US2105431A - Control system - Google Patents

Description

Jan. 11, 1938. F. MOI-ILER El AL 2,105,431

CONTROL SYSTEM Filed Oct. 11, 1953 2 Sheets-Sheet l Inventors I Francis Mohler; Leonid A. Un'1a1'1skg\ by NW4 .3%

Then- Attorney.

F. MOHLER ET AL CONTROL SYSTEM Filed Oct. 11, 1933 2 Sheets-Sheet 2 II'XVGI'TEOY'S Francis Mohlell Leonid A. Umanskg, b9 1+ 23 Their Atto r-ney.

Patented Jan. 11, 1 938 UNITED STATES PATENT oFFicE" CONTROL SYSTEM Francis Mohler and Leonid A. Umansky, Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application October 11,

11 Claims.

. This invention relates to control systems, more particularly to systems for controlling the operation of electric motors and it has for an object the provision of a simple, reliable and improved system of this character.

More specifically, the invention relates to control systems for motors required to operate over a wide range of speeds, such for example, as motors employed to drive apparatus for performing operations upon a moving strip of material and a more specific object of the invention is the provision of means for extending the operating range of the motors considerably beyond the range obtainable by field control.

In illustrating the invention in one form thereof, it is shown as embodied in a control system for motors employed to drive a reversing cold strip steel rolling mill in which the gauge of the strip is reduced by motor driven main mill rolls situated between a pair of motor driven reels, each of which alternately serves as an unwinding or winding-up reel depending upon the direction in which the strip is traveling during successive passes. The motor connected to the reel serving -as the unwinding reel during the particular pass under consideration, operates as a generator driven by the strip thus maintaining the strip under tension and returning energy to the line. At any given applied voltage, the speed or operating range of these reel motors must cover the speed range of the mill motor due to its field control, the build up of the coiled strip on the reels, and the draft or reduction in gauge of the strip effected by the mill rolls. For example, if the speedrange of the mill by field control of the mill'motor is 2 to l the speedrrange of the reel motors must be at least 2 to 1. If in addition the build up of the reels is 2 to 1, the reel motors must have at least a 4 to 1 speedrange and if inaddition the maximum draft of the strip taken by the mill rolls is the speed ratio on opposite sides of these rolls is 2 to 1 thus requiring a total speed range of at least 8 to 1 for the reel motors together with some additional margin to provide for proper functioning of the regulators. It is perhaps possible to design a dynamoelectric machine having an operating range by field control of 8 to 1 but the cost would be prohibitive and the overall operation would be unsatisfactory. Furthermore, there are installations of the type described above in which the operating range required of the reel motors is as great as 15 to 1. This range is unobtainable with field control. Thus a further object is this invention is the provision of a control system for generator I6 is driven at a speed which is pref- 1933, Serial No. 653,141

reversible apparatus operating on a. strip of material in which means are provided for extending the range of the reel motors to cover the speed range .by field control of the mill motor, the build up of the coil on the reels, and the draft taken by the mill rolls. Other objects will appear in the following specification. I a

In carrying this invention into effect in one form thereof a pair of dynamo-electric machines are connected to a load and means are provided for introducing a voltage into the armature circuit of one of the machines thereby extending the operatingrange of the machines beyond the range which is obtainable with field control. In a specific embodiment of this invention a dynamo-electric machine is mechanically connected to each reel of a cold strip reversing mill. These two dynamo-electric machines are controlled so that the machine connected to the unwinding reel operates as a generator and the machine connected to the winding reel operates as a motor, and an additional generator is connected in the circuit of the unwinding reel machine so that its voltage adds to that of the unwinding reel machine and opposes the voltage of the source.

For a better and more complete understanding of the invention reference should now be had to the following specification and to the accompanying drawings in which Fig. l is a simple diagrammatical representation of a control system embodying the invention, Fig. 2 is a modification of the system" of Fig. 1 and Fig. 3 is a slight modification of the system of Fig. 2.

Referring now to the drawings, a strip of material Hl such, for example, as cold strip steel, is unwound from the full reel II and is passed in the direction of the arrow to the mill rolls l2 by means of which the gauge of the strip is reduced the desired amount, and from the mill rolls the strip passes to the winding-up reel l3. In the following or next succeeding pass the direction of the strip is reversed and the reel, ll becomes the Winding-up or leading reel and the reel l3 becomes the unwinding or trailing reel.

