Application of CHE100 in Frequency Conversion Alteration of Air Compressor System Abstract: This paper describes application of CHE100 in frequency conversion alteration of the air compressor system and introduces in detail the principle of the special controller for air compressor and its interaction with the frequency inverter and application. Keywords: frequency conversion, Modbus, air compressor 1. Introduction Air compressor is a kind of equipment that uses a motor to compresses air in the compressing chamber and brings the compressed air to a certain pressure. As a basic industrial installation, the air compressor has wide applications in almost all industrial fields such as metallurgy, machine manufacturing, mine, electric power, textile, petrochemical, light textile, etc. The electric energy consumption of air compressors accounts for 15% of all large scale industrial equipment (fan, pump, boiler, air compressor, etc.). Most air compressors have obvious inherent technical weaknesses due to their structure and principle. When the output pressure exceeds a certain value, it automatically opens the relief valve and idles the asynchronous motor. This seriously wastes energy. Moreover, the asynchronous motor tends to start and stop frequently. This affects the service life of the motor. The compressor has high starting current in power frequency mode. This has a high impact on the power grid and causes serious wear of the motor shaft and high maintenance of equipment. The working conditions are harsh and the noise level is high. The automation level is low. The adjustment of output pressure is realized by manually regulating the valve opening. This has the disadvantages of low regulation speed, large fluctuation, low stability and low accuracy. 2. Operating principle 2.1 Original working manner Operating principle of screw compressor Two parallel mated female and male rotors (or screws) turn in the cylinder and cause constant, periodic volume variation of the air filling between the teeth and grooves of the rotors. The air is sent from the suction side to the output side along the axial of the rotors. The whole process of suction, compression and exhaust of the screw compressor is realized in this way. The inlet and outlet of the air compressor are located on two ends of the housing respectively. The grooves of the female rotor and the teeth of the male rotor rotate under the driving of the main motor. Operating principle of piston compressor The motor drives the pulley and directly drives the crank via the coupling. The crank drives the collecting rod and piston rod and causes reciprocation of the piston in the cylinder of the compressor. In this way the piston compressor completes its suction, compression and exhaust and other processes and boosts the pressure of non- or low-pressure gas and outputs it to the accumulator. The piston assembly, inner walls of the piston and cylinder and cylinder head form a work chamber of variable volume. Driven by the crank-link mechanism, the piston makes reciprocating in the cylinder to realize compression of the gas in the cylinder.
The main motor of the air compressor operates in full voltage mode after star-delta reduced voltage starting. The specific work flow of the air supply system: when the Start button is pressed, the control system switches on the starting coil and opens the oil cut-off valve. The compressor starts in unloading mode. At this moment, the suction valve is in closed position and the bleed valve is open to release pressure in the oil/air separator. After releasing pressure for n seconds (the time is controlled by the relay), the compressor starts loading operation and the system pressure begins to rise. If the system pressure rises to the upper limit of the pressure switch taking off pressure, the controller makes the suction valve to close, the oil/gas separator bleeds air, and the compressor operates at no load till the system pressure drops to the lower limit of the pressure switch online pressure. Then, the controller opens the suction valve, closes the bleed valve of the oil/gas separator, and the compressor to operates at full load. Unloading and loading tends to cause frequent variation of the pressure of the entire air network. In this way it is impossible to maintain constant working pressure and prolong the service life of the compressor. Even in the case that low flowrate is needed, with some adjustment methods of air compressor (for example, adjusting with valve, adjusting by unloading, etc.), as the rotational speed of the motor is constant, the reduction of the motor power is relatively low. Energy consumption analysis: Control by loading and unloading makes the pressure of the compressed air varies up and down between P min and P max. P min is the lowest pressure, i.e. the lowest pressure that can ensure normal work of the users. In general cases, the relationship between P min and P max can be expressed with the formula below: P max =(1+δ)P min δ is a percentage and its value is roughly between 15% and 30%. The energy wasted by the compressor in air supply control mode by loading and unloading consists of mainly 2 parts: (1) Electric energy consumed during loading When the pressure reaches the minimum, the original control method decides that the pressure will rise till the maximum. During pressurization, it will certainly dissipate more heat to the outside and thus cause loss of electric energy. On the other hand, the gas of pressure higher than the maximum needs to release pressure through the relief valve before entering the pneumatic elements. This process is also a energy consuming process. (2) Electric energy consumed during unloading When the pressure reaches the maximum, the compressor reduces pressure and unloads in this way: closes the suction valve and allows the motor to operate at no load, meanwhile, vents the redundant compressed air in the separating tank to the atmosphere through the atmos valve. This regulation method results in a large amount of energy waste. By our estimation, the energy consumed by the compressor during unloading accounts for 10% to 25% of that during operating at full load (this is just the case when the unloading time accounts for a low percentage). In other words, the compressor is at no-load state in 20% time and is doing work of no use. It is obvious that in the air supply mode by loading and unloading, there is large energy-saving space for the motor of the compressor. Other shortcomings
