METHOD FOR SELECTING A FREQUENCY CONVERTER FOR A REFRIGERANT COMPRESSOR UNIT

Abstract

In order to improve a method for selecting a frequency converter for a refrigerant compressor unit that includes a refrigerant compressor and an electric drive motor such that the frequency converter is selected in a manner for optimized use, it is proposed that a working state suitable for operation of the refrigerant compressor unit should be selected within an application field of an application graph of the refrigerant compressor, that an operating frequency for this selected working state should be selected, and that a working state operating current value that corresponds to the selected working state and the selected operating frequency should be determined from drive data, for operation of the refrigerant compressor unit.

Claims

1. A method for selecting a frequency converter for a refrigerant compressor unit, including a refrigerant compressor and an electric drive motor, a working state suitable for operation of the refrigerant compressor unit is selected within an application field of an application graph of the refrigerant compressor, an operating frequency for this selected working state is selected, and a working state operating current value that corresponds to the selected working state and the selected operating frequency is determined from drive data, for operation of the refrigerant compressor unit.

2. A method according to claim 1, wherein, using the working state operating current value, the frequency converter whereof the maximum converter current value is greater than or equal to the determined working state operating current value is selected from data of frequency converters available for selection.

3. A method according to claim 2, wherein the frequency converter whereof the maximum converter current value is as close as possible to the working state operating current value is selected.

4. A method according to claim 1, wherein the frequency converter is selected such that its maximum converter start-up current value is greater than or equal to a start-up current value of the refrigerant compressor unit.

5. A method according to claim 4, wherein a stored start-up current value is used for selecting the frequency converter.

6. A method according to claim 4, wherein the start-up current value is determined empirically.

7. A method according to claim 4, wherein the frequency converter is selected such that its maximum converter current value is as close as possible to the start-up current value.

8. A method according to claim 4, wherein the frequency converter is selected such that its maximum converter current value is as close as possible to the higher of the values of working state operating current value and start-up current value.

9. A method according to claim 1, wherein the drive data are determined empirically.

10. A method according to claim 1, wherein, in each working state of the refrigerant compressor unit, empirical drive data are stored for the possible operating frequencies to be selected.

11. A method according to claim 1, wherein the operating frequency that is to be selected lies in the range from 0 hertz to 140 hertz.

12. A method according to claim 1, wherein the drive data have the empirically determined power consumption for each working state in the application field at the different operating frequencies.

13. A method according to claim 12, wherein the working state operating current value at the selected operating frequency is calculated on the basis of the empirically determined electrical power consumption at the respective operating frequency, taking into account an equivalent circuit of the drive motor of the refrigerant compressor unit.

14. A method according to claim 1, wherein, for determining the working state operating current value, the impedance of the equivalent circuit of the drive motor is taken into account.

15. A method according to claim 14, wherein, for determining the working state operating current value, the empirically determined electrical power consumption of the refrigerant compressor unit is compared with the power consumption resulting from the equivalent circuit and the slip is determined therefrom.

16. A method according to claim 14, wherein the working state operating current value is determined from the determined slip and the impedance of the equivalent circuit of the drive motor.

17. A method according to claim 1, wherein the empirically determined electrical power consumption of each working state in the application field is recorded, in particular stored, at the respective operating frequency.

18. A method according to claim 1, wherein the working state operating current values calculated from the empirically determined electrical power consumption are recorded, in particular stored, for each working state and each operating frequency.

19. A method according to claim 1, wherein the working state operating current value for each working state and each operating frequency is determined empirically and recorded, in particular stored.

20. A method according to claim 1, wherein the working states in the application field that are associated with the maximum converter current value are determined on the basis of this maximum converter current value of the selected frequency converter at a selected operating frequency using the drive data.

21. A method according to claim 20, wherein the working states determined for the maximum converter current value are displayed in the application graph.

22. A method according to claim 1, wherein only frequency converters that include a frequency limiting unit are selected, wherein, at operating frequencies above a cut-off frequency, the frequency limiting unit limits the operating frequency such that the maximum converter current value of the frequency converter is not exceeded.

23. A method according to claim 22, wherein the working state operating current value of the frequency converter is continuously detected by the frequency limiting unit.

24. A method according to claim 22, wherein the working state operating current value of the frequency converter is compared with a current reference value and the operating frequency is limited to a limit frequency that applies when the current reference value is reached.

25. A method according to claim 24, wherein the frequency limiting unit takes into account as the current reference value both the maximum converter current value and the maximum compressor operating current value, and determines the limit frequency on the basis of the lowest of the maximum current values.

26. A method according to claim 1, wherein only a frequency converter in which a voltage adapter unit brings about an increase in the output voltage over the operating frequency independently of a fluctuation in a mains voltage is made available for selection.

27. A method according to claim 26, wherein a link voltage of the frequency converter is measured and, as a result of a comparison with at least one reference value, a voltage curve of the output voltage is corrected.