The main mill rolls I2 are driven by any suite able driving means such, for example, as the electric motor M which is illustrated as a direct current type motor having a separately excited field-winding 15. The motor I4 may be supplied from any suitable source, but is preferably supplied from the variable voltage generator IS with the armature of which the armature of the motor 14 is connected in a loop circuit. The supply by the two supply lines 2| to which the terminals of the field winding 20'are connected by one or the other of the reversing contactors 22, 23 toether with a rheostat 24 included in the circuit for varying the excitation of the generator l6, and thus varying the voltage of the generator and the speed of the mill motor l4 in proportion.

This connection of the mill motor H in a loop circuit with a variable voltage generator is known as a Ward-Leonard connection and has the advantage that a very wide range of motor speeds can be economically obtained. Selective operation oi the contactors 22, 23 controls the polarity of the field of the generator and consequently the direction oi rotation of the mill motor i4 and the direction of travel of the strip ill. A movable contact arm 24a. of the rheostat is rotated by any suitable means illustrated in the drawings as a handwheel 25. Instead of a manually operated device a small pilot motor may be employed, if desired. By turning the rheostat handle so that the contact arm 24a is rotated in a clockwise'direction, the voltage of the generator and the speed of the mill motorare raised, whilst conversely, by rotating the contact arm 24a in a counterclockwise direction'the generator voltage will be reduced substantially to zero and the mill motor will be brought to standstill. By reversing the contactors, 22, 23 and again rotating the rheostat arm in a clockwise direction, the mill is started and accelerated in the reverse direction.

As shown, the field i5 of the mill motor is supplied from a suitable source represented by the plus and minus signs, which source may be and preferably is the same as that indicated by the two supply lines 2i. A rheostat 26 is included in the circuit between the field winding i5 and its source for the purpose of controlling the speed of the mill motor l4 and thus further increasing its range. The rheostat 26 is mechanically connected to the rheostat 24 and the contact arm.

and resistance elements of these two rheostats are so arranged that when the handwheel is turned, the motor field is not weakened until the generator voltage is brought to its maximum.

A dynamo-electric machine 21 is connected to the reel ii and a similar dynamo-electric machine 28 is connected to the reel l3. When the reel H .is the trailing reel the dynamo-electric machine 21 operates as a generator driven by the strip Ill. and the dynamo-electric machine 28 operatesas a motor driving the reel l3 so as to wind up the strip. During the succeeding pass when the direction of the strip is reversed, the dynamo-electric machine 21 operates as motor to drive the then leading reel i I and the dynamoelectric machine 28 operates as a generator driven by the trailing reel l3. These two dynamo- electrio machines 21 and 28 are connected to any suitable source of supply, and in the embodiment illustrated they are connected to the. supply generator' l6 from which. the mill motor I4 is supplied. Thus when the dynamo-electric machine 21 is operating as a generator driven by the strip it returns energy to the system and maintains a back tension in the strip between the mill rolls l2 and the trailing reel -II and since at this time the dynamo-electric machine 28 is operating as a motor it applies and maintains a tension between the leading reel l3 and the mill roll l2. During the reverse pass the back tension is maintained by dynamo-electric machine 28 operating as a generator to return'energy to the system and similarly tension between the rolls and the leading reel is maintained by the dynamo-electric machine 21 operating as a motor.

In order that the quality and gauge of the strip shall be uniform and the strip free from wrinkles, it is desirable that these tensions be maintained substantially constant at all speeds from standstill to the top speed at which the mill operates. For this purpose the respective excitation of machines 21, 28 are under the control of suitable constant current regulators 30 and iii. Assuming the reel II to be the trailing reel and the dynamo-electric machine 21 operating as a generator the speed of the machine 21 will, of course, increase as the diameter of the reel decreases.

