water that is drained off is kept constant unless the speed of rotation of the impeller is altered as a result of a change in the frequency f of the motor supply voltage the present invention is directed to a full flow drainage process for the full flow drainage process the manufacturer defines or presets a plurality of possible frequencies f for the motor supply voltage limited by a maximum possible frequency fmax and a minimum frequency fmin the maximum and minimum frequencies fmax and fmin are selected in accordance with hydraulic and mechanical factors so as not to strain the pump unit 1 the preset frequencies f are therefore within a delimited range between the maximum and minimum frequencies fmax and fmin the maximum and minimum frequencies fmax and fmin being included between the preset frequencies f by way of example the maximum and minimum frequencies fmax and fmin may be 60 hz and 30 hz respectively and five intermediate frequencies f of 35 40 45 50 and 55 hz may be preset this example is not restrictive as other frequency f values may be selected and a different number of intermediate frequencies f the appliance 100 also comprises storage means 4 which correspond with a memory that may be integrated into.