REFRIGERANT COMPRESSOR DRIVEN BY VARIABLE SUPPLY FREQUENCY MOTOR

Abstract: Refrigerant compressor driven by an electric motor (10) whose supply frequency is selectively variable between a discrete number of values (f1, f2, fT) as a function of an input signal. The invention calls for the use of control means (7) which are integrated in the compressor and are adapted to monitor two or more quantities that are variable with time in a mutually independent manner, of which at least one is representative of the actual operating conditions of the appliance, said control means being further adapted to generate said input signal with a value that corresponds to a pre-established combination of the monitored quantities.

Description: REFRIGERANT COMPRESSOR DRIVEN BY VARIABLE SUPPLY FREQUENCY MOTOR DESCRIPTION The present invention refers to a compressor of the refrigerant medium of a refrigeration apparatus for home or similar use, namely of the type dnven by a variable supply -frequency electric motor The importance that is being ascribed also by public authorities to the limitation, le reduction m the energy usage of home appliances m general has led to the issuance of a number of Directives, eg by the European Commission, which, to the purpose of bringing about an overall reduction on the so-called greenhouse effect, practically forbid appliances to be put on the market which use too high an amount of energy to perform the duty they are required to ensure This applies in particular to compression-type refrigerating appliances, which are largely diffused in all countries worldwide In view of assuring compliance with these Directives, proposals are largely known to have been made concerning the use, in a generalized manner, of refngerant medium compressors driven by an electric motor which, instead of operating intermittently as this is the case m traditional appliances, operates continuously, but at a variable speed In theory, a motor of such a kmd is therefore capable of operating at anyone of the infinite values n comprised between a higher limit n, and a lower limit n, In order to constantly ensure an optimum storage condition of the foodstuffs stored m the appliance, the above cited limiting values n s and n^are pre-set in accordance with the thermal load which is generally known to be in a proportion with the difference of the temperature T e of the ambient where the appliance is installed (which, in the case of the climatic class defined as ST, is comprised between +10°C and +38°C) to the temperature T, of the room or compartment where the foodstuffs are stored (which, in the case of a so-called "cooler" refrigeration appliance, is compπsed between +2°C and +8°C) In theory, the ratio of n. to n^should be equal to 19 However, owing to a number of well-known practical difficulties, including the nsk of an inadequate lubrication at the low running speeds of the compressor motor, said ratio n s/ n, between the speed limits of the motor of the compressor actually amounts to approx 3 The preservation of the stored foodstuffs is therefore ensured in an optimum manner only under just a few ones of the actual operating conditions of the appliance, which in fact may deviate even considerably from reality A second major drawback deriving from the use of a compressor driven by a vanable-speed motor lies m the fact that this requires the refrigeration appliance to make use of a special thermostatic temperature control system that differs both as far as the hardware and the software are concerned from the systems currently used in traditional refngeration appliances, in which the motor of the compressor operates intermittently at a single speed A manufacturer of refrigeration appliances is thus forced to face a time-consuming and expensive job of re-designing and testing each single appliance model included in his manufacturing range A different type of compressor, disclosed m the publication EP-A-0 490 089, is driven by a motor that is capable of operating selectively at a discrete number of speeds (two or, at most, three) and has a volume displacement capacity that is relatively increased with respect to the one of equivalent compressors driven by a single-speed motor The first speed n, of the motor, which may be in the order of magnitude of 2 000 rpm is used in the operating penods of the appliance in which the need or opportunity arises for the energy usage of the same appliance to be minimized (le the so-called "preservation penods"), the second speed n 2 of the motor, which may be in the order of 2,800 rpm, and the possibly provided third speed n τ , which may amount to something as 3,200 rpm, are on the contrary used when the appliance needs to ensure a particularly sustained refngeration capacity, such as for instance in such transient penods as a starting after a prolonged pause, or is working under particularly heavy or demanding operating conditions Switching over from a speed to another one is performed, in the cases in which the motor is of the brushless type, by means of a device adapted to control the supply frequency It is a main purpose of the present invention to provide a compressor of refngerant medium of this second kind, whose electnc driving motor is capable of switching over in an automatic manner from a supply frequency to the other one to the purpose of minimizing, under any operating condition, the energy usage of the appliance A further purpose of the present invention is to enable the manufacturer to do away with the need of going through a function re-design of his range of refngeration appliances, thanks to the fact that, at least at the lowest speed n l thereof, the dnvmg motor of the compressor