1 Introduction
An air compressor is the main component of an air supply system. It converts the mechanical energy of a prime mover (usually an electric motor) into the pressure energy of gas, and is the device that generates compressed air. As a basic industrial equipment, air compressors are widely used in almost all industrial sectors, including metallurgy, machinery manufacturing, mining, power, textiles, petrochemicals, and light industry.
According to incomplete statistics, air compressors account for 15% of the electricity consumption of large industrial equipment (fans, water pumps, boilers, air compressors, etc.). In the face of the increasingly tight international energy situation, how to reduce the energy consumed by air compressor operation is of great practical significance in responding to the national energy conservation and emission reduction policy and improving the economic benefits of enterprises.
2. Problems with traditional air compressor loading and unloading air supply control methods
Traditional air compressor working diagram
High energy loss
Traditional air compressor loading and unloading control methods cause the compressed gas pressure to fluctuate between Pmin and Pmax. Pmin is the minimum pressure value that ensures normal operation for the user, while Pmax is the set maximum pressure value.
Generally, the relationship between Pmax and Pmin can be expressed by the following formula: Pmin=(1-δ)Pmax, where the value of δ is between 10-20%.
If variable frequency speed control technology is used to continuously adjust the gas supply, the pipeline pressure can always be maintained at the working pressure that meets the gas supply requirements, which is equal to the value of Pmin. Therefore, the energy wasted in the gas supply control method of adding and unloading mainly comes from the following parts:
a. The energy consumed when the compressed air pressure exceeds Pmin:
When the air pressure in the air tank reaches Pmin, the air supply control method for adding and unloading air needs to continue to increase the pressure until Pmax. This process requires power to supply energy to the air compressor, which is an energy loss.
b. Energy consumed by the pressure reducing valve to reduce pressure:
The rated air pressure of pneumatic components is around Pmin. If the gas pressure is higher than Pmin, it needs to be reduced to close to Pmin by a pressure reducing valve, which is also a form of energy loss.
c. Energy consumed due to improper adjustment methods during unloading:
Normally, when the pressure reaches Pmax, the air compressor reduces pressure and unloads by closing the intake valve so that the air compressor no longer needs to compress gas to do work. However, the air compressor motor still drives the screw to rotate. At this time, the air compressor does useless work, resulting in a serious waste of energy. It is estimated that the energy consumption of the air compressor during unloading is generally about 10-15% of its full-load operation.
Unstable pressure, low degree of automation
Traditional air compressors have a low degree of automation. The output pressure is regulated by adding and unloading valves, which are controlled by regulating valves. This results in slow regulation speed, large fluctuations, low accuracy, and unstable output pressure.
Large starting inrush current at power frequency
Although the main motor uses Y-Δ reduced-voltage starting, the starting current is still very large, which has a significant impact on the power grid, easily causing grid instability and threatening the safe operation of other electrical equipment. For self-generating power plants, a current surge several times the rated current may cause malfunctions in other equipment.
Large amount of equipment maintenance
Air compressors have a large starting current at industrial frequency, which can be as high as 5 to 8 times the rated current. The working mode determines that the loading and unloading valves will inevitably operate repeatedly, making the components prone to aging. High-speed operation at industrial frequency leads to large bearing wear and a large amount of equipment maintenance.
Harsh working conditions and high noise levels
Continuous high-speed operation at the power frequency, excessive pressure beyond the required working conditions, and repeated loading and unloading all directly lead to high noise levels during operation at the power frequency.
3. Air compressor modification principle
Variable frequency control principle
According to the formula for the speed of an asynchronous motor:
In the formula, f is the power supply frequency, S is the motor slip, and P is the number of pole pairs of the motor.
Once P and S are determined, the motor speed is directly proportional to the power supply frequency. Therefore, changing the power supply frequency can change the motor speed n, thereby achieving variable frequency speed control.
Variable frequency energy saving principle
Based on the operating characteristics of air compressors, it can be known that air compressors are basically constant torque loads. By using frequency conversion speed regulation to adjust the motor speed according to the amount of air supplied, the output power of the motor can be basically proportional to the speed (amount of air supplied).
When the gas consumption decreases, the exhaust port pressure increases, which is fed back to the frequency converter through a closed loop, causing the motor speed to decrease and reducing the shaft output power. When the gas consumption increases, the exhaust port pressure decreases, which is fed back to the frequency converter through a closed loop, causing the motor speed to increase and increasing the shaft output power.