28. A data processing unit for selecting a frequency converter for a refrigerant compressor unit, including a refrigerant compressor and an electric drive motor, the data processing unit has a display unit on which a working state suitable for operation of the refrigerant compressor unit is selected within an application field of an application graph of the refrigerant compressor, an operating frequency for this selected working state is selected, and the data processing unit determines from drive data stored in a memory a working state operating current value that corresponds to the selected working state and the selected operating frequency, for operation of the refrigerant compressor unit.

29. A data processing unit according to claim 28, wherein the data processing unit, using the working state operating current value, selects the frequency converter whereof the maximum converter current value is greater than or equal to the determined working state operating current value from stored data of frequency converters available for selection.

30. A data processing unit according to claim 29, wherein the data processing unit selects the frequency converter whereof the maximum converter current value is as close as possible to the working state operating current value.

31. A data processing unit according to claim 28, wherein the data processing unit selects the frequency converter such that its maximum converter start-up current value is greater than or equal to a start-up current value of the refrigerant compressor unit.

32. A data processing unit according to claim 31, wherein the data processing unit uses a start-up current value that is stored in a memory for selecting the frequency converter.

33. A data processing unit according to claim 31, wherein the start-up current value is determined empirically.

34. A data processing unit according to claim 31, wherein the data processing unit selects the frequency converter such that its maximum converter current value is as close as possible to the start-up current value.

35. A data processing unit according to claim 31, wherein the data processing unit selects the frequency converter such that its maximum converter current value is as close as possible to the higher of the values of working state operating current value and start-up current value.

36. A data processing unit according to claim 28, wherein the drive data are determined empirically.

37. A data processing unit according to claim 28, wherein, in each working state of the refrigerant compressor unit, empirical drive data are stored in the memory for the possible operating frequencies to be selected.

38. A data processing unit according to claim 28, wherein the operating frequency that is to be selected lies in the range from 0 hertz to 140 hertz.

39. A data processing unit according to claim 28, wherein the drive data have the empirically determined power consumption for each working state in the application field at the different operating frequencies.

40. A data processing unit according to claim 39, wherein the data processing unit calculates the working state operating current value at the selected operating frequency on the basis of the empirically determined electrical power consumption at the respective operating frequency, taking into account an equivalent circuit of the drive motor of the refrigerant compressor unit.

41. A data processing unit according to claim 28, wherein, for determining the working state operating current value, the data processing unit takes into account the impedance of the equivalent circuit of the drive motor.

42. A data processing unit according to claim 41, wherein, for determining the working state operating current value, the data processing unit compares the empirically determined electrical power consumption of the refrigerant compressor unit with the power consumption resulting from the equivalent circuit and determines the slip therefrom.

43. A data processing unit according to claim 41, wherein the data processing unit determines the working state operating current value from the determined slip and the impedance of the equivalent circuit of the drive motor.

44. A data processing unit according to claim 28, wherein the data processing unit stores the empirically determined electrical power consumption of each working state in the application field at the respective operating frequency.

45. A data processing unit according to claim 28, wherein the data processing unit stores, in particular, the working state operating current values calculated from the empirically determined electrical power consumption for each working state and each operating frequency.

46. A data processing unit according to claim 28, wherein working state operating current value for each working state and each operating frequency is determined empirically and stored by the data processing unit.

47. A data processing unit according to claim 28, wherein the data processing unit determines the working states in the application field that are associated with the maximum converter current value on the basis of this maximum converter current value of the selected frequency converter at a selected operating frequency using the drive data.

48. A data processing unit according to claim 47, wherein the data processing unit displays the working states determined for the maximum converter current value in the application graph on the display unit.

49. A data processing unit according to claim 28, wherein the data processing unit selects only frequency converters that include a frequency limiting unit, wherein, at operating frequencies above a cut-off frequency, the frequency limiting unit limits the operating frequency such that the maximum converter current value of the frequency converter is not exceeded.

50. A data processing unit according to claim 49, wherein the working state operating current value of the frequency converter is continuously detected by the frequency limiting unit.

51. A data processing unit according to claim 49,wherein the working state operating current value of the frequency converter is compared with a current reference value and the operating frequency is limited to a limit frequency that applies when the current reference value is reached.

52. A data processing unit according to claim 51, wherein the frequency limiting unit takes into account as the current reference value both the maximum converter current value and the maximum compressor operating current value, and determines the limit frequency on the basis of the lowest of the maximum current values.

53. A data processing unit according to claim 1, wherein the data processing unit makes available for selection only a frequency converter in which a voltage adapter unit brings about an increase in the output voltage over the operating frequency independently of a fluctuation in a mains voltage.

54. A data processing unit according to claim 53, wherein a link voltage of the frequency converter is measured and, as a result of a comparison with at least one reference value, corrects a voltage curve of the output voltage.

