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Application of high voltage frequency converter in air compressor

Jan 22, 2021

Application of high voltage frequency converter in air compressor

The waste of electric energy in the air supply control mode of the compressor loading and unloading


2 (1) The speed formula of AC asynchronous motor is:


n=60f(1-s)/p


Among them, n-motor speed f-operating frequency;


p-motor pole pair number s-slip rate;


2 (2) Problems existing in air compressor loading and unloading control methods


2.1 Energy consumption analysis


The loading and unloading control mode makes the pressure of the compressed gas change back and forth between Pmin and Pmax. Pmin is the lowest pressure value, that is, the lowest pressure that can guarantee the user's normal work. In general, the relationship between Pmax and Pmin can be expressed by the following formula:


Pmax=(1+δ)Pmin


δ is a percentage, and its value is roughly between 15% and 30%.


  In the air compressor under the control mode of loading and unloading air supply, the energy wasted is mainly in 2 parts:


(1) Power consumption during loading


   After the pressure reaches the minimum value, the original control method determines that the pressure will continue to rise until the maximum pressure value. During the pressurization process, more heat must be released to the outside world, which leads to power loss. On the other hand, before the gas above the maximum pressure enters the pneumatic components, its pressure needs to be reduced by a pressure reducing valve. This process is also an energy-consuming process. In addition, the air compressor itself automatically adjusts the intake valve by detecting the pressure, and part of the energy is consumed on the intake valve.


(2) Power consumption during unloading


   When the pressure reaches the maximum pressure, the air compressor uses the following methods to depressurize and unload: close the intake valve to make the motor idling, and at the same time vent the excess compressed air in the separation tank through the vent valve. This adjustment method will cause a lot of energy waste. According to our calculations, the energy consumption of the air compressor during unloading is about 10% to 25% of that when the air compressor is running at full load (this is still when the unloading time accounts for a small proportion). In other words, the air compressor is idle 20% of the time and is doing useless work. Obviously, there is a lot of room for energy saving in the air compressor motor under the automatic control mode of the intake valve and the loading and unloading air supply control.


2.2 Other shortcomings


  (1) The intake valve is adjusted mechanically, so that the air supply cannot be continuously adjusted. When the air consumption is constantly changing, the air supply pressure will inevitably produce large fluctuations. The accuracy of gas consumption cannot meet the process requirements. Coupled with frequent adjustment of the intake valve, it will accelerate the wear of the intake valve, increase the amount of maintenance and repair costs.


  (2) The vent valve is frequently opened and closed, and the durability of the vent valve cannot be guaranteed.


The design of constant pressure air supply control scheme


Motor model: Y450-2


Power factor: 0.87


Rated voltage: 10KV


Rated current: 35.1A


Rated power: 500KW


Rated speed: 2975rpm


Air compressor


Rated flow: 120 m3/min


Rated pressure: 0.3MPa


Control mode: PID constant pressure control


   In view of the many problems existing in the original air supply control method, a high-voltage frequency converter is used to control the above-mentioned air compressor with constant pressure. When using this scheme, we can take the pressure of the pipe network as the control object. The pressure transmitter converts the pressure P of the gas storage tank into an electric signal and sends it to the PID intelligent regulator, which is compared with the pressure set value P0, and based on the difference The value is calculated according to the established PID control mode, and the control signal is generated and sent to the frequency converter VVVF, and the working frequency and speed of the motor are controlled by the frequency converter, so that the actual pressure P is always close to the set pressure P0. At the same time, this scheme can increase the power frequency and frequency conversion switching function, and retain the original control and protection system. In addition, after adopting this scheme, the air compressor motor can be started by the frequency converter from stationary to rotating, realizing a soft start. Avoid the start-up shock current and the mechanical shock caused by the start-up to the air compressor.




In the above PID constant pressure control mode, we set the pressure setting value P0 to 0.25 Mpa according to the needs of the user site. When the user's production gas consumption increases and the pipe network pressure is lower than 0.25 Mpa, the inverter output frequency will increase , The motor speed increases, the air compression volume increases, and the pressure rises accordingly; when the production air volume decreases and the pipe network pressure is higher than 0.25 Mpa, the inverter output frequency decreases, the motor speed slows down, the air compression volume decreases, and the pressure Then it drops to keep the pressure around 0.25Mpa.


Transformation benefit


4.1 Power frequency operation parameter measurement


  Motor operating parameters: Voltage: 10KV, active power 385KW, annual operating time is about 7200 hours, electricity cost 0.8 yuan/kWh;


   Air compressor operating parameters: inlet valve opening is 40%, outlet valve opening is 100%, outlet pressure: 0.25MPa.



4.2 Frequency conversion operation parameter measurement


  Motor operating parameters: operating frequency 46HZ, active power 330KW, annual operating time about 7200 hours, electricity cost 0.8 yuan/kWh;


   Air compressor operating parameters: inlet valve opening is 80%, outlet valve opening is 100%, outlet pressure is 0.25 Mpa.



4.3 Economic benefits


Save electric power: 385-330=55 (kW)


Power saving rate: (385-330)÷385=14.28%


Annual energy saving: 55×7200÷10000=396,000 (ten thousand degrees)


Annual electricity savings: 39.6×0.8=31.68 (ten thousand yuan)



4.4 Additional economic benefits


1) Solve the problem of large pressure fluctuations and improve accuracy.


2) Solve the cost of valve wear and reduce maintenance.