US2436027A - Apparatus for controlling the electric heating of continuous metallic articles - Google Patents

Description

Feb. 17, 1948. E. E. VONADA ET AL APPARATUS FOR CONTROLLING THE ELECTRIC HEATING OF CONTINUOUS METALLIC ARTICLES 3 Sheets-Sheec 1 Filed Oct. 12, 1944 Feb. 17, 1948. E. VONADA ET AL 2 36K927 APPARATUS FOR CONTROLLING THE ELECTRIC HEATING OF CONTINUOUS METALLIC ARTICLES Filed Oct. 12, 1944 :s Sheets-Sheet 2 INVENTORS EDM/NV .54 PL I/O/V/IDA and ,W/LL/AM .E l WNTE/MAA 75A;

APPARATUS FOR CONTROLLING THE ELECTRIC HEATING OF CONTINUOUS METALLIC ARTICLES Filed Oct. 12, 1944 3 Sheets-Sheet 3 C A M a MW, fl n A B /I-////I/ I //V 4 Ill/ p] w m O w m w w w w w N I mGvK'B KSUQ 93x3? IO 3O 40 5O 7O 8O I00 //0 L/NE SPEED-Z E. MNTERHALTER,

Patented Feb. 17, 1948 APPARATUS FOR CONTROLLING THE ELEC- TRIC HEATING OF CONTINUOUS METAL- LIC ARTICLES Edwin Earl Vonada, Duquesne, and William E. Winterhalter, Dravosburg, Pa.

Application October 12, 1944, Serial No. 558,356

9 Claims.

This invention relates to apparatus for controlling theelectric heating of a continuous metallic article, and especially for controlling the bright melting of the tin coating on steel strip.

It is now common practice to apply a coating, such as tin, to continuous length strip material, such as iron or steel, by electrolytic means. After the strip is electrolytically tin coated, it is usually passed through a melting unit wherein the coating thereon is melted so as to provide a finished strip having a bright lustre and smooth, even finish. Sometimes the melting is done by passing the strip through hot oil, by electrical induction'heating, or by subjecting the strip to electrical resistance heating. In the resistance melting, the strip is usually disposed in a vertical plane during the melting of the coating thereon, passing over'one conductor roll then upwardlyover and around a deflection roll and then downwardly around another conductor roll into a tank containing a quenching medium with that portion of the strip disposed between the conductor rolls, at any particular time, heated by electrical resistance'so asto melt the coating thereon. Such a melting unit and quenching tank arrangement is disclosed in the copending application of Ewart S. Taylerson, Serial No. 447,966, filed June 22, 1942 (now abandoned).

As thestrip passes over and around the conductor roll of the melting unit, it will be understood that the speed of the strip will change from time to time due to operating conditions. Also, the cross-sectional area of the strip changes from time to time due to different widths and thicknesses of the strip. Accordingly, it' will be seenthat it is impossible to impress a constant voltage or current input to the conductor rolls and the strip for the reason that, at times, due to such changes in operating conditions, the strip would be heated far above the melting temperature of the tin coating thereby causing the same to flow therefrom resulting in spoiling the coating on that portion of the strip, and, at other times, the strip would not be heated sufiiciently to melt the tin coating thereon on other portions of the strip thereby defeating the purposes of the melting operation.

Thus, it will be seen that it is necessary to supply a variable source of alternating current so that the strip and coating thereon is properly heated to satisfy all operating conditions and it is to such a means for varying the supply of power to the strip that the present invention relates. Various means and equipment have been suggested and used for such a purpose, but usually they have been expensive and necessitated special'designing and construction for the particular application. In the present invention, such variable source of power is provided by standard equipment, by an improved electrical arrangement thereof.