55. A refrigerant compressor system, including a refrigerant compressor unit having a refrigerant compressor and an electric drive motor, and including a frequency converter for operating the electric drive motor, the frequency converter includes a frequency limiting unit that, at operating frequencies above a cut-off frequency, limits the operating frequency such that the maximum converter current value of the frequency converter is not exceeded.

56. A refrigerant compressor system according to claim 55, wherein the working state operating current value of the frequency converter is continuously detected by the frequency limiting unit.

57. A refrigerant compressor system according to claim 55, wherein the working state operating current value of the frequency converter is compared with a current reference value and the operating frequency is limited to a limit frequency that applies when the current reference value is reached.

58. A refrigerant compressor system according to claim 57, wherein the frequency limiting unit takes into account as the current reference value both the maximum converter current value and the maximum compressor operating current value, and determines the limit frequency on the basis of the lowest of the maximum current values.

59. A refrigerant compressor system, including a refrigerant compressor unit having a refrigerant compressor and an electric drive motor, and including a frequency converter for operating the electric drive motor, the frequency converter includes a voltage adapter unit that controls an increase in the output voltage over the operating frequency such that this increase is performed independently of a fluctuation in a mains voltage.

60. A refrigerant compressor system according to claim 59, wherein the voltage adapter unit measures a link voltage of the frequency converter and, as a result of a comparison with at least one reference value, corrects the increase in the output voltage.

61. A refrigerant compressor system according to claim 60, wherein the voltage adapter unit generates a proportionality correction factor which is used to correct the increase in the output voltage of the frequency converter.

62. A refrigerant compressor system according to claim 60, wherein the reference values used by the voltage adapter unit include at least one of the following values, for example: a reference frequency, a proportionality factor and a link voltage setpoint value.

63. A refrigerant compressor system according to claim 59, wherein the frequency converter has a frequency converter controller which, on the basis of a frequency request signal, generates a voltage control signal that, in addition to the frequency request signal, is supplied to an inverter stage controller of an inverter stage of the frequency converter, and in that the voltage adapter unit cooperates with the frequency converter controller for controlling the increase in the output voltage over the operating frequency.

64. A refrigerant compressor system according to claim 63, wherein the frequency converter controller has a proportional member which, on the basis of the frequency request signal, generates the voltage control signal, and in that the voltage adapter unit corrects a proportionality behavior of the proportional member.

65. A refrigerant compressor system according to claim 64, wherein the proportionality behavior of the proportional member is corrected using the proportionality correction factor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0079] FIG. 1 shows a schematic illustration of a refrigerant circuit having a refrigerant compressor unit, operated using a converter;

[0080] FIG. 2 shows a schematic illustration of an application graph of the refrigerant compressor unit, with an application field that is enclosed by an application limit and records the permitted working states of the refrigerant compressor unit;

[0081] FIG. 3 shows an illustration of a curve of an output voltage of the frequency converter over an operating frequency, and a curve of a working state operating current value over the operating frequency;

[0082] FIG. 4 shows a schematic illustration of a data processing unit for optimum selection of a frequency converter, corresponding to a first exemplary embodiment of the solution according to the invention;

[0083] FIG. 5 shows an illustration of an equivalent circuit of a drive motor of the refrigerant compressor unit, with indications of the equations for motor impedance, electrical power consumption and working state operating current value at a particular operating frequency;

[0084] FIG. 6 shows a schematic illustration of a method according to the invention for determining limits to the application field that are produced by the inventive selection of the frequency converter according to the first exemplary embodiment;

[0085] FIG. 7 shows a schematic illustration of a second exemplary embodiment of a method according to the invention for selecting a frequency converter;

[0086] FIG. 8 shows a schematic illustration of the second exemplary embodiment of the frequency converter according to the invention during determination of the restrictions on the application field;

[0087] FIG. 9 shows a schematic illustration of a frequency converter having a frequency limiting unit;

[0088] FIG. 10 shows a schematic illustration of a frequency converter having a voltage adapter unit;

[0089] FIG. 11 shows a drawing representing a voltage control signal for the frequency converter over the operating frequency; and

[0090] FIG. 12 shows a representation of output voltages of the frequency converter, similar to FIG. 3, in the case of a fluctuating mains voltage.

DETAILED DESCRIPTION OF THE INVENTION

[0091] A refrigerant circuit 10 that is illustrated schematically in FIG. 1 includes a refrigerant compressor unit 20 that has a refrigerant compressor 22 and an electric drive motor 24 driving the refrigerant compressor 22, wherein the refrigerant compressor 22 and the drive motor 24 may for example be integrated into one unit.

[0092] The refrigerant compressor 22 in the refrigerant circuit 10 compresses the refrigerant circulated around the refrigerant circuit 10, and the refrigerant is then supplied to a heat exchanger unit 12 that is on the pressurized side of the refrigerant circuit 10 and in which the compressed refrigerant is cooled, in particular condensed, by a discharge of heat W.

