EP2686930A1

EP2686930A1 - System and method of streamlining energy efficiency for application in cooling equipment compressors

Info

Publication number: EP2686930A1
Application number: EP12719246.6A
Authority: EP
Inventors: Günter Johann MASSS
Original assignee: Whirlpool SA
Current assignee: Whirlpool SA
Priority date: 2011-03-16
Filing date: 2012-03-15
Publication date: 2014-01-22
Also published as: BRPI1101387A2; CN102959819A; CN102959819B; WO2012122616A1

Abstract

The present invention refers to a system for streamlining energy efficiency, for application in cooling equipment compressors. Said system has at least a main circuit of the frequency inverter (10) and an electric energy power source (FAC) electrically associated to each other. The main circuit of the frequency inverter (10) comprises at least a wave rectifier (4). In addition, the main circuit of the frequency inverter (10) comprises at least a bus capacitor (CB), electrically associated in parallel to the wave rectifier (4) and to the compressor, electrically chargeable by a charge current. Additionally, the main circuit of the frequency inverter (10) comprises at least a passive component (P), electrically associated to the wave rectifier (4), capable of reducing a charge current of the bus capacitor (CB) and/or attenuating the harmonic content of an input current coming from the electric energy power source (FAC). Said system comprises at least a means of obtaining power in an input of the main circuit of the frequency inverter (10). Further, said system comprises at least a control unit (3) operatively associated to the means for obtaining power. Said main circuit of the frequency inverter (10) is provided with an active switch (K) electrically associable in parallel to the passive component (P), and the control unit (3) is arranged so as to permit the drive of the active switch (K) based on the power at the input of the main circuit of the frequency inverter (10). The present invention also refers to a method of streamlining energy efficiency, for application in cooling equipment compressors.

Description

Specification of Patent of Invention for "SYSTEM AND METHOD OF STREAMLINING ENERGY EFFICIENCY FOR APPLICATION IN COOLING EQUIPMENT COMPRESSORS".

The present invention refers to a system and to a method capable of providing streamlining in the efficiency of electric energy consumption of cooling equipment by reducing energy losses when operating at low power.

Description of the state of the art

Variable capacity compressors are normally used in domestic and commercial cooling equipment to assist in meeting the most demanding energy efficiency requirements known today. This cooling capacity variation is provided by the variation in rotation speed of an electric motor capable of pumping coolant gas to a cooling circuit. This variation in speed is permitted by means of a frequency inverter, which, along general lines, consists of an electronic circuit basically having two main stages responsible for processing the energy. In a first stage, an alternating current of the power grid is converted into a direct current voltage called CC bus. As can be seen in figures 1a and 1 b, this conversion is performed, in the most simplified manner, by way of a diode bridge rectifier 4' and a bus capacitor CB', responsible for storing the energy delivered by the grid in the semicycles of the alternating current. In a second stage, the di- rect current voltage of the CC bus is converted into an alternating current of variable amplitude and frequency, according to the rotation demand and power required by the electric motor.

It must be noted that, in relation to the first stage mentioned above, it is necessary to limit the electric current peak necessary to carry out the first charge of the bus capacitor (In-Rush current), in order to avoid damage to any of the components that conduct the input current. In addition, it is also necessary to permit attenuation of the harmonic content of the input current, in order to adjust the cooling equipment to domestic/international market requirements and norms.

A simple and low cost way of limiting the in-rush current and attenuating the harmonic content of the input current is by the use of a resistive ele- ment NTC (Negative Temperature Coefficient), represented in the drawings by the letter P', positioned in series with the path of the input electric current, as can be seen in figure 1a. Initially, this element presents a relatively high nominal resistance (resistance to cold) which limits the current peak during the first charge of the bus capacitor CB'. Over time, the resistive element heats due to the current circulating through it, and its resistance decreases to a condition sufficient for attenuating the harmonic content of the input current. It must be noted that the attenuation of the harmonic content is a requirement in some markets, such as in Europe. In addition, the existing norms, such as IEC61000- 3-2, establish maximum permitted values for the harmonics of the electric current drained from the public network by equipment, such as, for example, a refrigerator. Figure 1b illustrates a second possible positioning arrangement of the element NTC P', after the diode bridge rectifier 4'.

