Application of Variable Frequency Drives in Constant Pressure Control of Air Compressors
1. Variable Frequency Speed Control System and Principle of Air Compressor
An electric motor drives the air compressor to rotate to generate compressed air, which is stored in the air receiver. The air pressure inside the air receiver depends on the balance between the compressed air output capacity of the air compressor (i.e., the rotating speed of the motor in this system) and the air consumption of users. To guarantee stable air supply quality, the air pressure inside the air receiver shall be maintained at a set value. The pressure inside the receiver serves as the controlled variable. A pressure transmitter converts the pressure signal into an electrical signal and sends it to the PID unit of the variable frequency drive (VFD) for comparison with the pressure setpoint. According to the magnitude and direction of the deviation, the PID unit performs calculations following predefined PID algorithms and generates control signals to adjust the VFD output, thereby changing the motor speed and stabilizing the actual pressure inside the air receiver at the target value.
Meanwhile, a mains frequency power circuit is configured to avoid prolonged shutdown of the air compressor upon VFD faults. The built-in mains frequency switching function of the VFD is adopted to further improve motor operating efficiency.
2. System Configuration and VFD Parameter Setting
2.1 Configuration Principle
The main circuit consists of circuit breaker QF, contactors MC1, MC2, MC3 and thermal relay FR. Three-phase power is fed through circuit breaker QF. Contactor MC1 connects power to the VFD input terminal; MC3 connects the VFD output terminal to the motor; MC2 connects mains power directly to the motor. Thermal relay FR provides overload protection for the motor.
The ON/OFF status of terminals MRS, CS and STF determines the operating state of the VFD, which are connected to switch QS1, normally closed contact of relay KA0 and switch QS2 respectively. Terminal RT (X14) is connected to the normally open contact of relay KA3. When this signal is ON, the VFD activates PID control. The target value (pressure setpoint signal) is set via an external potentiometer and input between terminals 2 and 5. The measured signal from the pressure transmitter is input between terminals 4 and 5.
IPF, OL and FU are open-collector output terminals of the main unit for mains frequency switching function. They support a load of DC 24 V, 0.1 A. Relays KA1, KA2 and KA3 with 24 V coils control AC contactors MC1, MC2 and MC3 correspondingly, with freewheeling diodes equipped. When MC1 and MC3 are closed, the motor runs under VFD control; when MC2 is closed, the motor operates at mains frequency. For switching to mains frequency, MC3 opens first to disconnect the motor from the VFD. After a proper delay, MC2 closes to connect the motor to mains power.
Press button SB1 to energize contactor MC1. Turn on switches QS1 and QS2 to start the VFD (forward rotation), and the motor begins operation. A1, B1 and C1 are fault output terminals. When the VFD fails, conduction occurs between A1 and C1, triggering audible and visual alarms via buzzer HA and indicator lamp HL. Meanwhile, the normally closed contact of relay KA0 opens to turn the CS signal OFF, enabling automatic switchover to mains frequency operation. Operators can reset the alarm via button SB0 for subsequent VFD maintenance.
2.2 Variable Frequency Drive
Air compressors operate continuously for long periods with constant-torque and continuously variable loads. Mitsubishi FR-A700 series general-purpose VFD is adopted. This highly reliable product features convenient operation and simple maintenance. It supports customized V/F curves according to equipment torque characteristics, and is equipped with comprehensive PID functions, instantaneous power failure restart capability, frequency skip function, sequence control for mains frequency switching, and complete safety protection functions. It complies with multiple standards and features environmental adaptability.
Since the load does not run at low speed for extended durations, the VFD capacity matches the motor rated capacity. For example, a 30 kW motor is equipped with FR-A740-30K-CHT VFD. The VFD adopts V/F control mode. The compressor does not demand high control precision for various parameters, so vector control is unnecessary. One core purpose of variable frequency speed regulation is to boost motor efficiency under light load for energy saving, and the energy-saving function of the VFD only works under V/F control.
2.3 Pressure Transmitter
The ATE1151 capacitive pressure transmitter is selected. It adopts a two-wire system powered by 24 V DC with 4~20 mA output, and the measuring range is 0~2000 kPa. This instrument delivers stable performance, reliable quality, high precision, low cost, excellent explosion-proof, pressure-resistant and corrosion-resistant properties, and is fitted with a local indicator.
