Discussion on the output switching problem of frequency converter - and the soft start and stop control scheme of water pump group

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Discussion on the output switching problem of frequency converter - and the soft start and stop control scheme of water pump group [Copy link]

Discussion on the output switching problem of frequency converter - and the soft start and stop control scheme of water pump group

Abstract: In view of the misunderstandings on the output switching of frequency converters, this paper attempts to analyze and discuss the output switching of frequency converters from the perspective of technology and economic practicality, in order to clarify some erroneous views and establish an objective understanding. At the same time, a multi-pump constant pressure water supply and water pump group soft start and stop control scheme is proposed. Keywords: frequency converter; output switching; soft start; constant pressure water supply system

The current shock and torque shock generated by the direct start of AC asynchronous motors will have an adverse effect on the power supply system and the traction system, so a soft start solution is generally used for asynchronous motors with larger capacity. The commonly used method is step-down starting, but since the starting torque of the motor is proportional to the square of the applied voltage, step-down starting is only suitable for equipment that starts with no load or light load. Even so, during the step-down starting process, since the power supply frequency is not changed, the existence of excessive slip rate will inevitably cause a large overcurrent. For equipment that starts with heavy load, a variable frequency soft start solution is required, that is, the frequency converter is used to drive the motor to start from zero speed, gradually increase the voltage and speed until it reaches its rated speed. The advantage of variable frequency soft starting is that it does not generate overcurrent due to the use of voltage/frequency proportional control method; and it can provide a starting torque equal to the rated torque, so it is particularly suitable for equipment that needs to start with heavy load or full load.

If the inverter only undertakes the task of soft starting and does not operate at a speed regulation, then after the inverter drives the motor to reach the rated speed, the motor must be switched to the power grid for direct power supply, and the inverter can start other motors. The mother pipe multi-pump constant pressure water supply system is a typical example. When the water pressure is too high and the pump needs to be stopped, in order to avoid the "water hammer effect", it is not allowed to suddenly cut off the power supply of the pump, but it is required to gradually reduce the speed and stop slowly. At this time, it is necessary to switch the motor to the inverter to achieve deceleration and stop. In this way, it is inevitable to switch between the power grid and the inverter. The output switching problem of the inverter has not received enough attention at present, so there are still some misunderstandings in the understanding: one view is to treat the inverter as a general AC power supply, or like a soft starter, so the motor can be switched between the inverter and the power grid at will; another view is that due to the design principle of the inverter itself, the inverter is not allowed to switch during operation. Both of these views are biased. Therefore, in articles on the application of frequency converters in traction systems, when encountering the switching problem of frequency converters, they either deliberately avoid it and do not give a specific description; or simply mention it and use a simple sentence "switch to the grid operation" [3]. Even though some articles have made some explorations on the switching problem [1][2], they have not revealed the essence of the problem, giving people a feeling of failure [1]. This article attempts to further analyze and explore the problem of frequency converter output switching from the perspective of technology and economic practicality. If there are any inappropriateness, colleagues are welcome to criticize and correct it.