With the continuous development of power electronics technology, the new control theory has been continuously proposed and improved, and the frequency conversion technology has been rapidly developed. In recent years, with the continuous improvement and expansion of the function of the inverter, its application range has been expanding. It has now transitioned from the initial speed regulation of only fans and pumps to the application of various requirements for high precision and fast response. The industrial site of high performance speed regulation indicators. Designed and manufactured by ABB, the ACS150 series of inverters can meet the requirements of the mechanical industry through the optimized combination of optional components, mainly used for the control of fans, pumps, gates, material handling, conveyor belts, etc.
1 frequency conversion principle
n=60f(1-s)/p(1)
Where: n is the speed of the asynchronous motor;
f is the grid frequency;
s is the motor slip rate;
p is the number of pole pairs of the motor.
It can be seen from equation (1) that the rotational speed n is proportional to the frequency f, and the rotational speed of the motor can be changed by changing the frequency f. When the frequency f is varied within the range of 0 to 50 Hz, the motor rotational speed adjustment range is very wide. Frequency control is to adjust the speed of the motor by changing the frequency of the motor.
The frequency converter mainly adopts the AC-straight mode, that is, the power frequency AC power source is first converted into a DC power source through a rectifying device, and then the DC power source is converted into an AC power source whose frequency and voltage can be controlled to supply the motor. The circuit of the inverter is generally composed of four parts: rectification, intermediate DC link, inverter and control. The rectification part is a three-phase bridge type uncontrollable rectification, the inverter part is an IGBT three-phase bridge type inverter, and the output is a PWM waveform, and the intermediate DC link functions as filtering, DC energy storage and buffer reactive power.
The frequency converter changes not only the frequency in the application, but also has to change the voltage of the alternating current at the same time. If only the frequency is changed, the motor may be burned out. Because the torque of the asynchronous motor is generated by the interaction between the magnetic flux of the motor and the current flowing through the rotor. At the rated frequency, if the voltage is constant and only the frequency is reduced, the magnetic flux is too large, and the magnetic circuit is saturated. The motor will be burned. Therefore, the frequency and voltage should be changed in proportion, that is, the frequency of the inverter is controlled while changing the frequency, so that the magnetic flux of the motor is kept constant to avoid the occurrence of weak magnetic and magnetic saturation. However, the voltage is proportionally reduced at a low frequency, and since the AC resistance is small and the DC resistance is constant, the magnetic key waveform is distorted at a low speed, resulting in a decrease in torque.
2 inverter selection
The correct selection of the frequency converter is crucial for the normal operation of the electronic control system of the mechanical equipment. To select the frequency converter, firstly, according to the type of mechanical equipment, load torque characteristics, speed regulation range, static speed accuracy, starting torque and the requirements of the use environment, it is most appropriate to decide which control mode and protection structure are selected. The so-called suitable means to achieve the best cost performance of the inverter application under the premise of meeting the actual process production requirements and use occasions of the mechanical equipment.
2.1 square torque load
Fan and pump loads are square torque loads. Under normal circumstances, the inverter with U/f=const (U refers to the output voltage, f refers to the frequency of the output voltage) control mode can basically meet the requirements of this type of load. The following is based on the main features of this type of inverter. Need to pay attention to the problem when selecting the type.
2.1.1 Avoid overload
Fans and pumps are generally not easy to overload. When selecting the capacity of the inverter, it is guaranteed to be slightly larger than or equal to the capacity of the motor. At the same time, the overload capacity of the selected inverter is also low, but when the inverter is selected and preset. It should be noted that since the resistance torque of the load is proportional to the square of the rotational speed, when the operating frequency is higher than the rated frequency of the motor, the resistive torque of the load may exceed the rated torque, causing the motor to be overloaded. Therefore, it is necessary to strictly control the maximum working frequency not to exceed the rated frequency of the motor.
2.1.2 Matching the acceleration time and deceleration time of the inverter when starting/stopping
Since the load and inertia of the fan and the pump are relatively large, it is a very important problem to match the acceleration time and deceleration time of the inverter during starting and stopping. When selecting and applying the inverter, the acceleration time and deceleration time of the inverter should be calculated according to the load parameters to select the shortest time, so that no overcurrent trip occurs when the inverter starts and the overvoltage trip does not occur when the inverter decelerates. . However, sometimes in the production process, the start-up time of the fan and pump is very strict. If the above calculation time cannot meet the demand, the inverter should be redesigned and selected.
2.1.3 Avoid resonance
Since the inverter is energy-saving by changing the power frequency of the motor to change the motor speed, it may overlap with the resonance point and resonance frequency of the load shaft system at a certain motor speed, causing the vibration of the load shaft system to be tolerated, sometimes causing The equipment is out of service or the equipment is damaged. Therefore, when the inverter function parameters are selected and preset, the resonance frequency of the system should be avoided by setting the jump frequency point and width according to the resonance frequency of the load shaft system.
2.1.4 Pressure and water hammer effect
Pump load is prone to rolling and water hammer during actual operation. The inverter should be set separately for this problem during function setting.
1) When the pressure of the pump is at low speed, the pressure is increased due to the closing of the outlet valve, which causes the pump to cavitation. When the inverter function is set, the minimum speed of the pump flow can be limited by setting the lowest frequency of the inverter to avoid such a phenomenon.
2) Water hammer effect Pump type load When the power is suddenly turned off, the liquid in the pipe flows backward due to gravity. If the check valve is not strict or there is no check valve, the motor will be reversed, causing the motor to generate electricity and cause the inverter to malfunction or burn out. When designing the inverter system, the inverter should be stopped according to the deceleration curve. After the motor is completely stopped, the main circuit is disconnected, or the power-off deceleration stop function is set to avoid this phenomenon.
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