2.4 PID Action Setting
The objective of PID control is to achieve favorable static and dynamic control performance. Excessively large proportional gain may cause overshoot and reverse adjustment, resulting in system oscillation. To suppress overshoot and oscillation, appropriately reduce the proportional gain and activate the integral term to eliminate steady-state deviation effectively. For systems prone to oscillation, the proportional gain can only be set to a relatively low value. In this case, the controlled variable (pressure) cannot recover rapidly under abrupt air consumption changes. Derivative control responds quickly according to the trend of deviation variation and delivers corrective actions in advance.
The air compressor system has loose requirements on transition time, so PI regulation is adopted to reduce impact on the VFD.
2.5 VFD Function Parameter Settings
Upper and lower frequency limits
If the rotating speed exceeds the rated speed, the motor output power will surpass its rated capacity. Therefore, the upper frequency limit generally shall not exceed the rated frequency (50 Hz). Excessively low speed deteriorates the operating stability and mechanical performance of the compressor. Hence, the lower frequency limit shall not be less than 80% of the rated frequency (40 Hz). Settings: Pr.1=50 Hz, Pr.2=40 Hz.
Acceleration and deceleration time
Air compressors are not started frequently and have no strict requirements for speed change time. Appropriately prolonged acceleration/deceleration time can prevent overcurrent or overvoltage faults. Settings: Pr.7=30 s, Pr.8=25 s.
Acceleration and deceleration pattern
Faster frequency rise at startup facilitates quick commissioning. After reaching a certain speed, the air inside the receiver already builds considerable pressure, so the frequency rising rate shall be slowed down. The deceleration process follows the opposite logic: slow frequency drop at the initial stage, and faster deceleration after the speed decreases to a certain extent. Since the compressor seldom starts and stops, set Pr.29=4 or 2 or 0 to select S-shaped acceleration/deceleration C, S-shaped acceleration/deceleration B or linear acceleration/deceleration mode, which can satisfy general requirements.
V/F pattern selection
Air compressors belong to constant-torque loads. Set Pr.14=0.
Operation mode
Set Pr.79=2 to fix the VFD in external operation mode.
PID control
Set Pr.128=20 to activate reverse PID action. Set Pr.127 to a specified frequency (e.g. 45 Hz), so the motor operates under normal mode before reaching this frequency during startup.
Switchover functions
Enable the mains frequency sequence switching function: set Pr.135=1;
Set Pr.136=1.0 s for the interlock delay between relay KA2 and KA3;
Set Pr.137=1.0 s for startup waiting time;
Set Pr.138=1 to activate automatic switchover to mains frequency upon VFD faults;
Set Pr.139=50 Hz as the automatic switching frequency from variable frequency to mains frequency.
The switching threshold from mains frequency to variable frequency is determined according to system control precision requirements. For instance, set Pr.159=0.5 Hz to trigger switchover when the frequency command is below 49.5 Hz.
Set Pr.57=0.5 s as the free-running waiting time from instantaneous power failure to power recovery restart;
Set Pr.58=0.5 s as the voltage rising time during restart.
Input and output terminal function assignment
Pr.178=60: STF terminal for forward operation control;
Pr.186=6: CS terminal for instantaneous power failure restart control;
Pr.185=7: JOG terminal redefined as OH terminal for external thermal relay input;
Pr.183=14: RT terminal to enable PID control;
Pr.191=47: SU terminal indicates PID control in progress;
Pr.192=17, Pr.193=18, Pr.194=19: Assign IPF, OL and FU terminals to control mains frequency switching relays KA1, KA2 and KA3.
Other parameters
Set Pr.77=1 or 0 to prohibit parameter modification or only allow parameter writing under stop status, preventing accidental parameter changes.
Set Pr.78=1 to inhibit motor reverse rotation.
Set Pr.9=0: the motor uses an external thermal relay, and the electronic overcurrent protection of the drive is disabled.
