US2393884A - Voltage compensator - Google Patents

Description

Jan. 29; 1946. E. M. CALLENDER 9 2,393,884

VOLTAGE cbMPENsATon Filed April 19, 1943 Edwin MCQUzndQr.

Muted Jan. 29, 1946 UNITED STATES PATEN VOLTAGE COMPENSATOR Edwin Meyers Callender, Philadelphia, Pa alsignor to Edward G. Budd Manufacturing Company', Philadelphia, Pa., a corporation of Pennsylvania Application April19, 1943, Serial No. 483,006

7 Claim.

This invention relates to electrical control systems and it has particular relation to systems for automatically controlling the flow of power in a load circuit subject to line voltage variations.

In the copending application of Robert S. Phair, Serial No. 372,689, filed Dec. 31, 1940, now Patent No. 2,330,377 there is described a control system having particular applicability to electric resistance welding. As described in said application, resistance welding machines are frequently connected in blank to a single power source and at times the simultaneous draft of power causes a considerable voltage variation. Weld heat varies as the square of the current which in turn varies as the line voltage; hence, if welds are effected at such time, or at times when the line voltage is low for any other reason, these welds may be defective due to insufficient heat during.

the welding current flow. Manual means have not proven adequate for compensation of this line voltage variation.

It is an object of the present invention to im prove the compensation for voltage change in electrical systems subject to important load and power variations. A further object is to be provide correct compensation for instantaneous chopped line wave voltage drops peculiar to phase heat controlled resistance welding loads. Another object is to improve the electrical characteristics of the compensator control of the voltage of the peeking voltage transformer as used in welding circuits. More specifically, objects of the present invention are to improve in resistance welding the voltage compensation for variable current timer settings ranging from low to high current values; to reduce the distortion factor in the compensation under extremely heavy line voltage drops; and to reduce the normal current flow through the operating tubes of the compensator transformer circuit. Additional objects include increase in sensitivity and accuracy and such simplification of the control system as to reduce parts and costs.

The above and other objects of the invention, as will appear hereinafter, are effectuated by the control system detailed in the following description and described in the accompanying drawing in which:

Fig. l is a wiring diagram of the control circuits as applied to a resistance type welder;

Fig. 2 is a circle vector diagram illustrating a compared phase change arrangement;

Fig. 3 is a circle vector diagram illustrating a preferred phase change bridge arrangement; and

Fig. 4 is a diagram of the line voltage and load current during a welding cycle.

Referring to Fig. 1 of the drawing the numeral ill designates the welding transformer having the primary coil II and secondary coil i2; the secondary including the welding electrodes.

The ignitrons ll and II are of the conventional mercury pool type actuated by production of cathode spots in mercury If by the igniters ll under impress of substantial voltage from the firing tubes and 2!. The anode 22 of the ignltron II is connected to the pool cathode i1 of the tube I8, and anode 23 of ignitron I8 is connected to cathode ll of the other tube, to obtain the rectifying action of this inverse connection.

The firing tubes 20 and 2| are preferably of the tetrode type of gas filled tube or thyratron, the shield grids 24 having connection to ,the ignitron ignlters l8 and the control grids 2B and 25 having connection to the peaking and hold-off transformers as will appear hereinafter. These tubes function to apply voltage on the ignitron exciters l8 when the negative voltage on the thyratron control grids is below a critical value, and at the same time a positive voltage is applied to the anode 21, of these tubes.

Power is supplied the circuits from mains 28, tapes being made for the thyratron cathode transformers. The main circuit leads from points 29 and 30; from point 29 connecting the main to cathode I I and anode 23 of ignitrons i5 and i6 and from point 30 connecting the other main to primary II and through the primary to the cathode i1 and anode 22 of ignitrons l6 and I5.

Connection is also made from the mains 28 to the hold-off transformer 25 having a primary 36 connected across the mains and secondaries 31, 3B, 29 and 40, secondaries 31 and I! as well as secondaries II and 40 forming units with coils of opposite polarity. One end of each of these units is connected to the shield grid of the firing tubes 20 and 2| as shown. The other ends are connected symmetrically through separate coils of the secondaries ll and 42 of the peaking transformer u to the control grids 25 and 26 of the firing tubes as previously indicated. The prima'ry coil 44 and saturable core 48 completes the transformer 43.

