WO2008141407A1

WO2008141407A1 - Method and system for reducing electromagnetic interference in a refrigeration system

Info

Publication number: WO2008141407A1
Application number: PCT/BR2008/000139
Authority: WO
Inventors: Günter Johann MAASS; Roberto Andrich
Original assignee: Whirlpool S.A.
Priority date: 2007-05-17
Filing date: 2008-05-14
Publication date: 2008-11-27
Also published as: AR066618A1; BRPI0702291A2

Abstract

The present invention relates to a technique for reducing the levels of electromagnetic interference in refrigeration systems equipped with frequency inverter. The technique, known as Jittering, consists of cyclically varying the switching frequency of the bridge inverter. One of the ways in which the present invention may be embodied is by a method of reducing electromagnetic interference in a refrigeration system, said system comprising a variable speed electric motor (10) controlled by a bridge inverter (15) activated by a PWM modulation signal, said PWM modulation having a modulation period (TPWM) which varies between a maximum modulation period (TPWM-MAX) and a minimum modulation period (TPWM-MIN), said method comprising a step of continuously varying the modulation period (TPWM) between the maximum modulation period (TPWM-MAX) and the minimum modulation period (TPWM-MIN), SO as to gradually increase and decrease the period (TPWM) between the maximum and minimum values in each new PWM modulation period to attenuate the amplitude of the PWM signal at its fundamental frequency and its harmonic frequencies.

Description

Title: "METHOD AND SYSTEM FOR REDUCING ELECTROMAGNETIC INTERFERENCE IN A REFRIGERATION SYSTEM".

This application claims priority of Brazilian patent case No. PI0702291-3 filed on May 17, 2007, the disclosure thereof being herby incorporated by reference.

The present invention relates to a method and a system for attenuating electromagnetic interference generated by a refrigeration system equipped with a variable capacity compressor (VCC), by varying the switching frequency of the power inverter around a mean value (Jittering technique).

Description of the Prior Art

In order to meet the most demanding efficiency standards, domestic and commercial refrigeration systems have the option of using variable capacity compressors which, as the name suggests, enable the refrigeration capacity to be adjusted by varying the cooling gas pumping speed (that is, the mass flow), according to the need of the system.

Said variable capacity compressor goes from a minimum mass flow value to a maximum value by varying the rotation of its motor. The rotation variation is obtained by means of an electrical control called frequency inverter, which adjusts the voltage and the frequency applied to the motor.

Said frequency inverter uses pulse width modulation (PWM), that is, the voltage pulse width is transferred to the motor by a PWM modulator, and is defined in proportion to the desired rotation, which is also adjusted according to the load and voltage of the power line. The voltage applied to the motor is pulsed, with fixed mean value F_PWM frequency, usually of a few kHz.

The application of the modulation causes the electromagnetic interference (EMI), which is generated when the voltage and current switchings by the frequency inverter occur. Through the PHASE, NEUTRAL and GROUND connections, this interference can be transmitted by conduction to the electric line, and can affect the desirable operation of other equipment connected thereto or located nearby (in the latter case, affected by the interference irradiated from the refrigeration system or from the power cables themselves).

The electromagnetic interference may also affect the communication between two systems, one of which may be the refrigeration system itself, when the high voltage power line is used to send and receive data.

The levels of electromagnetic interference must be controlled according to several international standards, which set maximum limits for the frequency range of interest. The most common way of controlling EMI. levels is attenuation by means of passive components, which may be present in the noise generator circuit, as is the case with snubber circuits, or at the system power inlet (in this case, the frequency inverter), as is the case with differential and common mode filters. However, EMI attenuation by means of passive components has the disadvantage of impacting the final cost of the product, whether due to the cost of the component itself, to the physical space required or to the time required to develop the filter. Said impact increases considerably when the frequencies to be attenuated are lower frequencies (below 50OkHz), wherein the volume of the passive elements grows. Brief description and objects of the invention

The aims of the present invention are to use the Jittering technique in refrigeration systems equipped with variable capacity compressors and the PWM signal modulation technique for obtaining the Jittering.

- Using Jittering to reduce electromagnetic interference generated by variable capacity compressors (VCC) in refrigeration systems.

