Abstract:
A method for ground potential monitoring of a rectifier drive, having a capacitor which is connected with a voltage source via a switching device. The method including applying a test voltage between a connector of a capacitor of a rectifier drive and a mass or ground potential prior to connecting the rectifier drive with a voltage source that is connected with the capacitor via a switching device. The method also includes releasing the switching device if a potential of the connector changes by a predetermined amount after the applying the test voltage.

Description:
[0001]    Applicants claim, under 35 U.S.C. §119, the benefit of priority of the filing date of Sep. 27, 2001 of a German patent application, copy attached, Serial Number 101 48 740.1, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method for ground potential monitoring of a rectifier drive, having a capacitor which is connected with a voltage source via a switching device. The present invention further relates to a device for executing the method.  
           [0004]    2. Discussion of Related Art  
           [0005]    A ground potential detector includes a detector circuit, which is switched parallel to a load and a d.c. voltage source, is known from U.S. Pat. No. 4,253,056, the entire contents of which are incorporated herein by reference. The detector circuit has two voltage dividers, each with series-connected resistors, each of whose connections are made at an input of a voltage comparator. The comparator inputs are furthermore connected with a mass or ground potential by respective diodes, which are polarized in oppositely oriented conducting directions. A ground potential occurring in the d.c. circuit changes the polarity at the comparator inputs and calls up an output signal from the voltage comparator to indicate the ground potential.  
           [0006]    However, the known ground potential detector only indicates a ground potential in the charging circuit in case of a load already connected to the d.c. source, i.e. during an active operation, and not prior to connecting the load to the supply voltage source. But if a ground potential already exists prior to connecting the load to the supply voltage source, this can already lead to the destruction of components, in particular to the destruction of electronic components for triggering semiconductor power switches, because of high fault currents. This will be explained by an intermediate circuit frequency converter for supplying a three-phase motor with current from a three-phase supply network represented in FIG. 1.  
           [0007]    The three-phase intermediate circuit frequency converter  3 ,  4 ,  5  represented in FIG. 1 is composed of a charging rectifier  3  with charging diodes  31 ,  32  arranged in a three-phase bridge circuit, a d.c. intermediate circuit  4  with a charging resistor or a constant current source  41 , an intermediate circuit capacitor  40  and an intermediate circuit resistor  42  connected in parallel with the intermediate circuit capacitor  40 , as well as a semiconductor power switch  5  with semiconductor switches  51  to  56 , embodied as IGBT (Isolated Gate Bipolar Transistor) and also arranged in a three-phase bridge circuit, parallel to whose charging connectors recovery diodes  61  to  66  have been switched, which are polarized anti-parallel with the conducting direction of the semiconductor switches  51  to  56 . On the input side, the intermediate circuit frequency converter  3 ,  4 ,  5  is connected via a charge relay  2  to a current-supplying three-phase supply network with phases L 1 , L 2  and L 3 , and on the output side it supplies a three-phase motor  7  via the phases R, S and T.  
           [0008]    The intermediate circuit capacitor  40  is supplied with current via the charging resistor, or constant current source  41 , and smooths the output voltage of the charging rectifier  3  and stores the intermediate circuit energy. In this case, the charging resistor, or constant current source  41 , limits the inrush current, because in the uncharged state the usually very large intermediate circuit capacitor  40  acts like a short circuit, which causes a large inrush current. The latter would result in the destruction of components of the intermediate circuit frequency converter, inter alia of the charge relay  2 , of the rectifier  3 , of the intermediate circuit capacitor  40 , of one or several of the recovery diodes  61  to  66 , of upstream-connected fuses and/or of strip conductors of the printed circuit board.  
           [0009]    The electronic drive device connected to the control connections of the semiconductor power switches  51  to  56  determines the drive frequency and, via the current and voltage time surfaces, the motor voltage on the output side, or the motor current, wherein the driving of the individual motor phases R, S, T is performed by pulse width modulation. In this case the recovery diodes  61  to  66 , which are connected anti-parallel with respect to the conducting direction of the semiconductor power switches  51  to  56 , take over the current flow when the semiconductor power switches  51  to  56  which are assigned to them are switched off.  
