GB2281825A

GB2281825A - Preventing overload of a rectifier unit.

Info

Publication number: GB2281825A
Application number: GB9417704A
Authority: GB
Inventors: Guenther Boesche; Rupert Weber
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1993-09-09
Filing date: 1994-09-02
Publication date: 1995-03-15
Anticipated expiration: 2014-09-02
Also published as: IT1275011B; ITMI941834A1; DE4330537B4; ITMI941834A0; CH690015A5; DE4330537A1; GB9417704D0; GB2281825B

Description

2281825 Frequency converter and process for its operation State-of-the-Art

The invention is based on an arrangement and a process according to the form of the independent Claims 1 and 15. An arrangement of this type is disclosed in DE-OS 41 28 803. It consists of a mains rectifier which supplies a DC voltage link circuit to which several inverters are connected. The mains rectifier is designed so that its performance corresponds to the sum of the power levels simultaneously absorbed by the inverters. Chokes are arranged in the link circuit phases to smooth voltage peaks. These chokes also help to a small extent to prevent rectifier overload. In principle, however, this known arrangement, especially when designed for use in machines, assumes that the individual inverters are operated alternately with high and low power. If the time distribution of the power uptake by the inverters is not known in advance, or it is intended that several of the inverters are operated at the same time at maximum power, a rectif ier must be used that is designed for the sum of the maximum power values of the inverter. If a rectifier is used that is rated too low, this can usually withstand short-time overloads. By contrast, more prolonged overloads generally cause the shut-down of the complete system via existing protective devices. On the other hand, over-rating is undesirable for economic reasons.

The aim of the invention is to further develop a converter with several drives supplied by the same link circuit, so that the occurrence of prolonged overloads and short-time external load peaks is prevented in the rectifier.

This aim is achieved by an arrangement and a process with the characterising features of the independent Claims 1 and 15. An arrangement according to the invention enables overloading of the rectifier unit to be reliably avoided by monitoring the loading that is the ratio of the instantaneous power to the maximum possible power in each case. In addition, this prevents shut-down of the complete system due to overload. Consequently, the rectifier unit can be operated continuously at its power limit and thus be utilized to the optimum. This in turn permits a comparatively small and therefore cost-effective design of the rectifier unit.

Advantageous forms and practical further developments of the invention are revealed in the sub-claims.

It is an advantage if a signal corresponding to the calculated loading of the rectifier unit is transmitted to each inverter unit in order that this can match the power taken by it from the link circuit, or the power fed back into it, respectively, to the reported loading of the rectifier unit. In the case of an overload, matching is appropriate in that each rectifier unit reduces the power taken by it from the link circuit, or the power fed back into it, respectively. In a useful way, the matching of the rectifier units to the rectifier power is coordinated by a primary controller to which a warning signal is transmitted in the event of overload.

To determine the current respective loading of the rectifier unit, it is advantageous to use, over 2 or 3 phases, the current, the temperature of the heatsinks in the rectifier unit, the temperature of the environment and the switching frequency of the power semiconductors in the rectifier. Furthermore, it is recommended to take into account the junction temperature of the power semiconductors, which advantageously can be determined with the aid of a mathematical model. It is, moreover, especially advantageous to individually monitor the phase conductors of the supply connection for failure. In the case of a single-phase mains f ailure, f ailure of a power semiconductor or f ailure of several rectifiers connected in parallel, the system is not shut down but continues to be operated at reduced power.

Usefully, the means for determining the loading of the rectifier unit are arranged in the latter. The signal describing the loading is usefully a digital signal which is fed to the inverter units via a bus line. Here it is particularly advantageous that the load signal is pulse -durationcoded, where the length of the pulse corresponds to the loading.

The proposed arrangement is preferably used for supplying the drives of a numerically-controlled machine.

An embodiment of the invention is explained in more detail below with the aid of the drawing.

Drawing The figure shows a converter according to the invention.

Description

The converter illustrated in the Figure is connected via a rectifier unit I to a generally three-phase alternating current system 2. The output side of the rectifier unit I supplies a DC voltage link circuit 3. Smoothing means 4, in the Figure for example a capacitor, can be provided for stabilization of the link circuit voltage. Several inverters 6, each of which supplies at the output side a preferably brushless motor 7, particularly a servo drive, are connected to the link circuit 3. Rectifier unit 1 and inverter 6 are usefully constructed so that the converter can also be operated as an energy-recovering M-G set. The main constituents of the rectifier unit 1, such as the inverter 6, are power semiconductors, which in the case of the rectifier unit are triggered via the signal line 11, or in the case of the inverter 6 via the signal line 10. A control unit 8 is allocated to the rectifier unit 1 and a control module 5 to each of the inverters 6 to generate the drive signals. Rectifier unit 1 and associated control unit 8 are of ten designed as a constructional unit in the f orm of a power supply module VM, rectif ier 6 and control module 5 f orming one inverter unit. Usefully, control unit 8 and control modules 5 each have a device processing digital signals, for example in the form of a microprocessor or an application- specific integrated circuit for processing or generating the incoming and outgoing signals, respectively, that is described below as a microcomputer. Triggering of the inverter unit is preferably effected by pulsewidth-modulated signals. Control modules 5 and control unit 8 are interconnected via a bus line 12 as well as via a primary controller 9, particularly a numerical controller NC. The latter likewise contains its own microcomputer.