In controlling the power input to the conductor rolls of the melting unit, it has been found that the most accurate and reliable controlis obtained by accurately regulating the impressed heating voltage and maintaining the required voltage for any operating speed. The power (kw) requirements for heating a strip of any given cross-sectional area is directly proportional to the speed at which the strip passes through the meltingunit. By the formula, power (kw.) equals the current squared multiplied by the resistance, and since the resistance of the strip is unaffected by a change in operating speed, it will be seen that the voltage requirements for heating the strip must be varied with any change in speed thereof. However, it will be seen that the voltage is not directly proportional to the operating speed or the speed of the strip through the melting unit, but has a root mean square characteristic. In view of the-foregoing it is seen that if line speed is plotted against power a straight line will result. Since power (P) equals current (I) squared times the resistance (R) and E E 2 E I= P=( XR or where E is thevoltage requirement. Since the resistance is constant for any size strip the power is proportional to E and a straight line will result if E is plotted against strip speed. It is desired to use volts instead of volts squared as the basis of the control and therefore line speed is plotted against volts to obtain a curve showing the desired characteristic. This relationship is commonly referredto in alternating current heating as the root mean square characteristic.

In order to obtain the correct heating voltage for all operating speeds, it is necessary to secure a reference voltage having the same characteristics as the heating power voltage requirements. This reference voltage can then be used by means of a voltage regulator to maintain the desired impressed heating power voltage for all operating speeds.

present invention to establish a reference voltage for controlling the heating power voltage requirements of the metallic strip in a melting unit of the class described so that the voltage characteristics thereof are substantially the same as those of the reference voltage whereby the voltage supplied to the conductor rolls and the strip is maintained at the required impressed power voltage for properly heating the strip for any given condition.

It is a still further object to .provide continuous supervised control of heating current'with respect to speed by the use of a network of rectifiers, resistors, transformers, tachometer generators and/or reactors and capacitors when required to produce voltages of magnitudes and characteristics necessary to maintain essential representation of basic control elements to individual or combination rotating and/or static regulating units which perform combined functions of heating current/speed ratio maintenance, automatic amplification of forcing (more rapid change of power than is obtained by normal means) to overcome electrical inertia of controlled circuits, stabilization to prevent periodic oscillation of heating current with respect to .line speed,.and residual voltage control; all without resort to use of mechanical or electronic contact making devices, saturation of magnetiocircuitscr external regulation of the heating current supplycircuit to the end that a high degree. of uniformity .in heating is accomplished without incurring objectionable physical property variations, deformations and discoloration in product produced.

These and other objects will be more apparent after referring to the following specification and attached drawings, in which:

Figure l is a vertical section through a melting unit of an electrotinning linetogether with a schematic wiring diagram forcarrying out our invention;

Figure 2 is a schematic wiring diagram showing a second way of obtaining the reference voltaeetand...

Figure 3 is a curve showing the voltage input to the conductor rolls in relation to the speed, travel of the strip.

, Referring more particularly to the drawings, Sindicates a continuous length of coated metallic strip; as it is delivered from the electroplating bath of a continuous electrotinning, line with the strip traveling in the direction of the arrows, as shown in the drawings, and which is conveyed therethrough and therefrom by any suitable driving means. After the strip has been electrolytically coated, it ispassed through a heatingunit designed to melt the deposited tin or coating thereon with such heating being effected by any suitable means, but preferably by the electrical resistance ofthe strip. V

In the present instance there is shown a resistance; type melting unit which comprises, a pair of spaced apart energized conductor rolls 2 and 3 which are connected to a suitable source of power or transformer in a manner hereinafter to be described. There is provided an intervening upperdeflector roll 4 with the coated strip S passing between the rolls 2 and 3 in an elongated vertical loop over the roll 4, during which travel, the strip is heated until the tin or coating material disposed thereon reaches its melting point. The vertical loop of the strip as it travels along,

is enclosed preferably in an insulated substantially ll-shaped mulfia 5 having an entrance leg 6 and a discharge leg I which legs extend toward the inner periphery of the respective conductor rolls 2 and 3, and enclosing the strip to points adjacent to the latter. As the strip passes through the muffle '5, the heat generated therein due to the electrical resistance of the strip is designed to bring the tin or coated material on the strip to its melting point in a zone indicated at M adjacent the lower end of the discharge leg. I of the mufiie 5. Directly below the lower end of the dischargev leg 1 and into which the same extends, there is arranged a tank 8 containing a quenching medium and in which the conductor roll 3 is disposed. The strip passes from the muflie 5 into and through the quenching medium in tank 8 from which the strip passes to subsequent processing steps.