- Reducing of the level of electromagnetic noise or interference (produced by the inverter and conducted to the power line) by applying the Jittering technique to the PWM modulation of the frequency inverter, thus enabling communication with other equipment connected to the same power line, such as other household appliances and computers. - Possibility of reducing the electromagnetic interference passive filter both regarding values and physical size;

- Assisting in adapting systems that use the power line as a means of transmitting data to the standards for conducted electromagnetic interference;

- Easy and fast implementation;

- Repeatability of the EMI reduction effect, as it is digitally implemented (software).

- Reducing EMI by decreasing PWM amplitude in a single frequency, continuously alternating the T_PWM modulation period, attaining a

PWM signal distribution in several frequencies and a consequent attenuation of its amplitude.

In general lines, the aims of the present invention are achieved by varying the duty cycle of the inverter around its mean value, a method also known as Jittering. The maximum amplitude of the variation is defined and realized by consecutive increases and decreases to the value of the TPWM modulation period. Said variation is carried out cyclically, with or without constant increments, and may describe a linear, sinusoidal, ramp-like or continuously variable behavior, or another possible type of variation to distribute the electromagnetic interference levels originated by the PWM signal in different frequencies.

One of the aims to be achieved is assisting in the communication between a refrigeration system and a remotely positioned control using the high voltage line, for example, using a personal computer installed in a household to control the behavior of the domestic refrigerator, or using a suitable computer to control refrigerators in commercial environments, such as supermarkets and other similar places. Said communication is known as PLC (Power Line Communications) and enables, for example, data to be sent and received through the power supply line. This embodiment can be observed in figure 1 , which shows, as an example, a system "A" with PLC, a system "B" with PLC and a computer with PLC connected to the V_AC line 100, with an electric motor 10 of a compressor of a refrigerator 2 also connected to the same line, all of which systems communicate via PLC. It can be observed that a control circuit 3 is electrically connected to the V_Ac line, enabling the communication between systems "A", "B" and the computer, and the sending of control signals from the control circuit 3 based on decisions made by systems "A", "B" or by the computer. In other words, the present invention aims at implementing communication between a computer and the control central unit of a refrigerator or any air conditioning system, for example, which communication may be carried out directly from a computer with the refrigerator thermostat plate. But as it happens, the PWM inverters for these solutions must be according to the EMI standards, which set limits at much lower frequencies, tested for the range of from 9 kHz to 20 kHz, while the normal value would be from 150 kHz up. In this low frequency range, however, the electromagnetic interference levels generated by the PWM inverter switching may have very representative values, and very large passive elements having a very high inductance are required, which results in higher costs, making it impossible to meet the standard requirements other than by using Jittering combined with passive filters.

One of the ways in which the aims of the present invention may be embodied is by a method of reducing electromagnetic interference in a refrigeration system, said system comprising a variable speed electric motor controlled by a bridge inverter activated by a PWM modulation signal, said PWM modulation having a modulation period which varies between a maximum modulation period and a minimum modulation period, said method comprising the steps of: continuously varying the modulation period between the maximum modulation period and the minimum modulation period, so as to gradually increase and decrease the period between the maximum and minimum values in each new PWM modulation period in order to attenuate the amplitude of the PWM signal, or dividing the period of time between the maximum modulation period and the minimum modulation period into n intervals and continuously varying the modulation period from one interval to the next, at the ratio of a modulation variation every n intervals, said modulation variation being established between the maximum modulation period and the minimum modulation period.

In another embodiment, the aims are achieved through an electromagnetic interference reduction system in a refrigeration system, said system comprising a variable speed electric motor controlled by a bridge inverter activated by a PWM modulation signal, said PWM modulation having a modulation period which varies between a maximum modulation period and a minimum modulation period, said PWM modulator being configured to vary the modulation period between the maximum modulation period and the minimum modulation period, so as to gradually increase and decrease the period between the maximum and minimum values in each new PWM modulation period in order to attenuate the amplitude of the PWM signal at its fundamental frequency and at the harmonic frequencies.

