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Method for regulating a delivery variable of a pump
| Details |
Inventors: M.o slashed.ller, Eik Sefeldt;
Assignee: Danfoss Drives A/S (Graasten, DK)
Primary Examiner: Koczo; Michael
Assistant Examiner:
Attorney, Agent or Firm: Lee, Mann, Smith, McWilliams, Sweeney & Ohlson
In a known method for regulating a delivery variable of a pump, which is driven by an electric motor operated with alternating current of variable frequency, especially via a converter, the input power of the motor is measured as the actual value for the delivery variable and is regulated by comparison with a desired value. To avoid the influence of the temperature of the motor on the delivery variable at constant input power, according to the invention upon a change in the temperature in the motor a corresponding compensating variable is taken into account in the control for the purposes of correcting the input power. As an alternative, associated values of the input power and the speed of the motor at a predetermined desired pressure value are determined empirically and stored as a table, and during operation a value of the input power belonging to a measured or approximately calculated speed of the motor is retrieved from the table as desired for regulating the input power. |
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DETAILED DESCRIPTION The invention is based on the problem of improving a method of the kind mentioned in the introduction, without direct measurement of the pressure or flow rate of the pump, that is, without using a pressure sensor or flow rate sensor, but using the input power of the electric motor as control variable, to the effect that the desired operating point of the motor, and hence of the pump, remains stable.
According to the invention, that problem is solved in that upon a change in the temperature in the motor a corresponding compensating variable is taken into account in the control for the purposes of correcting the input power.
This solution is based on the realization that the change in the operating point is the consequence of a change in the ohmic resistance in the stator and rotor of the motor.
This change is in turn primarily the consequence of heating of the motor through electrical losses or convective heat, for example, through hot water that is being conveyed by the pump.
The electrical losses in the motor, and hence the slip, therefore increase, so that the output power of the motor, its speed and hence also the pressure of the pump, decrease.
Heating of the motor therefore has two effects: firstly, losses in the rotor are increased, with the result that less power is delivered to the shaft.
With regulation of the input power of the motor in which only the input power is measured, this power loss is not detected.
In the case of such regulation, there would therefore be no compensation for the power loss in the rotor.
A second effect of heating of the motor is that its slip increases.
This means that less power is transferred to the rotor.
The input power regulation interprets this erroneously as reduced power requirement and reduces the operating frequency of the motor.
The operating point of the pump therefore differs from the desired operating point.
The invention compensates for the temperature-dependent pressure fall, without the pressure being measured directly
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