A converter welding arrangement with a welding control for the operation on mains with different voltages, having a welding transformer which is designed for operation on a mains with the lowest voltage of different possible voltages and to which the alternating voltage which is generated by AC conversion from the rectified mains voltage is supplied directly. A saturation monitoring arrangement monitors the current in the transformer and breaks off or interrupts the current supply to the welding transformer (18) when the current (I.sub.P) flowing across the welding transformer (18) exceeds a threshold value (I.sub.T)

BACKGROUND OF THE INVENTION 
The invention relates to a welding arrangement as known, for example, from 
the publication "Semiconductor Databook", 1981/82, page A-18, of the 
INTERNATIONAL RECTIFIER company. In such a welding arrangement, and to 
facilitate smaller and lighter welding transformers, an alternating mains 
current is first rectified. Subsequently, an alternating voltage having a 
frequency that is higher than the mains frequency is again generated from 
the direct voltage by use of an inverter bridge circuit constructed with 
four power semiconductor devices. The alternating voltage is then fed to 
the primary winding of a welding transformer which reduces it to a desired 
welding voltage. 
Known arrangements of the above type are designed to be used with a 
specific fixed mains voltage. As a rule, they can also be operated on 
mains with a voltage that is smaller than the specified fixed 
predetermined (designated) voltage. However, in principle, it is not 
readily possible to operate them on mains having voltages that are higher 
than the specific fixed designated operating voltage. The reason for this 
is, in particular, that the welding transformer, owing to its magnetic and 
material properties, does not allow a transfer of power higher than the 
maximum power that is designated for its intended use. In cases where a 
welding arrangement designed for a lower operating voltage is to be 
operated on a mains with a higher voltage, it is therefore customary to 
connect a pretransformer or a voltage-reducing device upstream of the 
mains rectifier to reduce the higher mains voltage to the lower designated 
mains voltage. This solution is technically easy and effective and it can 
be realized as a modular element. This solution, however, causes 
correspondingly higher costs and, most of all, increases the space 
requirement for the welding arrangement. 
For a welding arrangement of the generic type, PCT published application 
No. WO 9308628 published Apr. 29, 1993, corresponding to U.S. Pat. No. 
5,570,254 issued Oct. 28, 1996, discloses the monitoring of the welding 
transformer with respect to saturation. Thus, unusually strong current 
rises in the primary circuit of the transformer are intended to be 
detected early. It is the object of the early detection of such current 
rises to effectively protect the expensive power semiconductor devices, 
that are used for realizing the inverter bridge circuit, against 
destruction due to oversized currents in the primary circuit. 
It is therefore the object of the present invention to provide a welding 
arrangement having a welding transformer that is demensioned to be as 
small as possible, which arrangement can be operated on mains with 
different higher voltages. 
SUMMARY OF THE INVENTION 
The above object generally is achieved according to a first aspect of the 
present invention by a frequency-converting welding circuit arrangement 
which comprises: a welding quality control circuit for operation of the 
arrangement on voltage mains having different voltages; a welding 
transformer designed for operation on a mains with a lowest voltage of the 
different possible voltages; a rectifier circuit for connection to a 
voltage mains for rectifying the mains voltage; an inverter circuit, which 
is controlled by the control circuit and is connected to the output of the 
rectifier circuit, for converting the rectified mains voltage to an AC 
voltage of a desired frequency and for directly supplying the AC voltage 
to a primary winding of the welding transformer; and, a saturation 
monitoring circuit arrangement which monitors the current flowing through 
the welding transformer and provides an output signal to the control 
circuit to cause interruption of the inverter circuit and of current 
supplied to the welding transformer when the current flowing across the 
welding transformer exceeds a threshold value (I.sub.T). Preferably, the 
saturation monitoring circuit arrangement measures at least the current in 
the primary winding of the transformer, and utilizes the primary current 
(I.sub.P) to determine whether to provide the output signal However, the 
saturation monitoring circuit arrangement may additionally measure the 
current in a secondary winding of the transformer and additionally utilize 
the secondary current (I.sub.S) to determine whether to provide the output 
signal. 