In other words, figures 1a and 1 b show a full circuit wave rectifier, with its basic elements. In these circuit arrangements, the alternating current FAC grid has an output impedance of zero, such that the format of the current during the first charge of the bus capacitor CB' is defined by the impedance of the circuit elements. These elements can be discrete, as in the case of the NTC P', or can be intrinsic (junction resistance of the diode rectifiers and equiv- alent resistance series of the bus capacitor). This same impedance is responsible for attenuating the harmonic content of the input current that circulates through the alternating current grid FAC. Therefore, the position of the passive component (in this case represented by NTC P'), before or after the diode bridge, produces the same effect.

Another possible form of carrying out the functions described previously is by the use of an inductive element, which may or may not be associated, to a resistive element such as the NTC itself.

However, the presence of a passive component in the circuit causes energy losses, since the entire input electric current is conducted thereby. These losses reduce the total efficiency of the cooling equipment, yet they are necessary for attenuating the harmonics of the current in this simpler and low cost method. There are other methods for reducing the harmonic content, such as, for example, the use of converter circuits operating at high frequency, yet the use of a passive component is less costly and complex for the range of power usually necessary for operating cooling equipment.

Put otherwise, the energy losses (Joule losses) due to the NTC element significantly influence the efficiency of the frequency inverter, as well as in that of the cooling equipment. However, when the passive component comprises an NTC element, the greater the relevance of these losses will be in a low power condition, where the input current is low and the resistance of the NTC has an intermediary value, lower than the nominal resistance to cold, yet greater than the resistance to heat, when the NTC element is around 100°C.

The choice of the NTC element for attenuating the harmonics of the input current takes into consideration the maximum operating power in which it is desirable to meet the limits established by Norms, such as, for example, IEC61000-3-2. Hence, the resistance of the NTC element is specified when the input current is that corresponding to this maximum power. Nevertheless, when the cooling equipment is operating at low power, the resistance of the NTC element increases, whereas the current will present a lower value (lower heating of the body of the NTC). Accordingly, the NTC element will present a resis- tance value above that needed to meet the limits of the harmonic content of the input current in a low power operating condition, which causes the energy loss mentioned previously, which is naturally undesirable.

A solution that considers the drive of a switch disposed parallel to an NTC input element is suggested in document WO2008/120928, with the aim of reducing the electric current that runs through the NTC after leaving the compressor, which reduces its temperature and, consequently, increases its conduction resistance to limit the current in a next re-driving of the compressor. Accordingly, the solution suggested in WO2008/120928 indicates the use of an electromechanical relay contact to reduce the current by the NTC element and obtain the expected gains. However, the art of the patent application WO2008/120928 is not capable of producing a significant increase in the effi- ciency of the cooling equipment, due to the Joule losses on the circulation path of the input electric current of the frequency inverter, since the drive of the relay requires a relatively high quantity of electric energy. Accordingly, the solution presented by WO2008/120928 meets the objective of reducing the temperature of the NTC element and increasing its resistance, in order to limit the charge current in a drive of the motor. However, this solution is not capable of increasing the efficiency of the frequency inverter, since there will be consumption of the relay bobbin to keep the contact closed.

Objectives of the invention

It is an objective of the present invention to provide a low cost implementation technique capable of permitting optimization of energy efficiency of cooling equipment, and, in addition, to meet the pre-established requirements relating to the harmonic content of an input current of a compressor of said cooling equipment.

It is also an objective of the present invention to provide a system and a method capable of providing a reduction in energy losses in frequency inverters used in driving variable capacity compressor motors of cooling equipment.

Brief description of the invention

One or more objectives mentioned above are achieved by way of a system for streamlining energy efficiency, for application in cooling equipment compressors, according to the art of the present invention. Said system has at least a frequency inverter circuit and an electric energy power source electrically associated to each other. The frequency inverter circuit comprises at least a wave rectifier. In addition, the frequency inverter circuit comprises at least a bus capacitor, electrically associated in parallel to the wave rectifier, electrically chargeable by a charge current. Additionally, the frequency inverter circuit comprises at least a passive component, electrically associated to the wave rectifier, capable of reducing a charge current of the bus capacitor and/or atten- uating the harmonic content of an input current coming from the electric energy power source. Said system comprises at least a means of obtaining power in an input of the frequency inverter. Further, said system comprises at least a control unit operatively associated to the means for obtaining power. Said frequency inverter circuit is provided with an active switch electrically associable in parallel to the passive component, and the control unit is arranged so as to permit the drive of the active switch based on the power at the input of the frequency inverter.