In another embodiment, the aims are achieved through an electromagnetic interference reduction system in a refrigeration system, wherein the variable speed electric motor is electrically connected to an alternating current line, the same line to which one or more remote processing units with PLC are connected, said PWM modulator being configured to vary the modulation period between the maximum modulation period and the minimum modulation period, so as to gradually increase and decrease the period between the maximum and minimum values in each new PWM modulation period in order to attenuate the amplitude of the PWM signal, or said PWM modulator being configured to apply Jittering varying the modulation period between the maximum modulation period and the minimum modulation period, the Jittering having a continuously variable Jit% modulation variation.

Brief Description of the Drawings

The present invention will be described in more detail below, based on the figures. The figures show:

Figure 1 - is an example of an embodiment of a refrigerator control system using the teachings of the present invention.

Figure 2 - is an electric diagram illustrating an inverter and its main elements, and the representation of the conducted electromagnetic interference flow;

Figure 3 - shows a rectangular signal having a defined pulse width, hereinafter referred to as PWM signal, compared to a PWM signal submitted to a modulation of its period, characterizing Jittering; Figure 4 - characterizes the term Tcic cycle period and shows the variation of the PWM period when Jittering is applied, with an "n" number of steps defined by a constant increase or decrease, referred to as ΔT_PWM modulation period variation;

Figure 5 - illustrates, by means of a flowchart, a simplified way of implementing Jittering using the bridge inverter control software;

Figure 6 - is a chart exemplifying the effect of Jittering on a defined PWM signal. The chart shows the components near the third harmonic of the fundamental frequency, obtained by Fourier analysis of the signal; and Table 1 - illustrates the percentage variation applied to the PWM in an embodiment of the present invention. Detailed Description of Drawings

Figure 2 illustrates a frequency inverter with its most basic components. The figure shows the bridge inverter 15 formed by six active switches, to which a command signal of the PWM type is applied. It further forms the basic circuit, an energy storage stage formed by the C_LINK capacitor, a stage with a bridge rectifier 20, and an electromagnetic interference filtering stage formed by the Cx, Cy and L components.

The frequency inverter or bridge rectifier 15 connects to the outside, and can be for an element internal or external to the refrigeration system. On the one hand, the frequency inverter receives electric power from a mono- or three-phase line, which may or may not have a grounding conductor. On the other, the frequency inverter supplies power to the load

(motor, for example), which is connected to the grounding of the refrigeration system.

As can be seen in figure 2, there are two ways of propagating conducted electromagnetic interference, one originated by differential mode currents and another originated by common mode currents.

The PWM modulation of the bridge inverter 15 has influence on both ways. The PWM of the bridge inverter 15 defines the time of transferring power from the CLINK capacitor to the load, which occurs in the form of current pulses which circulate through the parasitic resistance of CLINK (ESR - Equivalent Series Resistance), which consequently produces a voltage drop in the PWM frequency, which voltage is one of the origins of the differential mode current. The PWM of the bridge inverter 15 also defines the time of switching the voltage applied to the load. Since the switching lasts for a short period of time, there will be current circulation through the Cp parasitic capacitance of the load, which is one of the possible ways in which the common mode current originates. It is thus understood that the PWM signal directly impacts the electromagnetic interference levels produced by the inverter. Figure 3 compares PWM signals with and without Jittering and shows the variation of the switching period to obtain the Jittering effect.

The PWM signal without Jittering shown in figure 3 (PWM without Jittering) has a constant T_PWM modulation period, as well as constant t_ON (high level time interval) and toFF (low level time interval) times. The to_N and toFF intervals define the D duty cycle of the PWM signal according to the following equation:

^{D =} 7 , wherein: T_PWM = t_ON + t_OFF

¹ PWM

The electromagnetic interference produced by the differential mode and common mode currents is characterized by the energy levels (voltage and current) distributed in the frequency of the PWM signal and its respective harmonic frequencies.