[0093] The cooled, in particular condensed, refrigerant is supplied to an expansion member 14 in the refrigerant circuit 10, and the compressed, in particular condensed and pressurized, refrigerant is expanded there and then supplied in the refrigerant circuit 10 to a heat exchanger unit 16 in which the expanded refrigerant is able to take up heat W, in order in this way to perform its cooling action.

[0094] The refrigerant that is expanded in the heat exchanger unit 16 is then supplied to the refrigerant compressor 22 again and compressed by the refrigerant compressor 22.

[0095] The expanded refrigerant which has already taken up heat in the heat exchanger unit 16 is thus supplied to an input 32 of the refrigerant compressor 22 at a saturation temperature STE, is then compressed in the refrigerant compressor 22 and comes out of an output 34 of the refrigerant compressor at a saturation temperature STA.

[0096] As a result of its construction and refrigerant, the refrigerant compressor 22 operates without being damaged only with particular paired values of the saturation temperature STE at the input 32 and the saturation temperature STA at the output 34 of the refrigerant compressor 22, and these values are defined by an application graph 36 shown in FIG. 2, wherein the saturation temperature STE at the input 32 is shown on the X axis of the application graph 36 and the saturation temperature STA at the output 34 is shown on the Y axis.

[0097] Here, in the application graph 36, which is also predetermined in particular in dependence on the refrigerant, all the paired values of saturation temperature STE at the input 32 and saturation temperature STA at the output 34 of the refrigerant compressor 22 that are permissible for the refrigerant compressor 22 lie within an application field EF that is enclosed on all sides by an application limit EG. Application graphs of this kind for refrigerant compressors are explained for example in the book Lexikon der Kltetechnik [Dictionary of Refrigeration Engineering] by Dieter Schmidt (published by C. F. Mller), to which the reader is referred in this regard.

[0098] The paired values of saturation temperature STE from the input 32 and saturation temperature STA at the output 34 that are permissible within the application field EF each define a working state AZ of the refrigerant compressor 22 that can be implemented using the respective refrigerant compressor 22.

[0099] Because the refrigerant compressor 22 is driven by the electric drive motor 24, each working state AZ requires a certain electrical power consumption P.sub.AZ of the drive motor 24.

[0100] Here, the electrical power consumption value P.sub.AZ of the drive motor 24 is dependent on the one hand on the respective working state AZ in the application field EF and on the other on the speed of rotation of the refrigerant compressor 22.

[0101] If the refrigerant compressor 22 is operated at different speeds using a frequency converter 40, then the speed of the refrigerant compressor 22 is proportional to the operating frequency f supplied to the drive motor 24 by the frequency converter 40.

[0102] Thus, an electrical power consumption value P.sub.AZ is associated with each working state AZ within the application field EF at a certain operating frequency f.

[0103] However, the electrical power consumption value P.sub.AZ of the electric drive motor 24 depends not only on the working state AZ of the refrigerant compressor 22 but also on the type of electric drive motor 24 and the layout in which the windings thereof are connected to the frequency converter 40.

[0104] In the exemplary embodiment illustrated, the assumption is made that the electric drive motor 24 is an asynchronous motor or indeed a permanent-magnet motor whereof the windings are connected to the frequency converter 40 in a star layout.

[0105] As illustrated in FIG. 3, this layout of connecting the drive motor 24 to the frequency converter 40 has the result that, when the drive motor 24 is operated with the frequency converter, the output voltage U.sub.FU generated by the frequency converter 40 increases linearly as operating frequency f increases, from the operating frequency f=0 until a cut-off frequency f.sub.ECK is reached, above which the output voltage U.sub.FU no longer increases but has reached its maximum output voltage U.sub.FUMAX.

[0106] If there is a further increase in the operating voltage f to a maximum frequency f.sub.max, the output voltage U.sub.FUMAX at which the drive motor 24 is operated remains constant.

[0107] The maximum operating frequency f.sub.max of the frequency converter 40 for operating the electric drive motor is on the one hand affected by the construction of the electric drive motor 24 and on the other hand by the construction of the refrigerant compressor 22, and is conventionally around values of 80 hertz or less, while the cut-off frequency f.sub.ECK is conventionally in the range between 40 and 60 hertz.

[0108] In this mode of the electric drive motor 24, the operating current in the respective working state AZ is likewise dependent on the operating frequency f, resulting in working state operating current values I.sub.AZ that are constant between the operating frequencies f=0 and f.sub.eck but increase further at operating frequencies f above the cut-off frequency f.sub.eck, for example to the maximum operating frequency f.sub.max.

[0109] Here, the maximum output voltage U.sub.FUMAX that is available at the output of the frequency converter 40 for operating the drive motor 24 is proportional to the link voltage of the frequency converter 40 and thus proportional to the supply voltage of the frequency converter 40.

