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Comprehensive Benefit Analysis of Permanent Magnet Synchronous Motor Replacing Asynchronous Motor
Fecha:2019-12-30 Acceso:1240
In this issue, we start with the characteristics of the permanent magnet synchronous motor and combine the practical application to explain the comprehensive benefits of promoting the permanent magnet synchronous 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 used in the fields of petroleum, chemical industry, textile, mining, CNC machine tools, robots, etc., and is developing towards high power (high speed, high torque), high functionality and miniaturization.

The permanent magnet synchronous motor is composed of a stator and a rotor. The stator is the same as an asynchronous motor, and is composed of a three-phase winding and a stator core. The rotor is equipped with pre-magnetized (magnetized) permanent magnets, which can build a magnetic field in the surrounding space without additional energy, which simplifies the structure of the motor and saves energy. Based on the characteristics of permanent magnet synchronous motors, this paper describes the comprehensive benefits of popularizing permanent magnet synchronous motors in combination with practical applications.

Outstanding advantages of permanent magnet synchronous motors

(1) Because the rotor is made of permanent magnets, the magnetic flux density is high, no excitation current is needed, and the excitation loss is eliminated. Compared with asynchronous motors, the excitation current of the stator windings and the copper and iron losses on the rotor side are reduced, and the reactive current is greatly reduced. Because the stator and rotor magnetic potentials are synchronized, the rotor core has no fundamental iron loss, so the efficiency ( The relationship between active energy) and power factor (related with reactive energy) are higher than asynchronous motors. Permanent magnet synchronous motors are generally designed to have high power factors and efficiency even at light loads.


 

The figure above is the comparison curve of efficiency and power factor between a permanent magnet synchronous motor and an asynchronous motor of the same specification. From the perspective of the load change of the motor, the rare earth permanent magnet synchronous motor can maintain high power factor and high efficiency operation within the range of 25% to 120% of the rated power, while the general three-phase asynchronous motor is usually only rated at 60% to 100%. Power is maintained.

(2) Permanent magnet synchronous motors have relatively hard mechanical characteristics and have a strong ability to withstand motor torque disturbances caused by load changes. 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 times are much faster than the asynchronous motor. The high torque / inertia ratio makes the permanent magnet synchronous motor more suitable for fast response conditions than the asynchronous motor. Run.

 

(3) The size of the permanent magnet synchronous motor is greatly reduced compared to the asynchronous motor, and the weight is relatively reduced. Permanent magnet synchronous motors with the same heat dissipation conditions and insulation materials have more than twice the power density of three-phase asynchronous motors.

(4) The rotor structure is greatly simplified to facilitate maintenance and improve the stability of operation.

For AC permanent magnet servo motors, the magnetic steel on the rotor and the iron core are usually bonded together with high temperature glue. The glue curing time is generally long. During the motor's operation, the magnetic steel is repeatedly affected by temperature changes and impact forces. The risk of deformation or even shedding. In order to solve this technical problem, the CM500 series heavy-duty permanent magnet synchronous motor developed by Sifang changed the magnetic tile gluing process to a stainless steel sleeve mechanical fastening method, completely eliminating the risk of magnetic steel loosening. This innovative application technology has been applied for a patent, which is a major advancement in motor rotor technology.

         

The picture shows the rotor component structure of a permanent magnet DC motor

 

Due to the high power factor of the three-phase asynchronous motor, the air gap between the stator and the rotor must be made very small. At the same time, the uniformity of the air gap is also important for the safe operation and vibration noise of the motor. The requirements for the shape and position tolerances and assembly concentricity of components are relatively strict, and there is less freedom in the choice of bearing clearance. Asynchronous motors with larger bases usually use bearings with oil bath lubrication, which must be used in the specified work. Filling lubricating oil within time, leaking oil cavity or not filling in time will accelerate bearing failure. In the maintenance of three-phase asynchronous motors, bearing maintenance accounts for a large proportion. In addition, due to the presence 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.

Permanent magnet synchronous motors do not have such problems. 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 motors are not obvious on the synchronous motor. At the same time, the bearings of the permanent magnet synchronous motors are greased bearings with dust caps. Appropriate amount of high-quality grease has been sealed at the factory, which can be maintenance-free for life.

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 fixed displacement pump. During the operation of the injection molding machine, the motor needs to run for a long time. Through the energy-saving transformation of the servo oil pump drive system, the fast response of the permanent magnet synchronous motor and the large starting torque, the permanent magnet synchronous motor can be started when the injection molding machine needs to complete injection molding to drive the oil pump to generate pressure. During the process of blanking and mold clamping The main motor of the oil pump is in a shutdown state, achieving intermittent zero power consumption, and the energy saving rate can be up to 85% through comparison.

Under the control of the excellent servo motor algorithm, the noise is much lower than that of ordinary injection molding machines. In the ideal state of low-noise screw pumps, the noise of the injection molding machine is less than 70 decibels, which enables quiet operation and improves the working environment.

 

2.2 Energy saving application on iron ore dry separator

The use site is a dusty environment in a northwestern mine. This equipment uses a total of 125 motors. When the hoisting equipment is started, it needs a large starting torque. During normal operation, the motor runs at a light load of 30% to 50% of rated power for a long time. It is only about 65%, and the power factor is about 0.4 to 0.6, which causes serious waste of energy. The use of permanent magnet synchronous motors has a significant energy saving effect. The statistics of the power saving test are as follows:

 

 

User concerns

3.1 Motor life

The life of the entire motor depends on the service life of the bearing. The motor housing adopts IP54 protection level. Under special circumstances, it can be increased to IP65, which meets the requirements of most dust and humid environments. Under the condition that the coaxiality of the motor shaft extension is good and the radial load of the shaft is appropriate, the minimum service life of the motor bearing is more than 20,000 hours. The second is the life of the cooling fan. The fan uses a single-phase 220V shaded pole motor, which has a longer life than a capacitor-run motor. During long-term operation in a dusty and humid environment, it is necessary to periodically remove sticky substances attached to the fan to prevent the fan from being burned due to excessive load.

3.2 Failure and protection of permanent magnet materials

The importance of permanent magnet materials to permanent magnet motors is self-evident, and its cost accounts for more than 1/4 of the entire motor material costs. In this regard, Quartet formulated enterprise standards for permanent magnet materials, including corrosion resistance, consistency, high temperature demagnetization test, linear demagnetization curve test and other content. The current permanent magnet materials can run for a long time under the temperature allowance of the motor winding, and the demagnetization rate is within 2%. Conventional permanent magnetic materials require surface coatings to withstand salt spray tests for more than 24 hours. For environments with severe oxidation corrosion, users need to contact the manufacturer to select permanent magnetic materials with higher protection technology.

3.3 Coil burnout

The burn-out failure rate of traditional asynchronous motor coils is over 8%, which is mainly related to the thermal protection of the motor and the quality of electrical materials. The company's CM500 series permanent magnet synchronous motor has a thermal element embedded in the coil. When the temperature rise of the coil reaches the F-class insulation limit temperature rise of 105K, the driver implements overheat 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 processes, which greatly improves the qualification rate of the stator.



Conclusion

From the analysis of economic benefits: Permanent magnet synchronous motors are particularly suitable for heavy restart and light running applications. The promotion of permanent magnet synchronous motors has positive economic and social benefits and is of great significance for energy conservation and emission reduction. In terms of reliability and stability, permanent magnet synchronous motors also have valuable advantages. Selecting a high-efficiency permanent magnet synchronous motor is a long-term benefit process.