Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates -to a source of elec-trical energy
having a static inverter and a welding transformer for a resis-
tance-welding machine, in which use is made oE a first static
solid state frequency converter, the alternating output voltage
of which has a frequency and pulse shape which differ from the
frequency and pulse shape of the supply voltage fed to the inver-
ter.
A source of energy of this kind has already been des-
cribed in United S-tates Patent No. 3,553,567 which issued January
5, 1971 to Pesce et al. This source of energy produces high
frequency welding currents, the pulse shape of which resembles
a rectangle or square. However, the unit is satisfac-tory only
to a limited extent, since the pulse shape can be varied only
within narrow limits. In this known source of energy there is
no way of regulating the welding energy transferred.
It is the object of the invention to provide a source
of electrical energy in which the pulse shape, amplitude and
work to space ratio of the welding current can be varied within
wide limits and can meet the relevant requirements during welding.
According to the invention, there is provided a source
of electrical energy having a static inverter and a wel~ling trans-
former for a resistance-welding machine in which use is made
of a first static frequency converter, the alternating output
voltage of whichhas a frequency and pulse shape which differs
from the frequency and pulse shape of the supply voltage fed
to the inverter, at least a second frequency converter is connec-
ted in parallel t~ the first frequency converter, and the al-ter-
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nating output voltage of the at least second frequency converter
is arranged to be added in phase synchronism -to -the alterna-ting
output voltage oE the first frequency converter in the primary
circuit of the welding transformer, wherein each frequency conver-
ter consists of a transistor control stage and a thyristor power
stage, the transistor control stage being preceded by a direct
current bridge, the transistor control stage and the thyristor
power stage being connected to the outpu-t of a frequency regula~
tor which has an input for setting a set value signal of the
frequency of the welding current, the tra:nsistor con-trol stage
being in addition connected to the outpu-t of a welding current
regulator providi.ng an adjusting signal, the welding current
regulator having one input each for setting a set value signal
of the welding current and for an output signal, supplied by
a measured value transdueer, which is propor-tional to the rated
value of the weldi:ng eurrent.
The amplitudes of the output voltages of both frequeney
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converters may be constant or variable or only that of the first
frequency converter variable.
The pulse shape is rectangular in the preferred embodi-
ment and the work to space ratio is variable~
Providing the source of energy with at least two similar
frequency converters, the alternating output voltages of which
are adapted to be added, in phase synchronization, in the primary
circuit of a welding transformer, makes it possible to obtain
~i pulse shapes having very s~ sides, thus ensuring accurate weld
beginnings and accurate switching of weld endings. Furthermore,
the steepness of the sides of the welding current increase in the
secondary circuit of the welding transformer permits welding at a
very low frequency, even at high production speeds. The time
taken to reverse the welding current is negligable, and the
frequency therefore has almost no effect upon the quality of the
weld seam. If a low frequency welding current is used (below fs =
100 Hz), inductive eddy-current losses are largely eIiminated.
Accurate metering of welding energy can be achieved by providing
in either the primary or the secondary circuit of the transformer
a transducer having an output signal which is proportional to the
rated value of the welding current and which is fed to a welding
current regulator, and in that the output from the regulator is
connected to the frequency converters for adjusting the ~requency
converters.
Since the frequency converters are of similar design, it
is a simple matter to connect in parallel as many frequency
converters as are necessary to achieve rapid increase in current
and rapid reversal.
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The invention will now be described in greater de-tail
with reference to the accompanying drawings, in which:
Figure 1 is a block diagram of a power source according
to the invention;
Figure 2a is a graph of -transformer primary voltage
against time; and
Figure 2b is a graph of transformer secondary voltage
against time.
Figure 1 shows a source of electrical energy consisting
principally of a first frequency converter 10 and a second fre-
quency converter 11. The outputs from the two converters are
connected to the primary circuit of a welding transformer 12,
while the inputs thereto are connected to a 3 x 380 V -three-pilase
current supply, for example. The three-phase current passes
through isolating transformers 13, 14 to conventional rectifying
bridges 15, 16 which produce a direct voltage, by means of which
the actual inverter stages, each consisting of a transistor
control stage 17 or 18 and a thyristor power stage 19 or 2G,
are fed.
The design and operation of the two latter stages are
described in principle in Swiss Patent 559,~62, granted on January
15, 1975 to Marcel Etter.
The source of energy according to the invention also
comprises a welding current regulator 21 and a frequency regulator
22 which are components of an electronic control system not shown
in detail. Welding current regula-tor 21is connec-ted -to a feed-
back loop which permits determination of a rated current value,
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either in the secondary circui-t by a measured value transducer
23 or in the pr~mary current by a similar transducer 24.
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According to the setting, frequency regulator 22 determines the
frequency of the welding current which flows through electrode
rollers 25 and the workpiece therebetween, not shown in the
drawing.
The operating principle of the source of energy described
hereinbefore is explained hereinafter with the aid of Figure 2.
In Figure 2a, voltage ul appearing at the primary circuit
of welding transformer 12 is shown, while Figure ~b shows the
relevant welding current i2 in the secondary circuit of the
transformer. A voltage pul~e of specific length and amplitude may
be produced by one of the frequency converters 10, 11. The phase
synchronous addition of two of such voltage pulses gives, in the
present example, the point at which welding current i2 begins to
flow in the secondary circuit. The welding current very quickly
reaches its set amplitude and this is kept constant by welding
current regulator 21. In order to maintain the constant value of
the welding current during a half-wave, fre~uency converters 10,
11 produce alternate voltage pulses which produce, in the second-
ary circuit, slight modulations, according to amplitude, of
~elding current i2 with a relatively high frequency. Upon
completion of the first half-wave of rectangular welding current
i2, both frequency converters 10, 11 are connected, in phase
synchronism and additively, to the primary circuit of welding
transformer 12 (see Figure 2a), so that a reversal of the said
welding current takes place in a negligably short time.
The source of energy according to the invention makes it
possible to produce weldiny currents of an almost direct current
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nature in relation to the edges to be welded~ which makes the
present invention suitable for the execution of the method
described in our Canadian Patent Application Serial No. 370,688
filed February 11, 1981. In this connection, the invention is
characterized in that the welding transformer is designed for a
lower transferable limit frequency which is determined mainly by
the ratio between the speed of transfer and the length of the
edges to be welded, and in that the dimensions of the weakly
magnetic sheeting material are within a range such as to permit
determination of the operating point on the characteristic
magnetic curve in the B/H diagram at the beginning of the linear
section. Welding currents having higher frequencies, for example
mains frequency, may also be produced, the effect of welding
current frequency upon the quality of the weld seam, as a function
of production velocity (welding speed) being insigni:Eicant.