The conductor rolls 2 and 3 are energized by a variable source of power by means of an electrical arrangement now to be described. The electrical arrangement or circuit of our invention comprises, preferably, a pair of-transformers 9 and I9, although one may be used instead of two providing it has sufiicientcapacity. The secondary windings of these transformers are connected in parallel with each-other and to the load or the conductor rolls 2 and 3, in the present case, by means of lines l4 and IS. 'The primary windings of the two transformers 9 and iii are connected in parallel with each other and to one side of the line of a source of-alternating current supply by means of a connection :6, preferably, through a circuit breaker ll. If one transformer is used, it will be understood thatthe secondarywindings thereof are connected to theload or the conductor rolls with the primary windings thereof connected to one side of the line of the source of alternating current supply. 7 I I There is provided two saturable core reactors l8 and i9, the primary windings of which are connected in parallel with each other by means of the lines 20' and 2| and'in series with the primary windings of transformers 9 and ID by means of the line 22. The primary windings of both of the reactors l8 and 19 are connected to the other side of the line of the source of alternating current supply by means of the line '23 through the circuit breaker [1. In other"words,' the primarywindings' of the reactors l8 and 19' are connected in multiple with each'other. and those of the transformers 9 and I0 also are connected in multiple with each other. and two mul-. tiple 'groups' of primary windings 'of all of the transformers and reactors are connected in series with the primary source of alternating'current supply. It will be understood that the two reactors l8 and I9 are designed tocontrol the power output of the two transformers 9 and I0, and it will be seen by connecting the sameas above described, that the input current from the source of alternating current supply will pass through both of the primary windings of the reactors l8 and 19, through the connection 22 and thence through the primary windings of both of the transformers 9 and Ill.

The secondary windings of the two reactors l8 and I9 are connected in series with and in polarity opposition to each other, and with means, preferably an exciting generator 24, arranged in the series connection for supplying a direct cur-' rent to the secondary windings of the reactors l8 and IQ for saturating the cores thereof.

The means described above for supplying curren to the conductor rollsis not a part of the present invention, Thesaturablel), C. generator 24' ispreferabl rovided with apermanent solid grcund-connection-28 to prevent dangerous voltages *aboveground on the generator. The control'which'is the subject of this invention :exercises supervision over the supply of current to the-conductor rolls through the excitation of the field 230i the generator 24'. The field circuit of the saturable generator 24 is connected across the armature or output circuit of a Single unit amplifying regulating exciter 30 through a current limiting resistor 46. The amplifying regulating exciter 30 may beof the type manufactured by the General Electric Company and known as the Amplidyne which is provided with a plurality offield circuits wound on common pole pieces. This amplifying regulating unit may also be a combination of two or more cascade connected rotating r static unit-s. The armature or output'voltage of the exciter 30 is principally derivedf-romand controlled'by two of these field circuits having fields Hand 33, respectively. The voltage applied to the circuit including field 33 is the reference voltage described above and curves A and C in Figure 3 show how this reference or heating circuit-voltage will vary as the line speed varies. The reference voltage applied to the circuit including field 33 is obtained as follows:

The tachometer generator 34 is driven from the bridle roll 35 and thus its speed will vary in accordance with the speed of the strip and its output voltage which is employed as an excitation source will vary directly with the processing line speed. Installed in the circuit leading from the enerator 34 to the field 33 is a rheostat 36 which is driven from the motor operated rheostat 31 of the main variable voltage enerator for the processing line. The part of the voltage from the tachometer generator 34 appliedacross the field circuit includin field 33 is made to conform approximately to the root mean square characteristic of the required reference voltage by proportioning the resistance between the contact buttons of the rheostat 36. Once calibrated, the resistance will be automatically proportioned as the line speed varies by means of the connection between the rheostat 33 and the main line rheostat 31. Also installed in the circuit leading to the field 33 is a Vernier rheostat 38 which is used to accommodateheating requirements for various sizes of strip. If necessary, a resistance 39 is installed in the circuit to prevent excessive current in the field-33, which is connected to produce a positive flux which increases the output voltage of exciter 30- and consequently the heating power with an increase in processing line speed.

