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Comprehensive Benefit Analysis of Permanent Magnet Synchronous Motor Replacing Asynchronous Motor

Date:2022-06-08   Author:XINDA MOTOR

Compared with asynchronous motors, synchronous motors have the advantages of high power factor, high efficiency, measurable rotor parameters, large air gap between stator and rotor, good control performance, small size, light weight, simple structure, high torque/inertia ratio, etc. It has been increasingly widely used in the fields of petroleum, chemical industry, light textile, mining, vehicles, CNC machine tools, robots, and aerospace, and is facing high power (high speed, high torque), high functionality and miniaturization. develop.

The permanent magnet synchronous motor consists of a stator and a rotor. The stator is the same as an asynchronous motor and consists of three-phase windings and a stator core. Pre-magnetized (magnetized) permanent magnets are installed on the rotor, and a magnetic field can be established in the surrounding space without external energy, which can simplify the motor structure and save energy. This article starts from the characteristics of permanent magnet synchronous motor, combined with practical application to expound the comprehensive benefits of popularizing permanent magnet synchronous motor.

1 The outstanding advantages of permanent magnet synchronous motor

(1) Since the rotor is made of permanent magnets, the magnetic flux density is high, the excitation current is not required, and the excitation loss is eliminated. Compared with the asynchronous motor, the excitation current of the stator side winding and the copper and iron losses of the rotor side are reduced, and the reactive current is greatly reduced. Active energy) and power factor (reactive energy) are higher than asynchronous motors. Permanent magnet synchronous motors are generally designed to have high power factor and efficiency even in light-load operation. Figure 1 is a comparison curve of the efficiency and power factor of a permanent magnet synchronous motor and an asynchronous motor of the same specification. From the perspective of motor load changes, rare earth permanent magnet synchronous motors can maintain high power factor and high efficiency within the range of 25% to 120% of rated power, while general three-phase asynchronous motors can only operate at 60% to 100%. The rated power maintains a higher performance operation.

Figure 1 Comparison of efficiency and power factor of two motors

(2) The permanent magnet synchronous motor maintains a strict and constant fixed relationship between the speed and the frequency. Unlike the asynchronous motor, which has a large slip rate when running under load, the permanent magnet synchronous motor has relatively hard mechanical characteristics. The motor torque disturbance has a strong ability to withstand. The rotor core of the permanent magnet synchronous motor can be made into a hollow structure to reduce the rotor inertia, and the starting and braking time of the permanent magnet synchronous motor is much faster than that of the asynchronous motor. The high torque/inertia ratio makes the permanent magnet synchronous motor more suitable than the asynchronous motor. Operates under fast response conditions.

(3) The size of the permanent magnet synchronous motor is greatly reduced compared with that of the asynchronous motor, and the weight is also greatly reduced. The permanent magnet synchronous motor with the same heat dissipation conditions and insulating materials has more than 2 times the power density of the three-phase asynchronous motor. In big cities where an inch of land is like gold, the impact of equipment footprint on cost is gradually being realized by users.

(4) The rotor structure is greatly simplified, which is convenient for maintenance and improves the stability of operation. In AC permanent magnet servo motor, the magnet steel and iron core on the rotor are usually bonded together with high temperature glue. The curing time of the glue is generally long, which increases the production cycle and cost. Under the repeated action of impact force, etc., there is a risk of deformation or even falling off. In order to solve this technical problem, the CM500 series of heavy-duty permanent magnet synchronous motors developed by our company changed the magnetic tile gluing process to the method of mechanical fastening by stainless steel sleeves, as shown in Figure 2, which completely eliminated the magnetic steel. Risk of loosening. This innovative application technology has been applied for a patent, which is a major advancement in motor rotor technology.

The three-phase asynchronous motor must make the air gap between the stator and the rotor very small due to the design of high power factor. At the same time, the uniformity of the air gap is also very important for the safe operation of the motor and the vibration and noise. For this asynchronous motor The requirements for the shape and position tolerances and assembly concentricity of the components are relatively strict, and the freedom of bearing clearance selection is relatively small. The asynchronous motors with larger bases usually use oil-bath lubricated bearings, which must be in the specified work. Filling lubricating oil within a certain time, oil leakage in the oil cavity or untimely filling will accelerate bearing failure. In the maintenance of three-phase asynchronous motor, bearing maintenance accounts for a large proportion. In addition, due to the existence of induced current in the rotor of the three-phase asynchronous motor, the electrical corrosion of the bearing has also been concerned by many researchers in recent years, but there is no such problem in the permanent magnet synchronous motor. Due to the large air gap of the permanent magnet synchronous motor, the related problems caused by the small air gap of the above asynchronous motor are not obvious on the synchronous motor. An appropriate amount of high-quality grease has been encapsulated in the factory, which can be maintenance-free for life.

