Changes of permanent magnet variable frequency air compressor
In the past two years, permanent magnet variable frequency air compressors have become synonymous with pioneers in energy-saving work in the air compressor industry. Some air compressor manufacturers even use "air compressors without money" as a slogan to attract users' attention.
Driven by the national energy conservation and emission reduction policy, all industries are thinking about how to save energy. What changes have been made to the electric air compressors produced in the factory? In recent years, let's discuss it together. More and more people in the industry advocate the use of permanent magnet variable frequency air compressors.
At present, the country's evaluation of the energy efficiency of air compressors is mainly based on the specific power under full pressure and full load. High-efficiency motors and air compressors are reasonably arranged and designed with high-precision screw heads to achieve first-class energy efficiency. As a result of the power ratio, it is difficult for many screw compressors with frequency conversion control to achieve energy efficiency level 1 (even if it is a complete machine, no company can guarantee it).
In terms of energy-saving for permanent magnet motors, the same control method has a higher energy-saving effect than traditional asynchronous motors. The key to saving energy is that 80% of the air compressors are adjustable loads. According to the amount of gas, frequency conversion speed regulation is adopted, which can automatically meet the needs of users and reduce unnecessary losses. However, there are still 20% of cases where full voltage and load are used. Since the inverter itself is an IGBT, the frequency is controlled by switching, so in this case, the use of variable frequency to control the asynchronous motor consumes more energy than the direct start. Must lose 3%. Therefore, in this case, even if a permanent magnet motor is used, the energy saving effect is not obvious.
Therefore, energy-saving evaluation should be carried out according to the operating conditions of the variable-frequency air compressor, and the evaluation basis is not perfect. At the very least, the concept of variable working conditions is relatively simple. Regarding the energy efficiency level of qualitative frequency conversion (permanent magnet) air compressors, few companies have formulated the "Technical Conditions of (Variable) Variable Frequency Air Compressors" standard. Currently, there are two types of permanent magnet air compressors on the market, one is the more popular integrated type, and the other is the split type direct connection type.
The structural principle of the screw air compressor and the motor is: extend the shaft of the outer rotor ③ of the screw head out of the head, so that it is coaxially connected with the rotor ② of the motor. Then fix the housing of the stator and the motor to the housing of the screw head to form the whole of the motor and the head. The fan blades on the stator are cooled through the motor windings and the ventilation slots of the housing. This type of structure is the innovation and development direction of many electromechanical products, such as air-conditioning compressors, elevator traction, electric hoists, etc., and are deeply welcomed and loved by users. The screw-type all-in-one of the air compressor began to occupy the market due to its small size, simple structure, high system efficiency, and low price. But from the perspective of automotive experts, this design has many flaws and potential risks.
The main problem is: the motor temperature is too high: because the machine head is oil-cooled, the operating temperature must be between 65 degrees and 80 degrees, so the temperature of the motor's direct shaft and the machine head will be transferred to the permanent magnet rotor. The motor, and the body temperature is transmitted to the motor housing, so that the actual working environment temperature of the motor reaches about 65 degrees. Generally speaking, the reasonable temperature rise of the motor is 80K, so the winding temperature of the motor can be 145 degrees. The demagnetization (developed reversible demagnetization) test of 38UH electromagnetic steel shows that the demagnetization rate at 80 degrees is 2%. 100% 5; 120%; 150%; 12%. That is to say, as the temperature increases, the magnetic flux decreases. The magnetic flux of the electromagnetic steel is related to the efficiency of the motor, that is, the higher the temperature of the motor, the lower the efficiency, and the lower the efficiency of the motor. Back EMF and reduce the voltage given by the inverter. As the current of the motor increases, the temperature of the winding increases, the copper loss of the stator increases, and the resistance increases. When the total pressure is large, the actual efficiency of the integrated machine will drop again, and it will not meet the design requirements.
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