A restraining voltage proportional to primary heatin voltage opposes the reference voltage to maintain heating proportional to speed and is applied across the field circuit including field 32. An insulating transformer 40 is connected across lines l4 and I leading to conductor rolls 2 and 3 and is connected to the field 32 through a full wave rectifier 4|. This circuit is adjusted by current limiting resistor 42 and rheostat 43 to maintain the restrained flux at that percentage below the positive flux 33 necessary to maintain theproces'sin'g lineis deceleratedor the-primary heating voltage increases, the output voltage of exciter 30 decreases rapidly due to the decrease of excitation from field 33 which causes an immediate excess of restraint by field 32. These rapid changescause a fluctuating of the control as it seeks to establish the desired relationship between processing line speed and heating current. In order to effect a smooth transition to steady conditions from the transient conditions in which high forcing power has been applied by the exoiter 30 to the reactors l8 and. I3 through generator 24 a stabilizing circuit including field 44 is connected across the exciter 30 through direct current impulse transformer 45. A resistance 3| is employed in the circuit to adjust the magnitude of the secondary impulse from the transformer 45 to effect the stabilization desired. The relative time constant between field 44 and fields 32 and 33 in combination with the armature of exciter 3D and field circuit 29' are coordinated so that highly amplified instantaneous forcing power of exciter 301s smoothly tapered to steady conditions without the overshooting which causes oscillation of heating current in response to proc essing line speed. The polarity connections of field 44 are such that the raipd increase of the output voltage from exciter 30 induces a delayed D. C. impulse voltage in the secondary of transformer 45-which is applied to'circuit 44 in opposition to the increased total excitation of exciter 30, and tapers the output voltage to the steady conditions after the desired ratio of processing line speed and heating currents have been established. In like manner, a delayed D. C. impulse voltage having a polarity opposite to that associated with processing line acceleration is induced in the secondary of transformer which is applied-to field when there is rapid'decelera tion and this brings the output voltage of exciter 30 back to the steady conditions after the desired ratio between processing line speed and heating currents-has been established.

In order tocompensate for residual voltage of the generator 24 when the processing line is at rest, the exciter 30 is provided with a field 4! which is energizedfrom a source of constant D. C. voltage. The desired amount of correction is obtained by ad'ustment of resistor 48. The flux of this field-opposes that of field 33, but is so small that its effect is negligible when the processing lineisin operation. If desired, this field could be omitted and the primary circuit broken when the processin line is down.

