Method in connection with a pump driven with a frequency converter and frequency converter

1. A method of estimating an operation point of a pump driven with a frequency converter when a QH characteristic curve of the pump is known, wherein the method comprises:

controlling the pump with the frequency converter, wherein the controlling step comprises:

estimating a process curve when an operation point of the pump is in a nominal range, the process curve defining a head required by the process as a function of volumetric flow;

determining a rotational speed of the pump;

converting the QH characteristic curve of the pump to a current rotational speed of the pump;

(1) estimating the operation point of the pump with a QP calculation;

(2) estimating the operation point of the pump by determining the intersection point of the converted QH characteristic curve and the estimated process curve;

(3) calculating and storing a difference between corresponding operating points together with the rotational speed estimate;

repeating steps (1)-(3) and determining if the difference between the differently estimated operation points has changed; and

estimating the process curve if the difference between the estimated operation points has changed.

2. The method of claim 1 , wherein the estimation of the process curve comprises:

estimating using a QP calculation, a head (h QP ) produced with the pump and a volumetric flow (Q QP ) produced by the pump when the pump is operating in its nominal operation area,

storing estimated values of the produced head and the volumetric flow together with the rotational speed of the pump,

repeating for a predetermined number of times the steps of estimating the produced head and volumetric flow and storing the estimated values when the rotational speed of the pump changes, and

forming a curve fitted to the estimated values, the curve representing the process curve.

3. The method of claim 1 , wherein the rotational speed is determined by the frequency converter driving the pump.

4. The method of claim 1 , wherein the operation is in the nominal range of 80% to 120% of a nominal volumetric flow of the pump and of a nominal rotational speed.

5. The method of claim 1 , wherein the process curve is estimated when the pump is operated in the nominal range of 80% to 120% of the nominal volumetric flow of the pump and of the nominal rotational speed, and

the operation point of the pump is estimated by determining the intersection point of the QH characteristic curve and a process curve when the operation point of the pump is outside the range of 80% to 120% of the nominal volumetric flow of the pump and of the nominal rotational speed.

6. The method of claim 2 , wherein the rotational speed of the pump is determined by the frequency converter driving the pump.

7. The method of claim 2 , wherein the operation is in the nominal range of 80% to 120% of a nominal volumetric flow of the pump and of a nominal rotational speed.

8. The method of claim 2 , wherein the process curve is estimated when the pump is operated in the nominal range of 80% to 120% of the nominal volumetric flow of the pump and of the nominal rotational speed, and

the operation point of the pump is estimated by determining the intersection point of the QH characteristic curve and a process curve when the operation point of the pump is outside the range of 80% to 120% of the nominal volumetric flow of the pump and of the nominal rotational speed.

9. The method of claim 3 , wherein the operation is in the nominal range of 80% to 120% of a nominal volumetric flow of the pump and of a nominal rotational speed.

10. The method of claim 3 , wherein the process curve is estimated when the pump is operated in the nominal range of 80% to 120% of the nominal volumetric flow of the pump and of the nominal rotational speed, and

the operation point of the pump is estimated by determining the intersection point of the QH characteristic curve and a process curve when the operation point of the pump is outside the range of 80% to 120% of the nominal volumetric flow of the pump and of the nominal rotational speed.

11. The method of claim 4 , wherein the process curve is estimated when the pump is operated in the nominal range of 80% to 120% of the nominal volumetric flow of the pump and of the nominal rotational speed, and

the operation point of the pump is estimated by determining the intersection point of the QH characteristic curve and a process curve when the operation point of the pump is outside the range of 80% to 120% of the nominal volumetric flow of the pump and of the nominal rotational speed.

12. A frequency converter that estimates an operation point of a pump when a QH characteristic curve of the pump is known and the pump is adapted to be driven with the frequency converter, wherein the frequency converter comprises:

means for estimating a process curve when an operation point of the pump is in a nominal range, the process curve defining a head as a function of volumetric flow;

means for determining a rotational speed of the pump;

means for converting, based on affinity laws, the QH characteristic curve of the pump to a current present rotational speed of the pump;

wherein the frequency converter:

(1) estimates the operation point of the pump with a QP calculation,

(2) estimates the operation point of the pump by determining the intersection point of the converted QH characteristic curve and the estimated process curve,

(3) calculates and storing a difference between corresponding operating points together with the rotational speed estimate,

repeats steps (1)-(3) and determines if the difference between the differently estimated operation points has changed, and

estimates the process curve if the difference between the estimated operation points has changed.