3. Advantages of the System
With the above hardware configuration and VFD parameter settings, the system has the following merits:
VFD control greatly improves motor efficiency under light load and realizes energy saving. The VFD enables stepless speed regulation, soft start and soft stop of the motor. The system achieves high control precision and favorable dynamic performance, drastically reducing electrical and mechanical impact and improving operating characteristics.
The built-in automatic switching function between variable frequency and mains frequency of the VFD realizes seamless switchover between two operating modes. On one hand, mains power supplies the motor directly upon VFD faults to ensure continuous air compressor operation. On the other hand, the motor operates more efficiently at 50 Hz under mains frequency. Automatic switchover from variable frequency to mains frequency will be triggered when running at 50 Hz. The switching range can be defined according to process requirements to further boost overall system efficiency.
Reasonable external configuration and full utilization of VFD built-in safety protection functions provide comprehensive protection, ensuring safe and reliable system operation.
Simple peripheral circuits and optimized VFD parameter settings facilitate easy operation and commissioning, including system start/stop, frequency skip, instantaneous power failure restart, variable/mains frequency switching, parameter modification, setpoint preset and sensor output calibration. For example, Pr.127 sets the automatic switching frequency of PID control. The motor starts under normal control mode and switches to PID control after reaching the preset frequency. After entering PID closed-loop control, PID operation is maintained even if the output frequency drops below this threshold. This approach avoids abnormal conditions caused by integral windup during the startup of closed-loop PID control.
4. Key Issues in Installation and Commissioning
4.1 Installation Requirements
Three core requirements shall be followed during installation:
Monitor ambient temperature around the VFD during installation. The VFD and motor must be reliably grounded. The VFD uses an independent grounding terminal placed as close to the drive as possible, and ground cables shall comply with relevant standards. To suppress inductive noise from the VFD power cables, ground wires are recommended to be wired back to the VFD grounding terminal. For harmonic suppression, power factor improvement and power supply capacity above 1000 kVA, install an AC reactor on the VFD input side. A DC reactor must be equipped for drives above 75 kVA. Do not connect power capacitors, overvoltage absorbers or radio noise filters to the output side.
Keep the total wiring length within specified limits. Long wiring distance between the VFD and motor, especially under low-frequency output, leads to voltage drop in main circuit cables and reduced motor torque. Select wires with appropriate specifications. Signal cables susceptible to interference shall be routed away from the VFD and its input/output power cables. Avoid parallel wiring or bundled routing of signal cables and power cables.
Use shielded cables or twisted pairs for control circuit wiring. For micro signal contacts, adopt two parallel contacts or twin contacts. 0.75 mm² cables are recommended, and wiring length shall not exceed 30 m. Pay attention to terminal load capacity: open-collector outputs (IPF, OL, FU) support DC 24 V, 0.1 A load; relay outputs (A1-C1, B1-C1) support AC 230 V, 0.3 A load.
Contactors MC2 and MC3 must adopt both electrical interlock and mechanical interlock. Therefore, contactors with built-in mechanical interlocks are selected.
4.2 Commissioning
Verify insulation resistance of the circuit before energizing the VFD. Confirm the insulation resistance to ground and phase-to-phase insulation on the VFD output side thoroughly before power-on. Accurate presetting of the target pressure value is critical and can be conveniently adjusted via the external potentiometer. Calibration of sensor output shall be performed under PU mode when the VFD is stopped. If wiring needs modification after trial operation, watch out for hazardous high residual voltage on DC capacitors.
After completing VFD parameter setting and no-load testing, carry out system joint commissioning covering mains frequency operation and variable frequency operation. Variable frequency commissioning includes open-loop and closed-loop tests.
Open-loop commissioning: Observe frequency rising trend, operating sound of equipment, stability of pressure rise inside the air receiver, normal operation of the pressure transmitter and reliable shutdown performance.
Closed-loop commissioning: If open-loop testing passes, proceed to closed-loop debugging. Match the frequency variation speed of the VFD with pressure fluctuation of the air compressor to eliminate pressure oscillation. Identify mechanical resonance points and set frequency skip parameters to avoid resonance bands. Subsequently tune PID parameters. Adopt empirical setting: set proportional band P=70%, integral time Ti=60 s. Generate step signals by adjusting the setpoint and observe pressure response curves. Repeat tuning to obtain optimal P and Ti values, enabling ideal response characteristics: pressure fluctuates within a tiny range after several cycles of setpoint change, and expected anti-disturbance performance can be achieved.