In practice, the-amplifying exciter 30 may have an amplification factor of from 1,000 to 10,000 and the algebraic summation of voltage is therefore 0.1% to 0.01% of the output voltage of the regulator. If this output voltage is under steady conditions the algebraic summation of excitation voltages would be 0.02-volts for a regulator having an amplification factor of 5,000. Typical control voltages supplied to regulator control fields 32 and 33 are of the order of- 0.5 to 5 volts for 10% to 100% of processing line speed. It is therefore apparent that the algebraic summationof excitation voltages is a small percentage of the voltage to the control circuits of regulator 30. Variations in processing line speed or supply circuit voltage of 0.1% causes a departure of -055'volt from the adjusted algebraic summation ratio which would result in a 25 volt change in the output of exciter 30. The rate of change is approximately 2,000 volts per second which forces the reactors IB and l9 tomeet the new requirements and also supplies a D. C; impulse to control circuit 44 to taper this forcing to termination when the adjustedalgebraic summation ratio of the control circuits of exciter 30 has been re-established. The operation of the device is as follows: The control is first set up for a particular heating requirement, that is, the rheostat 36 is callbrated in order to produce the required reference voltage, the. vernier rheostat 38 is set according to the size of the material passing through the processing line, the resistance 39 is set to prevent excessive current from flowing to field 33 and the field 32 is adjusted by means of resistor 42 and rheostat 43 to maintain the restrained flux at thatpercentage below the positive flux of field 33 necessaryto maintain the desired heating current to the conductor rolls. The processing line is then started in operation and as the processing line speed increases, the speed increase is reflected by the position of rheostat 31, which in turn changes the position of rheostat 36, this tapering the output voltage of generator 34 applied to field 33 and causing the flux produced by field circuit 33 to increase non-lineally. This increases the output voltage of exciter 33 which increases the output of generator 24 to control the flow of current to the conductor rolls 2 and 3. This change is very rapid and there is a tendency for the heating current to increase more than is necessary. If it should increase more than necessary, then the flux of field 32 would increase to decrease the output voltage of exciter 3!). This change would also be rapid and the heating current would decrease to a value less than that required. This in turn would cause the flux of field 33 to increase the heating current. This causes oscillation of the heating current with respect to processing line speed, and in order to overcome these oscillations a stabilizing field 44 is provided. As the output voltage of exciter 39 increases due to increase of processing line speed, or due to a drop in the voltage of the primary heating circuit, a D. C. impulse voltage is induced in the secondary of transformer 45 and in the field 44 which creates a fiux that opposes the fiux of field 33 to efiect a smooth tapering to steady state conditions. Similarly as processing line speed decreases the flux of field 33 decreases to decrease the output voltage of exciter 33. This induces voltage in the secondary of transformer 45 and the field 44 which assists the flux of field 33 to'bring the output voltage back to the steady state condition after the desired ratio between processing line speed and the heating current has been established. As the processing line slows down the control operates in the reverse manner to decrease the flow of current to the rolls 2 and 3.

Figure 2 discloses a second way in which the reference voltage may be obtained. In this embodiment of the invention a conventional tachometer-generator49 is connected to be drive from the bridle roll 35 and its output voltage is used as an excitation source. The resistors 53, and 52 are adjusted to maintain maximum excitation of the field circuit 53 below saturation and its current within safe limits whengeneratingmaximum voltage and also when at rest. Theadjustment also provides for a limited current in field 33 when the generator 49 is at rest. In order to neutralize the output voltage of exciter 39 due to excitation of field 33 when at rest, the voltage supplied to fieldcircuit 41 is adjusted by resistor 48; The voltage supplied to the amplifying exciter field'33 is adapted to an approximate root mean square heating characteristic by an adjustment of resistance 54 which is employed to control the contour, and resistance 52 which is employed to control the slope of the curve A shown in Figure 3. It will be seen that the rate of excitation increase of field circuit 53 is thus made to decrease with increase in line speed, or to increase with decrease of line speed so that in combination with speed variation of the tachometer a root mean square voltage characteristic (Figure 3) is provided for the controlled field 33.

Referring again to Figure 2, assume that the 250 volt D. C. voltage source impresses '70 volts across resistor 5|, resistor 52, and the series combination of tachometer generator armature 49 and its field 53. With the processing line. and consequently tachometer generator armature 49 at rest, excitation of tachometer generator field 53 would be practically volts since the ratio of ohmic resistance of tachometer generator field 53 to tachometer generator armature 49 is very high. If the '70 volt excitation were maintained across field 53 with increasing speed, a tachometer generator armature voltage corresponding to the straight line characteristic 3 shown broken in Figure 3 would result. Actually the excitation across field 53 is, at full line speed, decreased to approximately 50% of that with the line stopped, or 35 volts, by the counter electromotive force action of tachometer generator armature 49. This transition is continuously progressive as processing line speed is increased from rest to full speed, and in combination with speed variation of tachometer generator armature 49 establishes the approximate root mean square voltage characteristic output of tachometer generator 49 which is applied to control field 33 of regulating exciter 30.