           [0010]    If, in the exemplary embodiment represented in FIG. 1, the intermediate circuit capacitor  40  is charged, a voltage of for example −280 V is applied to the negative connector −UZ of the intermediate circuit capacitor  40 , and to the positive connector +UZ of the intermediate circuit capacitor  40  a voltage of +280 V with respect to the mass or ground potential. If the three phase switches of the charge relay  2  are closed, and therefore the charging rectifier  3  is connected to the three-phase network  1  supplying the voltage, the charging rectifier  3  generates a voltage of approximately 560 V on the d.c. side, which drops completely at the charging resistor  41 , because the intermediate circuit capacitor  40  is still uncharged during switch-on and therefore acts as a short circuit. The charging resistor  41  limits the charge current shortly after the charge relay  2  has been switched on, and in this switch-on moment −280 V with respect to the mass or ground potential are applied to both connectors +UZ and −UZ of the intermediate circuit capacitor  40 . With increasing charging of the intermediate circuit capacitor  40 , the potential of the positive connector +UZ of the intermediate circuit capacitor  40  is increased to +280 V.  
           [0011]    If prior to or during the switch-on process a ground potential occurs in one phase or several phases R, S, T before the intermediate circuit capacitor  40  has been charged, the positive connector +UZ of the intermediate circuit capacitor  40  is more negative by up to 280 V than the mass or ground potential. Because of this, in the course of closing the charge relay  2  a short circuit current flows through one of the recovery diodes  57  to  62 , the intermediate circuit capacitor  40  and one diode of the diode branch  31 ,  32  of the charging rectifier  3 , i.e. a very high short circuit current flows between the positive connector +UZ of the intermediate circuit capacitor  40  and the mass or ground potential, which suddenly charges the intermediate circuit capacitor  40 , so that there is the previously described danger of the destruction of components of the intermediate circuit capacitor.  
           [0012]    [0012]FIG. 1 shows in dashed lines a ground potential of the phase T which, when the charge relay  2  is closed, results in the recovery diode  65  becoming conductive, so that a short circuit current flows over the recovery diode  65 , the intermediate circuit capacitor  40  and the charging diodes  32  of the charging rectifier  3 , which can result in the previously described destruction of the electronic drive device of the intermediate circuit capacitor.  
         OBJECT AND SUMMARY OF THE INVENTION  
         [0013]    It is an object of the present invention to disclose a method for ground potential monitoring of a rectifier drive of the species mentioned at the outset, which detects a ground potential prior to connecting the rectifier with a voltage source providing electrical current by a simple technical circuit.  
           [0014]    In accordance with the present invention, this object is attained by a method for ground potential monitoring of a rectifier drive, having a capacitor which is connected with a voltage source via a switching device. The method including applying a test voltage between a connector of a capacitor of a rectifier drive and a mass or ground potential prior to connecting the rectifier drive with a voltage source that is connected with the capacitor via a switching device. The method also includes releasing the switching device if a potential of the connector changes by a predetermined amount after the applying the test voltage.  
           [0015]    It is another object of the present invention to disclose a device for executing the method for ground potential monitoring of a rectifier drive.  
           [0016]    This further object is attained by a ground potential monitoring system including a voltage source and a rectifier drive that includes controllable semiconductor power switches, recovery diodes arranged parallel with respect to charging connectors of the semiconductor power switches and a capacitor connected via a switching device with the voltage source, wherein the capacitor includes a negative potential connector. At least one electric motor drive connected to the semiconductor power switches. A comparator that includes a first input connected to the negative potential connector, a second input connected to a reference voltage source and an output connected to a triggering device, wherein the negative potential connector is charged with a test voltage.  
           [0017]    The method of the present invention makes it possible to detect a ground potential at the output of the rectifier drive ahead of time by a simple electrical circuit, and to give out an appropriate signal indicating the ground potential, or blocking the closing of a network voltage switch.  
