Patent Publication Number: US-2020277719-A1

Title: Method for operating a laundry treatment machine and laundry treatment machine

Description:
The present invention concerns the field of laundry treatment techniques. 
     Specifically, the invention relates to a method for controlling a pump in a laundry treatment machine capable of performing a more efficient treatment cycle. 
     The invention relates also to a laundry treatment machine implementing such a method. 
     BACKGROUND ART 
     Nowadays the use of laundry treating machines capable of carrying out a washing and/or drying process on laundry is widespread. 
     Laundry treating machines generally comprise an external casing provided with a laundry container where the laundry to be treated is placed. 
     A loading/unloading door ensures access to the container for the insertion and removal of the laundry. 
     Laundry treating machines of known type comprise laundry washing machines, both “simple” laundry washing machines (i.e. laundry washing machines which can only wash and rinse laundry) and laundry washing-drying machines (i.e. laundry washing machines which can also dry laundry). 
     In the present description, therefore, the term “laundry washing machine” will refer to both a simple laundry washing machine and a laundry washing-drying machine. 
     Laundry treating machines of known type then comprise laundry drying machines, or dryer, i.e. laundry machines which only dry laundry. 
     Laundry washing machines generally comprise an external casing provided with a washing tub which contains said container, preferably a rotatable perforated drum, where the laundry is placed. A loading/unloading door ensures access to the drum. 
     Laundry washing machines then typically comprise a water supply unit and a treating agents dispenser, preferably equipped with a drawer, for the introduction of water and washing/rinsing products (i.e. detergent, softener, rinse conditioner, etc.) into the tub. 
     Known laundry washing machines are also typically provided with water draining devices that may operate during different phases of the washing cycle to drain water from the tub. 
     A water draining device of known type is constituted of a water outlet circuit suitable for withdrawing liquid, for example dirty water, from the bottom of the tub to the outside. The water outlet circuit is typically provided with a controlled draining pump. 
     Another water draining device of known type is constituted of a recirculation circuit adapted to drain liquid from the bottom region of the tub and to re-admit such a liquid into an upper region of the tub. The recirculation circuit is preferably provided with a terminal nozzle opportunely arranged so that the recirculated liquid is sprayed directly into the drum through its holes so that distribution of liquid over the laundry is enhanced. The recirculation circuit is typically provided with a controlled recirculation pump. 
     A further water draining device of known type is constituted of a recirculation circuit, or mixing circuit, which is adapted to drain liquid from a bottom region of the tub and to re-admit such a liquid (recirculated mixing liquid) into a region of the tub which corresponds substantially to the same bottom region of the tub. 
     The mixing circuit is preferably realized for transferring a portion of a liquid from a bottom region of the tub to the same bottom region for mixing and/or dissolution of the products, in particular for liquid and/or powder detergent. The recirculation circuit is typically provided with a controlled recirculation pump. 
     Laundry washing machines of known type are then provided with a water softening device, preferably arranged inside the cabinet. 
     The water softening device is structured for reducing the hardness degree of the fresh water drawn from the external water supply line. 
     The water softening device of known type comprises a water-softening agent container and a regeneration-agent reservoir. A controlled pump is then typically interposed between the water-softening agent container and the regeneration-agent reservoir and is structured for transferring/moving the brine (i.e. the salt water) from the regeneration-agent reservoir to the water-softening agent container. 
     Laundry drying machines generally comprise a container, preferably a rotatable drum, where the laundry is placed. A loading/unloading door ensures access to the drum. 
     Laundry drying machines of known type preferably comprise a sump arranged below the container where condensation water formed in the drying cycle is advantageously collected. 
     Condensation water collected in the sump is preferably drained to an extractable moisture tank located at the upper portion of the laundry drying machine so that it can be easily and periodically emptied by a user. 
     A controlled pump opportunely drains the condensation water from the sump to the tank. 
     As described above, therefore, laundry treatment machines are typically equipped with one or more pumps, each preferably operated by an electric motor. 
     Pumps of known type comprise an inlet and a rotating element, or impeller, which increases the pressure and the flow of the liquid towards an outlet of the pump itself. 
     Pumps differently work according to the liquid level at its inlet. 
     Pumps correctly work when there is enough liquid to be moved at its inlet. 
     However, a pump can work even in the presence of a low level of liquid and air at its inlet. 
     Nevertheless, in such situation, also indicated as air-water condition, the pump causes a great deal of noise, damage to components, vibrations, and a loss of efficiency. 
     According to known solutions, when an air-water condition arises the pump is typically switched off. 
     However, laundry washing machines of the known art pose some drawbacks. 
     A drawback of the laundry treating machines of the known art is the fact that the air-water condition is indirectly detected by using a dedicated water/liquid level sensor or using a water/liquid level sensor already installed in the laundry treating machine to control the washing liquid level (for example a pressure sensor installed in laundry washing machines). 
     For example, laundry washing machine comprises a water level sensor arranged at the bottom of the tub and the recirculation and/or the drain pump is switched off as soon as the water level reaches a prefixed minimum threshold level. 
     Nevertheless, detection of the water level through said sensor may not be accurate. For example, in laundry washing machine detection may not be accurate due in particular to oscillations of the water inside the tub, more in particular when the drum is rotating. Furthermore, the measure of the water level inside the tub only indirectly gives indication of presence of air at the pump inlet. Therefore, there is not absolute certainty that the pump is actually working in air-water condition. 
     The pump may therefore be switched off even if it is still working properly, thus reducing efficiency of the draining process and/or causing incompleteness of the draining process. 
     The object of the present invention is therefore to overcome the drawbacks posed by the known technique. 
     It is an object of the invention to provide a method for draining liquid in a pump of a laundry treating machine that makes it possible to drive the pump with a higher efficiency compared to known system. 
     It is another object of the invention to provide a method for draining liquid in a pump of a laundry treating machine which makes it possible to drive the pump independently from any water level sensor. 
     It is a further object of the invention to provide a method for draining liquid in a pump of a laundry treating machine that makes it possible to correctly drive the pump according to the effective presence of water/liquid at its inlet. 
     DISCLOSURE OF INVENTION 
     Applicant has found that by detecting/evaluating an air-water working condition in a pump of a laundry treating machine on the base of values of voltage and/or current of an electric motor of said pump, it is possible to reach the mentioned objects. 
     In a first aspect thereof the present invention relates, therefore, to a method for operating a laundry treatment machine comprising a container for housing a laundry load to be treated and a pump comprising an inlet for a liquid suction, said pump comprising an electric motor adapted to be powered by a motor voltage and a motor current; wherein an air-water working condition of said pump is detected/evaluated based on values of said motor voltage and/or said motor current of said electric motor. 
     Preferably, the air-water working condition of the pump is a condition caused by the presence of air at the inlet of the pump. 
     Preferably, the electric motor is connected or connectable to a mains power supply providing a sinusoidal mains voltage. 
     In a preferred embodiment of the invention, the motor voltage applied to the electric motor is a partialized version of the sinusoidal mains voltage. 
     According to a preferred embodiment of the invention, the air-water working condition occurs if the root-mean-square voltage of the motor voltage deviates from a threshold value. 
     Preferably, the air-water working condition occurs if the root-mean-square voltage of the motor voltage is above or below said threshold value. 
     In a preferred embodiment of the invention, the threshold value is set as a function of an expected target value of the root-mean-square voltage applied to the electric motor. 
     Preferably, said threshold value is proportional to the target value. 
     In a further preferred embodiment of the invention, the air-water working condition occurs if the activation time of the motor current deviates from a threshold value. 
     The activation time is preferably the period of time starting when the motor current increases or decreases from 0 and ending when said motor current crosses again 0. 
     According to a preferred embodiment of the invention, the air-water working condition occurs if the activation time is above or below the threshold value. 
     In a preferred embodiment of the invention, said threshold value is set as a function of an expected target value of the activation time. 
     According to a preferred embodiment of the invention, the threshold value is proportional to the expected target value. 
     In a further preferred embodiment of the invention, the air-water working condition occurs if the phase difference between the motor voltage and the motor current deviates from a threshold value. 
     In a preferred embodiment of the invention, said threshold value is set as a function of an expected value of the phase difference. 
     Preferably, said threshold value is proportional to the expected value. 
     According to a preferred embodiment of the invention, the laundry treating machine comprises a driving circuit for driving the electric motor, the driving circuit comprising a solid-state switch. 
     In a preferred embodiment of the invention, if an air-water working condition has been established then at least one action is taken. 
     Preferably, said action comprises one of the following actions:
         deactivation of said pump;   deactivation of said pump immediately after said air-water working condition has been established;   deactivation of said pump after a predetermined period after said air-water working condition has been established;   variation of the speed of said pump;   reduction of the speed of said pump;   temporary variation of the speed of said pump;   temporary reduction of the speed of said pump;   increasing the level of the liquid at said inlet of said pump.       