The above object is achieved according to a further aspect of the present 
invention by a method for operating a welding quality-controlled frequency 
changing welding circuit arrangement using a voltage mains having a 
voltage that exceeds the continuous rated voltage of the welding 
transformer, with the method comprising the steps of: applying an 
alternating voltage generated by AC conversion of a rectified mains 
voltage to a primary winding of the welding transformer; measuring a 
current flowing through the welding transformer; comparing the measured 
current to a predetermined threshold value (I.sub.T) and, switching off 
the applied alternating voltage to the welding transformer if the measured 
current flowing through the welding transformer exceeds the threshold 
value (I.sub.T). The step of measuring may include measuring only the 
current (I.sub.P) flowing in the primary winding, but preferably includes 
additionally measuring the current in a secondary winding of the 
transformer and combining the measured secondary current (I.sub.S) with 
the measured primary current to provide the measured current utilized 
during the step of comparing. 
A welding arrangement according to the invention only requires a welding 
transformer which is dimensioned for the lowest of the possible mains 
voltages to be used. Operation at the higher voltages is made possible by 
the transformer saturation monitoring which is provided by the invention. 
The proposed arrangement offers the advantage that only a comparatively 
small and thus space-saving and economical transformer is needed. 
Additional arrangements or devices for reducing an increased mains voltage 
to the voltage for which the welding transformer is designed can be 
omitted in an advantageous manner. Due to the fact that it can be used for 
a wide range of mains voltages that exceed the continuous rated voltage of 
a welding transformer, the welding arrangement according to the invention 
allows a reduction of the required number of models of the welding 
arrangement for different operation voltages and thus again contributes to 
cutting costs. 
An embodiment of a welding arrangement according to the invention is 
explained below in greater detail with reference to the drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The welding circuit arrangement shown in FIG. 1 is based on a known 
frequency-converting welding circuit arrangement including a DC voltage 
intermediate circuit 10 which is connected via a rectifier 12 to a 
three-phase network 11. A capacitor 13 serves to smooth the direct voltage 
that is present in the intermediate or linking circuit 10. With the 
assistance of four-bridge-connected power circuit switches 14 to 17, a 
pulse-width-modulated alternating voltage of a desired frequency is 
generated from the direct voltage in the circuit 10, with the alternating 
voltage being applied to the input or primary winding of a welding 
transformer 18. The selective actuation of the power circuit switches 14 
to 17 to produce the desired AC voltage is controlled by a welding control 
circuit 20. 
On the secondary side of the welding transformer 18, a low-voltage 
alternating voltage is provided which, usually after having been rectified 
with diodes 22, is supplied to welding tongs or electrodes 21. With 
respect to its turns ratio, the welding transformer 18 is designed for 
operation with the lowest voltage of several possible voltage mains on 
which it is to be operated. With respect to its insulation strength, the 
welding transformer 18 must be adapted to the highest possible voltage 
mains operating voltage. For conventional transformers, whose insulation 
strength usually is at least ten times that of the indicated mains 
voltage, this is ensured without necessitating additional technical 
measures 
Until now, welding arrangements of the afore-described type are usually 
designed for the operation on mains with a fixed voltage of, for example, 
400 volt. A specific dimensioning of the power circuit switches 14 to 17 
as well as of the welding transformer 18 is a function of the desired 
welding currents. Since, for cost reasons alone, the welding transformer 
18 is always precisely tuned or matched to an underlying mains voltage, it 
is not possible to operate the welding arrangement on mains having a 
higher voltage, and this is true, even when the energy amounts to be 
transferred do not increase during this process. A large number of 
conventional welding transformers can also be operated at voltages that 
are higher than the indicated continuous rated voltage if the duration of 
the voltage pulses to be transferred is shortened accordingly. Within the 
framework of a welding quality control, however control or adjustment 
interventions occur regularly which require an increased energy transfer 
in the form of a longer pulse duration at an elevated voltage. Such 
adjustment interventions inevitably lead to a saturation of the 
transformer and, as a consequence of the saturation, the current in the 
transformer primary circuit rises greatly and the power that is 
transferred via the transformer drops. 