           [0018]    The solution in accordance with the present invention can be basically employed with any type of rectifier circuit having a charging or intermediate circuit capacitor, i.e. not only in connection with such intermediate circuit frequency converters as explained above, but also in connection with single- or multi-phase inverters, as well as single- or multi-phase pulse inverters and frequency converters. The solution in accordance with the present invention is also not limited to drive systems, but can be employed with any single- or multi-phase load.  
           [0019]    The solution in accordance with the present invention is based on the knowledge that already prior to closing the switching device, i.e. prior to connecting the rectifier to the supply voltage source, a check is made whether the potential of the negative connection of the charging or intermediate circuit capacitor can be displaced in relation to ground potential. If a displacement of the negative connection of the charging or intermediate circuit capacitor is not possible, a ground potential exists in one of the load phases, while, with the charging connector intact, the test voltage superimposed on the potential of the charging or intermediate circuit capacitor leads to a corresponding displacement of the potential of the negative connection, so that the switching device for connecting the rectifier to the supply voltage source can be released.  
           [0020]    An advantageous embodiment of the method of the present invention provides for the comparison of the potential of the capacitor connector with a reference potential after the test voltage has been applied and, in case of a predetermined deviation of the potential of the capacitor connector from the reference potential, for blocking the switching device.  
           [0021]    In relation to an intermediate circuit frequency converter, whose input side is connected with an a.c. or a three-phase supply network and whose output side is connected with one or several electric motor drive(s), and in whose indirect circuit an intermediate circuit capacitor and a charging resistor or a constant current source are arranged, the negative connector of the intermediate circuit capacitor is charged with the test voltage and, following a predetermined change of the negative connection of the intermediate circuit capacitor in regard to the mass or ground potential, the switching device is released.  
           [0022]    With this special application of the method in accordance with the present invention it is possible, too, to apply a negative test voltage to the negative connector of the intermediate circuit capacitor and to compare it with a reference potential, so that the switching device is released when the potential of the negative connector of the intermediate circuit capacitor reaches or falls below the reference potential.  
           [0023]    It is alternatively possible to apply a positive test voltage to the negative potential connector of the intermediate circuit capacitor, to compare it with a reference potential and to release the switching device when the potential of the negative potential connector of the intermediate circuit capacitor reaches or rises above the reference potential.  
           [0024]    The negative or positive test voltage can be obtained in both methods in that the negative or positive half-waves from the a.c or three-phase current net supplying the electricity are applied via a high-impedance resistor to the negative connector of the intermediate circuit capacitor.  
           [0025]    A device for executing the method in accordance with the present invention for a rectifier drive with controllable semiconductor power switches having recovery diodes arranged parallel with respect to the charging connectors of the semiconductor power switches, having a capacitor, which is connected via a switching device with a voltage source, and having at least one electric motor drive connected to the semiconductor power switches, is distinguished in that the negative potential connector of the capacitor is charged with a test voltage and connected with a first input of a comparator, to whose second input a reference voltage source is connected, and whose output is connected with a triggering device.  
           [0026]    The triggering device can include an arrangement for releasing or blocking the switching device, and/or an indicator device, which are connected with the output of the comparator. To prevent erroneous switching and in this way to assure that no components of the intermediate circuit frequency converter are damaged or destroyed by too high a short circuit current, the release or blocking of the switching device for connecting the rectifier to the supply voltage source should be given preference wherein, in addition to the release or blocking of the switching device, an appropriate indicator is advantageous in order to give the user the option of error searching and error remedy. The indicator itself can include an optical or acoustic signal, or a combination of the two.  
           [0027]    In an intermediate circuit frequency converter having a charging rectifier, whose input is connected via a charging relay with an a.c. or three-phase current network for providing electricity, having an intermediate circuit capacitor, which is connected by a charging resistor or a constant current source with the charging rectifier, a first input of a comparator is connected via a voltage divider with the negative potential connector of the intermediate circuit capacitor, as well as with a charging resistor, and a diode circuit is connected with the a.c. or three-phase current network for providing electricity, and the second input is charged with a reference voltage.  
           [0028]    A negative or positive test voltage in the form of a d.c. voltage superimposition for detecting a ground potential can be generated by a diode circuit connected on the cathode side or the anode side with the a.c. or three-phase current network for providing electricity and applied to one input of the comparator, while a negative or positive reference voltage is applied to a second input of the comparator.  