By Fourier decomposition, the harmonic content of the PWM signal without Jittering can be found, according to the following equation: x(t)= a_Q + ∑a_k -cos(2 - π - f₀ - k - t)- b_k - sen(2 - /r - /₀ - k - t)

wherein: a_k = 2 - C_k - CQ*(θ_k ) b, = 2 - C, sen M

A-t_ON senfø)

C_k =

* PWM "k

θ_k = π - f₀ - t_ON - k

Zk = i ^ak^{2 + b}k² wherein:

"z_k" is the amplitude of the signal in the k-order harmonic; "k" is the harmonic order at which the z_k amplitude is to be determined;

"fo" is the fundamental frequency of the PWM signal, that is, the switching frequency of the bridge inverter;

"A" is the amplitude of the PWM signal. The PWM signal with Jittering illustrated in figure 3 (PWM with

Jittering) has different total T_PWM modulation periods between adjacent cycles, that is, there will always be an increase or a decrease in the period equal to an "i" or ΔT_PWM value, as shown in figure 4, wherein ΔT_PWM represents a variation in the modulation period. The t₀N and t_OFF times also vary in proportion to the variation of the switching period, so that the D duty cycle of the PWM signal is kept constant.

In order to determine the harmonic content of a PWM signal with

Jittering, the sum of the harmonic content of each switching period is calculated, considering its period and its shifting "α" for time zero, that is, the time interval between the beginning of the T_Cιc cycle period (see figure 4) and the beginning of the period for which the harmonic content is being calculated.

The following equations show how the harmonic content of a

PWM signal with Jittering is determined: f(t)<÷ F(f) f{t-a_n)+>F(f)-e-^j2«fa" wherein:

"n" is the number of the period relative to the origin (beginning of

Tcic cycle period); "a_n" is the shifting (time) of the "n" PWM cycle relative to the origin

(beginning of T_Cιc cycle period); wherein: e-JW^ac*-*. _{= cos}(_ 2-π-f_ac-k-a_n)+ ysen(- 2-π- f_ac-k-a_n)

and:

J CIC ^~

¹CIC

Considering that:

e-ii-n^.f_cιc-k-a_{n = cos}føj_ jsen(φ_n)

Then:

F{f)- e-w^∞*-^*' = k + Λ ]• [cosfe )- ysenfø, )]

Considering that:

b^ = -a_k ■ senfe, )+ b_k • cosfø_π ) the amplitude of the signal for each k harmonic is determined:

wherein:

"Z_k" is the absolute value of the amplitude in each "k" harmonic of the frequency determined by the T_Cιc cycle period.

Figure 4 illustrates a way of obtaining the Jittering effect. In this example, it can be noticed that the behavior of the switching period variation follows a triangular path. This path may be different, of the sinusoidal, sawtooth or even random type. Two characteristics of the Jittering can be identified in figure 4. The first is the variation rate of the T_PWM modulation period. It can be noticed that there is a maximum T_PWM modulation period and a minimum T_PWM modulation period, respectively referred to as T_PW_M-_MIN minimum modulation period and TPW_M-MAX maximum modulation period. This rate, herein referred to as Jit% modulation variation rate, directly impacts how much the amplitude of the harmonic frequencies is reduced, and Jit% may or may not be constant. A second characteristic is the "n" number of steps. A given Jit% modulation variation rate can be divided into "n" intervals. The increment between each "n" step, referred to as ΔTPWM, may or may not be constant. By fixing a constant number of "n" steps between the T_PWM-MIN minimum modulation period and the TPWM-_MAX maximum modulation period, the Tcic cycle period is determined, that is, how often the variation cycle of the T_PWM modulation period is repeated. By defining T_Cιc period as the period between the T_PWM-MAX maximum modulation period and the T_PWM-MIN minimum modulation period, it is also possible to define an "n" number of steps that is not constant but variable as to the amplitude and duration for each T_Cιc period.

If the Jit% and "n" values are variable, the following T_Cιc period illustrated in figure 4, for example, could be considered to have 12 steps of lower amplitude and shorter duration for each 1/4 of a cycle (n=12) when compared to the previous T_Cιc period with n=6 (in the figure, 1/4 of a cycle has n=6). It could also be considered that, in order to form the triangular drawing of figure 4, the "n" steps would be consecutively variable, with the result that the variation of the T_PWM modulation period is made by a continuously and consecutively variable ΔT_PWM modulation variation.