To overcome this difficulty, a monitoring of the saturation is carried out 
in the welding circuit arrangement according to the invention. In the 
alternating voltage-carrying primary circuit 19 of the welding transformer 
18, as well as in the secondary circuit, respective current sensors 23, 24 
are arranged to detect the primary current I.sub.P and the secondary 
current I.sub.S, respectively. The measuring signals received via the 
current sensors 23 and 24 are supplied to a saturation monitoring circuit 
arrangement 25 via connecting lines 26 and 27 which are connected with the 
welding control 20 via two further signal paths 28 and 29, respectively. A 
current limit value I.sub.T is supplied to the saturation monitoring 
arrangement 25 via a further signal path 33. The saturation monitoring 
circuit arrangement 25 transmits an error signal to the welding control 20 
via a further signal path 32 if a saturation has been detected. Advisably, 
a summing junction or circuit 30 is a component of the saturation 
monitoring circuit arrangement 25, which summing junction 30 adds the 
respective measuring signals present on lines 28 and 29 for the primary 
current I.sub.P and the secondary current I.sub.S preferably in a weighted 
manner. For this purpose a weighting member 34 is provided in the line 27. 
Downstream of the summing junction 30 is one input of a comparator 31, 
with the saturation threshold signal I.sub.T being fed to the second input 
of the comparator 30 via the line 33. The output of the comparator 31 is 
formed by the signal path 32 leading to the welding control 20. In a 
simplified design of the saturation monitoring circuit arrangement 25 
without summing junction 30, the measuring signal for the primary current 
I.sub.P is fed directly to the comparator 31. 
In order to detect a saturation condition, the fact is exploited that, in 
this case, the primary current I.sub.P rises above a maximum value I.sub.N 
reachable within the scope of a normal pulse applied to the primary 
winding of transformer 18, as is illustrated in FIG. 2, and demonstrates 
approximately the behavior shown by I.sub.X in FIG. 2. Therefore, a 
reference value I.sub.T is predetermined for the saturation monitoring 
circuit arrangement 25 by the welding control circuit 20, which reference 
value is just above the maximum current value I.sub.N that can be reached 
with normal pulses applied to the primary of the transformer 18. The 
saturation monitoring circuit arrangement 25 continuously compares the 
actual current values measured with the sensor 23 for the primary current 
I.sub.P with the reference circuit value I.sub.T. If the value measured 
for the primary current I.sub.P reaches the reference value I.sub.T, in 
FIG. 2 at the moment T.sub.3, the saturation monitoring circuit 
arrangement 25 emits a warning signal to the welding control circuit 20 
via the signal path 32. Based on the warning signal, the control circuit 
20 then breaks off or interrupts the current pulse then in progress. A 
saturation effect which, nevertheless, might have occurred already is 
compensated for by the next current pulse of opposite polarity in that the 
pulse is extended beyond the intended switch-off moment T.sub.2 until the 
current amplitude reaches the value of the threshold value I.sub.T, i.e., 
at the moment T.sub.4 in FIG. 2 On the basis of the developing flux of 
currents of opposite polarity of uneven length, the transformer is 
rebalanced magnetically. The procedure is repeated until the primary 
current I.sub.P no longer indicates saturation phenomena by prematurely 
reaching the current threshold value I.sub.T. 
To be able to better control, suppress or correct interference quantities, 
it is advisable to also integrate the value of the secondary current 
I.sub.S into the saturation monitoring. Advisably, this occurs such that 
the signals measured for the primary current I.sub.P and those measured in 
the secondary current I.sub.S are combined with a specific weighting ratio 
While adhering to the underlying concept, there is a wide design leeway in 
the implementation of the proposed welding circuit arrangement. Thus, the 
early detection of a saturation can be improved with regard to the 
response speed by evaluating time-derived signals instead of actual 
current signals I.sub.P and I.sub.S. Accordingly, a maximum current rise 
value must then be specified as the threshold value. It is expedient to 
implement the saturation monitoring circuit arrangement 25 and welding 
control circuit 20 in one device. Of course, the specific signal 
processing circuit proposed in the illustrated embodiment for the 
saturation monitoring circuit arrangement 25 can also be replaced by 
another equivalent circuit. It was proven in a practical test that a 
welding circuit arrangement according to the invention with a welding 
transformer designed for the operation on a 400 V mains can be readily 
operated on a 700 V mains. 
The invention now being fully described, it will be apparent to one of 
ordinary skill in the art that any changes and modifications can be made 
thereto without departing from the spirit or scope of the invention as set 
forth herein.