Therefore in order to maintain the strip tension between the mill rolls and the reel ll substantially constant, it is necessary gradually to reduce the excitation of the machine 21 so as to maintain the same armature current. This is accomplished .by the regulators 38 and 3| in a well known manner. Although these regulators may be of any suitable type they are shown as vibratory contact type regulators. Briefly, the regulator 30 comprises a stationary contact 32 and a movable contact 33 arranged for alternately short circuiting and removing the short circuit from a resistance 34 included in the field circuit of the dynamo-electric machine 21. member 35 is attached to one extremity of the contact arm 33 and is arranged within the turns of a solenoid 36 which in turn is connected so as to be responsive to the current flowing in the armature circuit of the dynamo-electric machine An adjustable resistance 31 is included in the circuit of the solenoid 36 for the purpose of adjusting the setting of the regulator so that the regulator will hold the current of the machine 2'! constant at any desired value. A spring 38 is attached to the'movable contact arm 33 so that its pull opposes the pull of the solenoid on the core. as follows: With the machine 21 operating as a generator the contacts of the relay 40 are closed .so that the stationary contact 32 of the regulator is connected with one terminal of the resistance 34 and a movable contact 33 is connected to the other terminal of the resistance. It the armature current of the machine 21 decreases below the desiredpredetermined value, the pull of the spring 38 overpowers the pull of the solenoid 36 thus actuating-the movable contact 33 into engagement with the stationary contact 32 to short circuit the resistance 34 and thus increase the excitation of the machine 21 until its armature.

A core The operation of the regulator is briefly rent is maintained substantially constant at a value that is dependent upon the ratio of the time that the contacts 32 and 33 are open to the time they are closed.

Since the machine 21 operates as a motor when the reel H is the leading reel, it is necessary that the regulator shall be able to distinguish between the operations of motoring and generating. vided with an upper contact 4| whilst a relay 42 is provided for connecting the stationary contact 4! with the left-hand terminal of the resistance 34 when the stationary contact 32 is disconnected therefrom. The operation of the regulator with the machine 21 operating as a motor is exactly the reverse of that above described. For example, when the armature current increases above the predetermined value the pull of the solenoid 36 overpowers the pull of the spring 38 to close the contacts 33 and M thereby short circuiting the resistance 34 to increase the excitation of the motor and thereby decrease its armature current. As the armature current decreases below the predetermined value the spring 38 overpowers the. solenoid and separates the contacts 33 and ti thereby reinserting the resistance 3t in the field circuit of the machine 21 to produce an increase in the armature current.

Since the regulator 3| is in all respects identical with the regulator 30 as to construction and operation, a detailed description of this device is unnecessary.

The arrangement thus far described will only give complete satisfaction within a rather narrow range of operation, e. g., when the speed range of the mill obtainable by field control of the motor it is rather narrow, when the build up or build down of the reel is limited and when the draft taken by the rolls is small. If, as previously assumed, the mill motor it has a 2 to 1 speed range by field control; the build up of the reel, i. a, ratio of maximum diameter to minimum diameter is also 2300 1 and in addition as much as 50% draft can be taken by the main rolls this calls for an 8 to 1 speed range of the dynamoelectric machines 27, 28 by means of field control. It is quite obvious that control of the speed of the machines 27, 28 by voltage of the generator it will be of no assistance because the generator voltage effects the speed of the mill motor Hi as well as the speed of the machines 21!, 2B. The three'factors specifically enumerated above entering into the speed range of the machines 2? and 28 are thus all in addition to any speed control that can be eifected by the voltage of the generator it.

With the present development of the art of steel rolling, reel motors of large capacity are involved. For example, reel motors as large as 800 H. P. have been employed. It would be very expensive, even if possible, to build machines of such large capacities for a speed range of 8 to 1 by means of field control.

in order to overcome this problem additional means are provided for introducing a voltage into the circuit of one or the other of the dyna- For this purpose the regulator is proof the machines 21, 28 is operating as a generator driven by the trailing reel. The connectionsare so controlled that the dynamo-electric machine driving the leading reel is connected directly across the terminals of the generator l6 whilst the dynamo-electric machine driven by the trailing reel has the voltage of the generator 43 interposed between its armature and that of the supply generator I6. The connections are further so controlled that the voltage of the generator 43 is additive with the voltage of the dynamo-electric machine driven by the trailing reel and subtractive with respect to the polarity of the supply generator, l6. :Thus the generator 43 may be termed a bucking generator since its voltage opposes the voltage of the generator it that is applied to the dynamo-electric machine driven by the trailing reel.