Several sensors are provided in the area of the power supply module VM to control the converter operation. Temperature sensors 14, 17 are located at at least one heatsink 16 used for cooling the power components in the rectifier unit 1, as well as in the space surrounding the power supply module M Furthermore, a device 13 for detecting the voltage in the link circuit is arranged at the link circuit 3, and a device 15 for monitoring the phases for failure, is arranged at the individual phases of the AC input 2.

The arrangement shown has the mode of operation described below. The power supply module VM continuously determines its loading and transmits a signal A corresponding to the loading thus ascertained to the control modules 5 of the inverters 6. To this end the control unit 8 first determines the current actual power value of the power supply module M It detects via sensors 15 the current flowing in the mains phases 2. If the link circuit voltage is considered to be constant, the measured value supplied by the sensors 15 corresponds to the instantaneous power of the rectifier unit 1.

Moreover, the control unit 8 determines the value of the maximum possible power of the rectifier unit 1 at an operational time instant. To do this it measures via one or more sensors 17 the temperature of at least one heatsink 16, by means of which the power semiconductors arranged in the rectifier unit 1 are cooled, as well as the ambient temperature via a sensor 14. Furthermore, it takes into account the stored power semiconductor data, especially characteristic values for the calculation of the junction temperature. The control unit 8 determines the value of the junction temperature of the power semiconductors based on the measured values obtained by the sensors 14, 17 and the stored data of the power semiconductors. Said control unit compares them with a semiconductorspecific, maximum value for the junction temperature usually specified by the manufacturer. The maximum permissible power at an operating time instant is directly given by the difference between the two values. Their value varies during operation, especially as a result of the increasing heating up of the power semiconductors present in the rectifier unit 1, which in turn is a consequence of the current flowing through the power semiconductors, or the high switching frequency, respectively. The loading of the rectifier unit 1 is now obtained by comparing the actual power value determined for the rectifier unit at any operating time instant with the value of the maximum power permitted for the rectifier unit 1 at this time instant.

The control unit 8 transmits the value determined for the loading in the f orm of a digital signal A via the data bus 12 to the control modules 5 assigned to the inverters 6. These incorporate the transmitted value in the generation of the drive signals. If the loading signal A indicates that the rectifier unit 1 is overloaded, the microcomputers present in the control modules 5 initiate measures to restore the overload condition to a normal condition. This can be achieved, f or example, by reducing the power taken by certain drives from the link circuit or supplied to it, respectively, by reducing the power of all drives uniformly or also by intentionally operating one drive as a generator. The coordination of the measures f or reducing an overload condition is usefully effected by the primary controller 9, to which the loading signal A is likewise transmitted. With regard to the power levels of the rectifier unit 1 and/or the inverter units 5, 6, it is practical to carry out trend detection, with the aim of detecting overload states or load peaks, respectively, when they occur. It is simply achieved by observing the change in power with reference to a specified time interval, or otherwise expressed by time differentiation of the power. If the rectifier unit I is already close to the maximum possible loading, and the power requirement nevertheless shows a tendency to increase, bef ore an overload condition actually occurs, the control modules 5 initiate steps to prevent the power in the rectifier unit 1 increasing further.

If reduction f an overload condition of the rectifier unit 1 cannot be achieved within a maximum time permitted f or the duration of such an overload condition, the drives 7 are brought to a safe stopped state, preferably under the control of the primary controller 9.

In order to operate a converter in the most optimum manner, that is close to its permissible rated power, it is advantageous to record as many as possible of the parameters determining the current performance of the rectifier unit 1. Further usefully recorded parameters and those taken into account during determination of the respective maximum possible performance, are for example the switching frequency of the power semiconductors and/or the link circuit voltage. Other useful, fixed parameters allowed for are primarily the thermal contact resistances between power semiconductors and heatsinks, and between heatsinks and the surrounding air.

Furthermore, it has proved particularly advantageous to also monitor the phases of the supply connection 2 of the rectifier unit individually for failure. Usefully, the current sensor 15 already present is used to determine the actual power value.

Equally, an independent voltage sensor can also be used. The control unit 8 then evaluates the signal output by the sensor 15 to ascertain if a specified minimum current flows or a minimum voltage is present in all phases of the AC supply connection 2. Since the failure of one mains phase results in a sudden reduction in the maximum possible power of the rectifier unit of about a third, and thus -usually immediately leads to a clear overload condition, in the case of a detected failure of a mains phase the control unit 8 usefully generates a separate signal, for example in the form of a special code, which it transmits to the primary controller 9 and to the control modules 5 assigned to the inverter units 6.

