Apparatus for instantly compensating for line voltage irregularities

An apparatus for compensating for line voltage variations in a pulsed welding arc supply. A circuit for drawing a current that is proportional to that part of the supply voltage in excess of a predetermined reference is drawn from the capacitor charging circuit in a UJT triggering device. Thus, the more the line voltage exceeds the desired level, the greater is the compensating current withdrawn from the capacitor charging circuit. As a result, the average welding current remains at the desired level despite supply-voltage variations.

BACKGROUND OF THE INVENTION 
In the art of arc welding, it has been discovered that welding in narrow 
grooves can be done very effectively by pulsing the current in the arc. 
This is typically done by allowing the flow of current in response to line 
voltages to occur only for short periods toward the end of each half cycle 
of the supply voltage. One of the difficulties attending this method, 
however, is the fact that it is particularly sensitive to line-voltage 
variations. It is desirable in these applications that the average current 
in each pulse be equal, but due to power supply variations, the average 
current produced by this method varies unless preventative steps are 
taken. 
The obvious response to this problem is to regulate the supply voltage, 
thereby maintaining a constant average current once the duty cycle is set. 
Unfortunately, means for accomplishing regulation of the AC supplies tend 
to be rather elaborate and expensive. An example is the motor-driven 
variable transformer. This is obviously rather elaborate and in addition 
has a slow response. The slow response, of course, makes the variable 
transformer relatively ineffective at insuring that each pulse has the 
same average current. A faster response is afforded by the 
constant-voltage transformer, but it also is large and expensive and tends 
to limit its usefulness to equipment that can tolerate its 
current-limiting action. As a result, what is needed is a fastacting and 
inexpensive means for compensating for line-voltage variations. 
SUMMARY OF THE INVENTION 
The present invention fulfills this need by adjusting the duty cycle of the 
arc pulse. Since the firing of the means for controlling the current 
usually occurs in response to the charging of the capacitor, the present 
invention uses the amplitude of the supply voltage to control the 
amplitude of a current signal that is subtracted from the charging 
current. More particularly, the current signal is roughly proportional to 
the amount by which the absolute value of the supply voltage exceeds a 
reference level.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The circuit of FIG. 1 receives 115-volt signal that is in phase with the 
voltage source that supplies power to the arc. This signal is received at 
terminals 11 and 13, and the circuit processes this signal to provide 
pulses at terminals 40, 41, and 42. These pulses are used to gate SCR's, 
not shown in the figures, that control the supply current in the arc 
welder. The 115-volt signal is stepped down to a 12-volt signal in 
transformer T10, which is rectified by bridge B12 and clipped at 12 volts 
by R14 and D18, which is a 12-volt zener. The resulting rectified and 
clipped sine wave is applied to the charging circuit composed of R20, R22, 
R24, R26, and C28. Thus, T10, B12, R14, and D18 comprise a charging-voltage 
source. The R20 - R22 combination is referred to in the claims as a "first 
series resistance." 
C28 begins charging at the beginning of each half cycle, and it continues 
charging until it reaches a trigger voltage equal to the turn-on voltage 
of Q32, a unijunction transistor. When the turn-on voltage is reached, C28 
quickly discharges through Q32, producing a pulse in T34. The pulse 
triggers silicon-controlled switch SCS38, allowing it to conduct through 
the rest of the half cycle. By adjusting 100-kilohm potentiometer R22, the 
operator can vary the amount of time it takes to charge up capacitor C28. 
This in turn controls the duty cycle of SCS38. 
Since SCS38 prevents current from flowing through the primary of T40 until 
it receives a gate signal from T34, the current is zero in T40 during the 
first part of each half cycle of the waveform appearing between terminals 
11 and 13. During this time, C36 charges through R16. Due to the short 
time constant of the R16 - C36 combination, the voltage across C36 is 
nearly equal to that across terminals 11 and 13. When C28 caused Q32 to 
fire, the resultant pulse turns on SCS38, permitting current to flow 
through T40. The quick jump in current is aided by C36, which discharges 
into T40, thereby quickly bringing the current in the primary of T40 up to 
the level dictated by the voltage at terminals 11 and 13 and the resistance 
of R16. SCS38 remains on during the rest of the half cycle and turns off 
when the voltage at terminals 11 and 13 nears zero. The result is a series 
of pulses of alternating polarity, each one of which begins toward the end 
of a half cycle. This signal is reproduced by T40 at terminals 40, 41, and 
42 and is used to trigger the SCR's in the welding-arc circuit. 
It was previously observed that the point in the half cycle at which the 
pulses produced by Q32 occur can be adjusted by means of potentiometer 
R22, which varies the charging time of C28. However, R22 is not the only 
means of varying the triggering time of Q32. Even though the signal at the 
output of bridge B12 is clipped by zener diode D18, and increased amplitude 
in the voltage occurring between terminals 11 and 13 would cause a change 
in charging time for C28 because a larger voltage would cause the 12-volt 
clipping potential to be reached sooner, shortening the charging time of 
C28. This increases the duty cycle, which is just the opposite of the 
desired effect; when the supply voltage increases, the duty cycle should 
decrease in order to maintain the desired average current. 