4.3 Operation Guidelines
Precautions during operation:
Do not switch off the control power supply when the main circuit power of the VFD is energized.
Start and stop the VFD only via the start signal (ON/OFF of STF signal). Do not use the input-side electromagnetic contactor for frequent start/stop.
During maintenance and inspection, wait at least 10 minutes after power cut. Use a multimeter to confirm that the DC voltage between main circuit terminals P(+) and N(-) drops below 30 V before any operation.
Pay attention to function changes after PID control is activated.
Modification of terminal functions via Pr.178~Pr.189 and Pr.190~Pr.196 may affect other functions. Verify terminal functions after parameter modification.
PID proportional band, integral time and derivative time are set via Pr.129, Pr.130 and Pr.134 respectively. These parameters can be adjusted online regardless of operating mode.
To reverse motor rotation, swap any two of the three output wires (or motor terminal wires). Changing wiring on the power input side is ineffective. An alternative simpler method is to swap the forward/reverse control terminals on the VFD.
Notes on closed-loop startup:
At system startup, the deviation is large, and integral calculation results rapidly reach the upper limit, triggering integral windup. This disables PID regulation temporarily, the motor accelerates quickly and may trigger overcurrent tripping. Two solutions are available: switch to open-loop control during startup; or utilize the built-in PID startup function of the VFD and configure dedicated PID acceleration/deceleration time for startup after PID activation. This system adopts the first solution.
5. Conclusion
The application of VFDs in constant-pressure air supply control of air compressors can well satisfy process requirements and offers numerous unparalleled advantages compared with traditional control schemes. This paper introduces the composition and working principle of the control system. Based on practical application experience, it investigates system configuration, VFD parameter settings and critical issues during operation. The functions of the VFD are expanded and optimized compared with conventional settings, achieving favorable application results.
With continuous technological advancement of variable frequency drives, their functions will be continuously expanded and enhanced with increasingly distinctive customized characteristics, promising broad application prospects in industrial production.
An electric motor drives the air compressor to rotate to generate compressed air, which is stored in the air receiver. The air pressure inside the air receiver depends on the balance between the compressed air output capacity of the air compressor (i.e., the rotating speed of the motor in this system) and the air consumption of users. To guarantee stable air supply quality, the air pressure inside the air receiver shall be maintained at a set value. The pressure inside the receiver serves as the controlled variable. A pressure transmitter converts the pressure signal into an electrical signal and sends it to the PID unit of the variable frequency drive (VFD) for comparison with the pressure setpoint. According to the magnitude and direction of the deviation, the PID unit performs calculations following predefined PID algorithms and generates control signals to adjust the VFD output, thereby changing the motor speed and stabilizing the actual pressure inside the air receiver at the target value.
Meanwhile, a mains frequency power circuit is configured to avoid prolonged shutdown of the air compressor upon VFD faults. The built-in mains frequency switching function of the VFD is adopted to further improve motor operating efficiency.
2. System Configuration and VFD Parameter Setting
2.1 Configuration Principle
The main circuit consists of circuit breaker QF, contactors MC1, MC2, MC3 and thermal relay FR. Three-phase power is fed through circuit breaker QF. Contactor MC1 connects power to the VFD input terminal; MC3 connects the VFD output terminal to the motor; MC2 connects mains power directly to the motor. Thermal relay FR provides overload protection for the motor.
The ON/OFF status of terminals MRS, CS and STF determines the operating state of the VFD, which are connected to switch QS1, normally closed contact of relay KA0 and switch QS2 respectively. Terminal RT (X14) is connected to the normally open contact of relay KA3. When this signal is ON, the VFD activates PID control. The target value (pressure setpoint signal) is set via an external potentiometer and input between terminals 2 and 5. The measured signal from the pressure transmitter is input between terminals 4 and 5.