     According to a preferred embodiment of the invention, if an air-water working condition has been established and if said laundry treatment machine is a laundry washing machine comprising a washing tub external to said container, said action comprises one of the following actions:
         introducing a quantity of water into said washing tub from an external water supply line;   increasing the level of the liquid at said inlet of said pump performing a spinning phase by rotating said container in order to extract liquid from said laundry, in case said container is not moving; or   increasing the rotating speed of said container, in case said container is already rotating.       

     In a further aspect, the present invention relates to a laundry treatment machine comprising at least one pump comprising an inlet for a liquid suction, said pump comprising an electric motor adapted to be powered by a motor voltage and a motor current, wherein said machine comprises a control unit which is designed for detecting/evaluating said motor voltage and/or said motor current during operation for determining an air-water working condition of said pump. 
     Preferably, the machine comprises a laundry washing machine or a laundry washing/drying machine or a laundry drying machine. 
     According to a preferred embodiment of the invention, the laundry treatment machine is a laundry washing machine comprising a washing tub external to the container. 
     Preferably, the pump is a pump of one or more of the following part of said machine:
         a water softening device;   a water outlet circuit for draining liquid outside the machine;   a recirculation circuit for draining liquid from a bottom region of said washing tub and to re-admitting such a liquid into said washing tub;   a treatment agents dispenser.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages of the present invention will be highlighted in greater detail in the following detailed description of some of its preferred embodiments, provided with reference to the enclosed drawings. In the drawings, corresponding characteristics and/or components are identified by the same reference numbers. In particular: 
         FIG. 1  shows a perspective view of a laundry treating machine implementing the method according to a first embodiment of the invention; 
         FIG. 2  shows a schematic view of the laundry treating machine of  FIG. 1 ; 
         FIG. 3  shows circuit elements of the laundry treating machine according to an embodiment of the present invention; 
         FIG. 4  shows qualitative waveforms of instantaneous electric parameters of the laundry treating machine working in a first operative condition; 
         FIG. 5  shows the evolution of an electric parameter of the laundry treating machine in a first operative condition during the execution of a washing cycle; 
         FIG. 6  shows the evolution of the electric parameter of  FIG. 5  in a second operative condition during the execution of a washing cycle; 
         FIG. 7  shows the qualitative waveforms of  FIG. 6  in a second operative condition; 
         FIG. 8  shows qualitative waveforms of instantaneous electric parameters of the laundry treating machine working in a first operative condition according to another preferred embodiment of the invention; 
         FIG. 9  shows the qualitative waveforms of  FIG. 8  in a second operative condition. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The method of the present invention has proved to be particularly advantageous when applied to laundry washing machines, as described below. It should in any case be underlined that the present invention is not limited to this type of application. On the contrary, the present invention can be conveniently applied to other type of laundry treating machines, like for example laundry washing and drying machines or laundry drying machines, equipped with one or more pumps. 
     With reference to  FIGS. 1 and 2 , a preferred embodiment of a laundry washing machine  1  according to the invention is described, in which a method according to a first embodiment of the invention is implemented. 
     The laundry washing machine  1  preferably comprises an external casing or housing  2 , a washing tub  3 , a container  4 , preferably a perforated washing drum  4 , where the laundry to be treated can be loaded. 
     The tub  3  and the drum  4  both preferably have a substantially cylindrical shape. 
     The housing  2  is provided with a loading/unloading door  8  which allows access to the drum  4 . 
     The drum  4  is advantageously rotated by an electric motor, not illustrated, which preferably transmits the rotating motion to the shaft of the drum  4 , advantageously by means of a belt/pulley system. In a different embodiment of the invention, the motor can be directly associated with the shaft of the drum  4 . 
     The drum  4  is advantageously provided with holes which allow the liquid flowing therethrough. Said holes are typically and preferably homogeneously distributed on the cylindrical side wall of the drum  4 . 
     The bottom region  3   a  of the tub  3  preferably comprises a seat  15 , or sump, suitable for receiving a heating device  10 . The heating device  10 , when activated, heats the liquid inside the sump  15 . 
     In different embodiments, nevertheless, the bottom region of the tub may be configured differently. For example, the bottom region of the tub may not comprise a seat for the heating device. The heating device may be advantageously placed in the annular gap between the tub and the drum. 
     A water supply circuit  5  is arranged in the upper part of the laundry washing machine  1  and is suited to supply water into the tub  3  from an external water supply line E. The water supply circuit of a laundry washing machine is well known in the art, and therefore it will not be described in detail. The water supply circuit  5  preferably comprises a controlled supply valve  5   a  which is properly controlled, opened and closed, during the washing cycle. 
     The laundry washing machine  1  advantageously comprises a treating agents dispenser  14  to supply treating agents into the tub  3  during a washing cycle. Treating agents may comprise, for example, detergents, rinse additives, fabric softeners or fabric conditioners, waterproofing agents, fabric enhancers, rinse sanitization additives, chlorine-based additives, etc. 