In order to reverse this effect, the circuit of FIG. 2 is connected to the 
circuit of FIG. 1. Terminals 44 and 46 of FIG. 2 are connected to 
terminals 11 and 13, respectively, of FIG. 1, and terminals 70 and 72 of 
FIG. 2 are connected to terminals 23 and 25, respectively, of FIG. 1. This 
connects R68 across R24, and the parallel combination of R24 and R68 is 
referred to in the claims as a "second series resistance." T48 steps the 
115-volt signal appearing between terminals 44 and 46 down to a 12-volt 
signal and applies it to the D50-D52-D54-D56 full-wave rectifying circuit. 
The full-wave-rectified signal is applied to R58 and R60, the wiper of R58 
being adjusted to produce equal amplitudes in the alternate half waves. 
D62, an 8.2-volt zener, performs a function just opposite that of zener 
D18 of FIG. 1. Rather than clipping off the tops of the half waves, as D18 
does, D62 clips off the bottoms, preventing current from flowing until the 
voltage across it reaches 8.2 volts. D62 will be recognized as a means for 
maintaining a relatively high incremental impedance between its terminals 
when the voltage drop from its first terminal to its second terminal is 
below a threshold value and for maintaining a relatively low incremental 
impedance between its terminals when the voltage drop from the first 
terminal to the second terminal is above the threshold value. 
Because the voltage in the T48 secondary is the same as that in the T10 
secondary, the potential difference across R24 never exceeds the potential 
difference across R60, and D62 is therefore never forward biased. This 
means that the only effect that the circuit of FIG. 2 has on the charging 
current through R20 and R22 when the turn-on voltage of D62 has not been 
reached is that caused by the fact that R68 is in parallel with R24. In 
other words, the part of FIG. 2 to the left of R68 can be thought of as 
one part of a loop, while the parallel combination of R24 and R68 can be 
thought of as the completion of the loop, and current flows around the 
loop only when D62 reaches its zener voltage. This occurs only when the 
supply voltage exceeds a threshold voltage determined by D62, by an amount 
that is greater than a value proportional to the uncompensated voltage drop 
across R24. More specifically, the threshold is roughly equal to the zener 
voltage of D62 multiplied by the turns ratio of T48, and the proportional 
value is roughly the uncompensated voltage across R68 multiplied by the 
turns ratio of T48. Furthermore, the average amount of loop current is 
greatest when the amplitude of the sinusoidal voltage across terminals 44 
and 46 is greatest, which is when the supply-voltage amplitude is 
greatest. It is for this reason that the part of FIG. 2 to the left of R68 
is referred to in the claims as a "means, connected across the second 
resistance and thereby forming a loop comprising itself and the second 
series resistance, for causing a current to flow in the loop that 
increases as supply voltage increases." 
The result of the loop currents is that when supply voltage increases, the 
potential difference across R24 is higher for a longer period of time. 
Since the total voltage drop across R20, R22, R24, and C28 is determined 
independently of the voltage drop across R24, a greater voltage drop 
across R24 results in a smaller drop across R20 and R22. It can be seen 
that the current flow through R20 and R22 is the charging current of C28, 
so an increase in potential in R24 results in a decrease in charging 
current to C28. Thus, the charging current of C28 tends to be decreased by 
the action of the circuit of FIG. 2 when the power-supply voltage 
increases. Conversely, when the power supply voltage decreases, there is a 
smaller potential difference across R24, allowing more current to charge 
C28. Thus, the circuit of FIG. 2 is a means for subtracting from the 
charging current a current whose average value increases with supply 
voltage. The effect of this subtraction of charging current is to delay 
the firing of SCS38 when the power supply voltage increases and to advance 
it when the power supply voltage decreases. This tends to compensate for 
supply-voltage variations. 
It has been found that the circuit of the preferred embodiment affords 
nearly exact compensation through a rather wide range. Pulsed-arc welding 
works best when the arc is triggered somewhere in the second half of each 
half cycle of the supply-voltage waveform; if it is triggered earlier, the 
narrow arc that is characteristic of pulsed-arc welding tends to broaden. 
Within the desired range, the circuit of the present invention also works 
best. The circuit of the preferred embodiment, for instance, is designed 
for a nominal supply voltage of 460 volts and a nominal triggering angle 
of 130.degree., and excellent results have been observed with line-voltage 
variations of as much as .+-.70 volts. Excursions outside of that range, 
however, tend to push the duty cycle into a range that is less favorable 
to the operation of the compensation circuit of the preferred embodiment, 
and overcompensation occurs at the low voltages, while high voltages cause 
undercompensation. Even outside its optimum range, however, a compensation 
circuit according to the present invention gives results that are better 
than those obtained with uncompensated circuits. 
In the practical operation of this circuit, certain initial adjustments 
must be made. In order to ensure the right amount of compensation, R64, 
which controls the amount of the FIG.-2-circuit action that is experienced 
by R24, is adjusted so that the FIG.-2-circuit effect is just enough to 
compensate for supply-voltage variations. In order to adjust to a given 
average current, a variable voltage is applied to the supply terminal, 
and, because the effects of R64 of FIG. 2 and potentiometer R22 of FIG. 1 
are not completely independent, both potentiometers must be adjusted until 
the welding current maintains a substantially constant average value 
throughout the expected range of input voltages. Experience has shown that 
two or three adjustments at either end of the expected voltage range are 
sufficient to completely compensate the apparatus. 
While the invention has been described in conjunction with a specific 
embodiment thereof, it is evident that many modifications will be apparent 
to those skilled in the art of the foregoing description. Accordingly, it 
is intended to embrace all such alternatives, modifications, and 
variations as fall within the broad scope of the appended claims.