readings are usually read in three planes: vertical, horizontal and axial. Vertical vibration may indicate an installation problem, horizontal vibration may indicate a balance problem, and axial vibration may indicate a bearing problem. The balance at the operating speed is important because the centripetal force of the bearing may also cause imbalance.

Optimum Design of Rotor Laminated Shows Sinusoidal Magnetic Field

Synchronous motors with high-performance permanent magnets have sinusoidal flux distribution and electromotive force. For distributed windings, the stator windings are usually the same as the asynchronous motor windings. It reduces vibration, noise and maintenance costs and improves overall performance.

Selection of rare earth and ferrite (ceramic) magnets

Neodymium, rare earth, samarium cobalt magnets or ferrite (ceramic) magnets are used in the motor. The strength of rare earth magnets is two to three times that of ferrite or ceramic permanent magnets, but the price is more expensive. Samarium cobalt magnets are the best choice for high temperature applications because of their high energy density, temperature resistance of 250 to 550°C, small reduction in parameters due to temperature increase, and oxidation protection. The choice of samarium cobalt or neodymium as motor magnets is based on operating temperature, corrosion resistance and required performance. If heated above 80°C, low-grade neodymium magnets may begin to lose "strength", and high-grade neodymium magnets work at temperatures below 220°C. Ferrite or ceramic magnets are widely recognized due to their strong resistance, good demagnetization, strong corrosion resistance, and low price. Magnetic losses occur when working at temperatures above 250°C, but they are restored when the magnet drops to a lower temperature. Unless the circuit is designed for extreme situations, a low temperature of -40°C may cause a permanent loss of permanent magnet strength.

Motor needs inverter

The inverter drive unit can be loss-free under no-load operation/static conditions. By replacing the existing line-powered three-phase drive device, it is expected that up to 30% of energy can be saved. The characteristics of the drive unit make it ideal for driving pumps and fans that run continuously. No additional components, such as an encoder, are required. Up to 25% of the floor space allows the machine to be designed more compact. The motor has good control performance, and combined with the sensorless drive controller unit, it has excellent real running performance even at low speeds, and has impressive dynamic characteristics under pulse load and speed changes.

Choose an inverter that provides sensorless operation

The drive can "self-detect" and track the permanent magnet position of the rotor. This is essential for the smooth start of the motor, while also allowing the best torque to be produced, resulting in the best efficiency. The lack of a position or speed sensor reduces cost and improves the reliability of the drive system. With the continuous improvement of efficiency, the importance of programming the controller settings of a specific motor to obtain the best efficiency is becoming more and more important.

Disadvantages of permanent magnet motor

A permanent magnet machine (PMM) generates torque through the interaction of stator currents with permanent magnets on or within the rotor. Small, low-power motors are common for surface rotor magnets in IT equipment, commercial machines, and automotive auxiliary equipment. Internal magnets (IPM) are common in large machines such as electric vehicles and industrial motors.

In permanent magnet machines, the stator may use concentrated (short pitch) windings if torque ripple is not considered, but distributed windings are common in larger permanent magnet machines.

Since the permanent magnet motor has no mechanical commutator, the inverter is very important to control the winding current. Unlike other types of brushless motors, permanent magnet motors do not require current to support their magnetic field.

Therefore, if it is small or light weight, a permanent magnet motor can provide the maximum torque and may be the best choice. No magnetizing current also means higher efficiency at the "sweet spot" load-that is, where the motor performance is best.

In addition, although permanent magnets bring performance advantages at low speeds, they are also technical "Achilles heel". For example, as the speed of the permanent magnet motor increases, the back EMF approaches the inverter supply voltage, so that the winding current cannot be controlled. This defines the basic speed of a general purpose permanent magnet machine, and in surface magnet designs typically represents the maximum possible speed for a given supply voltage.

At speeds greater than the base speed, the IPM uses active field weakening, where the stator current is manipulated to deliberately depress the magnetic flux. The speed range that can be reliably implemented is limited to around 4:1. As before, this limitation can be achieved by reducing the number of winding turns and accepting greater cost and power losses in the inverter.

The need for field weakening is speed dependent and produces associated losses regardless of torque. This can reduce efficiency at high speeds, especially at light loads.

In electric cars driving on highways, this is very serious. Permanent magnet motors are often favored by electric vehicles, but the efficiency benefits are questionable when calculated in actual driving cycles. Interestingly, at least one well-known electric car manufacturer has switched from PM to induction motors.

Other disadvantages include the fact that it is difficult to manage under fault conditions due to its inherent back EMF. Even if the frequency converter is disconnected, as long as the motor rotates, current will continue to flow through the winding fault, resulting in cogging torque and overheating, and both are dangerous.

For example, magnetic field weakening at high speeds can lead to uncontrolled power generation due to inverter shutdown, and the inverter's DC bus voltage can rise to dangerous levels.

With the exception of those permanent magnet motors equipped with samarium-cobalt magnets, operating temperature is another important limitation. High motor current due to inverter failure can cause demagnetization.

The maximum speed is limited by the holding force of the mechanical magnet. If the permanent magnet motor is damaged, repairing it usually requires returning to the factory because it is difficult to safely extract and dispose of the rotor. Finally, recycling at end-of-life is also cumbersome, although the high value of current rare earth materials may make such materials more economically viable.