     Preferably, the treating agents dispenser  14  comprises a removable drawer provided with various compartments suited to be filled with treating agents. 
     In a preferred embodiment, not illustrated, the treating agents dispenser may comprise a pump suitable to convey one or more of said agents from the dispenser to the tub. 
     In the preferred embodiment here illustrated, the water is supplied into the tub  3  from the water supply circuit  5  by making it flow through the treating agents dispenser  14  and then through a supply pipe  18 . 
     Furthermore, in the preferred embodiment here illustrated, a water softening device  170  is preferably arranged/interposed between the external water supply line E and the treating agents dispenser  14  so as to be crossed by the fresh water flowing from the external water supply line E. The water softening device  170 , as known, is structured for reducing the hardness degree of the fresh water drawn from the external water supply line E and conveyed to the treating agents dispenser  14 . 
     In a different embodiment, the water softening device  170  may be arranged/interposed between the external water supply line E and the washing tub  3 , so as to be crossed by the fresh water flowing from the external water supply line E and conveying it directly to the washing tub  3 . 
     Some elements and/or components of the water softening device  170  are well known in the art, and therefore will not be described in detail. 
     The water softening device  170  basically comprises a water-softening agent container  171  and a regeneration-agent reservoir  172 . 
     The water-softening agent container  171  is crossed by the fresh water arriving from the external water supply line E. The water-softening agent container  171  is filled with a water softening agent able to reduce the hardness degree of the fresh water flowing through the same water-softening agent container  171 . 
     The regeneration-agent reservoir  172  is fluidly connected to the water-softening agent container  171  and is structured for receiving a given quantity of salt or other regeneration agent which is able to regenerate the water softening function of the water softening agent stored inside the water-softening agent container  171 . 
     The water softening device  170  then preferably comprises an electrically-powered brine-circulating pump  180  which is interposed between the water-softening agent container  171  and the regeneration-agent reservoir  172  and is structured for transferring/moving, when activated, the brine (i.e. the salt water) from the regeneration-agent reservoir  172  to the water-softening agent container  171 . 
     Laundry washing machine  1  preferably comprises a water outlet circuit  25  suitable for withdrawing liquid from the bottom region  3   a  of the tub  3 . 
     The water outlet circuit  25  preferably comprises a main pipe  17 , a draining pump  26  and an outlet pipe  28  ending outside the housing  2 . 
     The water outlet circuit  25  preferably further comprises a filtering device  12  arranged between the main pipe  17  and the draining pump  26 . The filtering device  12  is adapted to retain all the undesirable bodies (for example buttons that have come off the laundry, coins erroneously introduced into the laundry washing machine, etc.). 
     The main pipe  17  connects the bottom region  3   a  of the tub  3  to the filtering device  12 . 
     In a further embodiment, not illustrated, the filtering device  12  may be provided directly in the tub  3 , preferably obtained in a single piece construction with the latter. In this case, the filtering device  12  is fluidly connected to the outlet of the tub  3 , in such a way that water and washing liquid drained from the tub  3  enters the filtering device  12 . 
     Activation of the drain pump  26  drains the liquid, i.e. dirty water or water mixed with washing and/or rinsing products, from the tub  3  to the outside. 
     Laundry washing machine  1  preferably comprises a first recirculation circuit  30 , or mixing circuit  30 . The mixing circuit  30  is adapted to drain liquid from the bottom region  3   a  of the tub  3  and to re-admit such a liquid (recirculated mixing liquid) into a first region of the tub  3 , which substantially corresponds to the same bottom region  3   a  of the tub  3 . 
     Preferably, the mixing circuit  30  is adapted to drain liquid from the bottom of the sump  15  and to re-admit such a liquid (recirculated mixing liquid) again into the sump  15 . 
     The mixing circuit  30  preferably comprises a first recirculation pump  31 , a first pipe  32  connecting the filtering device  12  to the first recirculation pump  31  and a second recirculation pipe  33 , preferably ending inside the sump  15 , as mentioned above. 
     In a further preferred embodiment, not illustrated, the mixing circuit may comprise a dedicated pipe connecting the bottom region of the tub to the recirculation pump; in this case, the mixing circuit is advantageously completely separated from the water outlet circuit, i.e. completely separated from the filtering device  12  and the main pipe  17 . 
     The mixing circuit is preferably realized for transferring a portion of a liquid from a bottom region of the tub to the same bottom region for mixing and/or dissolution of the products, in particular of the detergent. 
     Laundry washing machine  1  preferably comprises a second recirculation circuit  20  adapted to drain liquid from the bottom region  3   a  of the tub  3  and to re-admit such a liquid into a second region  3   b , or upper region, of the tub  3 . 
     The second recirculation circuit  20  preferably comprises a second recirculation pump  21 , a second pipe  22  connecting the filtering device  12  to the second recirculation pump  21  and a second recirculation pipe  23 , preferably provided with a terminal nozzle  23   a  arranged preferably at the upper region  3   b  of the tub  3 . The terminal nozzle  23   a  is opportunely arranged so that the liquid is sprayed directly into the drum  4  through its holes. 
     The terminal nozzle  23   a , therefore, enhances distribution of liquid over the laundry through the perforated drum  4 . 
     The liquid from the bottom region  3   a  of the tub  3  is conveyed towards the upper region  3   b  of the tub  3  by activation of the second recirculation pump  21 . 
