Abstract: Exemplary methods and systems of determining stall of a fan are disclosed, when the fan is controlled with a frequency converter. The method includes estimating a rotational speed (n) and the torque (T) of the fan. Transferring characteristic curves of the fan to the estimated rotational speed (n) of the fan, determining the stall region of the fan. Determining an operation point of the fan from the estimated rotational speed (n) and estimated torque (T) using the characteristic curves. Calculating RMS values of the low frequency components of the torque and rotational speed estimates (TRMS, nRMS) to obtain a low frequency parameter (S), and determining an occurrence of stall based on at least one of the operation point and the low frequency parameter (S).

Abstract: An exemplary method for determining a flow rate of a pump, includes measuring and storing the flow rate (QMean) when the pump is in operation, storing rotational speed (nMean) and torque (TMean) of the pump during measurement of the flow rate (QMean), determining torque (TManufacturer) corresponding to the measured flow rate (QMean), calculating a bias value of the torque (TBias) by comparing the torque determined from the characteristic curve (TManufacturer) and the stored torque (TMean) of the pump, and determining an operating point of the pump based on the bias value of the torque (TBias).

Abstract: A method and arrangement for determining the flow rate (Q) produced by a pump, when the pump is controlled with a frequency converter, which produces estimates for rotational speed and torque of the pump, and the characteristic curves of the pump are known. The method includes determining the shape of a QH curve of the pump, dividing the QH curve into two or more regions depending on the shape of the QH curve, determining on which region of the QH curve the pump is operating, and determining the flow rate (Q) of the pump using the determined operating region of the characteristic curve.

Abstract: An exemplary frequency converter and method are directed to estimating an operation point of a pump when the QH characteristic curve of the pump is known. The frequency converter controls the pump, and a process curve is estimated when a first operation point of the pump is in the nominal range. The process curve defines a head as a function of volumetric flow. A rotational speed of the pump is determined and a QH characteristic curve of the pump is converted to a current rotational speed of the pump based on affinity laws. A second operation point of the pump is estimated by determining an intersection point of the converted QH characteristic curve and the estimated process curve.