It will be clear that if a 50% draft is 'being taken by the mill rolls H the peripheral speed of the strip on the entering side of the rolls will be one-half of the speed on the leaving side of the roll, and this requires that the dynamo electric machine driven by the trailingx-reel' must operate at one-half the speed of the dynamoelectric machine driving the leading reel, neglecting the diflerence in speeds of these two machines due to build up and build down of the reels which can be taken care of by varying the excitation of these machines under the control of neglecting build up or build'down of the reels,

the dynamo-electric machine driven by the trailing reel must operate at one-half the speed of the dynamo-electric machine driving the leading reel. If a voltage is introduced into the circult of the dynamo-electric machine driven by the trailing reel opposing the voltage of the supply generator it and of such magnitude that the terminal voltage of the trailing reel machine is one-half the voltage of the leading reel machine,

the trailing reel machine will operate satisfactorily at half the speed of the leading reel machine.

If the voltage of the generator i6 is increased to increase the speed of the mill, it will be clear that this voltage ratio of the trailing and leading reel machine must be maintained and in order to do this it is necessary to vary the voltage of the bucking generator in accordance with variations in the voltage of the supply generator it. This may be accomplished in several ways. In the embodiment'illustrated in Fig. 1 the bucking generator Q3 is provided with a main field winding 418 supplied from an exciter 50 which in turn is driven by the main m ll motor Hi, the speed of which is proportional to the voltage of the supply generator l6. However, when the speed of the mill is changed by weakening the field of the mill motor i l the voltage applied to the dynamoelectric machines 2?, 28 does not'change and consequently if the voltage of the bucking generator 33, were allowed to increase when the speed of the mill motor M is increased by field weakening the ratio of the terminal voltages of the leading and trailing reel dynamo-electric machine would be altered. Obviously, the voltage of the bucking generator does tend to increase when the speed of the mill motor is increased by field weakening because its exciter is driven by the mill motor I4. In order to offset this tendency, a rheostat 5! is included in the field circuit of the exciter 50 and the movable'contact of this rheostat is rotated by the common control device 25 and is so arranged that when the rheostat 26 is operated to increase the speed of the motor i4 the rheostat 5i is operated to decrease the excitation of the exciter 50 so that its voltage and the voltage of the bucking generator remain constant irrespective of changes in the speed of the mill motor due to field weakening.

For any given voltage of the supply generator IS the voltage of the bucking generator 43 should be proportional to the draft taken by the main roll l2. For this 'purpose a draft compensating rheostat 52 is included in the circuit of the main field winding 48 of the bucking generator. As indicated in the drawings, this rheostat is provided with a calibrated scale to enable the operator to set the rheostat in accordance with the draft to be taken by the main mill rolls. Thus it will be seen that the main field of the bucking generator is energized in such a manner that its voltage is proportional to two factors: the

voltage of the main generator l6 and the percent draft taken on the strip by the main rolls. If

desired, the main field 48 of the bucking generator might be excited directly from the terminals of the supply generator l6. It is also possible, and sometimes desirable to, excite the field 48 of the bucking generator from the terminals of the supply generator it, through the rheostat 52.

The bucking generator 43 is further equipped with a relatively small auxiliary field winding 53 energized from the available constant potential source 2|. Thepurpose of this auxiliary field winding is to cause the bucking generator to generate a low voltage and thus to maintain a circulating current through the trailing reel dynamo-electric machine when the mill is brought to standstill by reducing the voltage of the supply generator IE to zero. When the mill is i brought to standstill neither the supply generator l6 nor the pilot exciter 50 generates any voltage and thus the addition of the auxiliary field 53 to the bucking generator causes the latter to generate a voltage sufilcient to energize the trailing reel dynamo-electric machine to maintain stalled back tension between the rolls and the trailing reel. This is desirable since it prevents loss of tension when the mill is stopped.