     The second recirculation circuit  20  is therefore advantageously activated in order to improve wetting of the laundry inside the drum  4 . 
     In general, the second recirculation circuit is properly realized for transferring a portion of a liquid from a bottom region of the tub, preferably from the sump, to an upper region of the tub in order to enhance absorption of the liquid by the laundry. 
     Preferably, laundry washing machine  1  comprises a device  19  suited to sense (or detect) the liquid level inside the tub  3 . 
     The sensor device  19  preferably comprises a pressure sensor which senses the pressure in the tub  3 . From the values sensed by the sensor device  19  it is possible to determine the liquid level of the liquid inside the tub  3 . In another embodiment, not illustrated, laundry washing machine may preferably comprise (in addition to or as a replacement of the pressure sensor) a level sensor (for example mechanical, electro-mechanical, optical, etc.) adapted to sense (or detect) the liquid level inside the tub  3 . 
     Laundry washing machine  1  advantageously comprises a control unit  11  connected to the various parts of the laundry washing machine  1  in order to ensure its operation. The control unit  11  is preferably connected to the water inlet circuit  5 , the water outlet circuit  25 , the recirculation circuits  30 ,  20 , the heating device  10  and the electric motor moving the drum  4  and receives information from the various sensors provided on the laundry washing machine  1 , like the pressure sensor  19 , a temperature sensor, etc. 
     In particular, the control unit  11  is preferably connected to the pumps  21 ,  26 ,  31 ,  180  so as to opportunely drive them during the washing cycle. 
     Laundry washing machine  1  advantageously comprises an interface unit  111 , connected to control unit  11 , accessible to the user and by means of which the user may select and set the washing parameters, like for example a desired washing cycle. Usually, other parameters can optionally be inserted by the user, for example the washing temperature, the spinning speed, the load in terms of weight of the laundry to be washed, etc. 
     Based on the parameters acquired by said interface  111 , the control unit  11  sets and controls the various parts of the laundry washing machine  1  in order to carry out the desired washing cycle. 
     As illustrated above, the laundry washing machine  1  is equipped with a plurality of pumps  21 ,  26 ,  31 ,  180 . 
     One of said pumps  21 ,  26 ,  31 ,  180  is schematically illustrated in  FIG. 3  and indicated with reference P. The pump P preferably comprises an inlet Pi, a rotating element R, or impeller, and an outlet Po. 
     Rotating element R increases the pressure and flow of the liquid from the inlet Pi towards the outlet Po. 
     Pumps used in laundry treating machines are well known in the art, and therefore will not be described in detail. 
     The pump P also preferably comprises an electric motor  140 , preferably an asynchronous electric motor operable by both AC and DC electric power supplies, adapted to be powered/fed for causing impeller rotation and hence activation of the pump P. 
     According to an aspect of the invention, the laundry washing machine  1  preferably comprises a driving circuit  155  for driving the electric motor  140 , i.e. for powering/feeding the electric motor  140  with an electric motor voltage, and a corresponding electric motor current, capable of causing impeller rotation. 
     Electric motor driving takes place according to a proper control signal VCTRL generated by the control unit  11 . The control unit  11  advantageously communicates with the driving circuit  155 . 
     Driving circuit  155  is preferably electrically connected to the control unit  11 . In further embodiments, driving circuit  155  may be wirelessly connected to the control unit  11 . 
     The laundry washing machine  1  is connectable/connected to a mains power supply providing an AC mains voltage VMAINS between a line terminal TL and a neutral terminal TN. 
     The driving circuit  155  preferably comprises a solid-state switch  205 , preferably a thyristor device, more preferably a TRIAC device, having a first anode terminal coupled (e.g. directly connected) to the neutral terminal TN of the mains power supply, a second anode terminal coupled (e.g. directly connected) to a first terminal of the electric motor  140 , and a gate terminal coupled (e.g. directly connected) to a triggering circuit  207  (e.g. part of the driving circuit  155  as well) adapted to generate triggering pulse signals for activating the solid-state switch  205  according to the control signal VCTRL. The electric motor  140  comprises a second terminal coupled (e.g. directly connected) to the line terminal TL of the mains power supply. 
     An AC-DC conversion circuit (only conceptually illustrated in the figure and denoted, as a whole, by the number reference  210 ) is provided (preferably arranged at control unit  11  side) that comprises transforming, rectifying and regulation components for receiving the mains voltage VMAINS across the line TL and neutral TN terminals of the mains power supply and providing a (DC) ground voltage GND and a (DC) supply voltage Vcc (e.g. a 3V, 5V or 12V DC voltage with respect to the ground voltage GND). The ground GND and supply Vcc voltages generated by the AC-DC conversion unit  210  are used, amongst other things, for supplying the electric and electronic components included in the triggering circuit  207 . 
     In the preferred embodiment here illustrated, the solid-state switch  205  is a TRIAC device. Nevertheless, in further preferred embodiments the solid-state switch may comprise a thyristor, a DIAC, an IGBT, etc. 
     In the following for sake of brevity the term “switch” will be used to indicate the solid-state switch. 
     The triggering circuit  207  for activating the TRIAC device  205  is preferably controlled by the control unit  11  that preferably generates control signals, such as the control signal VCTRL. Preferably, the control signal VCTRL is a digital voltage signal capable of selectively taking a high logic value (e.g. corresponding to the supply voltage Vcc) and a low logic value (e.g. corresponding to the ground voltage GND). 