[0110] As illustrated in FIGS. 2 and 3, the power consumption value P.sub.AZ1 in a working state AZ1 of the application graph 36 is for example greater than in a working state AZ2 of the application graph 36, which has the consequence, as in FIG. 3, that the working state operating current values I.sub.AZ1 have higher values than the working state operating current values I.sub.AZ2 in the working state AZ2.

[0111] Thus, FIGS. 2 and 3 illustrate the fact that the working state operating current values I.sub.AZ made available by the frequency converter 40 depend on the working states AZ and thus, depending on the working state AZ, the frequency converter 40 must be able to generate working state operating current values I.sub.AZ of different size.

[0112] The costs of the frequency converter 40 depend on the maximum converter current value I.sub.FUMAX that a frequency converter 40 can make available, and the greater the maximum converter current value I.sub.FUMAX the higher the costs.

[0113] If the selection of the frequency converter 40 according to the working state AZ.sub.s provided by the user of the refrigerant compressor unit 20 when the frequency converter 40 is used, which may be for example the working state AZ1 or AZ2, and the selected operating frequency f.sub.s are now optimized, then it is possible to optimize selection of the frequency converter 40 by taking into account the working state AZ.sub.s provided by the user and the operating frequencies f.sub.s in that the frequency converter 40 is selected taking into account the provided working state AZ.sub.s and operating frequency f.sub.s such that the frequency converter 40 is selected in such a way that the maximum converter current value I.sub.FUMAX is selected to be greater than the working state operating current value I.sub.AZfs required for the selected working state AZ.sub.s at the provided operating frequency f.sub.s.

[0114] For this, the working state operating current value I.sub.AZfs has to be determined.

[0115] The working state operating current value I.sub.AZfs with the respective operating frequency f.sub.s is determined, as illustrated in FIG. 4, using a data processing unit 50 that includes an input unit 52, in particular combined with a display unit 53, for displaying the application graph 36 and for selection of the working state AZ.sub.s and the operating frequency f.sub.s.

[0116] For this purpose, the data processing unit 50 uses empirically determined drive data to characterize the drive motor 24 of the refrigerant compressor unit 20.

[0117] For example, in a first exemplary embodiment it is provided for the power consumption values P.sub.AZ for the respective working states AZ in the application field EF of the refrigerant compressor unit 20 at the respective operating frequency f to be determined empirically, and to be stored in a memory 54 associated with the data processing unit 50, as empirical power consumption values P.sub.AZEXf, in the form of a power data field. These electrical power consumption values P.sub.AZEXf then provide the possibility, taking into account the Steinmetz equivalent circuit for the drive motor 24, which is illustrated in FIG. 5, and the known resistance values R and reactance values X, which are stored in a memory 56 associated with the data processing unit 50, of calculating the impedance Z according to formula (F1) of the drive motor 24 and then, comparing the empirically determined power consumption value P.sub.AZfs of selected operating frequency f.sub.s with the theoretical power consumption value P.sub.AZ of impedance Z, of determining the slip s iteratively from the formula (F2) and then using the slip s to determine the working state operating current value I.sub.AZfs from the formula (F3) with the respectively selected operating frequency f.sub.s.

[0118] The relationships and formulae presented in FIG. 5 may be varied slightly, depending on the approximations and assumptions made in the Steinmetz equivalent circuit.

[0119] Thus, a Steinmetz equivalent circuit with the associated formulae is described in the book THE PERFORMANCE AND DESIGN OF ALTERNATING CURRENT MACHINES, by M. G. Say, third edition, 1958, in Pitman Paperbacks, 1968, SBN 273 401998, pages 270 ff.

[0120] A similar Steinmetz equivalent circuit with the corresponding formulae can be found in Wikipedia in English, under Induction Motor, as at 4 Apr. 2016, and the references cited there.

[0121] Taking as a starting point this working state operating current value I.sub.AZfs, the appropriate frequency converter 40 is now determined in that the maximum converter current value I.sub.FUMAX made available by the frequency converter 40 must be greater than the working state operating current value I.sub.AZfs determined for the respective working state AZ at the selected frequency f.sub.s.

[0122] As a further constraint on the frequency converter 40 to be selected a start-up current value I.sub.ANLEX for the respective refrigerant compressor unit 20 is also used, which has likewise been determined empirically and stored in a memory 58 and may, where appropriate, be greater than the working state operating current value I.sub.AZfs.

[0123] For starting up the refrigerant compressor unit 20, the frequency converter 40 is constructed to be resistant to overload, with the result that a maximum converter start-up current value I.sub.FUANLMAX that is greater than the maximum converter current value I.sub.FUMAX is briefly available, for example being able to be 170% of the maximum converter current value I.sub.FUMAX for a period of 3 seconds.