Thus, along general lines, the system is provided with an active switch, electrically positioned in parallel to a passive component (inductance or resistance like that of an NTC element) used in an input of an electronic circuit of the frequency inverter, and the total impedance of this parallel association is lower than the impedance of the passive component, but, with a sufficient value for attenuating the harmonics of the input current of the frequency inverter. Said active switch is driven when the cooling equipment is operating at low power.

Particularly, the control unit active switch whenever the input power is lower than a preset reference value. In contrast, the control unit disables the active switch when the input power exceeds a reference value, as of which a greater impedance value is needed for attenuating the harmonic content of the input current.

One or more of the objectives mentioned above are also achieved by way of a method of streamlining energy efficiency, for application in cooling equipment compressors, according to the art of the present invention. Said compressor is electrically associated to an electric energy power source by means of a frequency inverter circuit provided with an active switch and a pas- sive component. The method comprises the following steps of:

- measuring the input power of the frequency inverter;

- electrically associating the active switch in parallel to the passive component when the input power of the frequency inverter is lower than a first preset reference value; and

- electrically dissociating the active switch to the passive component when the input power of the frequency inverter is higher than a second preset reference value, the second preset reference value being higher than the first preset reference value.

Accordingly, the method of the present invention considers the control of an active switch associated in parallel to a passive component used in the input electronic circuit of the frequency inverter for purposes of attenuating the harmonic content of the input current and/or limiting the charge current of the capacitors of the direct current voltage bus. In operating conditions where the presence of the passive component is not necessary, the active switch is driven to reduce the equivalent impedance of the parallel association, so as to reduce the losses caused by conduction of the input electric current of the frequency inverter.

Brief description of the drawings

The present invention will now be described in greater detail based on the accompanying drawings. The drawings show:

figure 1a - illustrates, in simplified form, the main elements of a CA-

CC rectifier employed in a frequency inverter of the state of the art, in a first arrangement;

figure 1 b - illustrates, in simplified form, the main elements of a CA- CC rectifier employed in a frequency inverter of the state of the art, in a second arrangement;

figure 2 - illustrates a system for streamlining energy efficiency for application in cooling equipment compressors according to a first preferred embodiment of the present invention;

figure 3 - illustrates a system for streamlining energy efficiency for application in cooling equipment compressors according to a second preferred embodiment of the present invention;

figures 4 and 5 - illustrates the electrical magnitudes measured to determine the power being processed by the frequency inverter;

figure 6 - illustrates an initial sequence of events when initializing the system according to a second preferred embodiment of the present invention; figure 7 - illustrates a sequence of events subsequent to the events of figure 6;

figure 8 - illustrates an example of levels of drive power or disconnection of the active switch of the system of the present invention, considering the existence of hysteresis;

figure 9 - illustrates a flowchart of the method of driving the active switch of the system of the present invention;

figure 10 - illustrates a comparative graph of the efficiency gains obtained when the MOSFET is connected and disconnected, in an implementa- tion of the system/method of the present invention;

figure 11 - illustrates a comparative graph of the efficiency of the inverter of the solution proposed in the present invention in relation to the technique known in the state of the art; and

figure 12 - illustrates a graph of the harmonic content of an input current of the frequency inverter of the system of the present invention.

Detailed description of the drawings

The system for streamlining energy efficiency, for application in cooling equipment compressors, according to a first and a second preferred embodiment of the present invention is schematically illustrated in figures 2 and 3, respectively. Said system has at least a frequency inverter having a main circuit 10 and an electric energy power source FAC electrically associated to each other. Preferably, the compressor consists of a variable capacity compressor, yet other types of compressor can be used.

As can be seen in figures 2 and 3, the main circuit of the frequency inverter 10 comprises at least a wave rectifier 4.

In addition, the main circuit of the frequency inverter 10 also comprises at least a bus capacitor CB, electrically associated in parallel to the wave rectifier 4, electrically chargeable by a charge current.

