Composition of traditional air compressor system
In the traditional air compressor control system, the main motor generally adopts the "Y - △" starting mode, and the pressure of the air compressor is controlled by the pressure transmitter installed on the rear outlet pipe. When the air compressor starts, the main motor starts with "Y - △". At this time, the loading solenoid valve is not opened, and the air compressor is in the "unloading" state. After the "Y - △" is completed, according to the delay time set by the system (such as 10s), the loading solenoid valve is opened, and the air compressor "loads" and runs. During the "loading" operation, if the air consumption of the back-end equipment is large and the pressure value detected by the pressure sensor can not reach the "unloading pressure" value, then the loading solenoid valve will continue to act, and the air compressor will keep "loading" state. If the air consumption of the back-end equipment decreases and the pressure value detected by the pressure sensor increases slowly to reach the "unloading pressure" value, the system will control the closing of the loading solenoid valve and the air compressor will enter the "unloading" state.
When the air compressor runs continuously, the temperature of lubricating oil will rise. When the temperature reaches a certain degree, the oil temperature fan will start to run to reduce the oil temperature. When the fan runs for a period of time, the lubricating oil temperature drops, it will stop automatically. Generally speaking, the control system will set the working temperature range of oil temperature fan as: 75 ℃ ~ 85 ℃.
The traditional screw air compressor control system has some obvious technical weaknesses
(1) The starting current impulse is large;
Although the starting mode is "Y - △" step-down starting, the starting impulse current will still reach more than 3 times of the rated current, which will cause great impact on the power grid, easily cause grid instability and affect the operation safety of other electrical equipment.
(2) The gas supply control mode of loading and unloading seriously wastes energy;
The air compressor loading and unloading control mode makes the pressure of compressed gas change back and forth between Pmin and Pmax, where Pmin is the minimum pressure value that can ensure the normal operation of users, and Pmax is the set maximum pressure value. Combined with the analysis of the loading and unloading principle of the air compressor, the energy waste is mainly reflected in three aspects under the traditional air compressor control mode
a. The energy consumed when the compressed air pressure exceeds Pmin;
b. The energy consumed when the pressure reducing valve depressurizes;
c. The energy consumed when the main motor is in no-load state during "unloading" operation.
(3) The stability of gas supply pressure is poor, and the production process is difficult to guarantee.
In traditional control mode, in order to avoid insufficient gas supply, the gas supply pressure will be higher than the demand value, which will increase the cost and energy consumption. In addition, in some occasions with small gas consumption and low gas frequency, the air compressor system will carry out frequent "loading" and "unloading" operations, so it is difficult to ensure the air pressure accuracy.

technological requirements
Screw air compressor frequency control system is mainly for the main motor control, oil temperature fan control demand is relatively small, specific process requirements are as follows:
(1) The starting torque is large;
(2) Stable start and small impact;
(3) Built in PID control function to meet the requirements of constant pressure control system;
(4) Strong overload capacity;
(5) The system has high stability and good EMC characteristics.

Application scheme
Taking a brand 30kW power frequency screw air compressor as an example, the application of vl6100 series vector general inverter in air compressor industry is introduced.
1. The original system uses "Y - △" starting, and the electrical schematic diagram after frequency conversion transformation is as follows:

Matters needing attention in engineering and frequency conversion transformation:
(1) Pressure sensor signal: in the original system, the pressure sensor signal is only connected to the air compressor controller. After the transformation, the pressure sensor signal is needed as the feedback signal of PID control, so the pressure sensor signal (4 ~ 20mA) needs to be connected in series to the controller and the inverter analog current input Ai1;
(2) Starting signal of frequency converter: on the premise of retaining the original "Y - △" starting circuit, the starting signal of the frequency converter is taken from the "no" point of the "△" contactor, which has been guaranteed to start the frequency converter after the completion of "Y - △";
(3) The power supply of transformer input side, oil temperature fan power supply and phase sequence detection circuit in the original system: after the inverter is connected to the original system, all the above-mentioned lines need to be moved to the input power side of the inverter.
System parameter setting:
The main motor of the air compressor on site: 30kW 380V 59A 50Hz 975r / min. according to the site conditions, Huayuan vl6100 series vector type general frequency converter is selected, which can be matched with 1:1.