According to the equation already shown, the spectrum of the PWM signal with Jittering will have harmonics at the frequency determined per Tcic cycle period. Figure 5 illustrates, by means of a flowchart, a possible way of implementing Jittering using software. In this case, a "Mult_JT" multiplier is applied to the nominal value of the TPWM modulation period. The values shown in the table follow an increment and decrement triangular function, with a variation rate of 2% of the nominal value of the switching period, forming a triangular cyclic variation, of "n=2" intervals with a fixed increment of 1 % of the nominal period value. Based on this example and using the table, higher "n" interval and "Jit%" rate values can be used to obtain a higher attenuation of the value of the amplitude of the PWM signal at its fundamental frequency and at the harmonic frequencies.

Specifically regarding the signal and the flowchart illustrated in figures 3, 5 and table 1 , it can be noticed that, according an example of an embodiment to provoke the Jittering effect, a configuration is used in which the PWM modulation is controlled in such a was as to generate a variation of the Tpw_M modulation period to achieve a triangular behavior. As can be observed, when the software algorithm routine is initiated, the value of the T_PWM modulation period is recalculated to a value submitted to the Jittering effect passing to a value of a modulation period altered by T_PWM_JT jittering, through the multiplication by a Mult_JT[i] jittering multiplier, which varies according to the desired percentage variation. As can be seen in figure 5, a constant percentage variation of 1% can be expected, for example, which defines a triangular variation and, more particularly, in the present example, two steps are used, that is, n=2, so that there will be eight steps defined in the variation between the minimum modulation period (T_PWM-_MIN) and the maximum modulation period (T_PWM-_MAX), with a sequence of eight jittering multipliers, Mult_JT[00] to Mult_JT[08], varying between 1.00 and 1 ,02 and between 1.00 and 0.98, with the result that the value of the T_PWM modulation period will be

As can be seen from the chart, the value of the T_PWM modulation period varies between the T_PW_M-MIN minimum modulation period and the Tpw_M-_MAx maximum modulation period in the course of the software algorithm routine. Specifically observing figure 3, it can be observed that, when Jittering is applied, the t_ON and t_0FF times, respectively corresponding to the time during which the switch is on and the time during which the switch is off, will vary constantly, as can be seen from the chart. Then, for the t_ON time to t_OFF time ratio to be kept constant, the duty cycle D is adjusted to a D_JT jittering duty cycle by multiplying it by the Mult_JT[i] jittering multiplier, then incrementing the value i to i+1 , and proceeding with the algorithm. In the present case, since eight multipliers are used, a verification is made, and if i > 8, then i is brought to zero for the routine to restart. On the other hand, if "i" is not higher than eight, the routine is repeated with an incremented i value for the T_PWM modulation to have the jittering effect.

The algorithm is analogous for the cases in which the number of intervals is different and for configurations in which the variation of the T_PWM modulation period has a sinusoidal, ramp-like or other behavior, and the required adjustments must be made to the percentage increments and to the T_PWM modulation periods.

Figure 6 illustrates the effect of Jittering on the frequency spectrum of a unit-amplitude PWM signal, third harmonic of the defined frequency (5kHz) and with 60% duty cycle (to_N to toFF ratio), when a twenty- step Jittering is applied. The example shows the effect on the 15kHz frequency range (third harmonic of the switching frequency). A reduction in the amplitude of the signal, the aim of the present technique, can be noticed (see in figure 6, in a darker shade, the 15 kHZ frequency without Jittering, and in a lighter shade, the spreading of the signal in different frequency ranges, that is, through the twenty steps), which means that there is a lower level of interference in this frequency range and that the aims of reducing the passive components while enabling and assisting data communication without causing EMI problems have been achieved. For the present invention to be implemented, the system should be provided with an electronic control circuit which is able to control an electric motor 10 of a variable capacity compressor (VCC) by PWM controlled by means of a bridge inverter 15. In order to attenuate the EMI by Jittering, the electronic control circuit should be configured to vary the TPWM modulation period between the T_PWM-MAX maximum modulation period and the Tpw_M-_Mi_N minimum modulation period, so as to gradually increase and decrease the T_PWM modulation period between the maximum and minimum values in each new PWM modulation period in order to attenuate the amplitude of the PWM signal at its fundamental frequency and at the harmonic frequencies, as shown in figures 3 and 6.