     According to this circuit implementation, which is however not limiting for the present invention, when the control signal VCTRL is at the low logic, the TRIAC device  205  is off and the electric motor  140  is not powered. When the control signal VCTRL is at the high logic value, the TRIAC device  205  is activated that enable an electric motor voltage VMOT to be fed across (and a corresponding electric motor current IMOT to flow through) the electric motor  140 . As will be better understood from the following description, the electric motor voltage VMOT fed across the electric motor  140  is preferably derived from the AC mains voltage VMAINS, whereas the electric motor current IMOT flowing through the electric motor  140  substantially (i.e. approximately) depends on the electric motor voltage VMOT, on an equivalent impedance exhibited by the electric motor  140  and on electric motor  140  components/parts introducing (capacitive and/or inductive) non-linearity. 
     In order to control the amount of electric motor voltage VMOT that is allowed to be fed across (and, accordingly, the amount of electric motor current IMOT that is allowed to flow through) the TRIAC device  205  and the electric motor  140 , the control signal VCTRL is such that activation of the TRIAC device  205  is triggered after a predefined delay time Tfire (corresponding to a predefined phase angle of the mains voltage VMAINS and also known as firing angle of the TRIAC) and over a predefined activation time TIon (corresponding to a predefined conduction angle of the mains voltage VMAINS). Therefore, the motor voltage VMOT waveform is a partialized version of the mains voltage VMAINS (as illustrated in  FIG. 4 ). 
     In a preferred embodiment of the invention, the mains voltage VMAINS is partialized so that the motor  140  is fed with a partialized motor voltage VMOT according to requested design parameters. 
     For example, the motor  140  is fed with a motor voltage VMOT so that the root-mean-square voltage VMOTrms applied thereto is lower than the typical 230V root-mean-square voltage value VMAINSrms of the VMAINS supply voltage (for example VMOTrms is preferably set to 155V). 
     In further preferred embodiments of the invention, the mains voltage VMAINS is opportunely partialized so that the motor  140  is fed with a partialized motor voltage VMOT when the VMAINSrms, for some reasons, goes higher than 230V. In this situation, advantageously, the motor is protected from overheating. 
       FIG. 4  shows qualitative ideal waveforms of the mains voltage VMAINS, of the electric motor voltage VMOT across the electric motor  140 , of the motor current IMOT flowing through the electric motor  140  and of the control signal VCTRL when the electric motor  140  is fed with a partialized motor voltage VMOT. 
     As visible in this figure, the mains voltage VMAINS is an alternating voltage having a full-wave periodic, e.g. sinusoidal, waveform (and, as usual, a VMAINSrms amplitude of 230V or 125V and a frequency of 50 Hz or 60 Hz). The motor voltage VMOT is an alternating voltage defined by a sequence of positive-slope and negative-slope sinusoidal portions of (or, when possible non-idealities are considered, approximately matching the waveform/trend of) the mains voltage VMAINS, and the motor current IMOT is an alternating current having a sequence of positive and negative sinusoidal (or substantially sinusoidal) waveforms. The sinusoidal portions of (or approximately matching the waveform/trend of) the mains voltage VMAINS that define the motor voltage VMOT derive from activation of the TRIAC device  205  according to the control signal VCTRL. Instead, the substantially sinusoidal waveform of the motor current IMOT is due, ideally (i.e. without taking into account delay time intervals and non-idealities introduced by electric motor  140  components/parts, as herein assumed), to inductive nature of the electric motor  140  (so that the motor current IMOT waveform results from the full-wave sinusoidal waveform of the mains voltage VMAINS over the activation time intervals, or otherwise stated, from the positive-slope and negative-slope sinusoidal portions of the mains voltage VMAINS). 
     The motor current IMOT corresponds to the current ITRIAC flowing through the TRIAC. 
     In the figure, the following parameters may be depicted. 
     Firing angle Tfire: the time from the zero crossing ZC of the VMAINS and the firing point of the TRIAC caused by the control signal VCTRL. 
     Time TIon: activation time of the TRIAC starting at the firing angle and ending when the IMOT, or ITRIAC, crosses 0. 
     The activation time TIon is also preferably defined as the period of time starting when the motor current IMOT increases, or decreases, from 0 and ending when the motor current IMOT crosses again 0. 
     In a preferred embodiment, the control signal VCTRL is a pre-fixed sequence of digital signals according to the VMOTrms requested. In particular, the firing angle Tfire has a pre-fixed value according to the VMOTrms requested. 
     The activation time TIon of IMOT is then substantially due to inductive nature of the electric motor  140 , as said above, and its expected value TIont can be therefore evaluated in advance according to the parameter of electric motor  140  actually used. 
       FIG. 5  is an exemplary diagram showing the evolution of the root-mean-square voltage VMOTrms applied to motor  140  of the second recirculation pump  31  as a function of the time in laundry machine  1  during the execution of a washing cycle. 
     In the preferred embodiment of the washing cycle here illustrated the second recirculation pump  31 , and therefore the motor  140 , is activated four times during the washing cycle. It is clear that in different embodiments the second recirculation pump  31 , and therefore the motor  140 , may be activated a different number of times. 
     Preferably, the motor  140  is intermittently activated three times at the beginning of the washing cycle, preferably during a phase of the washing cycle where the laundry is being wetted and/or completely soaked with addition of a washing detergent. 
     The motor  140  is then activated a fourth time in a washing phase during which the drum  4  is rotated and the water contained therein is heated to a predetermined temperature based on the washing cycle selected by the user. During this washing phase, the drum  4  is preferably rotated, so as to apply also a mechanical cleaning action on the laundry. 
