Method for controlling the defrost of an evaporator in a refrigeration appliance

Abstract

A method for controlling evaporator defrost in a refrigeration appliance having a cooling circuit with a variable cooling capacity compressor is disclosed. The method includes assessing the cooling capacity of the cooling circuit and calculating the defrost time on the basis of such cooling capacity. Defrost time is preferably assessed on the basis of the inverter frequency driving the compressor and/or the percentage of insertion of the compressor.

Claims

1. A method for controlling a defrost of an evaporator in a refrigerator comprising: providing a refrigerator comprising a cooling circuit with a variable cooling capacity compressor; assessing an average cooling capacity of the cooling circuit over a predetermined time period based on inverter frequency driving the compressor, a percentage of insertion of the compressor, or both the inverter frequency driving the compressor and the percentage of insertion of the compressor; calculating a defrost initiation time on a basis of the average cooling capacity; and initiating a defrost of the evaporator based in part on the calculated defrost initiation time; wherein the step of assessing cooling capacity is assessed according to the following formula:
CC=f(freq)A.sub.0+A.sub.1.Math.freq where freq is the frequency of the inverter driving the compressor, and A0 and A1 are experimental constants.

2. The method according to claim 1, wherein the step of calculating the defrost initiation time is calculated proportional to an amount of ice accumulated on the evaporator.

3. A refrigerator comprising: a cooling circuit with a variable cooling capacity compressor; and a control unit for driving the compressor; wherein the control unit is adapted to assess a cooling capacity of the cooling circuit over a predetermined time period, to calculate a defrost initiation time on a basis of such cooling capacity, and to initiate a defrost procedure based in part on the calculated defrost initiation time; wherein the control unit is adapted to assess cooling capacity on a basis of an inverter frequency driving the compressor, or a percentage of insertion of the compressor, or both the inverter frequency driving the compressor and the percentage of insertion of the compressor; and wherein the cooling capacity is assessed according to the following formula:
CC=f(freq)A.sub.0+A.sub.1.Math.freq where: freq is the frequency of the inverter driving the compressor, and A0 and A1 are experimental constants.

4. The refrigerator according to claim 3, wherein the defrost initiation time is calculated proportional to an amount of ice accumulated on an evaporator.

5. A method for controlling a defrost of an evaporator in a refrigerator comprising: providing a refrigerator comprising a cooling circuit with a variable cooling capacity compressor driven by an inverter motor; assessing an average inverter motor frequency over a predetermined time period; predicting ice formation over the evaporator based in part on the average inverter motor frequency; and initiating a defrost procedure of the evaporator based in part on the predicted ice formation exceeding a predetermined threshold; wherein the step of assessing the average inverter motor frequency is assessed according to the following formula:
freq=freq.sub.mean=1/Tfreqdt where: freq is the frequency of the inverter driving the compressor, and T is a predetermined time period.

6. The method according to claim 5, wherein the step of predicting the ice formation over the evaporator is predicted according to the following formula:
iceA.sub.0+A.sub.1.Math.freq where: freq is the frequency of the inverter driving the compressor, A0 and A1 are experimental constants; and T is a predetermined time period.

7. The method according to claim 5, wherein the step of predicting the ice formation is calculated proportional to an amount of ice accumulated on the evaporator.

8. The method according to claim 5, wherein the step of assessing the average inverter motor frequency over a predetermined time period is based on both the inverter frequency driving the compressor and a percentage of insertion of the compressor.

9. The method according to claim 1, wherein the step of assessing the average cooling capacity is based on a temperature measurement of the evaporator and a temperature measurement of a condenser.

10. The refrigerator according to claim 3, wherein the control unit is adapted to assess cooling capacity based on a temperature measurement of an evaporator and a temperature measurement of a condenser.

11. The method according to claim 5, wherein the step of predicting the ice formation over the evaporator is based in part on a temperature measurement of the evaporator and a temperature measurement of a condenser.

12. The method according to claim 1, wherein A0 and A1 experimental constants are appliance calibration constants for the refrigerator.