keeps operatmg intermittently Owing to the fact that the present invention does not actually require any modification to be made on the thermostatic temperature control associated to the food storage compartment of the refrigeration appliance in which the compressor is installed, the same system may thus be come down even to a simple, well-known thermostat of the fluid-expansion type According to the present invention, these and further aims are reached m a control system having the characteristics as recited in the appended claims Anyway, features and advantages of the present invention can more readily be understood from the descnption of a preferred embodiment that is given below by way of non-ømitmg example with reference to the accompanying drawings, m which - Figure 1 is a schematical overall view of the compressor assembly, - Figure 2 is an electnc wiring diagram relating to the connection of a driving motor for the compressor used in a refrigeration appliance of the so-called "cooler' type with automatic cycuc defrost operation In a per se well-known manner, a compressor of refngerant media for use in a home-type refngeration appliance consists substantially of a metal sealed casing 1 that encloses both the actual compressor, of the reciprocating type, and the electnc dn ig motor 10 provided coaxially thereto From the outer surface of said sealed casing 1 there are protruding three metal tubes 2, 3, and 4 acting as the suction pipe, the delivery pipe and the service pipe, respectively The same outer surface of the sealed casing 1 acts as the support for a cover 5 that encloses and protects the terminal box for the connection of a plug-cable (not shown) ensuring the power supply from the electnc mains Said terminal box is furthermore connected via a first wiring system 9 to a microprocessor-based control unit 7 (shown in Figure 2 only), which performs m the manner that is explained further on, is enclosed in a proper protective box 6, and is in turn adapted to be connected to the thermostat 12 (which is also only shown in Figure 2) of a refngeration appliance via a second wiring system 8 In the wiring and connection diagram given to exemplifying purposes in Figure 2, and which refers to the use of the compressor according to the present invention in a refngeration app-iance of the so-called "cooler" type with automatic cyclic defrost operation, further to the conductor leads L and N of the power supply plug-cable, the compressor dnvmg motor 10 and the control unit 7 are for reasons of simplicity connected directly to each other both as far as signal and power are concerned via the above mentioned first wiring system 9 The above mentioned second wiring system 8 connects the thermostat 12, which is m turn connected to the line conductor lead L, to a point 13 acting as the interface with the already cited microprocessor-based control unit 7 It should be nght away noticed that the thermostat 12, which in this example of an application is used to control the temperature T, in the food storage room of the refngeration appliance m which the compressor is installed, is not necessarily of a solid-state type, but can in an advantageous manner be also of the fluid-expansion type. Inside the box 6, to the interface point 13 there are connected two conductor leads 14 and 15 in a parallel arrangement The first conductor lead 14 leads to the common contact 16 of a changeover switch 17 that is m rum adapted to selectively connect the interface point 13 with a first terminal contact 18 and with a second terminal contact 19 of the control unit 7 via the conductor leads 20 and 21, respectively Along the second conductor lead 15, which is brought to the interface point 13 and ends at a third terminal contact 22 of the microprocessor-based control unit 7, there is provided a normally open switch 23 In the preferred embodiment illustrated in Figure 2, but not necessarily required for the implementation of the present invention, further terminal contacts 25, 26 and 27 of the control unit 7 constitute the inputs of further signals 28, 29 and 30, as this will be described in greater detail further on For instance, the signal 28 that reaches the contact 25 may be the measurement signal of the actual temperature prevailing mside the food storage room, m the case that the refngeration appliance is provided with an appropriate sensor (not shown), whereas the signal 29 reaching the contact 26 may refer to the opening rate and or duration of the door of the food storage room and, finally, the signal 30, which is sent to the contact 27, may be the one relating to the storage room defrost operation According to an important feature of the present invention, the control unit 7 performs following tasks - it actuates the changeover switch 17 and causes it to switch over to connect the interface point 13 with either the first terminal contact 18 or the second terminal contact 19 of the control unit 7, so as to cause the compressor driving motor 10 to be supplied respectively at either a first and lower electric frequency f, or a second and higher electnc frequency £,, which are both lower than the frequency f of the power supply current (50 or 60 Hz), le m the line conductor leads L and N It will be readily appreciated that a pre-set speed of the motor 10 corresponds to each one of said frequencies, eg a speed of 1 ,600 rpm may correspond to the frequency f 1( and a speed of 2,400 rpm may correspond to the frequency f 2 , - it actuates the normally open switch 23 to close on the thirds terminal contact 22 of the control unit 7 so as to short-circuit the above cited first and second terminal contacts 18 and 19 of the same control unit 7 In this