I would like to ask: why does a large fluctuation in flow rate occur when switching from an inverter-driven pump to a conventional frequency-driven pump? How can this phenomenon be understood?

The question isn’t clear – are the variable-frequency pump and the fixed-frequency pump meant to serve as backups for each other? What is the frequency of the variable-frequency pump during switching?

I think it’s necessary for you to explain how you switch between them, and then we can decide on what to do next...

In this situation, it can be seen from the description that the power at variable frequency does not decrease, while for the power at the industrial frequency, the opening degree of the valve at the outlet of the high-speed pump directly determines the amount of feed. Therefore, pump operation is primarily focused on maintaining stable flow rates.

This post was last edited by longziyun on 2020-8-25 at 11:39. It’s a matter of combining theory with experience. This situation usually occurs when switching from an variable-frequency pump to a fixed-frequency pump. I think it can be understood in this way: generally, we set the frequency conversion at levels below 90%; if it’s set higher, the initial investment may not be recouped within ten or twenty years, in which case the advantage of using a frequency converter is minimal. For example, an ordinary refrigerator costs 2,500 yuan, while a frequency-conversion refrigerator costs 4,000 yuan. Ads claim that it can save 1 kilowatt-hour of electricity per day, with the cost of 1 kilowatt-hour being 0.5 yuan; thus, the cost savings can be recovered over 3,000 days. By the time the refrigerator reaches the end of its useful life after 8 years, that’s 8×365 = 2,920 days, and still no electricity has been saved – the same principle applies here. That’s a bit of a digression. To get back to the topic, variable frequency drives adjust the motor’s output power by controlling its frequency and voltage, thereby changing the pump’s capacity to suit new flow rates and head pressures. When variable frequency pumps are in use, the changing frequency alters the pump’s operating curve; as a result, the head pressure usually decreases, while the flow rate remains unchanged. In my view, the frequency converter functions like a control valve, and on the pump outlet pipeline, a control valve assembly is also installed as required by the process. The power-frequency pump system has one control valve, while the variable-frequency pump system is equivalent to having two control valves in the pipeline ； The outlet head of a variable-frequency pump is low; therefore, in order to maintain a constant flow rate, the opening degree of the control valve in the pipeline must be greater than that which would be required for a normal-frequency pump operating at a higher head. In this way, during the transition from a variable-frequency pump to a fixed-frequency pump, if the opening degree of the backflow control valve is not adjusted in advance (i.e., not reduced), then after switching to the fixed-frequency pump, the pump’s head increases. With the opening degree of the control valve remaining unchanged, the flow rate will inevitably increase, resulting in the large fluctuations in flow rate that you mentioned when switching to the fixed-frequency pump. By adjusting the opening degree of the control valve in a timely manner during the switching process, flow fluctuations can be reduced, thereby maintaining operational stability.

The simplest and most effective approach is to set the frequency converter to 100 before switching to fixed frequency, control the flow rate using a control valve, and then switch to fixed frequency.

Ours are all stable; there have been no major fluctuations

In not all cases will installing a frequency converter on a mechanical pump save energy. To be blunt, your situation might well indicate that such operating conditions are not suitable for installing an inverter in the first place; it is actually clear evidence of a failed renovation attempt.