IPF, OL and FU are open-collector output terminals of the main unit for mains frequency switching function. They support a load of DC 24 V, 0.1 A. Relays KA1, KA2 and KA3 with 24 V coils control AC contactors MC1, MC2 and MC3 correspondingly, with freewheeling diodes equipped. When MC1 and MC3 are closed, the motor runs under VFD control; when MC2 is closed, the motor operates at mains frequency. For switching to mains frequency, MC3 opens first to disconnect the motor from the VFD. After a proper delay, MC2 closes to connect the motor to mains power.
Press button SB1 to energize contactor MC1. Turn on switches QS1 and QS2 to start the VFD (forward rotation), and the motor begins operation. A1, B1 and C1 are fault output terminals. When the VFD fails, conduction occurs between A1 and C1, triggering audible and visual alarms via buzzer HA and indicator lamp HL. Meanwhile, the normally closed contact of relay KA0 opens to turn the CS signal OFF, enabling automatic switchover to mains frequency operation. Operators can reset the alarm via button SB0 for subsequent VFD maintenance.
2.2 Variable Frequency Drive
Air compressors operate continuously for long periods with constant-torque and continuously variable loads. Mitsubishi FR-A700 series general-purpose VFD is adopted. This highly reliable product features convenient operation and simple maintenance. It supports customized V/F curves according to equipment torque characteristics, and is equipped with comprehensive PID functions, instantaneous power failure restart capability, frequency skip function, sequence control for mains frequency switching, and complete safety protection functions. It complies with multiple standards and features environmental adaptability.
Since the load does not run at low speed for extended durations, the VFD capacity matches the motor rated capacity. For example, a 30 kW motor is equipped with FR-A740-30K-CHT VFD. The VFD adopts V/F control mode. The compressor does not demand high control precision for various parameters, so vector control is unnecessary. One core purpose of variable frequency speed regulation is to boost motor efficiency under light load for energy saving, and the energy-saving function of the VFD only works under V/F control.
2.3 Pressure Transmitter
The ATE1151 capacitive pressure transmitter is selected. It adopts a two-wire system powered by 24 V DC with 4~20 mA output, and the measuring range is 0~2000 kPa. This instrument delivers stable performance, reliable quality, high precision, low cost, excellent explosion-proof, pressure-resistant and corrosion-resistant properties, and is fitted with a local indicator.
2.4 PID Action Setting
The objective of PID control is to achieve favorable static and dynamic control performance. Excessively large proportional gain may cause overshoot and reverse adjustment, resulting in system oscillation. To suppress overshoot and oscillation, appropriately reduce the proportional gain and activate the integral term to eliminate steady-state deviation effectively. For systems prone to oscillation, the proportional gain can only be set to a relatively low value. In this case, the controlled variable (pressure) cannot recover rapidly under abrupt air consumption changes. Derivative control responds quickly according to the trend of deviation variation and delivers corrective actions in advance.
The air compressor system has loose requirements on transition time, so PI regulation is adopted to reduce impact on the VFD.
2.5 VFD Function Parameter Settings
Upper and lower frequency limits
If the rotating speed exceeds the rated speed, the motor output power will surpass its rated capacity. Therefore, the upper frequency limit generally shall not exceed the rated frequency (50 Hz). Excessively low speed deteriorates the operating stability and mechanical performance of the compressor. Hence, the lower frequency limit shall not be less than 80% of the rated frequency (40 Hz). Settings: Pr.1=50 Hz, Pr.2=40 Hz.
Acceleration and deceleration time
Air compressors are not started frequently and have no strict requirements for speed change time. Appropriately prolonged acceleration/deceleration time can prevent overcurrent or overvoltage faults. Settings: Pr.7=30 s, Pr.8=25 s.
Acceleration and deceleration pattern
Faster frequency rise at startup facilitates quick commissioning. After reaching a certain speed, the air inside the receiver already builds considerable pressure, so the frequency rising rate shall be slowed down. The deceleration process follows the opposite logic: slow frequency drop at the initial stage, and faster deceleration after the speed decreases to a certain extent. Since the compressor seldom starts and stops, set Pr.29=4 or 2 or 0 to select S-shaped acceleration/deceleration C, S-shaped acceleration/deceleration B or linear acceleration/deceleration mode, which can satisfy general requirements.
V/F pattern selection
Air compressors belong to constant-torque loads. Set Pr.14=0.