way, the motor 10 is supplied at a frequency f τ that is higher than the other two and may for instance be equal to the frequency f^ of the power supply line, so as to enable the motor 10 to rotate at a speed of 3,000 rpm, - it monitors, via the line 9, both the absolute duration of operation of the compressor dnving motor 10 and the utilization index (generally known as operating percentage) thereof, which are two quantities that vary with time m a mutually independent manner It is of course also capable of calculating both the absolute values and the possible combinations of said vanable quantities and/or functions thereof, such as for instance the consecutive number of operating cycles of the motor 10 which have a determined operation or utilization index, - it compares the absolute values and/or the above mentioned combinations of said variable quantities with pre-set values to the purpose of issuing, when necessary, an input signal to cause the changeover switch 17 to switch over from the first to the second terminal contact 19, 20 of the control unit 7, or vice-versa, or even to cause the normally open switch 23 to close on the third contact 22 of the same control unit 7 this to the purpose of varying the electric supply frequency of the motor 10 from any one of the three above cited values f^ f ? and f τ to any other one and, as a result increasing or reducing the operating speed thereof, - it also monitors the on and off tnppings of the thermostat switch 12 which, as anyone m the art is well aware of is not a part of the compressor itseif but is anyway inherently provided in all refngeration appliances Some examples of operation of the compressor according to the present invention are given below, as referred to the use of such a compressor in a "cooler 1 -type refngeration appliance and, therefore, with an electnc wiring and connection diagram as the one illustrated in Figure 2 Example no 1 - First starting of the appliance upon installation Considering that the switch 12 of the refngeration appliance is clearly closed, so that the interface point 13 is set under voltage, the control unit 7 causes the normally open switch 23 to close on the third terminal contact 22 so as to enable the compressor dnvmg motor 10 to be supplied with power at the highest frequency f τ As this has already been set forth earlier m this description, such a frequency may be the frequency £j of the power supply line (le 50 or 60 Hz, as the case may be), which causes the motor 10 to operate at a speed of 3,000 rpm Such an operating condition is maintained all along the time that is necessary for the thermostat 1 to tnp for the first time, thereby interrupting of course the power supply coming from the fine conductor lead L Example no 2 - Regular food storage and preservation operation Via the wiring line 9, the control unit 7 is able to find out that the utilization mdex of the motor 10 is low, le it has namely failed to exceed a pre-set threshold value (which might be, say, m the order of 50%) throughout a pre-determmed number of consecutive tnppmg cycles (for instance, 5 cycles) of the thermostat 12 At this pomt, the same control unit 7 causes the changeover switch 17 to switch over m such a manner that when the thermostat 1 is closed, the mterface pomt 13 is capable of applying voltage to the first terminal contact 19 via the conductor lead 20 so as to enable the compressor dnvmg motor 10 to be supplied with power at the lowest frequency f, and, as a result, to go on operatmg at a speed of just 1,600 rpm As a result from this moment on (and as long as the conditions do not change, as described m the following examples), the energy usage of the refngeration appliance, as caused by the operation of the same compressor dnvmg motor 10, is at a mmimum smce it is actually reduced to just the amount of energy that is precisely required to keep the food storage compartment of the appliance at either the temperature selected by the user (eg +4°C) or a possibly set default temperature Example no. 3 - Frequent use of the refrigeration appliance for loading and removing foodstuffs Via the wiring line 9, the control unit 7 is capable of detecting that the compressor driving motor 10 has been operating at a high utilization index, ie. an index that is higher than the afore mentioned threshold value (eg. 50%), throughout a number (eg. 5) of consecutive cycles of the thermostat 12. At this point, the same control unit 7 causes the changeover switch 17 to switch over in such a manner that, when the thermostat 12 is closed, the interface point 13 is capable of applying the voltage of the line conductor lead L to the second terminal contact 19 via the conductor lead 1 so as to enable the compressor driving motor 10 to be supplied with power at the frequency f^ and, as a result, to increase the operatmg speed thereof to 2,400 rpm. As a result, the energy usage of the refrigeration appliance increases, but only for the period of time that is necessary for the conditions described in Example no. 2 above to be restored. Example no. 4 - Prolonged opening of the door The control unit is capable of ascertaining whether the door of the refrigeration appliance is kept open for an unusually long period of time, eg. owing to an inattention of the user, on the basis of at least one of the afore mentioned signals 28, 29 that reach the terminal contacts 25, 26 thereof. This causes the compressor driving motor 10 to operate through a prolonged period of time, m particular a period of time that is in excess of a pre-set threshold duration of 90 minutes. With the thermostat 12 in its closed position, it is therefore ensured that the changeover switch 17 is kept