[0124] In this way, for selection of the frequency converter 40, illustrated schematically in FIG. 4, it is relevant that the maximum converter current value I.sub.FU is greater than the working state operating current value I.sub.AZfs, and the maximum converter start-up current value I.sub.FUANLMAX is greater than the start-up current value I.sub.ANLEX of the refrigerant compressor unit 20, as illustrated for example in FIG. 3. The maximum converter current I.sub.FUMAX and the maximum converter start-up current I.sub.FUANLMAX should, however, be as close as possible to the working state operating current value I.sub.AZfs and the start-up current value I.sub.ANLEX in order to select a frequency converter with as small as possible a maximum converter current value I.sub.FUMAXs, which represents the lowest-cost solution.

[0125] With a frequency converter 40 selected in this way, because of the selection method it is ensured that the frequency converter is able to operate the refrigerant compressor unit 20 in the selected working state AZ.sub.s, but a frequency converter 40 selected in this way does not ensure that the refrigerant compressor 22 can consequently be operated in all the working states AZ within the application field EF.

[0126] Rather, this procedure and the selection of the frequency converter 40 such that the frequency converter need only be able to supply the working state operating current I.sub.AZfs and the start-up current value I.sub.ANLEX have the effect of restricting the application field EF.

[0127] In order to display to a user the restriction of the application field EF that results from the selection made of the frequency converter 40, then as illustrated for example in FIG. 6, and on the basis of the maximum converter current value I.sub.FUMAXs of the selected frequency converter 40s, the working states AZ in the application field EF that are associated with this maximum converter current value I.sub.FUMAX are determined for the selected operating frequency f.sub.s or indeed at other operating frequencies f.sub.s, using the equivalent circuit of the drive motor 24 illustrated in FIG. 4 with the known resistance values R and the known reactance values X from the memory 56, and using the formulae for electrical power consumption and working state operating current I.sub.AZ that are illustrated in FIG. 5 and are associated with the equivalent circuit of the drive motor 24, taking into account the power consumption values P.sub.AZEX stored in the memory 54 for the different working states AZ in the application field EF at the respectively selected operating frequencies f.sub.s.

[0128] For this purpose, the maximum converter current value I.sub.FUMAXs of the selected converter 40s is used for the current I.sub.AZfs according to the formula F3, the slip s is determined from this, and the formula F2 is used to calculate the power consumption value P.sub.AZCAL, and then, using the empirical power consumption values P.sub.AZEX stored in the memory 54, all the working states AZCAL(fs) that correspond to the calculated power consumption value P.sub.AZCAL at the selected operating frequency f.sub.s are determined.

[0129] The sum of these working states A.sub.ZCALfs gives a boundary line G.sub.fs in the application graph 36, as illustrated in FIG. 2.

[0130] This calculation results in the boundary lines G.sub.fs illustrated in FIG. 2 and FIG. 6 for different selected operating frequencies f.sub.s; for example the boundary line G.sub.fs represents the boundary line for the application field EF at the operating frequency f.sub.s that is selected for selection of the frequency converter 40, the boundary line G.sub.fr represents for example a boundary line for the limit of the application field EF at a smaller operating frequency fr than the selected operating frequency fs, and the boundary line G.sub.frr represents for example a boundary line of the application field EF for an operating frequency frr selected to be even smaller, and these are displayed by the data processing unit 50 on a display unit 53 together with the application graph 36.

[0131] Thus, a user of the method according to the invention is also at the same time provided with information on the restrictions resulting from the selection of the frequency converter 40 in accordance with the selection method described above, and a user can check whether these restrictions of the application field EF do or do not rule out possible potential working states AZ that could where appropriate also be applicable for use of the refrigerant compressor unit 20.

[0132] In a second exemplary embodiment, as illustrated in FIG. 7, as an alternative to the first exemplary embodiment it is provided for the current I.sub.AZf to be determined in the manner described in conjunction with the first exemplary embodiment, using the data processing unit 50 for each empirically determined power consumption value P.sub.AZEXf with the respective operating frequency f, using the resistance values R and reactance values X of the Steinmetz equivalent circuit that are known from FIG. 5 for each individual working state AZ, and to be stored in a memory 54 such that when a user makes a selection of the working state AZ.sub.s and the selected operating frequency f.sub.s, the corresponding working state operating current value I.sub.AZfs may be accessed directly in the memory 56, and this working state operating current value I.sub.AZfs corresponding to the selected working state AZ.sub.s can be read off directly without further action, and, using the empirically determined start-up current value I.sub.ANLEX the selection of the frequency converter 40s can be performed, using the maximum frequency converter currents I.sub.FUMAX stored in the memory 62, in the manner already explained in conjunction with the first exemplary embodiment.

[0133] Similarly, in the second exemplary embodiment, once the frequency converter 40s has been established, the maximum frequency converter current I.sub.FUMAXs may be used to determine the working states AZCAL.sub.fs associated with this current value in the memory 54, and to display the sum of all these working states AZCAL.sub.fs as the respective boundary line G.sub.fs for example on a display unit 64, as described in conjunction with the first exemplary embodiment.