- Control of contour by adjustment of resistor 54 is efiected through changein the ratio of distribution of tachometer generator armature virtual voltage between armature 49 and control field 33.

Control of slope by adjustment of resistor 52 to a lower value of resistance, results in reductien of base voltage across the series combination of tachometer generator armature 49 and its field 53 (Figure 2), with the result that the slope of the approximate root mean square voltage characteristic of tachometer generator armature as which is applied to control field 33 of regulating exciter 38 is lowered from that indicated in curve A to that indicated in curve C in Figure 3.

While two embodiments of the invention have been shown and described, it will be apparent that other adaptations and modifications may be made without departing from the scopeof the following claims. r

We claim:

1. In combination with a conductor roll-for supplying current to a continuous metallic article as it passes thereover, means for driving the article so as to pull the same over the conductor roll, means for supplying alternating curapair of field circuits for controlling the output voltage of the exciter, said reference voltage being pplied to one of said field circuits to produce a flux to vary the output voltage of said exciter, means for rectifying the heating power voltage, said rectified voltage being applied to the second of said field circuits to produce a flux in opposition to the flux of the first field circuit whereby the heating necessary to maintain a predetermined temperature is appliedinthe strip as its speed varies.

'2: In combination with a conductor roll for supplying alternating current to a continuous metallic article as it passes thereover. means for driving the article so as to pull the same over the conductor'roll, means for supplying current to said conductor roll and said article for heating the same, means for establishing a reference voltage, an amplifying exciter for controlling the current supplying means to var the voltage supplied to the article as its speed varies, a pair of field circuits for controlling the output voltage of the exciter, said means for establishing a reference voltage including a tachometer generator driven by the article driving means and connected to the first of said field circuits, a rheostat installed in the line leading to the first field circuit and means for varying the resi ta ce of said rheostat in proportion to the speed of the article, said reference voltage being applied to the field circuit to produce a flux to vary the output voltage of said exciter, and means for rectifying the heating power voltage. said rectified voltage being applied to the second of said field circuits to produce a flux in opposition to the fiux of the first field circuit whereby the heating necessary to maintain a pre determined temperature is applied in the strip as its speed varies.

3. In combination with a conductor roll for supplying alternating current to a continuous metallic article as it passes thereover, means for driving the article so as to pull the same over the conductor roll, means for supplying current to said conductor roll and said article for heating the same, means for establishing a reference voltage. an amplifying exciter for controlling the current supplying means to vary the voltage supplied to the article as its speed varies, a pair of field circuits for controlling the output voltage of the exciter, said means for establishing a reference voltage including a tachometer generator driven by the article driving means and connected to the first of said field circuits and a variable resistance in the field circuit of the generator for varying the ratio of distribution of the tachometer generator armature virtual voltage between the generator armature and the first of said field circuits, said reference voltage being applied to the first field circuit to produce a flux to vary the output voltage of said exciter, and means for rectifying the heating power voltage, said rectified voltage being applied to the second of said pair of field circuits to produce a fiux in opposition to the flux of the first field circuit whereby the heating necessary to maintain a, predetermined temperature is applied in the strip as its speed varies.

4. In combination with a conductor roll for supplying alternating current to a continuous metallic article as it passes thereover, means for driving the article so as to pull the same over the conductor roll, means for supplying current to said conductor roll and said article for heating the same, means for establishing a reference 10 voltage, an amplifying exciter for controlling the current supplying means to vary the voltage supplied to the article as its speed varies, a pair of field circuits for controlling the output voltage of the exciter, said means for establishing 2. ref erencevoltage including a tachometer generator driven by the article driving means and connected to the first of said field circuits, a variable resistance in the field circuit of the generator for varying the ratio of distribution of the tachometer generator armature virtual voltage between the generator armature and the first of said field circuits and means for varying the base voltage across the series combination of generator armature and field. said reference voltage being applied to the first field circuit to produce a fiux to vary the output voltage of said exciter, and means for rectifying the heating power voltage, said rectified voltage being applied to the second of said pair of field circuits to produce a flux in opposition to the flux of the first field circuit whereby the heating necessary to maintain a predetermined temperature is applied in the strip as its speed varies.