It is pointed out that the transformer assembly includes the coil 8 in operative connection with the welding time control unit (not shown), this coil also forming the primary of a transformer unit including the secondaries 39 and 40 of transformer 85.

The functioning of the transformer system described is as follows. Transformer 35 provides the necessary hold-oil bias for the firing tubes 20 and 2|. As is usual in resistance welding use is made of a variable portion of a given half cycle of current in accordance with the heat requirements of the particular weld. The length of time of weld application used is set by the timer connected to time transformer primary 6. The actual .heating cycle is established by the peaking transformer 43 which by virtue of the nounced peak formation of sufficient value to impress a positive voltage on the nring tube control grids 25 and 25 above the critical value. This occurs when the timer primary It actuates secondaries 39 and It to neutralize secondaries 31 and 38 of transformer 35 thereby permitting the peaking transformer secondaries 4| and 42 to be effective on the firing tubes.

Under constant load and line conditions the above described circuits would be adequate for proper welding but under actual operative conditions both of these factors are subject to considerable variation. In order to compensate for such variations there is provided the phase change bridge and phase change compensatory control which will now be described. A phase change bridge circuit 49 is utilized comprising the fixed condenser 50 manually variable resistance 5|, primary 52 of transformer 53, and adjustable resistance 54. The condenser 50 and variable resistance 5! are connected in series between points 55 and 56. Also between the same points the primary coil 52 and resistance 54 being connected in series-parallel. Connections to the main lines l8 are made at points and 55. Also at oints 51, between the condenser and variable resistance, and point 58, between the primary coil and fixed resistance, connection is made to the peaking transformer primary ll through condenser 59, resistance 80, and choke coil 8|.

It should be perceived at this point that with fixed phase relation between points and 58 by virtue of rigid voltages developed between points 56 and 58 and also between points 58 and 55 of the bridge circuit a reduction of line voltage will cause a reduction in welding current in the welding secondary i2 thus tending to cause weld failure. This is because although the controlled ignition firing angle of the firing tubes and 2i has not been changed, the magnitude of current passed by the ignitrons l5 and it, being proportional to the voltage, will be reduced by the voltage line drop,

In order to overcome the above mentioned difficulties the inductance primary 52 of transformer 53 is made to function as a variable resistance under control of a circuit sensitive to line voltage changes and operative to vary this effective resistance in such a manner as to compensate both for line voltage and external load variations. To this end the secondary 10 of control transformer '53 is connected at its ends to the anodes of vacuum triode regulating tubes H and 12, the cathodes of which are interconnected. Likewise, the grids 13 and H of these tubes are interconnected, this circuit including resistances 15. Power for the tube filaments is supplied from the transformer 16 including the primary l1 and secondary 18, the mid-point of which is connected to the mid-point 69 of secondary '10 through variable resistance 19, or alternately directly to 69 as indicated by dotted line Ill, the ends of secondary 18 having connection to the filament circuit of tubes H and 12 as shown.

A differential voltage, variable with line voltage, is applied to the grids T3 and 14 of tubes H and 12 by the following circuit. Direct current rectified from the power mains through transformer secondary 80, anode tube 8| and inductance filter 8283, is passed through the adjustable potential potentiometer 84. One end 85, and the adjustable contact 85, of the potentiometer are connected to opposite points 81 and 53 Of a saturable core 45, develops a voltage wave of provoltage reference bridge ll having four bridge arms ll, OI, I! and II. Bridge arms II and II connected in series between points OI and I1 in-v clude respectively in circuit the glow discharge voltage regulator tube :4 and fixed resistance as.