     As it can be appreciated from the diagram, at the beginning of each activation of the motor  140  the voltage VMOTrms applied to the motor  140  for a short period of time has a value corresponding to the VMAINSrms, i.e. 230V, that is then reduced to the target value Vt of 155V thanks to the partialization of the mains voltage VMAINS through the triggering circuit  207  (as explained above). 
       FIG. 5  represents the evolution of the root-mean-square voltage VMOTrms applied to motor  140  in a washing cycle where the second recirculation pump  31  is working properly, in particular it is not working in an air-water condition. 
     The applicant has proved that when the second recirculation pump  31  is working in air-water condition, i.e. presence of air at its inlet, the evolution of the root-mean-square voltage VMOTrms applied to the motor  140  deviates from the expected target value Vt. 
     In particular, in air-water condition the root-mean-square voltage VMOTrms applied to the motor  140  appears to increase up to a higher level Vf than the expected target value Vt. 
     This situation is shown in  FIG. 6  which represents the evolution of the root-mean-square voltage VMOTrms applied to motor  140  in a washing cycle whereas the second recirculation pump  31  is working in an air-water condition starting from time t-aw. 
     According to an advantageous aspect of the invention, therefore, in order to detect the air-water condition of the pump  31 , the root-mean-square voltage VMOTrms applied to the motor  140  is monitored and is compared with the expected target value Vt. 
     If the root-mean-square voltage VMOTrms applied to the motor  140  deviates from the target value Vt it is determined that the pump  31  is working in air-water condition. 
     In the preferred embodiment here illustrated and described, in particular, if the root-mean-square voltage VMOTrms applied to the motor  140  is above the target value Vt it is determined that the pump  31  is working in air-water condition. 
     Preferably, the root-mean-square voltage VMOTrms applied to the motor  140  is compared with a threshold value TV and if the root-mean-square voltage VMOTrms applied to the motor  140  is above the threshold value TV it is determined that the pump  31  is working in air-water condition. 
     The threshold value TV is preferably set as function of the target value Vt: 
       TV= f ( Vt ) 
     Preferably, the threshold value TV is proportional to the value of the target value Vt: 
       TV=1.08* Vt    
     i.e. the threshold value TV is 108% of the target value Vt. 
     In a further preferred embodiment, the threshold value TV may be set as: 
       TV= Vt+ΔV.    
     In different embodiments, nevertheless, the threshold value TV may be differently set. 
     Referring in particular to  FIG. 6 , the target value is Vt=155V and the threshold value TV is set to 170V. 
     According to the invention, therefore, at time t-aw it is detected that the second recirculation pump  31  is working in air-water condition. 
     According to a further advantageous aspect of the invention, in order to detect the air-water condition of a pump  31 , the evolutions of the instantaneous values of electric motor voltage VMOT across the electric motor  140  and/or of the motor current IMOT flowing therethrough are monitored. 
       FIG. 4  above described shows qualitative ideal instantaneous waveforms of the mains voltage VMAINS, of the electric motor voltage VMOT across the electric motor  140  of the second recirculation pump  31 , of the motor current IMOT flowing through the electric motor  140  and of the control signal VCTRL when the electric motor  140  is fed with a partialized motor voltage VMOT and the second recirculation pump  31  is working properly, in particular it is not working in an air-water condition. 
     Instead,  FIG. 7  shows the same waveforms of  FIG. 4  when the second recirculation pump  31  is working in an air-water condition. 
     The applicant has proved that when the second recirculation pump  31  is working in air-water condition, i.e. presence of air at its inlet, the evolutions of the electric motor voltage VMOT applied to the motor  140  and of the motor current IMOT flowing therethrough deviates from the expected values. 
     According to an advantageous aspect of the invention, in order to detect an air-water condition of a pump  31 , the activation time TIon of the TRIAC is monitored and is compared with the expected value TIont. 
     If the detected activation time TIon deviates from the expected value TIont it is determined that the pump  31  is working in air-water condition. 
     In the preferred embodiment here illustrated and described, in particular, if the detected activation time TIon is above the expected value TIont it is determined that the pump  31  is working in air-water condition. 
     This situation can be appreciated by comparing  FIG. 4  and  FIG. 7 . 
     In  FIG. 4  where the pump  31  is properly working, i.e. not in air-water condition, the activation time TIon repeats regularly with a value equal to, or substantially equal to, the expected value TIont. 
     In  FIG. 7  where the pump  31  is working in air-water condition, the activation time TIon varies from the expected value TIont, in particular the activation time TIon′ increases with respect to the expected value TIont starting from time t-aw. 
     From the same  FIG. 7  it can be also appreciated that the root-mean-square voltage VMOTrms of the motor voltage VMOT in air-water condition increases from time t-aw, according also to what previously described with reference to  FIG. 6 . 
     Preferably, the detected activation time TIon is compared with a threshold value TI and if the detected activation time TIon is above the threshold value TI it is determined that the pump  31  is working in air-water condition. 
     The threshold value TI is preferably set as function of the expected value TIont: 
         TI=f ( TIont ) 
     Preferably, the threshold value TI is proportional to the value of the expected value TIont: 
         TI= 1.15 *TIont    
     i.e. the threshold value TI is 115% of the expected value TIont. 
     Preferably, the threshold value TI is between 1.05*TIont and 1.5*TIont, more preferably between 1.1*TIont and 1.3*TIont (i.e. the threshold value TI is preferably between 105% and 150% of the expected value TIont, more preferably between 110% and 130% of the expected value TIont). 
     In a further preferred embodiment, the threshold value TI may be set as: 
     