In order to implement a system which can remotely control the refrigeration system, the present invention provides an electromagnetic interference reduction system in a refrigeration system to achieve the objectives of the invention, wherein the variable speed electric motor 10 is electrically connected to a remote processing unit through the alternating voltage line 100, for the electronic control circuit 3 to transmit and receive data by V_Ac line PLC, for which a personal computer or similar device which controls the behavior characteristics of the refrigeration system can be used, whether in a household or in larger environments, to control refrigerators or even air conditioning systems, for example.

The methodology for controlling the system which is the subject of the present invention should comprise a step of continuously varying the T_PWM modulation period between the T_PWM-M_AX maximum modulation period and the T_PWM-M_IN minimum modulation period, so as to gradually increase and decrease the T_PWM modulation period between the maximum and minimum values in each new PWM modulation period to attenuate the amplitude of the PWM signal at its fundamental frequency and its harmonic frequencies.

In order to attenuate the PWM signal, the following steps can also be carried out: dividing the period of time between the T_PWM-M_AX maximum modulation period and the TPWM-MIN minimum modulation period into n intervals and continuously varying the T_PWM modulation period from one interval to the next, at the rate of a ΔT_PWM modulation variation every n intervals, said ΔT_PWM modulation variation being established between the TP_WM-MA_X maximum modulation period and the T_PWM-MIN minimum modulation period. Thus, the present invention achieves the aim of reducing EMI without using additional filtering components, therefore reducing the construction and maintenance costs of electromagnetic interference reduction systems in refrigeration systems.

The description above refers to an example of a preferred embodiment; however, it should be understood that the scope of the present invention includes other possible variations and is limited only by the content of the attached claims, including therein the possible equivalents.

Claims

1. A method of reducing electromagnetic interference in a refrigeration system, said system comprising a variable speed electric motor (10) controlled by a bridge inverter (15) activated by a PWM modulation signal, said PWM modulation having a modulation period (T_PWM) which varies between a maximum modulation period (T_PWM-MAX) and a minimum modulation period (T_PVVM-MIN), said method being characterized by comprising a step of continuously varying the modulation period (T_PWM) between the maximum modulation period (TPWM-M_AX) and the minimum modulation period (TPWM-MIN), so as to gradually increase and decrease the modulation period (T_PWM) between the maximum and minimum values in each new PWM modulation period to attenuate the amplitude of the PWM signal at its fundamental frequency and its harmonic frequencies.

2. A method, according to claim 1 , characterized in that the variation of the modulation period (T_PWM) exhibits a triangular behavior.

3. A method, according to claim 1 , characterized in that the variation of the modulation period (TPW_M) exhibits a sinusoidal behavior.

4. A method, according to claim 1 , characterized in that the variation of the modulation period (T_PWM) exhibits a ramp-like behavior.

5. A method, according to claim 1 , characterized in that the variation of the modulation period (T_PWM) exhibits a random behavior.

6. A method, according to claim 1 , characterized in that the variation of the modulation period (T_PWM) is carried out by a constant modulation variation (ΔT_PWM)-

7. A method, according to claim 1 , characterized in that the variation of the modulation period (TPWM) is carried out by a continuously variable modulation variation (ΔTPWM).

8. A method, according to claim 7, characterized in that the variation of the modulation period (T_PWM) is carried out by a continuously and consecutively variable modulation variation (ΔT_PWM)-

9. A method of reducing electromagnetic interference in a refrigeration system, said system comprising a variable speed electric motor (10) controlled by a bridge inverter (15) activated by a PWM modulation signal, said PWM modulation having a modulation period (T_PWM) which varies between a maximum modulation period (T_PWM-M_AX) and a minimum modulation period (T_PWM-MIN)- said method being characterized by comprising the steps of: - dividing the period of time between the maximum modulation period (T_PWM-MAX) and the minimum modulation period (T_PWM-MIN) into n intervals; - continuously varying the modulation period (T_PWM) from one interval to the next, at the ratio of a modulation variation (ΔT_PWM) every n intervals, said modulation variation (ΔT_PWM) being established between the maximum modulation period (T_PWM-_MAX) and the minimum modulation period (T_PWM-MIN)-

10. A method, according to claim 9, characterized in that the cyclic variation of the modulation period (T_PWM) occurs within a full PWM modulation cycle (T_Cιc)-

11. A method, according to claim 9, characterized in that the variation of the modulation period (T_PWM) is carried out by a constant modulation variation (ΔT_PWM)-

12. A method, according to claim 9, characterized in that the variation of the modulation period (T_PWM) is carried out by a continuously variable modulation variation (ΔT_PWM).