Bridge arms II and 52,. in series between points II and I1 and in parallel with series circuit II. II. include respectively fixed resistance It and glow discharge voltage regulator tube II. The glow discharge tubes are two element gas-diode tubes of the one way type having the positive electrode of area comparatively large with reference to the other electrode. The diode is designed to pass current at approximately volts, this voltage remaining constant with varying impressed voltages. The resistances are designed to pass current which at normal line voltages gives a resistance voltage drop substantially the same as the diode tube. Thus, if a potentiometer or voltage divider as It! be connected between points it and I", between the diodes and resistance no current will flow under normal line voltage conditions. Should the line voltage as rectified and applied from variable voltage potentiometer I to bridge circuit terminals 88 and 81, be lowered. the voltage dro across resistances It and II will diminish and hence a voltage difference is effective at points 99 and Ill. Utilization of this bridge voltage change is made by connecting the grids l3 and ll of regulating tubes II and I! to the movable arm III of a potentiometer III! connected across bridge points It and Ill; and by connecting the mid-point I! of transformer secondary III to the movable arm III of potentiometer I connected across glow discharge tube N.

The operation of the compensator circuit follows. Assuming functioning of the main power ignitrons, the timer and peaking transformer under normal load and line voltage conditions, the voltage drops in the reference voltage bridge 89 between points 81 and I8 and points N and Ill are approximately the same. Consequently the current in the anode circuit of transformer secondary I0 is low and at a value which establishes the normal cycle of the peaking transformer ll. However, should there occur a drop in line voltage there would simultaneousl occur a drop in the voltage of variable voltage potentiometer between points 88 and II which would at once establish a differential voltage on the reference voltage bridge at points 99 and I", and impress a less negative potential on grids l3 and H of the regulating tubes II and I2. This lowers the effective resistance of the secondar ll of the transformer 53 by increasing the anode current and disturbs the phase balance of the bridge 4!, shifting the phase of the peaking transformer cycle to cause an earlier peak on succeeding cycles thus automatically lengthening the cyclic section of power application in the welding circuit.

Should the line voltage increase over the preestablished normal, a reverse action would follow, a more negative diiferential potential developing at reference bridge 59 to increase the negative potential between points I 00 and I" and at grids 13 and H. In this manner the effective resistance of secondary HI is increased causing a phase shift in the phase of the peaking transformer to delay the time of peaking, thus compensating for the increased line voltage. In brief whenever the line voltage drops the phase of the peaking transformer is advanced; and whenever the line voltage rises the phase of the peaking transformer is retarded.

An important feature of the invention will now s,sos,sea 3 be described, reference being made to Figs. 2, 3 and 4 in this connection. Fig. 2 illustrates a voltage vector circle diagram in which the resistance 04 is directly connected to O-angle point 00, and impedance to point 00. With this arrangement of the bridge it is apparent that in orden to increase the phase angle on the peaking transformer circuit with decrease in line voltage the elective resistance of impedance 02 must be Increased. Hence, the electronic control of 1121- X0 pedance 02 must be adapted to operate with increased current flow with a linear reduction on decrease in line voltage. This arrangement therefore normally requires appreciable current flow through the regulating tubes II and 12. lo

Moreover, in resistance welding, on occurrence of abnormally heavy line voltage drops as'indicated at I20, Fig. 4, the wave shape distortion factor increases in importance.

To improve the above mentioned factors the 20,

position of the inductance and resistance on the bridge circuit is reversed, as illustrated in Fig. 3. the impedance 02 now being connected to O-angle point 00. Also by the regulating control as previously described, the effective resistance of impedance 02 is decreased with line voltage drop, thus securing the desired increase in phase angle. Additionally the regulating tubes Ii, I2 are normally biased to give good wave form with a lower current value in the regulating tubes hence the is the improvement in compensation for all variations in timer heat settings. In Fig. 2 there is indicated by way of example a bridge arrangement where the voltage vectors of inductance 02 and resistance 04 are balanced at the mid-point IIO. It is apparent from inspection that the angle al for a given source voltage drop at high heat setting IIO, of the resultant voltage vector I I0 is less than the angle a2 for medium heat setting H8 and for the same source voltage drop, a change opposite to that desired, since less com pensation is required at lower heat and more compensation at higher heat.

