Application of the hottest inverter soft start in

2022-08-19
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Application of frequency converter soft start in large and medium-sized water pumps

generally speaking, it has the advantages of stable operation, high efficiency and energy saving, high degree of automation and easy operation. The main components of high-performance control system include frequency converter, soft start and programmable controller with Mongolian side. However, the function of the starter in the high-performance control system is to make the process of starting and stopping the equipment more stable, avoid the effect of water hammer, and then save investment, reduce costs, and obtain a substantial improvement in the performance of the equipment. This technology has great promotion value in large and medium-sized water pumps

generally, the control system of water supply equipment is composed of frequency converter, controller, low-voltage electrical apparatus and pressure sensor, which can complete the closed-loop control of water supply pressure. When the flow of water supply pipe changes, the purpose of stabilizing the outlet pressure of water supply pipe can be achieved by adjusting the speed of variable-frequency pump and changing the number of water pumps put into operation. Figure 1 shows the block diagram of a typical constant pressure variable frequency water supply system. The control object of this control system is the outlet pressure of the water supply pipe. The pressure sensor collects the outlet pressure signal of the water supply pipe, compares this pressure signal with the set

constant pressure signal, and the difference value enters the CPU for PID calculation. The calculation result controls the output frequency and output voltage of the frequency converter, so that the speed of the water pump can change continuously with the fluctuation of the pressure of the water supply pipe, Thus, the outlet pressure of the pipe is stabilized. If the pipe flow changes greatly, when only adjusting the attention maintenance and protection speed of the variable-frequency pump can not meet the requirements of stable pipe outlet pressure, the controller will send a command to meet the requirements of stable pipe outlet pressure by changing the number of pumps put into operation. In the above process, when the variable-frequency pump reaches the maximum set speed, it indicates that the water consumption of the pipe is large, and the pressure at the outlet of the pipe cannot be stabilized only by adjusting the speed of the variable-frequency pump. After a certain time delay, if the pump still operates at the maximum set speed, the control system will start a water pump. In the water supply system with large pump capacity, a technology called circulating soft start is often used, That is, the motor running at full speed driven by the frequency converter is powered, and the frequency converter drives the next motor to start with frequency conversion, in order to reduce the mechanical and electrical impact in the starting process. This technology, called circulating soft start, has a fatal weakness, because in this process, the water pump just separated from the frequency converter rotates at high speed under the action of inertia, and there is still a large current in the motor rotor. The magnetic field formed by this current induces a high voltage in the motor stator, which is different in frequency and phase from the voltage. Therefore, the motor cannot be switched on immediately at this time, The general method is to determine a time delay according to the motor capacity. The motor can be switched on only after the rotor current is attenuated to a certain value, and then the frequency converter drives the next motor to run. As mentioned above, the motor disconnected from the frequency converter can only be connected to the power after a certain time delay. For medium-sized motors, this time delay is about seconds. During this period, the water pump loses power, and the water level can prevent the water pump from continuing to rotate. The rotation speed of the water pump decreases rapidly. When the water pump is connected to the power, the rotation speed of the motor has decreased very low. When the motor is connected to the power, there will be a large electrical and mechanical impact. If the process of motor switching from frequency conversion to power frequency is not handled properly, it will cause major accidents to electricity and water supply pipes, so many experts do not advocate the use of this circulating soft start technology in large and medium-sized water supply equipment. The smooth switching between frequency conversion and power frequency has become an urgent problem to be solved in large and medium-sized water supply equipment

in order to solve the above problems, another power electronic product soft starter is adopted. Its basic principle is to change the conduction angle of the thyristor and change the output voltage, so that the terminal voltage can gradually change in a preset way during the starting and stopping of the motor, so as to make the starting and stopping process stable. If a motor is started, the soft start will gradually increase the conduction angle of the thyristor, gradually increase the voltage at the motor end, and the water pump will steadily speed up to complete the starting process. If a motor is shut down, the conduction angle of the thyristor in the soft start will gradually decrease from large, and the output voltage will be gradually reduced, so that the running motor will stop smoothly. The high-performance soft start and control system allows one soft start sequence to drive multiple motors to complete soft start and soft stop operations