In a variant of the proposed converter, the instantaneous power requirement of the rectifier unit 1 is not determined on the basis of the detection of the current or voltage present in the mains phases 2. Instead, the control modules 5 transmit to the control unit 8 a signal value A' corresponding to the output current and thus the power of the associated inverter 6. By simple addition of the transmitted signal values, the control unit 8 supplies the instantaneous actual power value of the rectifier unit 1. Usefully, the transmission of the power levels of the inverter 6 to the control unit is in the form of pulsewidthmodulated, or time-coded signals, respectively. Here the duration of a signal corresponds to the power taken up from the link circuit 3 by the transmitting inverter unit. The transmission of the power signals from the inverter units 6 to the power supply module VM is usefully effected in each case at a set time instant, for example at the current regulator timing.

It is particularly advantageous if the control modules 5 determine the power signal values in advance, based on internal information, and by taking into account operating setpoints which they receive via the bus line 12, for example from an NC 9, and transmit to the control unit 8. Consequently, the power requirement of the complete system is known to the said control unit prior to its actual physical occurrence. Such advance calculation of the power requirement can advantageously be used for precontrol of the rectifier unit.

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Claims

Claims

Frequency converter with a mains rectifier unit for supplying a DC link voltage circuit, to which at least one inverter unit is connected, characterised in that during the operation at any time means permit the determination of the loading, that is the ratio of the instantaneous to the maximum possible power of the rectifier unit (1) at the time instant of the determination.
2. Converter according to Claim 1, characterised in that a signal A corresponding to the loading of the rectifier unit (1) is provided f or each inverter unit (5, 6), which on the basis of this signal matches the power taken by it f rom the link circuit (3) or power supplied to the latter, to the loading of the rectifier unit (1).
3. Converter according to Claim 2, characterised in that at least one rectifier unit (5, 6) reduces the power taken from the link circuit (3) or supplied to the latter, if the loading of the rectifier unit exceeds a limiting value.
4. Converter according to Claim 3, characterised in that the rectifier unit (1) checks the time characteristic of the power converted in it and that at least one inverter unit (5, 6) reduces the power taken f rom the link circuit (3) or supplied to the latter, respectively, if the power converted in the rectifier unit (1) has a tendency to increase, and the loading is above a specified threshold value.
5. Converter according to Claim 1, characterised in that a device (15, 8) is provided for monitoring of the mains phases (2) for failure, and that in the event of failure of a mains phase, the inverter units (5, 6) reduce the power taken f rom the link circuit or supplied to the latter, respectively, until the rectifier unit (1) is no longer overloaded. -
6. Converter according to Claim 1, characterised in that the means for determining the loading of the rectifier unit (1) have at least one current or voltage sensor (15) as well as a device (17) for detecting the temperature of at least one heatsink (16) provided in the rectifier unit (1) for cooling the power components.
7. Converter according to Claim 5, characterised in that the means for determining the loading of the rectifier unit (1) furthermore have a device for detecting the ambient temperature (14) and/or the switching frequency of the power semiconductors.
8. Converter according to Claim 1, characterised in that the means for determining the loading of the rectifier unit (1) are arranged in the latter.
9. Converter according to Claim 1, characterised in that rectifier unit (1) and inverter (6) are interconnected via a bus line (12), via which the loading signal A is transmitted.
10. Converter according to Claim 9, characterised in that the loading signal A is a signal coded as a pulse-duration signal, whereby the length of a pulse corresponds to the power taken from the link circuit (3) by the transmitting inverter unit (5).
11. Converter according to Claim 1, characterised in that each inverter unit (5, 6) transmits a signal A' to the rectifier unit (1), which corresponds to the power taken from the link circuit (3) or supplied to the latter by the inverter (6).
12. Converter accordingto Claim 11, characterised in that at least one inverter unit (5, 6) determines in advance the power taken by it from the link circuit (3) or supplied to the latter, respectively, on the basis of operating setpoints.
13. Converter according to one of the preceding Claims, characterised in that at least one inverter unit (5, 6) is used to supply a drive (7) that is a constituent part of a numericaily-controlled machine.
14. Converter according to Claim 9, characterised in that the inverter modules control the drives into safe rest positions if an overload of the rectifier unit (1) exceeds a specified limiting value.
15. Process for the operation of a frequency converter, with a mains rectifier for supplying a DC voltage link circuit to which at least one inverter unit is connected, characterised by the following steps:

Determination of the power of the rectifier unit (1) at a determination time instant, Determination of the maximum permissible power of the rectifier unit (1) at the determination time instant, Determination of the loading of the rectifier unit (1) by comparing the value obtained for the actual power value with the value obtained for the maximum permissible power.
16. Process according to Claim 15, characterised by the following steps: Transmitting a signal (A) corresponding to the loading of the rectifier unit (1) to at least one inverter unit (5, 6), 4 Matching the power taken by the inverter unit or units (6) f rom the link circuit (3) to the loading of the rectifier unit (1).
17. Any of the frequency converters substantially as herein described with reference to the accompanying drawing.
18. A process for the operation of a frequency converter substantially as herein described with reference to the acompanying drawing.

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