13. An electromagnetic interference reduction system in a refrigeration system, said system comprising a variable speed electric motor

(10) controlled by a bridge inverter (15) activated by a PWM modulation signal, said PWM modulation having a modulation period (T_PWM) which varies between a maximum modulation period (T_PWM-M_AX) and a minimum modulation period (T_PWM-MIN). said system being characterized in that the PWM modulator is configured to vary the modulation period (T_PWM) between the maximum modulation period (T_PWM-MAX) and the minimum modulation period (T_PWM-MIN), so as to gradually increase and decrease the modulation period (T_PWM) between the maximum and minimum values in each new PWM modulation period to attenuate the amplitude of the PWM signal at its fundamental frequency and its harmonic frequencies.

14. A system according to claim 13, characterized in that the

PWM modulator is configured for the variation of the modulation period (TPW_M) to have a continuously variable behavior.

15. A system according to claim 13, characterized in that the PWM modulator is configured for the variation of the modulation period (T_PWM) to have a triangular behavior.

16. A system according to claim 13, characterized in that the PWM modulator is configured for the variation of the modulation period (T_PWM) to have a sinusoidal behavior.

17. A system, according to claim 13, characterized in that the variation of the modulation period (T_PWM) is continuously variable.

18. A system, according to claim 13, characterized by being configured to divide the modulation period (T_PWM) into n intervals to vary the modulation period (T_PWM) from one interval to the next, at the ratio of a modulation variation (ΔT_PWM) every interval, said modulation variation (ΔT_PWM) being established between the maximum modulation period (TPWM- MA_X) and the minimum modulation period (T_PWM-MIN).

19. An electromagnetic interference reduction system in a refrigeration system, said system comprising a variable speed electric motor (10) controlled by a bridge inverter (15) activated by a PWM modulation signal, said PWM modulation controlling a voltage of an alternating voltage line (100) rectified by a bridge rectifier (20), said PWM modulation having a modulation period (T_PWM) which varies between a maximum modulation period (T_PWM-MAX) and a minimum modulation period (TPWM-MIN), said system being characterized in that the variable speed electric motor (10) is electrically connected to a remote processing unit through the alternating voltage line (100), in which said remote processing unit transmits and receives data to and from systems external to the refrigerator by PLC communication, said PWM modulator being configured to vary the modulation period (T_PWM) between the maximum modulation period (T_PWM-MAX) and the minimum modulation period (T_PWM-MIN), SO as to gradually increase and decrease the modulation period (TPWM) between the maximum and minimum values in each new PWM modulation period to attenuate the amplitude of the PWM signal at its fundamental frequency and its harmonic frequencies.

20. An electromagnetic interference reduction system in a refrigeration system, according to claim 19, characterized in that the remote processing unit is a computer.

21. An electromagnetic interference reduction system in a refrigeration system, characterized in that the behavior characteristics of the refrigeration system may be controlled from the computer.

22. An electromagnetic interference reduction system in a refrigeration system, said system comprising a variable speed electric motor

(10) controlled by a bridge inverter (15) activated by a PWM modulation signal, said PWM modulation controlling a voltage of an alternating voltage line (100) rectified by a bridge rectifier (20), said PWM modulation having a modulation period (T_PWM) which varies between a maximum modulation period (TPWM-MAX) and a minimum modulation period (T_PWM-MIN), said system being characterized in that the variable speed electric motor (10) is electrically connected to a remote processing unit through the alternating voltage line (100), in which said remote processing unit transmits and receives data to and from systems external to the refrigerator by PLC communication, said PWM modulator being configured to apply Jittering varying the modulation period (T_PWM) between the maximum modulation period (T_PWM-MAX) and the minimum modulation period (T_PWM-MIN), wherein the Jittering exhibits a continuously variable modulation variation (Jit%).