Operation mode
Set Pr.79=2 to fix the VFD in external operation mode.
PID control
Set Pr.128=20 to activate reverse PID action. Set Pr.127 to a specified frequency (e.g. 45 Hz), so the motor operates under normal mode before reaching this frequency during startup.
Switchover functions
Enable the mains frequency sequence switching function: set Pr.135=1;
Set Pr.136=1.0 s for the interlock delay between relay KA2 and KA3;
Set Pr.137=1.0 s for startup waiting time;
Set Pr.138=1 to activate automatic switchover to mains frequency upon VFD faults;
Set Pr.139=50 Hz as the automatic switching frequency from variable frequency to mains frequency.
The switching threshold from mains frequency to variable frequency is determined according to system control precision requirements. For instance, set Pr.159=0.5 Hz to trigger switchover when the frequency command is below 49.5 Hz.
Set Pr.57=0.5 s as the free-running waiting time from instantaneous power failure to power recovery restart;
Set Pr.58=0.5 s as the voltage rising time during restart.
Input and output terminal function assignment
Pr.178=60: STF terminal for forward operation control;
Pr.186=6: CS terminal for instantaneous power failure restart control;
Pr.185=7: JOG terminal redefined as OH terminal for external thermal relay input;
Pr.183=14: RT terminal to enable PID control;
Pr.191=47: SU terminal indicates PID control in progress;
Pr.192=17, Pr.193=18, Pr.194=19: Assign IPF, OL and FU terminals to control mains frequency switching relays KA1, KA2 and KA3.
Other parameters
Set Pr.77=1 or 0 to prohibit parameter modification or only allow parameter writing under stop status, preventing accidental parameter changes.
Set Pr.78=1 to inhibit motor reverse rotation.
Set Pr.9=0: the motor uses an external thermal relay, and the electronic overcurrent protection of the drive is disabled.
3. Advantages of the System
With the above hardware configuration and VFD parameter settings, the system has the following merits:
VFD control greatly improves motor efficiency under light load and realizes energy saving. The VFD enables stepless speed regulation, soft start and soft stop of the motor. The system achieves high control precision and favorable dynamic performance, drastically reducing electrical and mechanical impact and improving operating characteristics.
The built-in automatic switching function between variable frequency and mains frequency of the VFD realizes seamless switchover between two operating modes. On one hand, mains power supplies the motor directly upon VFD faults to ensure continuous air compressor operation. On the other hand, the motor operates more efficiently at 50 Hz under mains frequency. Automatic switchover from variable frequency to mains frequency will be triggered when running at 50 Hz. The switching range can be defined according to process requirements to further boost overall system efficiency.
Reasonable external configuration and full utilization of VFD built-in safety protection functions provide comprehensive protection, ensuring safe and reliable system operation.
Simple peripheral circuits and optimized VFD parameter settings facilitate easy operation and commissioning, including system start/stop, frequency skip, instantaneous power failure restart, variable/mains frequency switching, parameter modification, setpoint preset and sensor output calibration. For example, Pr.127 sets the automatic switching frequency of PID control. The motor starts under normal control mode and switches to PID control after reaching the preset frequency. After entering PID closed-loop control, PID operation is maintained even if the output frequency drops below this threshold. This approach avoids abnormal conditions caused by integral windup during the startup of closed-loop PID control.
4. Key Issues in Installation and Commissioning
4.1 Installation Requirements
Three core requirements shall be followed during installation:
Monitor ambient temperature around the VFD during installation. The VFD and motor must be reliably grounded. The VFD uses an independent grounding terminal placed as close to the drive as possible, and ground cables shall comply with relevant standards. To suppress inductive noise from the VFD power cables, ground wires are recommended to be wired back to the VFD grounding terminal. For harmonic suppression, power factor improvement and power supply capacity above 1000 kVA, install an AC reactor on the VFD input side. A DC reactor must be equipped for drives above 75 kVA. Do not connect power capacitors, overvoltage absorbers or radio noise filters to the output side.