2 Typical applications of permanent magnet synchronous motors

2.1 Application scheme of electro-hydraulic servo injection molding machine

The traditional injection molding machine uses a three-phase asynchronous motor with a quantitative pump. During the working process of the injection molding machine, the motor needs to run continuously for a long time. Through the energy-saving transformation of the servo oil pump drive system, the fast response and large starting torque of the permanent magnet synchronous motor, the permanent magnet synchronous motor can be started to drive the oil pump to generate pressure when the injection molding machine needs to complete the injection molding. , the main motor of the oil pump is in a shutdown state, achieving intermittent zero power consumption, and the energy saving rate can reach up to 85% by comparison.

Under the control of excellent servo motor algorithm, the noise is much lower than that of ordinary injection molding machines. Under the ideal state of low-noise screw pump, the noise of the injection molding machine is lower than 70 decibels. Achieve quiet operation and improve the working environment.

2.2 Energy-saving application of iron ore dry separator

The use site is a mining farm in the northwest, with a dusty environment. The equipment uses a total of 125 motors. The hoisting equipment needs a large starting torque when starting, and the motors run at a light load state of 30% to 50% of the rated power for a long time during normal operation. The efficiency of the three-phase asynchronous motor originally used It is only about 65%, and the power factor is about 0.4 to 0.6, which causes serious energy waste. The permanent magnet synchronous motor is used, and the energy saving effect is very obvious. The power saving test statistics are shown in Table 1. Figure 4 is a photo of the scene before and after the replacement.

Table 1125 comparison of energy-saving parameters between permanent magnet synchronous motors and high-efficiency three-phase asynchronous motors

3 Several issues that users are more concerned about

3.1 Motor life

The CM500 series motors use a sturdy resolver or an absolute magnetic encoder as an angle measuring device, which can meet the high-precision control requirements of the motor, and has higher reliability than optical encoders. When the control accuracy is slightly lower and there is no overload requirement, the sensorless vector control method can be used to save the cost of the angle measuring device. The service life of the entire motor depends on the service life of the bearing. The motor housing adopts the protection level of IP54, which can be increased to IP65 in special environments, which can meet the requirements of most dust and humid environments. The minimum service life of the motor bearing is more than 20,000 hours under the condition that the coaxiality of the motor shaft extension is good and the radial load of the rotating shaft is appropriate. The second is the life of the cooling fan. The fan uses a single-phase 220V shaded pole motor, which has a longer service life than the capacitor running motor. When running for a long time in a dusty and humid environment, it is necessary to regularly remove the viscous substances attached to the fan to prevent the fan from being burnt due to excessive load.

3.2 Failure and protection of permanent magnet materials

The development of permanent magnet motors is closely related to the development of permanent magnet materials. Since the beginning of the 21st century, with the continuous improvement and improvement of the performance of permanent magnet materials, high remanence density, high coercivity, high magnetic energy product and linear demagnetization curve The excellent magnetic materials are especially suitable for the manufacture of motors, especially the improvement of thermal stability and corrosion resistance of NdFeB permanent magnets and the gradual reduction of prices, which makes the research and use of permanent magnet motors have achieved rapid development.

The importance of permanent magnet materials to permanent magnet motors is self-evident, and its cost accounts for nearly 1/4 of the material cost of the entire motor. In this regard, our company has also specially formulated enterprise standards for permanent magnet materials, which focus on corrosion resistance, consistency, high temperature demagnetization test, linear demagnetization curve test and so on. The current permanent magnet materials can run for a long time under the maximum allowable temperature rise of the motor winding, and the demagnetization rate is within 2%. Conventional permanent magnet materials require the surface coating to withstand more than 24 hours of salt spray test. For environments with severe oxidation and corrosion, users need to contact the manufacturer to select permanent magnet materials with higher protection technology.

3.3 Coil burnout

The traditional asynchronous motor coil burnout failure rate is as high as 8%, which is mainly related to the thermal protection of the motor and the quality of electrical materials. The traditional asynchronous motor has almost no thermal protection components, and the manufacturers who pursue the output are also greatly discounted in the motor insulation treatment. After the maintenance, the insulation treatment of the motor is even weaker. The company's CM500 series permanent magnet synchronous motor coils are embedded with thermal elements. When the temperature rise of the coil reaches the F-class insulation limit temperature rise of 105K, the driver implements overheating protection for the motor through the temperature signal collected by the thermal element to ensure the safe operation of the motor. At the same time, the stator of the motor adopts two vacuum dipping process, which greatly improves the qualified rate of the stator.

4 Conclusion

From the analysis of economic benefits: permanent magnet synchronous motor is especially suitable for the occasion of heavy start and light operation. The promotion and use of permanent magnet synchronous motor has positive economic and social benefits, which is of great significance to energy saving and emission reduction. Permanent magnet synchronous motors also have valuable advantages in terms of reliability and stability. Therefore, the selection of a high-efficiency permanent magnet synchronous motor is a one-time investment and long-term benefit process.