(1) The suction valve is adjusted with a mechanical method and the air supply volume cannot be adjusted continuously. When the air consumption varies constantly, it is not avoidable that the air supply pressure fluctuates greatly. The accuracy of the supplied air cannot meet the process requirement. Moreover, frequent adjustment of the suction vale can quicken its wearing and increase maintenance works and cost. (2) The bleed valve is opened and closed frequently. The durability of the bleed valve cannot be ensured. 2.2 Frequency conversion operation mode Rotational speed control refers to adjust flowrate by varying the rotational speed of the compressor while keeping the opening of the valve unchanged (keeping at maximum opening). When the rotation speed of the compressor varies, the lift characteristic of the air supply system varies with it while the pipe resistance characteristics keeps unchanged. In this control mode, the rotational speed of the compressor motor is changed with frequency conversion speed regulation technology. The air supply flowrate of the compressor changes with the air consumption flowrate and the real balance between supply and demand is reached in this way. It can enable the whole system to get optimal work efficiency while saving energy. The frequency inverter is based on the principle of AC-DC-AC power conversion. It can output an AC voltage with constantly adjustable frequency according to the demands of the controlled object. The rotational speed of the motor is proportional to the frequency of the power supply. So, by outputting an AC voltage with adjustable frequency as the power voltage of the compressor motor, the frequency inverter can conveniently change the rotational speed of the compressor. 2.3 Constant pressure air supply principle Flowrate is the basic controlled object of the air supply system. Air supply flowrate shall meet the air consumption flowrate at all times. In the air supply system, the air pressure in the reservoir pipe can fully reflect the relation between the air supply capacity and air consumption demand: If the air supply flowrate is higher than the air consumption flowrate, the air pressure in the reservoir pipe will rise. If the air supply flowrate is lower than the air consumption flowrate, the air pressure in the reservoir pipe will drop. If the air supply flowrate is equal to the air consumption flowrate, the air pressure in the reservoir pipe will not change. So, keeping constant pressure in the pipe can ensure the air supply capacity at the point just meets the air consumption demand. This is the aim of the constant pressure air supply system. The frequency conversion speed regulation system takes the pipeline pressure as the controlled object. The pressure transducer fitted at the outlet of the reservoir pipe converts the pressure in the accumulator into electrical signal and sends it to the PID regulator inside the controller. It compares the signal with the pressure setting, conducts operation with preset PID control mode according to the difference, and generates a control signal to control the output voltage and inversion frequency of the frequency inverter, and thus regulates the rotational speed of the motor so that the actual pressure is always maintained at the pressure setting. Additionally, after this solution is adopted, the frequency inverter can realize soft starting from stationary to stable speed of the compressor motor. This can avoid high starting current and mechanical shock to the air compressor caused by starting. In normal cases, the air compressor operates at the frequency inverter speed control mode. Once the frequency inverter becomes faulty, production process does not allow stopping of the air compressor. So the system is designed with switching function between power frequency and variable frequency. In this way, when the frequency inverter becomes faulty, the air compressor will still operate normally with the power supply of power frequency directly supplied via the contactor. The whole control process is as the follows:
Air consumption demand pipeline pressure difference between pressure setting and feedback value PID output frequency inverter output frequency rotational speed of compressor motor air supply flowrate pipeline pressure tends to be stable. Feedback value Pressure tolerance Setting Fig 2: Pressure and frequency PID curves Special attention should be paid that, within the pressure tolerance range, the PID of the controller is not adjusted, i.e. the output frequency is kept unchanged. 3. Solution configuration Hereunder we take the application of CHE100-075G/090P-4 in alteration of the compressor of a company in Shanghai as an example. According to the experience in other previous projects, attention should be paid to the following during frequency conversion alteration of air compressors: 1) Air compressor is a load of high moment of inertia. The starting feature is very likely to cause over-current protection of the frequency inverter in V/F control mode during starting. So the vector frequency inverter with high starting torque and without speed sensor (CHE100 series) is recommended to ensure continuity of constant pressure air supply as well as reliable, stable operation of the equipment. 2) Air compressor is not allowed to operate at low frequency for long period. When the rotational speed of an air compressor is too low, on one hand, the operation stability of the compressor will become poor, and on the other hand, the cylinder lubrication will also become poor and this will quicken the wearing. Therefore, the lower limit of the operation frequency should not be lower than 20Hz. 3) In order to effectively filter the high-order harmonic components in the output current of the frequency inverter and reduce the electromagnetic interference caused by the high-order harmonics, output AC reactor is recommended. It can also reduce operation noise and temperature rise of the motor and improve the motor stability. The power of the compressor motor in this air compressor system is 55kW. The power frequency/variable frequency compatible special controller drive solution is designed for the system according to the operating conditions on the field. CHE100 series vector frequency inverter of high starting torque is selected. The specific model is CHE100-075G/090P-4. The fan is driven by a CHF100 general frequency inverter. This solution is suitable for applications with high requirements for accuracy and reliability of the control system. Power frequency/variable frequency compatible double redundant circuits are adopted for the
main motor and the fan of the main motor. They operate at variable frequency in normal operating mode and are switched to power frequency when being overhauled or when the frequency conversion circuit becomes faulty. The control mode also changes at the same time.
