With the continuous development of power electronics technology, the new control theory has been constantly proposed and improved, and the frequency conversion technology has been rapidly developed. In recent years, along with the continuous improvement and expansion of the inverter's function, its application range is also expanding, and it has now transitioned from the initial speed control that can only be used for fans and pumps to various types of applications requiring high precision and fast response. The high performance speed regulation indicator for the industrial site. The ACS150 series inverter is designed and manufactured by ABB. It can meet the requirements of the machinery industry in the widest range through the optimized combination of options. It is mainly applied to the control of fans, pumps, gates, material handling, conveyor belts and so on.
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 motor pole pair number.
From the formula (1) we can see that the speed n is proportional to the frequency f, as long as the frequency f can be changed by changing the motor speed, when the frequency f in the range of 0 ~ 50Hz, the motor speed adjustment range is very wide. Frequency conversion speed regulation is to achieve speed regulation by changing the frequency of the motor power supply.
The frequency converter mainly uses the direct-crossing method, that is, the AC power supply of the industrial frequency is first converted into a DC power supply through a rectifier device, and then the DC power supply is converted into an AC power supply of which the frequency and voltage can be controlled to supply the motor. The inverter's circuit is generally composed of four parts: rectification, intermediate DC link, inverter and control. The rectifying part is a three-phase bridge type uncontrollable rectification. The inverter part is an IGBT three-phase bridge type inverter, and the output is PWM waveform. The intermediate DC link functions as filtering, direct current energy storage and buffering reactive power.
The frequency changer in the application changes not only the frequency, it also must change the alternating current voltage at the same time, if only changes the frequency, the electrical machinery possibly burns out. Because the torque of the asynchronous motor is generated by the interaction between the magnetic flux of the motor and the current flowing in 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 must be changed proportionally, that is, the inverter output voltage should be controlled while changing the frequency, so that the magnetic flux of the motor can be kept constant to avoid the generation of weak magnetic and magnetic saturation phenomena. However, the voltage is still reduced proportionally at low frequencies. As the AC impedance becomes smaller and the DC resistance does not change, the magnetic key waveform will be distorted at a low speed, resulting in a reduction in torque.
2 Frequency Converter Selection
The correct selection of the frequency converter is crucial for the normal operation of the electronic control system of machinery and equipment. To select the inverter, first determine which control method and protective structure of the inverter is most suitable according to the type of mechanical equipment, load torque characteristics, speed range, static speed accuracy, starting torque and requirements of the use environment. The so-called "fitness" refers to the realization of the best performance-to-price ratio for the application of the frequency converter under the premise of satisfying 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, inverters with U/f=const (U is the output voltage, f is the frequency of the output voltage) control mode can basically meet the requirements of this kind of load. The following describes the main features of this kind of inverter. Problems to be noted when selecting a model.
2.1.1 Avoid Overloading
Fans and pumps are generally not easily overloaded. When selecting the capacity of the inverter, ensure that it is slightly greater than or equal to the capacity of the motor. At the same time, the overload capacity requirements of the selected inverter are also lower, but when the inverter function parameters are selected and preset It should be noted that since the resistive 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 exceeds the rated torque, overloading the motor. Therefore, strictly control the maximum operating frequency can not exceed the motor rated frequency.
2.1.2 Matching of Inverter Acceleration and Deceleration Time at Start/Stop
Because the load inertia moment of the fan and the pump is relatively large, matching the acceleration time and the deceleration time of the inverter with the start and stop is a very important issue. In the selection and application of the inverter, the shortest time should be selected according to the load parameters to calculate the acceleration time and deceleration time of the inverter, so that no over-current tripping occurs when the inverter starts and no over-voltage trip occurs when the inverter decelerates. . But sometimes in the production process, the start-up time requirements for the fan and the pump are very strict. If the above calculation time does not meet the demand, the frequency converter should be redesigned and selected.
2.1.3 Avoidance of Resonance
Because the inverter changes the speed of the motor by changing the power frequency of the motor to achieve energy saving, it may coincide 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 operation or the equipment is damaged. Therefore, when selecting and presetting the function parameters of the inverter, the jump frequency point and width should be set according to the resonance frequency of the load shaft system to avoid resonance in the system.
2.1.4 Pressure and Water Hammer Effect
In the actual operation of the pump load, the pressure and water hammer effects are prone to occur, and the inverter must be set separately for this problem when setting the function.
1) When the load of the ram pressure pump is running at a low speed, the pressure is increased due to closing the outlet valve, thereby causing cavitation of the pump. When the function of the inverter is set, the minimum speed of the critical point of the pump flow rate can be limited by setting the minimum frequency of the inverter, which can avoid such phenomena.
2) Water Hammer Effect When the pump load suddenly loses power, the liquid in the pipe flows back 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 power and causing the inverter to malfunction or burn out. During the design of the inverter system, the inverter shall be stopped according to the deceleration curve. After the motor is completely stopped, the main circuit is disconnected, or the power-off deceleration and stop function is set to avoid this phenomenon.
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