Additionally, the main circuit of the frequency inverter 10 comprises at least a passive component P, electrically associated to the wave rectifier 4, capable of reducing a charge current of the bus capacitor CB and/or attenuating the harmonic content of an input current coming from the electric energy power source FAC. Preferably, the passive component P consists of an NTC element.

Further, the main circuit of the frequency inverter 10 is provided with an active switch K electrically associable in parallel to the passive compo- nent P. Preferably, the active switch K consists of a semiconductor component of the MOSFET transistor type.

The system of the present invention comprises at least a means for obtaining or measuring power at an input of the main circuit of the frequency inverter 10.

Further according to figures 2 and 3, the system of the present invention also comprises at least a control unit 3, operatively associated to the means for obtaining power, arranged so as to permit the drive of the active switch K based on the power at the input of the main circuit of the frequency inverter 0. Said means for obtaining or measuring power comprises at least a current sensor arranged to measure an IB bus current, as can be seen in figure 4. In addition, the means for obtaining or measuring power also comprises at least a voltage sensor arranged to measure a VB bus voltage corresponding to a voltage on the bus capacitor CB, as can be seen in figure 5.

Particularly, the control unit 3 is arranged to calculate an active power delivered to the compressor based on the measurement of the IB bus current and the measurement of the VB bus voltage. More specifically, the control unit 3 is also arranged to calculate the power at the input of the main circuit of the frequency inverter 10 adding the active power delivered to the compressor with the losses of power in the wave rectifier 4, in the passive component P and/or in the active switch K.

Additionally, the system comprises at least a drive circuit 5 operatively associated to the control unit 3 and to the active switch K, and the control unit 3 is also arranged to send a command to the drive circuit 5 to drive the active switch K when the power measured at the input of the frequency inverter is lower than a first preset reference value. In contrast, the control unit 3 is also arranged to send a command to the drive circuit 5 to disconnect the active switch K when the power measured at the input of the frequency inverter is higher than a second preset reference value. The second preset reference value is greater than the first preset reference value.

It must be noted that the block referenced by the numerical indica- tion 1 in the drawings represents a combination formed by the compressor and the entire remainder of the electronic circuit that makes up the frequency inverter. Said circuit can be a three-phase or single-phase inverter bridge, etc.

Next, the benefits and efficiency gains obtained by the present invention can be better understood by means of graphs and tables resulting from its implementation by way of the first and second preferred embodiments already mentioned previously.

First preferred embodiment

Figure 2 illustrates a first preferred embodiment of the system of the present invention. In this preferred embodiment, active switch K consists of a MOSFET semiconductor provided with a source terminal S connected to reference REF B.

It must be noted that the voltage drop between references REF A and REF B does not prevent the correct operation of the MOSFET since it is given by the voltage drop in the parallel association of the NTC and MOSFET whose equivalent resistance has a relatively low value.

The control unit 3 will drive the MOSFET whenever the presence of the resistance of the passive component P is irrelevant for attenuating the harmonic content of the input current. One of the possible ways of managing this drive is by reading the electric power processed by the main circuit of the fre- quency inverter 10 and delivered to the compressor. Considering that the level of the electric current drained from the FAC electric energy power source is proportional to the processed power, the harmonic content of the input current with this power will also be proportional. Hence, the control unit 3 is capable of driving the MOSFET whenever a power delivered to the compressor is below a certain value (first preset reference value), stipulated experimentally for a certain impedance value of the passive input component P of the main circuit of the frequency inverter 10 (in this case, by the resistance of the NTC element).

As can be seen in figure 2, the MOSFET is driven when the output voltage of the control unit 3 (voltage on the resistor R6) is at a high level, placing the transistors Q1 and Q2 in conduction, providing an applied voltage be- tween trigger G and source S terminals of the MOSFET. The output of the control unit 3 may assume two status during the time period in which the first charge of the bus capacitor CB occurs: low level (zero voltage on R6) or High Impedance status. Both cases prevent conduction of Q1 and consequent drive of the MOSFET during the in-rush current period. After initializing the control unit 3, it is possible to drive the MOSFET by applying a voltage on the resistor R6.