5. In combination with a conductor roll for supplying current to a continuous metallic article as it passes thereover, means for driving the article so as to pull the same over the conductor roll, meansv for supplying current to said conductor roll and said article for heating the same, means for establishing a reference voltage proportional to heating power voltage and having an approximate root mean square characteristic. said last named means including a tachometer generator driven by the article driving means, a rheostat in series with the generator, and means for varying the resistance of said rheostat in proportion to the speed of the article whereby the heating necessary to maintain a predetermined temperature is applied in the strip as its speed varies.

6. In combination with a conductor roll for supplying current to a continuous metallic article as it passes thereover, means for driving the article so as to pull the same over the conductor roll, means for supplying current to said conductor roll and said article for heating the same, means for establishing a reference voltage proportional to heating power voltage and having an approximate root mean square characteristic, said last named means including a tachometer generator driven by the article driving means, a rheostat in series with the generator having its resistance proportioned to vary as the line speed varies, a motor operated rheostat for controlling the speed of the article, and a connection between the motor operated rheostat and the first named rheostat whereby the resistance of the first rheostat will vary as the article speed varies whereby the heating necessary to maintain a predetermined temperature is applied in the strip as its speed varies.

'7. In combination with a conductor roll for supplying current to a continuous metallic article as it passes thereover, means for driving the article so as to pull the same over the conductor roll, means for supplying current to said conductor roll and said article for heating the same. means for establishing a reference voltage proportional to heating power voltage and having an approximate root mean square characteristic, a control field on which the reference voltage is impressed, said last named means including a tachometer generator driven by the article driving means, and a variable resistance in the field circuitof the generator for varying the ratio ofdistribution of the tachometer generator armature virtual voltage between the, generator armature and the control field whereby the heating necessary to maintain a predetermined temperature is applied in the strip as its speed varies.

- 8.-In combination with a-conductor roll for supplying current to a continuous metallic article as ,it passes thereover, means for driving the article so as to pullvthe same over the conductor roll, means for supplying current to said conductor roll and said article for heating the same, means for establishing a reference voltage proportional to heating power voltage and having an approximate root mean square characteristic, a control field on which the reference voltage is impressed, said last named means including a tachometer generator driven by the article-driving means, a variable resistance in the field circuit of the generator for varying the ratio of distribution of the tachometer generator armature virtual voltage between the generator armature and the control field, and means for varying the base voltage across the series combination of generator armature and field whereby the heating necessary to maintain'a predetermined temperature is applied in the strip as its speed varies.

9. Apparatus for supplying alternating current heating power to a continuous metallic article comprising means for establishing a reference 25; Number voltage proportional to heating power. voltage and having an approximate root mean square characteristic, an amplifying exciter for control ling the current supplying means to vary the voltage supplied to the article as its speed varies, a pair of excitation circuits for controlling the output voltage of the exciter, said reference voltage being applied to one of said excitation circuits to produce a fiux to vary the output voltage of said exciter, and means for rectifying the heating power voltage, said rectified voltage being applied to the second of said excitation circuits to produce a flux in opposition to the flux of the first excitation circuit whereby the heating necessary to maintain a predetermined temperature is applied in the strip as its speed varies.

- EDWIN EARL VONADA. v

WILLIAM E. WINTERHAL'I'ER.

REFERENCES crren The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date 2,285,195 2,294,775 2,325,401 2,352,619 2,352,620

Edwards et a1. Sept. 1, 1942 Garr July 4, 1944 Edwards June 2, 1942 Hurlston July 27, 1943-

Images