What is the minimum frequency of a centrifugal pump?
In fact there are two hurdles:
- The motor: a standard motor is cooled by the fan built into the shaft. In continuous mode, you don’t go below 30 Hz, in intermittent mode you can go down to 25 Hz. If lower speeds are required, you will have to use a self-ventilated motor that ensures nominal cooling airflow whatever the speed. It is a specific kind of motor.
- Pump: the manufacturer determines this. You can ask him to provide the curve Total Head (TH) = f(Q) in 5 Hz increments from 50 to 25 Hz on which it will draw the acceptable minimum flow rate. You will then need to verify the whether the minimum speed/minimum frequency/TH is appropriate to your installation
You will need to know these parameters should you have to incorporate a variable speed booster.
Below is a block diagram of the Lattes booster, 3 pumps can run in tandem. To achieve a recommended pressure of 4.3 bars you can see that the minimum flow rate is in the range of 25 m3/h for a frequency of 43 Hz.
The green circle represents the operating point, it is dynamically positioned depending on the flow rate and the calculated TH
If the set pressure had been 2.5 bars, mini flow would have been 20 m3/h to about 32 Hz. This shows that frequency and minimum flow rate depend on the application in question.