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TBQD series electromagnetic voltage regulation compensation soft start device
The three-phase induction motor is an inductive load. The power factor is very low at the initial start (usually about 0.3), and gradually increases with the increase of the speed. When the rated speed is reached, the power factor is the highest, as shown in the curve of section a in the right figure. The capacity of the motor is very large when starting, and it can reach 5~7 times the rated capacity at full voltage. In the case of low power factor, there will be a very large inductive reactive current. Therefore, the method of connecting the compensation capacitor in parallel at the motor terminal during starting can reduce the starting current.
At present, large-scale motors all use the soft start method. When the starting conditions are met, try to limit the power during starting, that is, reduce the initial voltage at the motor terminal during starting. In this way, the starting current is greatly reduced, and the voltage drop of the power supply bus will also be greatly reduced. Generally speaking, when the design of the power supply system is reasonable, the soft start mode can be used to ensure that the voltage drop of the power supply bus is within an appropriate range.
When the capacity of the power supply system is too small (for example, when a new load needs to be added to the old power supply system), in order to ensure that the voltage drop of the system is controlled within an appropriate range when starting, as an optional solution, add a Install a temporary compensation capacitor to further reduce the starting current. During the starting process, it will be cut off gradually as the power factor rises.
The electromagnetic voltage-regulating soft starter adopts the scheme of parallel capacitors at the inlet end of the starter and its equivalent circuit schematic diagram and phasor analysis are shown in the figure. According to the equivalent diagram, Iq is decomposed into two components, and the active component IR = IqcosΦ1 , The reactive component IL = IqsinΦ1, make a vector. After paralleling the capacitors in the original soft start system, ignoring a small change in Um, the start parameters of the soft starter remain basically unchanged. Since the soft starter and the motor are equivalent to a load with a power factor angle of Φ1 when starting, the starting current is Iq, and the phase angle of the busbar outlet voltage Um is set as, then the angle at which Iq lags Um is Φ1.
Through vector diagram analysis, it is obvious that the current I output by the busbar at startup will decrease with the increase of the Ic of the parallel compensation capacitor (when the absolute value of Ic is less than Iq), that is to say, it will decrease with the increase of the capacitance of the capacitor. small. If the output current I of the busbar is 3 times of the rated current when starting without the capacitor, through graphical analysis, it is easy to reach about 1.5 times or even lower after adding the compensation capacitor. However, in order to reduce unnecessary excessive investment, an appropriate value of Ic should be designed according to the value of the starting current and the voltage drop of the power supply bus system required by the simulation calculation of the motor starting, and the appropriate capacitance should be selected to ensure the rationality of the technical and economic instructions. When designing the system, you should understand the electrical system and mechanical system in detail, carefully calculate and verify repeatedly, select appropriate parameters, so that the starting current can meet the requirements of the voltage drop of the power grid, and the motor can be started successfully to achieve the optimal effect. .