Second preferred embodiment

If the control unit 3 does not have the capacity to define its logical output status as High Impedance, it is possible to employ the circuit exemplified in figure 3, which consists of the second embodiment of the present invention. In this example, the drive of the MOSFET is performed by the transistor Q1 , commanded by the control unit 3. To prevent the MOSFET from being unduly driven during the first charge of the bus capacitor CB, the capacitance C1 is added between trigger G and source S terminals of the MOSFET. The pair of components C1 and R1 , suitably sized, provides that the voltage between trigger G and source S is near zero, regardless of the initialization of an auxiliary source 2 (15V) and of the command output of Q1 , coming from the control unit 3. Once the initialization period of the control unit 3 has elapsed, a voltage is applied on resistor R6, so as to block the transistor Q1 and keep the MOSFET disconnected.

Measuring the input power of the frequency inverter

To define the moment to connect or disconnect the MOSFET, it is necessary to measure directly or indirectly the input power of the main circuit of the frequency inverter 10. A possible arrangement to perform this measure- ment is indicated in figure 4. The control unit 3 uses average return current values of the charge and voltage on the bus capacitor CB to calculate the power or a value proportional thereto. Based on this power value and following a hysteresis rule pursuant to figure 8, the MOSFET parallel to the passive component P is commanded.

A possible variation of the current reading is illustrated in figure 5. In this case, a reading of the rectified input current is taken, which returns to the FAC electric energy power source (mains) by the diode bridge rectifier.

Time Diagrams (letters)

Figure 6 illustrates a sequence of events as of the moment in which the initial charge of the bus capacitor CB occurs, as per the second preferred embodiment (figure 3). In this sequence, it is noted that the active element K should not be driven in this interval so that the entire In-Rush current can circulate through the impedance of the passive component P (in this case, an NTC element).

In other words, figure 6 illustrates the first instants after powering up the input rectifier circuit. It is noted, as would be expected, that the charge current of the bus capacitor CB (In-Rush current) only circulates in the passive component P (NTC). The voltage between trigger G and source S terminals of the MOSFET slowly increases due to the presence of the pair R1 and C1 illustrated in figure 3. After fully charging the bus capacitor CB (in this case, after 60ms) the trigger voltage of the MOSFET continues below the minimum level to begin conduction.

Figure 7 illustrates a sequence of events subsequent to those of figure 6, where the command circuit of the main circuit of the frequency inverter 10 is initialized, a reading of the processed power is taken and, the decision whether or not to drive the MOSFET is made.

In other words, figure 7 demonstrates a sequence of events subsequent to those of figure 6. In this example, the voltage between trigger G and source S of the MOSFET reaches the minimum level to begin conduction (approximately 4V) in 0.4s. Bearing in mind that the bus capacitor CB is complete- ly charged and that the control unit 3 maintains the inverter inactive (compressor stopped), there is no relevant input current circulation in the MOSFET and passive component P. In the instant 2s (2 seconds), the control unit 3 drives the transistor Q1 (of figure 3), reducing the trigger voltage of the MOSFET. In instant 3s, the compressor is driven, raising the input current of the rectifier. It is important to note that the current circulates through the passive component P while the control unit 3 calculates the power being delivered to the compressor. If the power is below a reference value (see figure 8 which exemplifies the power levels in which the active switch K is driven or disconnected, considering a hysteresis to prevent intermittent drives of the active switch K), the MOSFET is driven (ON) so that the input current also circulates there through (instant 6s), reducing the conduction losses previously present in the passive component P.

As already mentioned previously, the drive of the active switch K only occurs when the equivalent impedance of the association is sufficient to attenuate the harmonic content of the input current in an adequate manner. Accordingly, the control unit 3 is capable of recognizing this level of power or any other variable proportional to the power. The input and the blockage of the active switch K may follow a hysteresis, as exemplified in figure 8. In this example, the active switch K is driven when the input power falls to a value below 40W and, is deactivated when this power exceeds 45W.