Therefore, the variable frequency air compressor system, through pressure closed-loop control, can track changes in the air supply system load in real time, adjust the speed of the air compressor motor, and ensure constant exhaust pressure, allowing the compressor motor to operate in the most economical state. This avoids the frequent loading and unloading, and frequent starting and stopping of the air compressor in the original control method, significantly reducing the amount of electrical energy absorbed from the power grid.
4. Air compressor retrofit plan
The HD30 series vector control frequency converter offers two retrofit options. The first utilizes the built-in general-purpose PID control function, while the second uses the HD30-H, specifically developed by our company for air compressors. The difference between the two options is that the first option requires no optional accessories, while the second requires the HD30-EIO expansion card. Compared to the first option, the second option optimizes the constant pressure control function and adds a range pressure control function, enabling operation at different frequencies in different pressure ranges. The operating frequency can be freely set according to the customer's actual needs and on-site working conditions. The setpoint and feedback in the PID control are represented by more intuitive pressure values. Most importantly, it can achieve smooth automatic and manual switching between industrial and frequency conversion, with the peak inrush current at the moment of switching being only 2-2.5 times the rated current.
Control Method Description
The variable frequency speed control system uses the output pressure of compressed air from the air compressor as the control object. It consists of a frequency converter, pressure sensor, motor, air compressor, etc., forming a closed-loop constant pressure control system.
The working pressure value is determined by the PID control channel of the frequency converter. The field pressure is detected by the sensor and converted into a 0-10V voltage signal (or a current signal) and fed back to the frequency converter. The frequency converter compares and calculates through its built-in PID controller to adjust its output frequency, thereby adjusting the motor speed and thus the motor output power.
Therefore, by adjusting the motor speed, the output power of the motor shaft can be adjusted, which in turn adjusts the input power of the air compressor, so that the air volume produced by the air compressor matches the air volume used, thereby achieving the purpose of constant pressure air supply and energy saving.
The following is a brief explanation of the wiring diagrams, parameter settings, and debugging operation of the two schemes.
4.1 Option 1
Control wiring diagram
Debugging instructions
Before operation, F08.00 (rated power), F08.02 (rated current), and F08.04 (rated speed) need to be modified in sequence according to the actual situation.
Functional Parameter Setting Summary
Parameter number | Parameter name | Setting value | Functional Notes |
|---|---|---|---|
F00.01 | Speed control method selection | 0 | V/f control without PG |
F00.11 | Command settings for channel selection | 1 | Terminal operation command channel |
F03.01 | Acceleration time | 40.0 | S |
F03.02 | deceleration time | 40.0 | S |
F04.00 | Process PID control selection | 1 | Effective PID control |
F04.01 | Given channel selection | 0 | Given numbers |
F04.02 | Feedback Channel Selection | 0 | AI analog quantity feedback |
F04.03 | Given digital quantity setting | 5.00 | V |
F04.13 | PID controller upper limit | 50.00 | Hz |
F04.14 | PID controller lower limit | 20.00 | Hz |
F04.16 | Integral term adjustment selection | 1 | Continue scoring once the maximum number of points has been reached. |
F04.18 | PID output inversion selection | 0 | PID control cannot reverse |
F08.00 | Motor 1 Rated Power | Set according to the parameters on the motor nameplate. | kW |
F08.01 | Motor 1 Rated Voltage | V | |
F08.02 | Motor 1 rated current | A | |
F08.03 | Motor 1 rated frequency | Hz | |
F08.04 | Motor 1 Rated Speed | Rpm | |
F08.05 | Motor power factor | ||
F15.00 | DI1 terminal function selection | 2 | Forward function |
F16.02 | AI2 function selection for analog input | 5 | Process PID feedback |
4.2 Option Two
Control wiring diagram
Debugging instructions
Before operation, F08.00 (rated power), F08.02 (rated current), and F08.04 (rated speed) need to be modified in sequence according to the actual situation.
In air compressor mode, for the frequency converter to operate, both function 3 of DI terminal A02 (system operation command) and function 2 of DI terminal F15 (frequency converter forward operation) must be enabled simultaneously. Furthermore, both the frequency converter contactor feedback signal and the power frequency contactor feedback signal must be connected. Otherwise, a contactor malfunction will be reported during operation. It is recommended that the frequency converter and power frequency contactors be interlocked. If contactor feedback signals are not required, functions 8 (frequency converter contactor feedback signal) and 9 (power frequency contactor feedback signal) selected on DI terminal A02 can be disabled. At the factory, pins 2 and 3 of CN6 are shorted, meaning the default AI2 input signal is a current signal.