           [0029]    Alternatively to the formation of a negative or positive test voltage for detecting a ground potential by a d.c. voltage superimposition, it is also possible to provide an a.c. voltage as the test voltage for detecting a ground potential by an a.c. voltage superimposition. In this embodiment, the first input of the comparator is connected via a voltage divider to the cathode of a diode, whose anode is connected with the negative potential connector of the intermediate circuit capacitor and via a resistor with the a.c. or three-phase current network for providing electricity, and the second comparator input is charged with a positive reference voltage, wherein a capacitor is switched parallel with the one resistor of the voltage divider connected with the mass or ground potential.  
           [0030]    The method and the device for executing the method in accordance with the present invention are particularly suitable for detecting a ground potential in on or several phases of a multitude of output elements connected to a circuit of an intermediate circuit capacitor, in particular of a.c. or three-phase current motors, since the method in accordance with the present invention tests all phases of the load connector simultaneously for ground potential without one of the load phases having to be energized.  
           [0031]    The idea on which the present invention is based will be described in greater detail by exemplary embodiments represented in the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 shows an embodiment of a circuit for ground potential monitoring of an intermediate circuit capacitor, having a ground potential detection circuit by negative d.c. voltage superimposition in accordance with the present invention;  
         [0033]    [0033]FIG. 2 shows an embodiment of a circuit with a ground potential detection circuit by positive d.c. voltage superimposition in accordance with the present invention; and  
         [0034]    [0034]FIG. 3 shows an embodiment of a circuit having a ground potential detection circuit by a.c. voltage superimposition in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    [0035]FIG. 1 shows a three-phase current intermediate circuit frequency converter  3 ,  4 ,  5 , which is connected to a current-supplying three-phase supply network  1  via a charge relay  2  for supplying a three-phase motor  7  with phases R, S. T, such as was described above by FIG. 1 for explaining the problem at the root of the present invention.  
         [0036]    A framed ground potential detection circuit  20  according to the present invention is attached to the phases L 1 , L 2 , L 3  of the network  1 . The circuit  20  is composed of a diode circuit  9  for generating a test voltage, a comparator  8 , a voltage divider  11 ,  12 , and a reference voltage connector  13 . The negative test voltage, which in the exemplary embodiment in accordance with FIG. 1 is negative, is generated by the diode circuit  9 , which includes three diodes  91 ,  92 ,  93 , which are connected at the cathode side to the phases L 1 , L 2 , L 3  of the current-supplying three-phase supply network  1 . The anodes  91 ,  92 ,  93 , which are connected with each other, are connected via a high-impedance resistor  10  with the negative connector −UZ of the intermediate circuit capacitor  40 , which at the same time is a connecting point for a voltage divider formed from two series-connected resistors  11 ,  12 .  
         [0037]    The voltage divider  11 ,  12  is connected with the mass or ground potential, and the connectors of the two voltage divider resistors  11 ,  12  with a first input E 1  of the comparator  8 . A reference voltage −Uref of −5 V, for example, is applied to the second input E 2  of the comparator  8 . The output of the comparator  8  is connected with a triggering device, not shown in detail, for example a control circuit for the charge relay  2  and/or an indicator device.  
         [0038]    The function of the ground potential detection circuit in accordance with FIG. 1 will be explained in greater detail in what follows:  
         [0039]    The diodes  91 ,  92 ,  93  of the diode circuit  9  apply the negative half-waves of the supply voltage of the current-supplying three-phase supply network  1  via the high-impedance resistor  10  to the negative connector −UZ of the intermediate circuit capacitor  40 . This causes a current flow of a few milliampere which, however, if there is no ground potential, must draw the potential of the negative connector −UZ of the intermediate circuit capacitor  40  towards more negative values. In the ideal case the potential of the negative connector −UZ of the intermediate circuit capacitor  40  is −280 V when the test current is applied. However, for reasons of circuitry technology, the voltage at the negative connector −UZ of the intermediate circuit capacitor  40  lies below this in one of the phases R, S, T of the three-phase motor  7 , even if no ground potential exists, so that, for example, it is possible to predetermine the criteria that, with a potential of approximately −50 V and lower at the negative connector −UZ of the intermediate circuit capacitor  40 , no ground potential exists.  