[0134] In a third exemplary embodiment, as an alternative to the first and second exemplary embodiments, it is provided, in an analogous manner to the second exemplary embodiment, for the working state operating current values I.sub.AZf to be determined empirically in the memory 54 and stored in the memory 54 such that in the third exemplary embodiment, in a similar manner to the second exemplary embodiment, selection of the frequency converter 40s can take as a starting point the values in the memory 54.

[0135] Similarly, and conversely, when determining the boundary lines G.sub.fs, the data processing unit 50 can proceed in accordance with the second exemplary embodiment, in which case the empirically determined working state operating current values I.sub.AZf are stored in the memory 54 and are then used to determine the boundary line G.sub.f with the maximum converter current value I.sub.FUMAXs established by the selected frequency converter 40s.

[0136] Preferably, the frequency of the frequency converter 40s used is controlled by a frequency control unit 70, which on one side detects the saturation temperature STE or indeed, as an alternative, detects the saturation pressure at the input 32 of the refrigerant compressor 22 and supplies it to a comparator member 74, across which on the other side a temperature specifying signal TV is applied.

[0137] Depending on how much the saturation temperature STE deviates from the temperature specifying signal TV, a proportional regulator 76 is triggered, and this generates a frequency request signal FAS that is supplied to a frequency converter controller 78 which then, in a manner corresponding to the frequency request signal FAS, specifies the frequency f of the frequency converter 40s at which the drive motor 24 is then operated.

[0138] If selection of the frequency converter 40s is made in accordance with one of the exemplary embodiments described above, then, as illustrated in FIG. 3, when the refrigerant compressor unit 20 is operated, as in FIGS. 2 and 3 a working state AZ3 may occur in which the working state operating current I.sub.AZ3 as illustrated in FIG. 3 is sufficiently high for it to happen, at frequencies f above the cut-off frequency f.sub.ECK, that the maximum converter current value I.sub.FUMAX is already reached at a limit frequency f.sub.L, wherein the limit frequency f.sub.L is lower than the operating frequency f.sub.s provided for example for the working state AZ1.

[0139] This would have the result, in a conventional construction, that the frequency converter 40s would switch off because of overload.

[0140] For this reason, with a frequency converter 40 according to the invention, as illustrated in FIG. 9 a frequency limiting unit 80 is provided that limits the operating frequency f of the frequency converter 40 when it is above the cut-off frequency f.sub.ECK such that the working state operating current value I.sub.AZ does not exceed the maximum converter current value I.sub.FUMAXs but at most reaches the maximum converter current value I.sub.FUMAXs.

[0141] This ensures that the frequency converter 40s does not switch off even in working states that, at operating frequencies f above the cut-off frequency f.sub.ECK, could result in a current of the frequency converter 40 exceeding the maximum converter current value I.sub.FUMAX.

[0142] As illustrated in FIG. 9, the frequency limiting unit 80 includes a current sensor 84 that is arranged in a supply line 72 leading from the frequency converter 40s to the drive motor 24 and that measures the actual working state operating current value I.sub.AZ and supplies it to a comparator member 86 which compares the actual working state operating current value I.sub.AZ with the maximum converter current value I.sub.FUMAX as a predetermined value and supplies the comparison result to a limit regulator 92, for example a proportional regulator, which if the working state operating current I.sub.AZ actually measured by the current sensor 84 is greater than the maximum converter current value I.sub.FUMAX serving as a reference value generates a frequency limiting signal for a frequency limiting member 94 that acts on the frequency request signal FAS and prevents a further increase in the operating frequency f.

[0143] Preferably, there is additionally provided a comparator member 88 coupled to the current sensor 84 which compares the working state operating current value I.sub.AZ measured by the current sensor 84 with a maximum compressor operating current value I.sub.VMAX and triggers a limit regulator 98, for example a proportional regulator, when the working state operating current value I.sub.AZ actually measured by the current sensor 84 approximates to the maximum compressor operating current value, I.sub.VMAX. A frequency limiting signal is generated and transmitted to the frequency limiting member 94.

[0144] Preferably, the frequency limiting signals of the limit regulators 92 and 98 are compared with one another in a minimizing member 102, and in each case the frequency limiting signal that leads to the lowest limit frequency f.sub.L is supplied to the frequency limiting member 94.

[0145] Further, preferably there is transmitted to the frequency limiting member 94 as a reference value the cut-off frequency f.sub.ECK, which represents the minimum frequency at which frequency limitation is performed by the frequency limiting member 94.

[0146] For optimum operation of the frequency converter 40, the increase in the output voltage U.sub.FU of the frequency converter 40 over the frequency fin the range from f=0 to f=f.sub.ECK is significant, since the increase in the output voltage U.sub.FU over the frequency f of the frequency converter 40 is relevant for forming the flow in the drive motor 24.