The special controller has many kinds of built-in control logics. It can drive different combinations of switch state. The user can select switch combinations according to the demand to drive different control modes. Additionally, the control logics have been pre-configured and can be changed with one keying. (1) Variable frequency driving of the main motor: the PID operation regulation part of the controller samples the feedback signal from the pressure transducer, performs PID operation with the pressure setting, and outputs the regulation signal to regulate the output frequency of the main frequency inverter. After being switched to power frequency mode, the controller will adjust the discharge pressure by opening or closing the suction valve. (2) Driving of the fan is also selectable between variable frequency and power frequency If the fan adopts variable frequency control mode, the controller samples the feedback signal of the head temperature sensor, performs PID operation with the temperature setting, and outputs the regulation signal to regulate the output frequency of the fan frequency inverter and thus to continuously adjust the fan speed in stepless mode. In this way the temperature of the compressor head is accurately maintained at the set temperature point so that the lubricant of the compressor is kept in optimal effect. If the fan is set in switch control mode, the controller determines the corresponding switch state according to the sampled exhaust temperature to start or stop the fan motor and thus maintains the exhaust temperature within the set temperature range.
Main motor of the compressor Fan Y- starting 1. KM5 ON, Y starting 2. KM4 ON, starting 3. KM4 and KM5 shall be interlocked 4. When the main motor is set to direct starting at power frequency, KM4, KM5 and related circuits can be eliminated. 1. The main motor of the compressor can be switched between power frequency and variable frequency: (1) Variable frequency: KM1 & KM3 ON, KM2 OFF and KM5 ON (2) Power frequency: KM2 ON, KM5 ON, Y starting KM2 ON, KM4 ON, starting The sequence of power frequency starting: Y starting, and after certain delay, starting, starting completed, after a delay, open the suction valve, the compressor starts operating. (3) KM2 and KM3 are interlocked. KM4 and KM5 are interlocked. 2. The compressor fan can be switched between power frequency and variable frequency: (1) Variable frequency: KM6 & KM7 ON, KM8 OFF, operates at variable frequency (2) Power frequency: KM8 ON, KM6 & KM7 OFF, operates at power frequency 3. The controller interacts with the two frequency inverters directly with communication. The state of all contactors is determined by the special controller. 4. General description of the special controller The special compressor controller is specifically developed for compressor application. It has many flexible control modes. It can monitor and control many kinds of compressor operation data in real time, provide protection and early-warning functions as well as maintenance reminding functions. The terminal functions are defined as follows: Symbols Definitions Remarks PT1+,PT1- Input of the main motor temperature sensor Detect main motor temperature in real time and use it as the PID regulation signal of the fan and provide feedback data for main motor temperature protection
I1+ I2+ Symbols Definitions Remarks V3+,V34- V4+,V34- Input of R-phase current sensor of the main motor Input of T-phase current sensor of the main motor Input of ST-phase voltage sensor Input of pressure transducer I1 shall be shorted with V1+, a phase current transformer is fitted on the input side. I2 shall be shorted with V2+, a phase current transformer is fitted on the input side. A voltage transformer is fitted on the input side. Pressure PID regulation input of the main motor P24V 24V auxiliary power supply X0 Input of the pressure switch X1 Air filter signal input X2 Oil filter signal input X3 Aerosol separator signal input X4 E-stop signal NC COM Common terminal Y0 Y1 Y2 Y3 Y4 Y5 Y6 KM1 KM2 KM3 KM4 KM5 KM6 KM7 COM0 Common terminal 0 Common terminals for Y0~Y1 COM1 Common terminal 1 Common terminals for Y2~Y4 COM2 Common terminal 2 Common terminals for Y5~Y7 RS485 (+ \-) 485 wiring terminal RS485 terminal for communication, supports Modbus communication protocol The compressor controller is designed according to the control process requirement of the rotary screw compressor with LCD display and intuitive, easy UI in both Chinese and English to realize various functions needed by the rotary screw compressor:
AC power voltage and phase sequence detection function to avoid reversed rotation of the motor, can be set with under-voltage protection function; PID constant pressure control, the output pressure is stable; With many built-in combination control solutions of variable frequency driving and power frequency driving, the user can set according to field demand if frequency conversion or switching between power frequency and variable frequency is used. The driving mode can be switched with the external setting switch. In this way the operation reliability of the air compressor system is improved. Early-warning and stop protection functions at over-temperature. Over (air) pressure protection ensures safety of air pipeline equipment of the user. Light load starting, smart control of stop after heat dissipation, immediately switch off all control outputs at emergency stop; Temperature and pressure sensors failure detection, immediately protection and warning. Can detect and display the operation current of the main motor and the fan, and can separately set motor overload and phase broken protection; Can detect many types of blockage signals and give early warning. Trouble conditions State display Compressor state Air filter is blocked Air filter is blocked Alarm but not stop Using time of the air filter is out Remained using time of the air filter Alarm but not stop Oil filter is blocked Oil filter is blocked Alarm but not stop Using time of the oil filter is out Remained using time of the oil filter Alarm but not stop Aerosol separator is blocked Aerosol separator is blocked Alarm but not stop Using time of the aerosol separator is out Remained using time of the aerosol separator Alarm but not stop Using time of the lubrication grease is out Motor bearing needs grease filling Alarm but not stop Exhaust temperature is higher than the Alarm due to high temperature Alarm but not stop alarm value exhaust Exhaust temperature is higher than the Stop due to high temperature Alarm and stop stop value exhaust The exhaust temperature is lower than Too low temperature Alarm and stop the limit Exhaust pressure is higher than the limit Stop due to too high pressure Alarm and stop Broken wire of temperature sensor Broken wire of temperature sensor Alarm and stop Broken wire of pressure transducer Broken wire of pressure transducer Alarm and stop Overload of motor The main motor has stopped due to overload Alarm and stop Will delay when starting Overload of fan The fan has stopped due to Alarm and stop overload Wrong phase sequence, lack of phase Stopped due to wrong sequence Alarm and stop Two kinds of interlock control modes are provided to control sequential starting and unloading of the unit. The alternation time is adjustable. The display interface has a simple structure and is easy to understand. The menu is displayed in several levels. The operation is simple and needs no special understanding or remembering; By providing background software for background monitoring, remote monitoring from upper PCs can be realized; Automatic sleep function in the condition of low air consumption can save energy and reduce equipment wearing;
Built-in Chinese/English bilingual display, can be switched online; Visual view of operation parameters and states of the air compressor; Real-time perpetual calendar; Can remind various maintenances 100 hours before the planned time according to various maintenance plans; Failure logging can be inquired and is tagged with time mark; Seamless connection with CHE frequency inverter is provided. There is no need to set the function parameters of the frequency inverter; Protected with several levels of passwords, parameters are set on different levels of screens to avoid unauthorized operation, and a spare universal password is provided. Conclusion After the frequency conversion alteration, the air compressor system operates stably and the overall energy-savings rate of the system is 31.8%. In a word, as a high, new technology, basic technology and energy-saving technology, frequency conversion speed regulation technology has been used in various technical departments of all industries. Various problems may occur during application in the air compressor industry, which are waiting for us to solve. We can promote the application of frequency conversion speed regulation technology in air compressor equipment only by fully considering the actual operating conditions and demands and continuously optimizing the solutions. References (1) Shenzhen INVT Electric Co., Ltd. Second Volume of INVT Application Cases. [M], 2009 (2) Shenzhen INVT Electric Co., Ltd. Operation Manual of CHE100 Vector Frequency Inverter. [M], 2009