To overcome this deficiency and to increase the compensation with increase in heat setting by substantially linear values, the connections of Fig. 3 are utilized and the Junction point 00 shifted to the left of the mid-point. In other words. the bridge section 6850 is unbalanced by adjusting resistor I04 connecting bridge 00 so as to reduce voltage drop on resistor 00 compared to that of coil 02. I

With the unbalanced circuit as described, it is apparent from Fig. 3 that the angle a indicating shift of resultant vector IIO forvdecrease in source voltage to I00 increases as an approximately linear function of the heat setting for the same linevoltage drop, with a maximum value at high heat IIO. Also, for a uniform increase in line voltage to point III at the different 5 heat setting, low I I1, medium H0, and high N9, the phase angle 11 increases uniformly. Thus, by unbalancing the resistance-impedance bridge section as indicated the desired linear variation in compensation for varying heat settings is'se- 7 point, although on unusually high power factors 5 ance 04 of about 240 volts.

the unbalance is preferably increased, varying as high as 20%. Response on an 1'1 recorder indicates that with the described mode of compen sation a uniform heat output is provided within 5% of optimum as against 30% for compensated circuits; and within 10% as against 70% for uncompensated circuits. This applies to low, medium and high loads. The described compensation with the indicated adjustments assures complete control of the heat output, and such adiustments as may be necessary are required only The eflectlveness of the described compensation system Is indicated by shear pull tests where with a normal weld strength of 2250 lbs. for line drop with compensation, without compensation the weld strength was only 1350 lbs, this being far below normal.

In describing the control system emphasis has been laid on circuits and position of elements in said circuits. In general the electrical values of the electrical elements are not critical. However it is desirable that the resistance of resistor 54 in the phase change' bridge 40 be appreciable, preferably over 3000 ohms. For operating voltage of 150 volts of the glow discharge diodes a value of ten thousand ohms is desirable in the resistances 00 and of the reference bridge. Preferably ten thousand ohms in each of the grid circuits of regulator tubes II and I2 is desirable. The variable voltage potentiometer functions satisfactorily on ten thousand ohms. Values of forty thousand ohms and five hundred ohms are appropriate inpotentiometer 94 and variable resistance I0. The reference bridge I02 in bridge 00 is normally set for zero voltage output and its swing on line voltage drop subtracts or adds to the voltage bias of resistance I04, usually within a range of 40 volts.

In preparing the described compensating equipment for use the contact 00 is adjusted on resistance 84 to obtain approximately e300 volt drop between points 00 and 01 and'a less than 5 volts drop between points 00 and I00, a zero reading being ideal. Then with a fixed heat setting I00 is adjusted on resistance I04 to give an alternating current voltage across bridge resist- IOI is now set to midpoint on resistance I02, to be later more closely adjusted on ascertainment of the particular compensation requirement of the welding control panel.

It is pointed out that the reference voltage bridge 00 establishes a true proportion between the reference and variable voltages i. e. between points 00 and I00, the deviation depending only on the extent of variation of line source voltage from a normal assigned value. Consequently the range of compensation is enlarged, compensation being correctly maintained for large voltage changes of as high as 35% or even as high as 50% under some conditions of operation.

Another important factor in the phase control circuit is the utilization of the inductance input filter 02-00, which insures an output based on the average voltage occurring over the full voltage cycle.

Advantage is taken of a fast time constant for condenser 00 and resistor 84 so as to allow the direct current output voltage of the rectifier filter to follow closely all fluctuations of the line supply.

Consequently, at low heat settings in welder operation occurring at over 90 in the voltagetime cycle, Pr p r compensation is secured. This is an improvement over the capacitance type input filter which averages the input voltage only between and 90 under ideal conditions and hence fails to average correctly the line voltage for low heat settings of the welding machine.

It is noted, also, that since the regulating tubes II and 12 are operated on alternating current it is desirable that self bias resistor 18 be used in conjunction with the direct current bias. This provides a refinement in tube operation that is not entirely essential and may be omitted where desirable by using the by-pass H indicated in dotted line.

Variations of the details of the invention may of course be made, the scope of the invention being determined and defined by the claims hereto appended.