[0147] Provided the maximum output voltage U.sub.FUMAX is constant, this has the consequence that the cut-off frequency f.sub.ECK can also be constant, with the result that the increase in the output voltage U.sub.FU over the frequency f is likewise always constant.

[0148] If, however, with a frequency converter 40s the supply voltage fluctuates, for example as a result of a poor-quality mains network, then the maximum output voltage U.sub.FU of the frequency converter 40 at its output is not constant, so with a constant cut-off frequency f.sub.ECK the increase in the output voltage U.sub.FU would necessarily vary in the frequency range between f=0 and f=f.sub.ECK.

[0149] In order to keep the increase in the output voltage U.sub.FU over the frequency constant, even with fluctuations in the mains network that are more than negligible and thus fluctuation in the maximum output voltage U.sub.FUMAX of the frequency converter 40 that is more than negligible, it is also necessary to vary the cut-off frequency f.sub.ECK in a manner corresponding with the variation in the maximum output voltage U.sub.FUMAX.

[0150] A known frequency converter 40, illustrated in FIG. 10, includes a rectifier stage 112, an inverter stage 114 and a link 116 that is provided between the rectifier stage 112 and the inverter stage 114, across which the link voltage U.sub.Z is applied as a DC voltage.

[0151] Here, the link voltage U.sub.Z depends on the mains voltage U.sub.N supplied to the rectifier stage 112, and fluctuates in proportion to the mains voltage U.sub.N.

[0152] Here, the inverter stage 114 of the frequency converter 40 is controlled by the frequency converter controller 78, to which the frequency request signal FAS is supplied.

[0153] Here, on the basis of the frequency request signal FAS and with the aid of a proportional member 118, the frequency converter controller 78 generates a voltage control signal SSS, which is supplied in addition to the frequency request signal FAS to an inverter stage controller 122 that, on the basis of the frequency request signal FAS and the voltage control signal SSS, which specifies for example percentage values of the maximum output voltage U.sub.FUMAX, generates the output voltage U.sub.FU.

[0154] For adapting to drastically fluctuating mains voltages U.sub.N, there is thus associated with the frequency converter 40 a voltage adapter unit 130 that uses a voltage measuring unit 132 to measure the link voltage U.sub.Z in the link 116 and supplies this link voltage U.sub.Z to a dividing member 134, to which a reference frequency f.sub.REF is also supplied.

[0155] The reference frequency f.sub.REF is of a size such that, with a setpoint value U.sub.ZS of the link voltage U.sub.Z, the result is the proportionality factor that is desired for the increase in output voltage U.sub.FU of the converter 40 over the frequency F.

[0156] The result from this dividing member 134 is supplied to a further dividing member 136 to which on the other hand there is supplied the desired proportionality factor PF for the increase in output voltage U.sub.FU of the frequency converter 40 over the operating frequency f, which corresponds to the link voltage setpoint value U.sub.ZS divided by the reference frequency f.sub.REF.

[0157] The result from the second dividing member 136 is a proportionality correction factor PKF which is equal to one if the result from the first dividing member 134 that is supplied to this dividing member 136 corresponds to the desired proportionality factor, and is not equal to 1 if the link voltage U.sub.z differs from the link voltage setpoint value U.sub.ZS.

[0158] If the proportionality correction factor PKF generated by the dividing member 136 is now supplied to the proportional member 118, then it can be used to vary the proportionality behavior PV provided in the proportional member 118 between the operating frequency f of the frequency request signal FAS and the voltage control signal SSS.

[0159] FIG. 11 illustrates for example how the proportionality between the operating frequency f of the frequency request signal FAS and the voltage control signal SSS varies.

[0160] Here, the function of the voltage adapter unit 130 is that, when the link voltage U.sub.Z corresponds to the link voltage setpoint value U.sub.ZS as illustrated in FIG. 11, the cut-off frequency corresponds to the setpoint cut-off frequency f.sub.ECKSO, which is for example 50 hertz.

[0161] If the link voltage U.sub.Z differs from the link voltage setpoint value U.sub.ZS by the value , for example giving smaller voltage values, then the voltage control signal SSS of 100% will reach lower operating frequencies f than the setpoint cut-off frequency f.sub.ECKSO.

[0162] If by contrast the link voltage U.sub.Z is greater than the link voltage setpoint value U.sub.ZS by the value , then the voltage control signal SSS of 100% will reach higher operating frequencies f than the setpoint cut-off frequency f.sub.ECKSO.

[0163] As illustrated in FIG. 12, this results in the cut-off frequency f.sub.ECK, that is to say the frequency at which the maximum output voltage U.sub.FUMAX is reached at the output of the frequency converter 40, varying in particular in accordance with the deviation in the link voltage U.sub.Z from the link voltage setpoint value U.sub.ZS, with the result that the maximum output voltage U.sub.FUMAX of the frequency converter 40 also varies.