         [0040]    With the aid of the voltage divider formed from the voltage divider resistors  11 ,  12 , the voltage at the first input E 1  of the comparator  8  is reduced by a factor of 10 and is compared with the reference voltage −Uref of −5 V, for example, at the second input E 2  of the comparator  8 . If a lower potential exists at the first input E 1  of the comparator  8  than at its second input E 2 , the comparator  8  sends a release signal to the charging relay  2 .  
         [0041]    But if in the case of a ground potential in one of the three phases R, S, T at the output of the intermediate circuit frequency converter  3 ,  4 ,  5 , the negative connector −UZ of the intermediate circuit capacitor  40  cannot follow the negative test voltage, i.e. switch to a negative potential, since in the case of a ground potential in the phase T and with the intermediate circuit capacitor still not charged at the switch-on time, no more than the diode voltage can drop via the recovery diode  65 , so that the switching criteria of, for example, −50 V can no longer be attained as long as the intermediate circuit capacitor  40  remains uncharged. In this case, the comparator  8  sends an appropriate signal to the triggering device, for example a blocking signal to the charge relay  2  as well as possibly a signal to an optical and/or acoustic indicator device.  
         [0042]    In the circuit in accordance with FIG. 2, a positive test voltage is generated in the ground potential detection circuit  20 ′, and accordingly the negative connector −UZ of the intermediate circuit capacitor is pulled to a positive potential. This is created by reversing the connectors of the diode circuit  9 , i.e. the modified diode circuit  9 ′ has three diodes  91 ′,  92 ′,  93 ′, which are connected on the anode side to the phases L 1 , L 2 , L 3  of the current-supplying three-phase network  1 , whose cathodes are connected via the high-impedance resistor  10  with the negative connector −UZ of the intermediate circuit capacitor  40 .  
         [0043]    In this embodiment the ground potential detection even functions with a charged intermediate circuit capacitor  40  since, with a positive potential at the negative connector −UZ of the intermediate circuit capacitor  40 , the recovery diode  66  (in case of a ground potential in the phase T) is conductive, and only the diode voltage of the recovery diode  66  can be reached again.  
         [0044]    In connection with this embodiment of a ground potential detection circuit it is disadvantageous that a voltage of approximately 560 V (−280 V from the negative connector −UZ of the intermediate circuit capacitor  40  and +280 V from the positive test voltage, i.e. the voltage at the anodes of the diodes  91 ′,  92 ′,  93 ′) exists at the high-impedance resistor  10 , so that the high-impedance resistor  10  continuously consumes energy. However if the potential on both sides of the high-impedance resistor  10  is −280 V, for example, no voltage is applied to the high-impedance resistor  10 , so that no energy is consumed in this case.  
         [0045]    In a further simplified ground potential detector circuit  20 ″ in accordance with FIG. 3, the diode circuit  9  is omitted. An a.c. voltage is applied to the negative connector −UZ of the intermediate circuit capacitor  40  via a resistor  14  directly connected to the phase L 1  of the current-supplying three-phase network  1 . The positive portion of the resulting a.c. voltage potential is selected by a diode  15  connected on the anode side to the negative connector −UZ of the intermediate circuit capacitor  40  and is applied to the first input E 1  of the comparator  8  via a voltage divider formed from two voltage divider resistors  16 ,  17 .  
         [0046]    In this circuit a capacitor  18 , which is connected parallel with the second voltage resistor  17 , is used for smoothing of the signal applied to the first input E 1  of the comparator  8 .  
         [0047]    As in the above described exemplary embodiments, a reference voltage  13  applied to the second input E 2  of the comparator  8  is used as the criteria for sending a release or blocking signal, or a signal to a downstream-connected indicator device, from the comparator  8  of the ground potential detector circuit to the charge relay  2 .  
         [0048]    Within the scope of the present invention, further embodiment variations of course also exist besides the explained example.