Keep the total wiring length within specified limits. Long wiring distance between the VFD and motor, especially under low-frequency output, leads to voltage drop in main circuit cables and reduced motor torque. Select wires with appropriate specifications. Signal cables susceptible to interference shall be routed away from the VFD and its input/output power cables. Avoid parallel wiring or bundled routing of signal cables and power cables.
Use shielded cables or twisted pairs for control circuit wiring. For micro signal contacts, adopt two parallel contacts or twin contacts. 0.75 mm² cables are recommended, and wiring length shall not exceed 30 m. Pay attention to terminal load capacity: open-collector outputs (IPF, OL, FU) support DC 24 V, 0.1 A load; relay outputs (A1-C1, B1-C1) support AC 230 V, 0.3 A load.
Contactors MC2 and MC3 must adopt both electrical interlock and mechanical interlock. Therefore, contactors with built-in mechanical interlocks are selected.
4.2 Commissioning
Verify insulation resistance of the circuit before energizing the VFD. Confirm the insulation resistance to ground and phase-to-phase insulation on the VFD output side thoroughly before power-on. Accurate presetting of the target pressure value is critical and can be conveniently adjusted via the external potentiometer. Calibration of sensor output shall be performed under PU mode when the VFD is stopped. If wiring needs modification after trial operation, watch out for hazardous high residual voltage on DC capacitors.
After completing VFD parameter setting and no-load testing, carry out system joint commissioning covering mains frequency operation and variable frequency operation. Variable frequency commissioning includes open-loop and closed-loop tests.
Open-loop commissioning: Observe frequency rising trend, operating sound of equipment, stability of pressure rise inside the air receiver, normal operation of the pressure transmitter and reliable shutdown performance.
Closed-loop commissioning: If open-loop testing passes, proceed to closed-loop debugging. Match the frequency variation speed of the VFD with pressure fluctuation of the air compressor to eliminate pressure oscillation. Identify mechanical resonance points and set frequency skip parameters to avoid resonance bands. Subsequently tune PID parameters. Adopt empirical setting: set proportional band P=70%, integral time Ti=60 s. Generate step signals by adjusting the setpoint and observe pressure response curves. Repeat tuning to obtain optimal P and Ti values, enabling ideal response characteristics: pressure fluctuates within a tiny range after several cycles of setpoint change, and expected anti-disturbance performance can be achieved.
4.3 Operation Guidelines
Precautions during operation:
Do not switch off the control power supply when the main circuit power of the VFD is energized.
Start and stop the VFD only via the start signal (ON/OFF of STF signal). Do not use the input-side electromagnetic contactor for frequent start/stop.
During maintenance and inspection, wait at least 10 minutes after power cut. Use a multimeter to confirm that the DC voltage between main circuit terminals P(+) and N(-) drops below 30 V before any operation.
Pay attention to function changes after PID control is activated.
Modification of terminal functions via Pr.178~Pr.189 and Pr.190~Pr.196 may affect other functions. Verify terminal functions after parameter modification.
PID proportional band, integral time and derivative time are set via Pr.129, Pr.130 and Pr.134 respectively. These parameters can be adjusted online regardless of operating mode.
To reverse motor rotation, swap any two of the three output wires (or motor terminal wires). Changing wiring on the power input side is ineffective. An alternative simpler method is to swap the forward/reverse control terminals on the VFD.
Notes on closed-loop startup:
At system startup, the deviation is large, and integral calculation results rapidly reach the upper limit, triggering integral windup. This disables PID regulation temporarily, the motor accelerates quickly and may trigger overcurrent tripping. Two solutions are available: switch to open-loop control during startup; or utilize the built-in PID startup function of the VFD and configure dedicated PID acceleration/deceleration time for startup after PID activation. This system adopts the first solution.
5. Conclusion
The application of VFDs in constant-pressure air supply control of air compressors can well satisfy process requirements and offers numerous unparalleled advantages compared with traditional control schemes. This paper introduces the composition and working principle of the control system. Based on practical application experience, it investigates system configuration, VFD parameter settings and critical issues during operation. The functions of the VFD are expanded and optimized compared with conventional settings, achieving favorable application results.
With continuous technological advancement of variable frequency drives, their functions will be continuously expanded and enhanced with increasingly distinctive customized characteristics, promising broad application prospects in industrial production.
