Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2002-09-27
2004-06-08
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S523000, C324S509000, C363S050000, C361S059000
Reexamination Certificate
active
06747458
ABSTRACT:
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
1. Field of the Invention
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.
2. Discussion of Related Art
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.
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
.
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.
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.
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.
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.
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.
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
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.
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 connec
Huber Norbert
Ritz Franz
Deb Anjan K.
Johannes Heidenhain GmbH
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