I. Introduction
The vacuum pump is a rotary variable volume vacuum pump, which can be used only with the cooperation of the fore-stage pump. Its function is to extract gas molecules from the vacuum chamber, reduce the gas pressure in the vacuum chamber, and make it reach the required vacuum degree. It has a larger pumping speed in a wider pressure range and is insensitive to the dust and water vapor contained in the gas being pumped. Vacuum pumps are widely classified, including WLW series vacuum pumps, W type vacuum pumps, 2X type vacuum pumps, ZJ type vacuum pumps, 2SK type vacuum pumps, SK type vacuum pumps, 2BV type vacuum pumps, MH-2 type vacuum pumps, TLZ type vacuum pumps, SL type Roots blowers, JZJX vacuum pump, JZJS vacuum pump, JZJWLW vacuum pump, JZJP vacuum pump, RPP vacuum pump, 2XZ vacuum pump and other models.
Second, the control problem of the traditional vacuum pump
The load of the vacuum pump decreases as the degree of vacuum increases. When the air resistance in the container is large, the vacuum pump often works in an overload state. As the vacuum increases, the current drops and the motor works normally. When the container seal is normal, the motor will work for a short time under overload and will not cause the motor to burn. When the container seal is lowered, the motor will work longer when overloaded, and the motor will burn.
In order to solve the problem of pump overload, the following methods have traditionally been used:
1) Adopt mechanical automatic pressure regulating bypass valve.
The bypass valve is installed on the bypass line between the outlet and inlet of the Roots vacuum pump. This valve controls the pressure difference between the pump inlet and outlet not to exceed the rated value. When the pressure difference reaches the rated value, the valve automatically opens by the pressure difference, so that the outlet and inlet of the roots vacuum pump are connected, so that the pressure difference between the inlet and the outlet is quickly reduced. At this time, the roots vacuum pump works under a load with almost no pressure difference. When the pressure difference is lower than the rated value, the valve closes automatically, and the gas is pumped by the foreline pump through the roots vacuum pump. The Roots vacuum pump with bypass relief valve can be started at the same time as the foreline pump, making the unit easy to operate.
2) Adopt hydraulic coupling
The use of hydraulic coupling can also prevent the overload phenomenon of the pump, so that the pump can work under high pressure difference. The hydraulic coupling is installed between the pump and the motor. Under normal working conditions, the hydraulic coupling transmits the rated torque from the motor end to the pump. The maximum pressure difference of the Roots vacuum pump is determined by the maximum torque transmitted by the hydraulic coupling, and the maximum torque that can be transmitted by the hydraulic coupling is adjusted by the amount of liquid in it. When the pump works under the high-pressure difference or starts at the same time as the foreline pump, a speed difference, that is, slippage occurs in the liquid coupling, and only a certain torque is transmitted to make the pump decelerate. As the pumping progresses, the gas load decreases and the Roots vacuum pump gradually accelerates to the rated speed.
3) Use vacuum electrical components to control pump inlet pressure
Place pressure sensitive components such as vacuum membrane box relays or electric contact vacuum pressure gauges at the inlet pipeline of the Roots vacuum pump. After the vacuum system is started, when the pressure at the entrance of the roots vacuum pump is lower than the given value (the pump allows the starting pressure), the pressure sensitive component sends a signal to start the roots vacuum pump through the electrical control system (if the vacuum system is equipped with a roots vacuum pump When the pipeline is connected, the valve of the bypass pipeline is closed at the same time). If the inlet pressure of the pump is higher than the specified value, the Roots vacuum pump is automatically closed (or the valve of the pump bypass line is opened at the same time), thereby ensuring the reliable operation of the Roots vacuum pump.
Obviously, the above method has a complicated structure, high cost, poor control accuracy, and serious energy waste. It can no longer meet the requirements of equipment development.
Three, ordinary inverter application program
In order to protect the motor, some vacuum pump manufacturers use frequency converters, use current transformers and current transmitters to detect the actual current of the motor, and then input the converted analog signal to the frequency converter, and let the frequency converter do PID calculation to control the current of the motor And the speed of the motor, finally achieve the purpose of protecting the motor.
It can be seen from the figure on the left that the control loop is complicated. Although the frequency converter can achieve closed-loop PID control of the current, the control accuracy is not high and the conversion is complicated. Unable to distinguish the motor stall current and normal load current, it may cause the motor to work in the stall state for a long time, causing the motor to burn. The control effect is not ideal.
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