The operation is as follows: It is assumed that the driving motor l1 has been started and is driving the supply generator i6 and the bucking generator 43 at the proper speed. It is further assumed that the reel l l is full and the reel I3 is empty and that the strip II is to be passed through the rolls 12 in the direction of the arrow. With the aid of the calibrated scale the operator sets the rheostat 52 in accordance with the draft that is to be taken by the mill rolls 12. The switch 54 is then operated to its left-hand position into engagement with the stationary contact member 5411. This completes an energizing circuit for the operating coil of the relay 44 extending from the positive side of the supply source (preferably the same as 2|) through the operating coil of the relay to the negative side of the supply source. Relay 44 in responding to its energization closes its contacts to connect the stationary contacts 32 of the regulator to the left-hand terminal of the resistance 34. As previously explained, this connection provides for regulating the generator action of the machine 21. At the same time an energizing circuit is established for the operating coil of relay 55 which causes the latterto close its contact to connect the upper contact 56 of the regulator 3| to the right-hand terminal of the resistance 51 thereby providing for regulating the motoring operation of the machine 28. Energizing circuits are also established for the operating coils of contactors 45 and 41. These energizing circuits are obvious and require no tracing; Contactors 45 and 41 close in response to their energization and connect the dynamo-electric machine 28 across the terminals of the supply'generator I5 and also connect the armature of the machine 21 across the terminals of the supply generator IS with the armature of the bucking generator 43 interposed in the circuit. The circuit for the machine 28 is traced from the right-hand terminal of the supply generator It by conductors 60 and GI through the armature of machine 28, contactor 41 in the closed position thereof and thence by conductors 62 and 63 to the left-hand'terminal of the generator It. The circuit for the machine 21 extends from conductor 60, through the ar mature of machine 21, conductor 64, lower contact of contactor 45, conductors 65, armature of bucking generator 43, and thence by conductors 66 and the upper contacts of contactor 45, to the conductor 63. The closing of switch 54 also establishes an energizing circuit for the operating coil of field contactor 22, thereby connecting the field 20 of supply generator l6 and auxiliary field 53 of bucking generator 43 to the supply line 2|. The. arm 24.; of the rheostat 24 is in its lowermost position so that all the resistance of the rheostat is in circuit with the field of the generator l5. Consequently the voltage of the generator 16 is substantiallyzero and the mill is at rest. To accelerate the mill the operator turns the handwheel 25 in such a direction that the contact arm. of rheostat 24 is rotated in a clockwise direction to decrease the resistance in the field circuit of the supply generator and thereby increase its voltage. the supply generator increases the mill motor I 4 begins to rotate and drives the mill rolls l2 and likewise the dynamo-electric machine 28 operates as a motor driving the leading reel l3 to take up the strip as it emerges from the rolls. As the strip I0 is drawn from the unwinding reel II by the action of the rolls, the reel ll drives the machine 21 causing it to operate as a generator and returns energy to the system thereby maintaining a back tension between the rolls and the unwinding reel.

The mill motor l4 drives the exciter 55' so that As the voltage of the latter generates a voltage and energizes the 21 is proportional to the ratio of the speeds at which the reels l3 and II are, required to operate due to the draft taken by the rolls [2.

As the operator continues to rotate the hand Due to the setwheel 25 the voltage of the generator 18 and the speeds of the mill motor and dynamo- electric machines 21, 28 all increase proportionately. Likewise the ratio between the terminal voltages of the dynamo- electric machines 21, 28 is maintained constant-since the voltage of the bucking generator 43 also increases proportionately as the voltage of the generator 16 is increased; this being due to the fact that the bucking generator 43 is excited by means of an exciter driven by the mill motor I4.

After all of the rheostat 24 has been short circuited. Further turning of the hand-wheel 25 gradually inserts resistance 28 in the field of the mill motor 14 to increase its speed still further. The speeds of the dynamo- electric machines 21, 28 increase due to the action of the regulators 30 and 3! in maintaining the current in these machines constant. When the speed of the mill is increased by weakening the field of the mill motor Hi the speed of the exciter 50 is likewise increased. This, of course, tends to increase the voltage of the bucking generator 43 which, of course, would tend to disturb the ratio of the terminal voltages of the machines 21 and 28, since the voltage of the generator it has not I changed. However, as the resistance is gradually inserted in the field of the mill motor the resistance is gradually inserted in the field of the exciter 50 thereby compensatingfor its increase in speed so that the voltage of the bucking generator 43 remains constant and the ratio of the terminal voltages of the machines 21 and 28 remains undisturbed.

