The Application Of PLC And Inverter In Energy-saving Modification Of Refrigerating Unit

Refrigeration unit is the heart of the refrigeration system. To reduce the energy consumption and improve the level of automatic control of refrigeration unit is the direction and the theme of refrigeration industry product.At present, the technologies such as frequency control of motor speed technology, computer control technology, power electronic technology and related automation technology have been used in the refrigeration industry widely. It has made the refrigeration system get great progress in the following two aspects:(1) Improve the energy efficiency of refrigeration system. According to the temperature of the controlled environment, refrigerating capacity can be supplied on demand. It can help us save electric energy.(2) Improve the control level of the refrigeration system. The control system makes the refrigeration unit computerized, digitalization and visualization gradually.Introduced the frequency conversion technology to the refrigeration unit regulating the refrigerating capacity of the refrigeration system can meet the requirement of accurate temperature control, and improve the efficiency of the refrigeration system. At the same time we use the PLC (an industrial controller) to control and monitor the whole refrigeration system.Through analyzing several methods we are using in the refrigeration unit control system now, we found it necessary to realize frequency conversion operation and automatic control in the refrigeration unit. With the aid of refrigeration theory, Pressure enthalpy diagram, we analyzed operating parameters of the various components of refrigeration system, and concluded that to realize the energy saving operation refrigeration unit must be carried out on the refrigerant flow control.In terms of the unit control, the characteristics of refrigerant thermodynamic properties determine the compressor’s suction pressure for the control object of the refrigeration unit. The suction pressure signals are send to PLC industrial controller through the pressure sensor. The pressure signals are turned into4-20mA output signal to frequency converter after comparing with the setting parameters. In order to achieve the purpose of the refrigerant flow on-demand, the speed of the motor of compressor is controlled by a frequency converter. Through T chart programming, PLC controller collected suction pressure signal and exhaust pressure, suction temperature and exhaust temperature signal. Through the other protection signal the PLC controller collected, operation condition of the refrigeration unit can be real-time monitoring. When refrigeration unit is in an abnormal condition the warning signals would be triggered at the same time and record the parameters of operation condition of the unit. To make the refrigeration unit has better handling character, we set a touch screen as the upper machine, the unit operation parameters can be set by the touch screen. It can also display alarm screen and record unit operation curve.Through the design of semi-closed compressor control system, installation, operation test, we got a conclusion that the refrigerating capacity of the refrigeration system can be adjusted by controlling rotational speed of compressor motor. Refrigeration unit provides the required refrigerating capacity under the condition of different operating load. It can improve the energy efficiency rate of the refrigeration unit under partial load condition.The whole design of the control system can improve the control level of the refrigeration system and optimize the control method of the system. It can improve the control precision of the refrigerating system and enlarge the application range of refrigeration units in various fields to satisfy the increasing variety of technological requirements.