What is claimed is:

1. An electric power transfer system comprising a load circuit, power supply electronic tubes, firing means for said power tubes, phase change means including an impedance for varying the firing point of said firing means, a source of alternating current voltage and control means for controlling the phase change means to compensate for source voltage change, said control means including electronic regulating grid-biased tubes connected to said impedance, means for rectifying said source voltage, means for establishing a. fixed reference value of said rectified voltage and means for establishing a voltage in bridge relation to said reference voltage and variable directb with source voltage, said rectifying means averaging the full cycle variation of the transmitted source wave.

2. An electric power transfer system comprising a load circuit, power supply electronic tubes, firing means for said power tubes, phase change means including an impedance for varying the firing point of said firing means, a source of alternating current voltage and control means for controlling the phase change means to compensate for source voltage change, said control means including electronic regulating grid-biased tubes connecting to said impedance, and voltage means for decreasing the bias on the grids of said regulating tubes with decrease of source voltage, said voltage mean including a voltage bridge circuit having fixed voltage branches, variable current branches and a bridge connected to the grids of said regulating tubes.

3. An electric power transfer system comprising a load circuit, power supply electronic tubes, firing means for said power tubes, phase change means including an impedance for varying the firing point of said firing means, a source of alternating current voltage and control means for controlling the phase change means to compensate for source voltage change, said control means including electronic regulating grid-biased tubes connected to said impedance, and voltage means for decreasing the bias on the grids of said regulating tubes with decrease of source voltage, said voltage means including a voltage bridge circuit having fixed voltage branches, variable current branches and a bridge connected to the grids of said regulating tubes, and a source of rectified voltage variable with source voltage connected to said voltage bridge circuit.

4. An electric power transfer system comprising a load circuit, power supply electronic tubes, firing means for said power tubes, phase change means including an impedance for varying the firing point of said firing means, a source of alternating current voltage and control means for controlling the phase change means to compensate for source voltage change, said control means including electronic regulating grid-biased tubes connected to said impedance, and voltage means for decreasing the bias on the grids of said regulating tubes with decrease of source voltage, said voltage means including a source of rectified voltage variable directly with alternating source voltage, and a voltage bridge connected across said rectified voltage, said bridge comprising two parallel branches connected across said rectified voltage, each branch having a constant voltage tube and resistance connected in series, but in inverse relationship, a bridge connecting the Junction points of tube and resistance of each branch including a voltage divider and variable contact means connecting said voltage divider to the grids of said regulating tubes.

5. In a welding circuit, a load, an alternating current voltage source, means for transmitting power from the source to the load, a phase change means including an impedance for changing automatically the time of power transmission; and control means for controlling said phase change means, said control means comprising a potential bridge balanced for approximately zero potential at normal source voltage, means for establishing a substantially constant voltage in said bridge, means for developing voltage on said bridge in direct ratio to source voltage change, resistance means for imparting resistance attributes to said impedance, and circuit connections between said resistance means and said potential bridge.

6. In a welding circuit, a load, an alternating current voltage source, means for transmitting power from the source to the load, a phase change means including an impedance for changing automatically the time of power transmission; and control means for controlling said phase change means, said control means comprising a potential bridge balanced for approximately zero potentie] at normal source voltage, means for establishing substantially constant voltage in said bridge, means for developing voltage on said bridge in direct ratio to source voltage change, resistance means for imparting resistance attributes to said impedance, including electronic tubes each having an anode and grid, and circuit connections between the tube anodes and said impedance and between the tube grids and said potential bridge.

7. In a welding circuit, a load, an alternating current voltage source, means for transmitting power from the source to the load, a phase change means includin an impedance for changing automatically the time of power transmission; and control means for controlling said phase change means, said control means comprising a potential bridge balanced for approximately zero potential at normal source voltage, means for establishing a substantially constant voltage in said bridge, means for developing voltage on said bridge in direct ratio to source voltage change, resistance means for imparting resistance attributes to said impedance, including two electronic tubes each having an anode and grid, said impedance being connected between said tube anodes, a connection between said tube grids and one side of said bridge and a connection between the other side of said bridge and the mid-point of said impedance.

EDWIN MEYERS CALLENDER.

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