       
      
       TI=TIont+ΔT  
      
     
     Preferably, ΔT is between 0.5 and 3 ms, preferably between 0.8 and 2 ms, more preferably equal to 1 ms. 
     Said preferred values for ΔT refers to a main voltage frequency of 50 Hz. 
     In different embodiments, nevertheless, the threshold value TI may be differently set. For example, the threshold value TI may be calculated as a function of the main voltage frequency. 
     As described above, air-water condition of the pump is therefore preferably determined if:
         the root-mean-square voltage VMOTrms applied to the motor is above a threshold value TV; or   the detected activation time TIon is above a threshold value TI.       

     The two conditions, as explained and illustrated above, are linked one to the other and substantially depend on the type of pump/motor actually used. 
     In a further preferred embodiment, and according to the invention, air-water condition of the pump may be determined if:
         the root-mean-square voltage VMOTrms applied to the motor is below a threshold value TV; or   the detected activation time TIon is below a threshold value TI.       

     According to this further preferred embodiment, threshold values TV and TI may be opportunely set. 
     The threshold value TV may be preferably set as function of the target value Vt: 
       TV= f ( Vt ) 
     Preferably, the threshold value TV may proportional to the value of the target value Vt: 
       TV=0.85* Vt    
     i.e. the threshold value TV is 85% of the target value Vt. 
     Preferably, the threshold value TV is between 0.5*Vt and 0.95*Vt, more preferably between 0.65*Vt and 0.90*Vt (i.e. the threshold value TV is preferably between 50% and 95% of the target value Vt, more preferably between 65% and 90% of the target value Vt). 
     In a further preferred embodiment, the threshold value TV may be set as: 
       TV= Vt−ΔV.    
     Preferably, ΔV is between 5 and 50V, more preferably between 10 and 30V, more preferably equal to 15V. 
     The threshold value TI may be preferably set as function of the expected value TIont: 
         TI=f ( TIont ) 
     Preferably, the threshold value TI is proportional to the value of the expected value TIont: 
         TI= 0.9 *TIont    
     i.e. the threshold value TI is 90% of the expected value TIont. 
     Preferably, the threshold value TI is between 0.5*TIont and 0.95*TIont, more preferably between 0.65*TIont and 0.9*TIont (i.e. the threshold value TI is preferably between 50% and 95% of the expected value TIont, more preferably between 65% and 90% of the expected value TIont). 
     In a further preferred embodiment, the threshold value TI may be set as: 
     
       
      
       TI=TIont−ΔT  
      
     
     Preferably, ΔT is between 0.5 and 3 ms, preferably between 0.8 and 2 ms, more preferably equal to 1 ms. 
     Said preferred values for ΔT refers to a main voltage frequency of 50 Hz. 
     According to a preferred embodiment of the present invention, the motor voltage VMOT across the electric motor  140  and the motor current IMOT flowing therethrough are detected by means of proper sensors. From said electric parameters, the activation time TIon may be also be detected/determined, according to techniques well known in the art, and therefore not described in detail. 
     According to the preferred embodiments above described, the motor  140  is fed with a motor voltage VMOT which is a partialized version of the mains voltage VMAINS. 
     According to a further preferred embodiment of the invention, the motor  140  may be fed with an alternating motor voltage VMOT having a full-wave periodic, e.g. a sinusoidal waveform. For example, the motor  140  may be directly connected to the mains voltage VMAINS. The motor current IMOT, due to inductive nature of the electric motor  140 , is also an alternating current having a full-wave periodic, e.g. a sinusoidal waveform. 
       FIG. 8  exemplary shows qualitative ideal instantaneous waveforms of the electric motor voltage VMOT across the electric motor  140  of the second recirculation pump  31  and of the motor current IMOT flowing through the electric motor  140  when the second recirculation pump  31  is working properly, in particular it is not working in an air-water condition. 
       FIG. 9  exemplary shows the same qualitative ideal instantaneous waveforms of  FIG. 8  when the second recirculation pump  31  is working in an air-water condition. 
     As visible in  FIG. 8 , VMOT and IMOT both have an alternating evolution, preferably a full-wave periodic evolution, more preferably a sinusoidal waveform, and exhibit a phase difference Td. 
     The phase difference Td is due to inductive nature of the electric motor  140 , as said above, and its expected value Tdt can be therefore evaluated in advance according to the parameter of electric motor  140  actually used. 
     The applicant has proved that when the second recirculation pump  31  is working in air-water condition, i.e. presence of air at its inlet, the evolutions of the electric motor voltage VMOT applied to the motor  140  and of the motor current IMOT flowing therethrough deviates from the expected values. 
     In particular, the phase difference Td′ between the electric motor voltage VMOT and the motor current IMOT when the second recirculation pump  31  is working in air-water condition changes with respect the phase difference Td between the electric motor voltage VMOT and the motor current IMOT when the second recirculation pump  31  is working properly, not in air-water condition. 
     Preferably, the detected phase difference Td′ is compared with a threshold value TT and if the detected phase difference Td′ is above the threshold value TT it is determined that the pump  31  is working in air-water condition. 
     The threshold value TT is preferably set as function of the expected value Tdt: 
         TT=f ( Tdt ) 
     Preferably, the threshold value TT is proportional to the value of the expected value Tdt: 
         TT= 1.15* Tdt    
     i.e. the threshold value TT is 115% of the expected value Tdt. 
     Preferably, the threshold value TT is between 1.05*Tdt and 1.5*Tdt, more preferably between 1.1*Tdt and 1.3*Tdt (i.e. the threshold value TT is preferably between 105% and 150% of the expected value Tdt, more preferably between 110% and 130% of the expected value Tdt). 
     In a further preferred embodiment, the threshold value TT may be set as: 
     