Functions 1 and 2 of the DI terminal in group A02 are variable frequency operation commands and power frequency operation commands, respectively, enabling smooth switching between power frequency and variable frequency. Function 6 is for receiving inverter fault signals. Once the inverter malfunctions, it will immediately switch to power frequency operation.
Since many parameters are already set to factory defaults, few parameters need to be set in actual applications, making operation simple and controllable.
Functional Parameter Setting Summary
Parameter number | Parameter name | Setting value | Functional Notes |
|---|---|---|---|
F00.04 | Universal expansion card selection | 1 (Factory default) | HD30-EIO Expansion Card |
F00.05 | Extended application functionality | 2 (Factory default) | Air compressor extended functions |
F00.06 | Maximum output frequency of frequency converter | 51.00Hz (factory default) | Maximum output frequency of frequency converter |
F00.08 | Maximum operating frequency | 51.00Hz (factory default) | Maximum operating frequency |
F00.11 | Command to set channel | 1 (Factory default) | Terminal to run command |
F15.00 | DI1 terminal function selection | 2 | Inverter forward rotation command |
F23.00 | Carrier frequency setting | 6k (factory default) | carrier frequency |
A00.00 | Pressure setpoint channel selection | 0 (factory default) | Pressure given channel |
A00.01 | Pressure setpoint | 800 (to be set according to actual working conditions) | Pressure setpoint |
A00.09 | Pressure sensor range selection | 1 (This setting needs to be based on the actual range of the pressure sensor) | Pressure sensor range |
A00.10 | Industrial frequency conversion switching mode selection | 0 (Set according to actual situation) | Manual mode |
A00.13 | Delay time of power frequency contactor closing when switching from frequency converter to power frequency | 10ms | The setting needs to be adjusted according to the contactor's mechanical delay, as this value will affect the switching performance when switching from frequency converter to power frequency. |
A00.14 | Frequency converter closing delay time when switching from power frequency to frequency converter | 10ms | It needs to be set appropriately according to the contactor's mechanical delay. |
A00.17 | Pressure control mode selection | 0 (factory default) | Constant pressure control |
A02.03 | DI4 Function Selection | 3 (Factory default) | System operation |
A02.07 | DI8 terminal function | 8 (Factory default) | Variable frequency contactor feedback signal |
A02.08 | DI9 terminal function | 9 (Factory default) | Power frequency contactor feedback signal |
A02.10 | AI2 input signal type | 1 (Factory default) | 4-20mA current signal (Pin 2 and 3 of CN6 need to be shorted) |
A02.17 | DO1 Function Selection | 1 (Factory default) | Output indicator signal during frequency conversion operation |
A02.18 | DO2 Function Selection | 2 (Factory default) | Output indicator signal when operating at power frequency |
A02.19 | RLY1 Function Selection | 3 (Factory default) | Variable frequency drive control signal |
A02.20 | RLY2 Function Selection | 4 (Factory default) | Power frequency operation control signal |
4.3 Detailed Description of Control Flow
When gas consumption decreases, the gas output from the outlet valve decreases, the pressure in the gas tank increases, the voltage signal value of the pressure sensor increases, and the feedback signal sent to the frequency converter increases. When the PID feedback pressure is greater than or equal to the PID setpoint pressure, the frequency converter will decelerate through the PID adjustment function, the motor speed will decrease, and the air intake valve will reduce the air intake.
When the lower limit frequency of the PID controller is reached, the system will operate at a constant speed at that lower limit frequency.
When the gas consumption increases, the gas output from the outlet valve increases, the pressure in the gas tank decreases, the voltage signal value of the pressure sensor decreases, and the feedback sent to the frequency converter decreases. When the PID feedback pressure is less than the PID set pressure, the frequency converter will accelerate, the motor speed will increase, and the air intake valve will increase. When the upper limit frequency of the PID regulation is reached, it will run at a constant speed at the upper limit frequency.
5. Advantages of air compressor modification
Reduced operating costs
Electricity costs account for approximately 72% of the compressor's operating costs. Inverters reduce electricity consumption by about 18%, and the reduced impact on equipment during variable frequency startup also lowers maintenance and repair requirements, resulting in significantly lower operating costs and energy savings.