for example, when starting 1# motor, the initial conduction angle of soft start thyristor is zero, close km11, and then the conduction angle of soft start thyristor changes from small to large, and the motor terminal voltage gradually increases to the electrical voltage,

one soft start realizes the soft start and soft stop control of multiple motors. The main circuit diagram of motor can rise steadily?? With the same frequency and phase, the thyristor of the soft starter is completely connected, and its output voltage is close to the electrical voltage. In this way, km21 can be closed to bypass the soft starter, and then km11 can be disconnected, and the soft starter will exit the operation

during this process, a relatively stable voltage is always maintained on the motor terminal, so the whole starting process is stable without impact. After the soft start exits the operation, prepare to accept the next start or shutdown operation instruction. If the next operation command is to start another motor, the soft start will close the thyristor on the soft starter, then close the corresponding contactor, and then repeat the above process. If the next operation is to shut down a motor, such as 1# motor shutdown, the soft start first makes the thyristor fully conductive, and the output voltage is close to the electrical voltage, then km11 closes, integrates the soft start into the running 1# motor, and then disconnects the contactor km21 directly connected with the electrical connection of 1# motor. At this time, the soft start alone drives 1# motor to operate. The soft start gradually reduces the output voltage, and the motor speed gradually decreases until shutdown, After the soft stop operation is completed, km11 is disconnected. In the above process, the control system will timely connect or exit the soft start circuit. The soft starter that uses a soft start sequence to drive multiple motors to complete the soft start and soft stop operations should have the cascade function. The main function of the cascade function is to prepare the status of the soft start before each operation, and send a signal to make the soft start exit the operation in time after the completion of the operation. For example, to start the motor, the soft start thyristor must be in the off state, the output voltage is zero, and then enter the start operation. If it is to perform the shutdown operation, the soft start thyristor must be in the on state, gradually reduce the output voltage, and complete the shutdown operation. After each operation, the soft start must exit the running line. The protection function of soft start itself is complete, but when a soft start is equipped with multiple motors, the soft start must exit the operation after the start and stop operation is completed, so the motor protection should be set separately, and the soft start can only protect during the start and stop process. A water supply control composed of frequency converter, soft starter, programmable controller and low-voltage electrical appliances has good operation performance

a frequency converter drives a pump to run at variable speed, and a soft starter completes the operation of starting and stopping the other pumps. The frequency conversion pump can rotate regularly to balance the operation time of each pump. This control system can not only adjust the speed of the variable frequency pump according to the outlet pressure of the pipe, but also timely connect or exit the soft start circuit to complete the operation of starting and stopping the pump. This system overcomes the electrical and mechanical impact caused by the frequency conversion to power frequency switching of the frequency conversion pump in the frequency converter control system. This control system has the function of soft start and soft stop, which can avoid the water sag effect caused by the sudden change of the pump when starting and stopping, and ensure the safety of equipment and pipes. This performance is particularly important for large and medium-sized water supply pump stations. To sum up, the control system mainly composed of frequency converter, soft start and programmable controller is adopted in large and medium-sized pump stations, which integrates modern power electronics technology, microelectronics technology and control technology. The use of long glass fiber composites reduces the cost and weight of vehicles, and provides a high-performance control system that meets the needs of large and medium-sized water supply pump stations. This system has the advantages of stable operation, high efficiency and energy saving, high automation and easy operation. Due to the use of soft start, the process of starting and stopping the equipment is stable and the water sag effect is avoided. Compared with the ordinary frequency conversion control system, this control system increases the soft start, and the soft start cost is relatively low. This control system only increases less investment, which can greatly improve the equipment performance. This technology is of great promotion and application value in large and medium-sized water pump stations

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