       
      
       TT=Tdt+ΔT  
      
     
     Preferably, ΔT is between 0.5 and 3 ms, preferably between 0.8 and 2 ms, more preferably equal to 1 ms. 
     Said preferred values for ΔT refers to a main voltage frequency of 50 Hz. 
     In different embodiments, nevertheless, the threshold value TT may be differently set. For example, the threshold value TT may be calculated as a function of the main voltage frequency. 
     As described above, air-water condition of the pump is determined if the detected phase difference Td′ is above the threshold value TT. 
     The condition, as explained and illustrated above, depends on the type of pump/motor actually used. 
     In a further preferred embodiment, and according to the invention, air-water condition of the pump may be determined if the detected phase difference Td′ is below the threshold value TT. 
     According to this further preferred embodiment, the threshold value TT is preferably set as function of the expected value Tdt: 
         TT=f ( Tdt ) 
     Preferably, the threshold value TT is proportional to the value of the expected value Tdt: 
         TT= 0,85* Tdt    
     i.e. the threshold value TT is 85% of the expected value Tdt. 
     Preferably, the threshold value TT is between 0.5*Tdt and 0.95*Tdt, more preferably between 0.65*Tdt and 0.9*Tdt (i.e. the threshold value TT is preferably between 50% and 95% of the expected value Tdt, more preferably between 65% and 90% of the expected value Tdt). 
     In a further preferred embodiment, the threshold value TT may be set as: 
     
       
      
       TT=Tdt−ΔT  
      
     
     Preferably, ΔT is between 0.5 and 3 ms, preferably between 0.8 and 2 ms, more preferably equal to 1 ms. 
     Said preferred values for ΔT refers to a main voltage frequency of 50 Hz. 
     According to a further advantageous aspect of the present invention, if it is evaluated that a pump is working in an air-water condition then one or more actions are preferably taken, as will be better described later. 
     Hereinafter, possibly actions that can be preferably performed are described. 
     An action that can be performed after detection of an air-water condition for a pump preferably comprises deactivation of the pump, i.e. preferably the deactivation of the motor of the pump. 
     In a preferred embodiment of the invention, deactivation is carried out immediately after detection of the air-water condition. 
     In a further preferred embodiment of the invention, deactivation is carried out after a predetermined period from the detected air-water condition, for example after some seconds. 
     In another preferred embodiment of the invention, the action that can be performed preferably comprises a variation of the speed of the pump, more preferably a reduction of the speed of the pump (preferably a variation of the speed of the motor of the pump, more preferably a reduction of the speed of the motor of the pump). 
     In a further preferred embodiment of the invention, the action preferably comprises a temporary variation of the speed of the pump, more preferably a temporary reduction of the speed of the pump (preferably comprises a temporary variation of the speed of the motor of the pump, more preferably a temporary reduction of the speed of the motor of the pump). 
     Another action that can be performed after detection of an air-water condition preferably comprises a step of increasing the level of the liquid at the inlet of pump so that to reduce the presence of air. 
     In a preferred embodiment of the invention, in particular if the pump refers to one of the first or second recirculation circuit above described, the step of increasing the level of the liquid at the inlet of pump comprises a phase of introducing a quantity of water into the tub from the external water supply line. 
     In a further preferred embodiment of the invention, in particular if the pump refers to one of the first or second recirculation circuit above described, the step of increasing the level of the liquid at the inlet of pump comprises a phase of:
         performing a spinning phase by rotating the drum in order to extract liquid from the laundry, in case the drum is not moving; or   increasing the rotating speed the drum in order to extract more liquid from the laundry, in case the drum is already rotating.       

     Advantageously, furthermore, said actions, in particular if the pump refers to one of the first or second recirculation circuit above described, avoids formation or foam or allows reduction of foam. 
     Advantageously, the method according to the invention allows the detection of an air-water condition of a pump in laundry treating machine. 
     Advantageously, the method according to the invention allows the prompt detection of an air-water condition of the pump as soon as it happens. 
     It follows that one or more proper actions may be promptly performed to avoid malfunctioning of the pump. 
     Furthermore, detection of the air-water condition is advantageously carried out without necessity of installing a liquid level sensor. 
     Therefore, the method of the invention allows detecting of an air-water condition of a pump in a laundry treating machine having reduced complexity and/or size compared to laundry treating machines of known type. 
     It follows that the laundry treating machine performing the method of the invention has higher reliability compared to laundry treating machines. 
     Still advantageously, the method according to the invention assures a more correct functioning of the pumps installed in the laundry treating machine which drain liquid from the washing tub and, in turn, reduces the noise caused by the pump working in air-water condition. 
     It has thus been shown that the present invention allows all the set objects to be achieved. In particular, it makes it possible to provide a method for draining liquid in a pump of a laundry treating machine that makes it possible to drive the pump with a higher efficiency compared to known system. 
     It is underlined that the laundry washing machine illustrated in the enclosed figures, and with reference to which some embodiments of the method according to the invention have been described, is of the front-loading type; however it is clear that the method according to the invention can be applied as well to a top-loading laundry washing machine, substantially without any modification. 
     Furthermore, the method according to the invention can be applied to any type of pump in laundry treating machine. 
     While the present invention has been described with reference to the particular embodiments shown in the figures, it should be noted that the present invention is not limited to the specific embodiments illustrated and described herein; on the contrary, further variants of the embodiments described herein fall within the scope of the present invention, which is defined in the claims.