Improve pressure control accuracy
Because the variable frequency control system has a constant pressure air supply control mode, the air tank can receive a relatively stable air supply pressure, which is beneficial to the safe operation of air-using equipment. It also reduces energy consumption caused by unnecessary pressure increases. The output air volume of the variable frequency air compressor changes with the motor speed. Due to the improved accuracy of the variable frequency motor speed control, it can keep the system pressure variation in the pipeline network within 0.2 MPa, effectively improving the quality of operation.
Extend the service life of mechanical equipment such as motors and air compressors
A frequency converter starts the air compressor from 0Hz, and its start-up acceleration time can be adjusted, thereby reducing the impact on the electrical and mechanical components of the air compressor during startup, enhancing system reliability, protecting the motor and extending its service life, and effectively increasing the installed capacity of the power grid. It also extends the service life of machinery and molds. Furthermore, frequency converter control can reduce current fluctuations during unit startup, which can affect the power grid and other equipment. The frequency converter can effectively reduce the peak starting current to a minimum.
Reduced air compressor noise
Based on the operating requirements of the air compressor, the motor speed was significantly reduced after the frequency converter speed control modification, thus effectively reducing the noise during air compressor operation. On-site measurements showed that the noise level decreased by approximately 5-10 dB compared to the original system, the equipment operated more stably, and the environment was improved.
Stable output pressure
By adopting a frequency conversion control system, the pressure of gas in the gas supply pipeline can be monitored in real time, keeping the pressure of gas in the gas supply pipeline constant, thereby improving production efficiency and product quality.
Easy to operate and has comprehensive protection functions
No adjustments to the energy-saving equipment are required during production. The entire process is automatically tracked and controlled, and it has multiple protection functions (overcurrent, overvoltage, undervoltage, motor overload, motor overheating, etc.). It is simple, safe, and convenient.
Low equipment maintenance
The variable frequency air compressor has a low starting current, less than twice the rated current. The loading and unloading valves do not need to operate repeatedly. The variable frequency air compressor automatically adjusts the motor speed according to the air consumption. The low operating frequency and slow speed result in less bearing wear, extended equipment service life, and reduced maintenance workload.
6. Precautions
Install
During installation, the wiring between the control cabinet and the compressor must not exceed 30m and must be kept a certain distance from the control lines, running them separately. Shielded twisted-pair cable should be used for the control circuit wiring, with a connection distance of no more than 20m. Because the frequency converter generates a lot of heat, a cooling fan must be installed inside the control cabinet, and the frequency converter's grounding terminal must not be mixed with the power grounding terminal.
debug
After completing the function settings and no-load operation of the frequency converter, system linkage debugging can be carried out.
Open loop
At this stage, the main observations are the frequency rise of the inverter, i.e., whether the equipment operation sound is normal, whether the air compressor pressure rise is stable, whether the pressure sensor display is normal, and whether the equipment shutdown is normal. If everything is normal, closed-loop debugging can be performed.
closed loop
The main principle is to match the speed of frequency rise and fall of the inverter with the pressure rise and fall of the air compressor to avoid pressure oscillation. Additionally, it's crucial to observe the mechanical resonance point and ensure that frequencies near the resonance point are skipped (this can be achieved by correctly setting the relevant parameters for skip frequencies in F05.17-F05.20). Air compressors are high-inertia loads, and this starting characteristic easily triggers overcurrent protection on the inverter during startup. Using a frequency converter with high starting torque and no speed vector control ensures both continuous constant-pressure air supply and reliable, stable equipment operation. Air compressors should not operate at low frequencies for extended periods. Excessively low compressor speeds reduce stability and lower lubricating oil pressure, leading to poor cylinder lubrication and accelerated wear. Therefore, the lower operating frequency should not be lower than 20Hz.
To effectively filter out high-order harmonic components in the inverter output current and reduce electromagnetic interference caused by high-order harmonics, it is recommended to use an output AC reactor. This can also reduce motor operating noise and temperature rise, and improve motor operating stability.
It is recommended to select a frequency converter with a power rating one level higher than that of the air compressor to avoid frequent tripping during air compressor startup.
7 Conclusion
By adopting the above renovation plan, a relatively ideal energy-saving effect can be achieved, thereby creating considerable economic benefits for customers.
To develop the most reasonable and feasible renovation plan, the historical data of the equipment's power frequency operation should be carefully analyzed, the energy-saving potential should be calculated, and the circuit principle of the equipment should be carefully analyzed during the renovation process.
References
User Manual for Shenzhen Haipumont Technology Co., Ltd.'s HD30 Series Vector Control Frequency Inverter (V1.0)