Method of streamlining energy efficiency

It is also an object of the present invention a method of streamlining energy efficiency for application in cooling equipment compressors which comprises the following steps of:

- measure the input power of the main circuit of the frequency inverter 10;

- electrically associate the active switch K in parallel to the passive component P when the input power from the main circuit of the frequency inverter 10 is lower than a first preset reference value P₀N', and

- electrically dissociate the active switch K to the passive component P when the input power from the main circuit of the frequency inverter 10 is higher than a second preset reference value POFF, and the second preset reference value POFF is greater than the first preset reference value P₀N- In the flowchart illustrated in figure 9, it can be noted that the circuit remains inactive (MOSFET disconnected) while the compressor is not working. After the start-up of the compressor, the power value is measured, comparing it with a reference value PO_FF (upper value of the hysteresis). If this power is greater than P₀F_F, the MOSFET is kept disconnected, as it is necessary for the conduction resistance of the passive component P to be in the circuit for attenuating the harmonic content of the input current. In contrast, the MOSFET is only connected when the compressor is working and when the input power is lower than the lower value of the hysteresis P_ON- The MOSFET will remain connected while the compressor is working and the power read is not higher than the P_OFF value.

Performance comparisons

Figure 10 illustrates experimental results obtained by implementing a main circuit of the frequency inverter 10 having an input rectifier circuit similar to that illustrated in figure 2 (first preferred embodiment), where the active switch K consists of a MOSFET model IRF840AS and the passive component P consists of an NTC with a value at 25°C of 10Ω (model B57237S0100M). Figure 10 presents a graph that contains two curves that relate the efficiency gain of the main circuit of the frequency inverter 10 with its input power, under deac- tivated MOSFET status (MOSFET OFF) and driven MOSFET (MOSFET ON) status.

Based on these two curves of the graph, it can be concluded that the addition of the conduction resistance of the MOSFET in parallel to the NTC (MOSFET status driven or MOSFET ON) provides an increased in the efficiency of the inverter between 0.5 and 0.6% for the range of 20 to 60 W of input power in relation to the deactivated MOSFET status (MOSFET OFF), which proves the efficiency gains obtained in a practical implementation of the proposed art of the present invention.

Figure 11 allows a comparison of the efficiency gain value (as per- centage of the input power) for the arrangement of the art shown in application WO2008/120928 with that of the present invention (figure 2 - first preferred embodiment). The relay used was model F3AA012E, with fixed bobbin consumption of 240 mW, and the MOSFET used was the component model STD12NM50N, with conduction resistance value of 0.46 Ω to the junction temperature of 50°C.

Based on observations of figure 11 , it can be concluded that the solution with relay proposed in patent application WO2008/120928, employing a contact of a relay in parallel to a passive component, does not provide the desirable efficiency gains, that is, it does not meet the efficiency gain objectives achieved by the solution of the present invention in a satisfactory manner.

Table 1 below shows the reference status for drawing the curves of figure 11. In this table, it is noted that the conduction losses in the passive component P (NTC) in the original arrangement (without implementing any active switch, be it relay or MOSFET) are proportional to the input current and to the conduction resistance.

Table 2 below illustrates the total loss when the solution with the relay F3AA012E is used. It is noted that the difference to the original arrangement (without implementing any active switch, be it relay or MOSFET) is irrelevant for the input power of 30 W and even negative for lower power values.

* Additional losses: 240mW to drive the relay, and additional losses in the bus capacitor due to the greater value of the RMS input current.

Table 3 below shows details of the total losses in the implementation of the solution proposed in figure 2 (first preferred embodiment), as well as the different to the original arrangement (without implementing any active switch, be it relay or MOSFET) and the efficiency gain obtained. It can be perceived that the loss basically depends on the conduction resistance of the parallel association between the passive component P (NTC) and the conduction channel of the MOSFET.

* Additional losses: 23mW to drive the Mosfet, and additional losses in the bus capacitor due to the greater value of the RMS input current.

Harmonics attenuation

Figure 12 illustrates the graph of the harmonic content of the input current obtained in a practical implementation of the present invention, in which it is evident that, as of a certain input power, the active switch K should be discon- nected so that the input impedance increases and the attenuation on the harmonics is greater.

Particularly, figure 12 enables a comparison of the limits of the harmonic content of the input current according to Norm IEC6 000-3-2 and the level of the harmonics when the MOSFET is connected and when the input power is 50 and 75 W. It is noted that the limit value of the 15^th harmonic is exceeded when the power is 75 W. It is thus demonstrated that the drive of the active switch K should be performed only for powers below a value in which the harmonic content of the input current comes close to the limits (it must be noted that, by consequence of the reduction of the equivalent resistance of the association MOSFET and the NTC, an increase of the harmonic content of the input current occurs).