closed on the second terminal contact 19 of the unit 7. It shall of course be appreciated that the compressor according to the present invention proves equally advantageous when used in connection with other types of refrigeration appliances, eg. freezers or fridge-freezer combinations, by introducing appropriate variants in the operating logic of the microprocessor- based control unit 7. The advantages of the present invention may be summarized as follows: - the microprocessor-based control unit is an integral part of the compressor, - no functional re-design is required for any of the refngeration appliances m which the compressor is go g to be installed, not even as far as winngs are concerned, - it is on the contrary possible for even such time-proven, low-cost thermostats as the fluid-expansion ones to be further used in the above refngeration appliances, - the energy usage of the appliances is precisely and automatically adapted to the actual operatmg conditions of the same appliances and, therefore, is is reduced to a mmimum under the standard, le regular operatmg conditions that are used as a reference for the energy efficiency data stated m the energy label accompanying the appliances themselves, - similarly, the level of the noise generated by the appliances during operation is kept under control and, m practice, such a noise is kept at a certainly low level for It will be further appreciated that the compressor according to the invention may be implemented m a number of manners differing from the afore described embodiment In particular, the power supply frequencies (and, therefore, the operatmg speeds) of the compressor dnvmg motor may have both absolute and relative values differing from the afore mdicated ones, m particular, none of the three frequencies may be equal to the power supply line frequency (50 or 60 Hz) Furthermore, other time-variable quantities may be used as a reference, such as for instance the current oput of the dnvmg motor It should also be noticed that, if desired, the manufacturer can add, on the control panel of his refrigeration appliances, appropnate manually operated means adapted to actuate the normally open switch and/or the changeover switch, le to double the functions thereof In these cases, the microprocessor-based control unit is ovemdden and the refngeration appliance requires the user to intervene manually m order to vary the operating speed of the compressor It should be finally noticed that it is in all cases possible for the operating logic of the microprocessor-based control unit 7 to be "personalized" m view of bemg able to duly take mto account the actual installation conditions and or any possible particular construction or design feature of the refrigeration appliance (eg if the latter is of the type with more than two food storage compartments and, of course, as many food storage temperatures) m which the compressor is installed

Claims: CLAIMS 1. Compressor of refngerant medium for a home-type or similar refrigeration apparatus, dnven by a motor (10) whose power supply frequency is selectively vanable between a discrete number of values (f,, f 2 , f τ ) as a function of an mput signal, charactenzed m that it integrally comprises control means (7) which are adapted to monitor two or more quantities that are vanable with time m a mutually mdependent manner, of which at least one is representative of the actual operating conditions of the appliance, said control means bemg further adapted to generate said mput signal with a value that corresponds to a pre-established combmation of the monitored quantities 2. Compressor according to claim 1 , characterized m that it also integrally comprises means for processmg said mput signal, which are from time to time adapted to pnvilege any of said time-variable quantities over the other ones as a function of the absolute value that such quantities take m a respective pre-set scale 3. Compressor according to claim 1 or 2 for a refngeration apparatus having at least one storage compartment whose temperature is adjustable by means of a thermostat (12), charactenzed m that also said thermostatic control system is associated to said monitoring means (7) 4. Compressor according to any of the preceding claims, charactenzed in that one of said vanable quantities is constituted by the utilization index of the dnvmg motor (10) thereof 5. Compressor according to any of the preceding claims, charactenzed m that one of said vanable quantities is constituted by the absolute duration of the operation of the dnvmg motor (10) thereof. 6. Compressor according to any of the preceding claims, charactenzed m that one of said vanable quantities is constituted by the electnc current mput of the dnvmg motor (10) thereof 7. Compressor according to any of the preceding claims, charactenzed in that the ratio of the highest power-supply frequency (f τ ) to the lowest power-supply frequency (f,) of the dnvmg motor (10) thereof is compnsed between 1 4 and 2 1 and is preferably anywhere near 2 8. Compressor according to any of the preceding claims, charactenzed in that the means that are adapted to generate said mput signal are programmable by the user so as to be able to take the mstaUation conditions and/or any particular design feature of the refngeration appliance mto due account.

Inventors: BERTOTTI TIZIANO, CARLI FABRIZIO, PERUZZO ROBERTO, BERTOTTI, TIZIANO, CARLI, FABRIZIO, PERUZZO, ROBERTO
Applicants: BERTOTTI TIZIANO, CARLI FABRIZIO, PERUZZO ROBERTO, ZANUSSI ELETTROMECC, BERTOTTI, TIZIANO, CARLI, FABRIZIO, PERUZZO, ROBERTO, ZANUSSI ELETTROMECCANICA S.P.A.
Application year: 2000
Main classification: F25B49
IPC codes: F04B49, F25B1, F25B49, F25D11