13. The refrigerator according to claim 3, wherein A0 and A1 experimental constants are appliance calibration constants for the refrigerator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a control process according to prior art;

(2) FIG. 2 is a schematic view similar to FIG. 1 in which a control process according to the present invention is shown; and

(3) FIG. 3 show how the frost generation process is controlled in a refrigerator using a variable cooling capacity compressor and a control algorithm according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) The control algorithm according to the invention starts from the idea that the cooling capacity (CC) is a function of inverter motor frequency driving the compressor (freq), temperature of the evaporator (T.sub.evap) and temperature of condenser (T.sub.cond):
CC=f(freq,T.sub.evap,T.sub.cond)

(5) The most important variable in the above relationship is the frequency. So, as first approximation, we can consider:
CCf(freq)

(6) According to very general conditions (that we can assume being satisfied without loss of generality of the consequences), it is known that a function can be represented by a Taylor series:
CC=f(freq)=.sub.h=0.sup.A.sub.h.Math.freq.sup.h

(7) Where h is the sum index (non negative integer) and the A.sub.h are constants that can be analytically evaluated by

(8) ${A_h=\frac{1}{h!}\frac{{}^h\mathrm{CC}}{{\mathrm{freq}}^h}.Math.}_{\mathrm{freq}=0}$
if the function CC=f(freq) is known.

(9) It is also known that a function satisfying that assumption can be approximated by the first N terms of the series (where N determines the error of the approximation). In particular, we assume N=1
CC=f(freq).sub.h=0.sup.NA.sub.h.Math.freq.sup.hA.sub.0+A.sub.1.Math.freq

(10) A.sub.0 and A.sub.1 can be both analytically and experimentally evaluated . . . . That is:
CCA.sub.0+A.sub.1.Math.freq

(11) Then, the first and most important point is that this approach is able to reduce the number of the inputs to the algorithm.

(12) Applying the concept to defrost operation, where the target is to estimate ice formation over the evaporator, the solution according to the invention is based on the general principle:

(13) $\mathrm{ice}\stackrel{\_}{\mathrm{CC}}=\frac{1}{T}\mathrm{CC}t~A_0+A_1.Math.\stackrel{\_}{\mathrm{freq}}$
Where:

(14) $\stackrel{\_}{\mathrm{CC}}={\mathrm{CC}}_{\mathrm{mean}}=\frac{1}{T}\mathrm{CC}t$$\stackrel{\_}{\mathrm{freq}}={\mathrm{freq}}_{\mathrm{mean}}=\frac{1}{T}\mathrm{freq}t$

(15) The average cooling capacity and the average frequency are evaluated, respectively, over the time span T. This relationship works for variable cooling capacity compressor and single speed compressor as well. It is clear that, by using the frequency as input, it works in case of variable cooling capacity compressor, where variable frequency is the main control parameter. Let's see how it works and how it could be modified in case of single speed compressor.

(16) In case of single speed compressor, in fact, constant input frequency is equal to:

(17) $\stackrel{\_}{\mathrm{freq}}=\frac{1}{T}\mathrm{freq}t=\frac{{\mathrm{freq}}_0}{T}\mathrm{Comp\_OnOff}t=\mathrm{Insertion}*{\mathrm{freq}}_0$
Where: Insertion stands for the time fraction during which the compressor is ON (time fraction respect on T); freq.sub.0 is the only working frequency of the single speed compressor.
Then, the formula can be simplified in:
ice CC=A.sub.0+k.sub.1.Math.insertion
where:
k.sub.1=A.sub.1.Math.freq.sub.0

(18) With reference to FIG. 3, with A it is indicated the cooling capacity (right Y axis, in Watt), with B the total frost generated (left Y axis, in gram), with C the defrost threshold according to the algorithm of the invention, on the X axis time (in minutes) being reported.

(19) The control approach according to the invention is an improvement also because it extends the approach based on ice CC=A.sub.0+k.sub.1.Math.insertion that works only in case of single speed compressor to a new class of compressor (variable speed compressors or variable cooling capacity compressors) by the formula ice CCA.sub.0+A.sub.1.Math.freq.