In any case, figure 12 demonstrates that the present invention is capable of reducing the value of its input impedance, in order to reduce losses by conduction of the input current, whilst attenuating the harmonic content of the input current. In other words, by reducing the resistance of the NTC element, adding a resistive element parallel to it so that the equivalent value of the resistance is the minimum necessary, the harmonic content is adequately attenuated in a low power status of the cooling system. By raising the power required by the system, the parallel element is withdrawn so that only the resistance of the NTC is again present in the circuit.

Accordingly, the present invention is capable of, with the use of a

MOSFET semiconductor and in a low power status required by the compressor, reducing the resistance of the parallel association NTC and MOSFET, reducing the Joule losses on the circulation path of the input electric current of the frequency inverter and, consequently, boosting the efficiency of the cooling system. Moreover, the art described in patent application WO2008/120928 is not capable of achieving the objectives of the present invention principally due to the high consumption needed to drive the relay. Having described an example of a preferred embodiment, it must be understood that the scope of the present invention encompasses other possible variations, being limited only by the content of the accompanying claims, potential equivalents being included therein.

Claims

1. A system for streamlining energy efficiency, for application in cooling equipment compressors, the system having at least a main circuit of the frequency inverter (10) and an electric energy power source (FAC) electrically asso- ciated to each other, the frequency inverter circuit ( 0) comprising at least a:

- wave rectifier (4);

- bus capacitor (BC) electrically associated in parallel to the wave rectifier (4), the bus capacitor (BC) being electrically chargeable by a charge current; and

- passive component (P) electrically associated to the wave rectifier

(4), the passive component (P) being capable of reducing a charge current of the bus capacitor (BC) and/or attenuating the harmonic content of an input current coming from the electric energy power source (FAC);

the system being characterized by comprising at least:

- means of obtaining input power from the main circuit of the frequency inverter ( 0); and

- a control unit (3) operatively associated to the means for obtaining power,

the frequency inverter circuit being provided with an active switch (K) electrically associable in parallel to the passive component (P), the control unit (3) being arranged so as to permit the drive of the active switch (K) based on the power at the input of the main circuit of the frequency inverter ( 0).

2. A system according to claim 1 , characterized by comprising at least a drive circuit (5) operatively associated to the control unit (3) and to the active switch (K), the control unit (3) being arranged to send a command to the drive circuit (5) to drive the active switch (K) when the power measured at the input of the frequency inverter is lower than a first preset reference value (P_ON), the control unit (3) being arranged to send a command to the drive circuit (5) to disconnect the active switch (K) when the power measured at the input of the frequency inverter is higher than a second preset reference value (P_OF_F), the second preset reference value (POF_F) being higher than the first preset reference value (PO_N).

3. A system according to claim 1 or 2, characterized wherein the means for obtaining power comprises at least:

- a current sensor arranged to measure a bus current (IB); and

- a voltage sensor arranged to measure a bus voltage (VB) corresponding to a voltage on the bus capacitor (BC),

and the control unit (3) is arranged to calculate an active power delivered to the compressor based on the measurement of the bus current (IB) and the measurement of the bus voltage (VB), the control unit (3) being also arranged to calculate the power at the input of the main circuit of the frequency inverter (10) adding the active power delivered to the compressor with the losses of power in the wave rectifier (4), in the passive component (P) and/or in the active switch (K).

4. A system according to any of the preceding claims, characterized wherein the active switch (K) consists of a semiconductor component of the MOS- FET transistor type.

5. A system according to any of the preceding claims, characterized wherein the passive component (P) consists of an NTC element or an inductive element or association of both.

6. A method of streamlining energy efficiency, for application in cooling equipment compressors, the compressor being electrically associated to an electric energy power source (FAC) by means of a frequency inverter circuit (10), the main circuit of the frequency inverter (10) being provided with an active switch (K) and a passive component (P), the method being characterized by comprising the following steps:

- measure the input power of the main circuit of the frequency inverter

(10);

- electrically associate the active switch (K) in parallel to the passive component (P) when the input power from the main circuit of the frequency inverter (10) is lower than a first preset reference value (PON); and

- electrically dissociate the active switch (K) to the passive component (P) when the input power from the main circuit of the frequency inverter (10) is higher than a second preset reference value (POFF), the second preset reference value (POFF) being higher than the first preset reference value (PON)-