During all this time the regulators 30 and 3i function in the previously 'described'manner to maintain the current in the machines 27, 28 substantially constant and thus to maintain the tension in the strips between the rolls l2 and the reels II and I3 substantially constant.

If the bucking generator were not used, and the voltages at terminals of machines 21 and 28 were equal, although the strip speeds at both reels were different due to the draft taken by the main rolls |2,it would be. necessary to have .the field -of the trailing machine suitably strengthened to compensate for too high a termi nal voltage. As noted previously, this field strengthening would mean an additional and expensive fieldcontrol range of the machines 21 and 28; but even if this speed range were feasible and available, thecurrent input to the machine 21 would not be proportional to the strip tension, which is the case when the relative excitation strength of machines 21 and 28 are adjusted only in proportion to the momentary values of the diameters of the" reels II and, I2.- This is another important advantage of the bucking and opened and the previously opened relays and contactors 42, 61, 44, 45, and '23 are obviously closed. The closing of relays 42 and 61 reverses the operation of the regulators 30 and-3l so that the regulator'30 now functions to regulate the motoring action of the machine 21 whilst the regulator 3| functions to regulate the generating action of the machine 28. The closing of contactor 44 connects the dynamo-electric machine 21 directly across the terminals of the supply generator I6 whereas the closing of the contactor 46 connects the machine 28 across the terminals of the supply generator It with the armature of the bucking generator 43 interposed in circuit so that its voltage opposes the voltage of the genorator l6. Contactor 23 in closing reverses the polarity of the field of the generator l6 thereby reversing the polarity of its voltage and the direction of rotation of the mill motor l4. The remainder of the operation in the reverse direction is similar with that previously described and need not be repeated.

The modification shown in Fig. 2 is in most respects similar with the system of Fig. 1. This modification is particularly applicable when an extremely wide range'of mill speed, reel build up, and draft is required. For example, consider an equipment in which the main mill motor has a speed range of 3 to 1 by field control, the reel build up is 2 to 1 and the maximum draft is 50%, This requires a total speed range of the reel for any given voltage of the main supply generator of 15 to 1. Even if a bucking generator is provided to take care of the draft compensation, the remaining speed range of the reel motors would still be 7% to 1. In order to, provide the necessary'speed range, the reel dynamo electric machines 10 and H are supplied by means of a separate generator 12 which may be driven by ably driven by the same driving motor 13 as that which drives the bucking generator 14 and the A generator I5 which supplies the main mill motor 16. Generator I2 is provided with a field winding 11 which is separately excited from the supply source 18. A rheostat 80 is included in the field circuit of the generator 12 and a movable contact arm of this rheostat is mechanically connected to the movable contact arms of the rheostats 8| and 82 so that the voltage of the generator 12 will be increased in proportion to the speed of the mill motor 16, irrespectively of whether this increased mill speed is accomplished by increasing the voltage of the generator 15 or weakening the field of the mill motor 16.

The bucking generator 14 is" connected in the circuit of the trailing reel dynamo-electric machine by means of the contactors 83, 84 and the voltage of the bucking generator is so controlled that it is proportional to the mill speed and to the draft taken by the mill rolls 85. With this arrangement the speed range of the dynamo-electric machines 10 and H which must be eflected by means of' field control is limited to the amount made necessary by reel build up or build down only, which in the example assumed is 2%; to 1, since the remainder of the total required speed range of 15 to 1 is taken care of by varying the voltage of the generator 12 from which the machines 10 and II are supplied. As in the system of Fig. 1 the bucking generator 14 is again pro- 'vided with an auxiliary field winding 88 for thepurpose of maintaining the standstill stalled back tension in the strip 88 between the mill rolls and the trailing reel.. i

In order that the voltage of the bucking gen-a erator 14 shall be automatically controlled in accordance with the mill speed and the percent= draft of the strip taken by the main roll two - pilot exciters 90 and 9| are provided. As shown, these two exciters are respectively connected to the idler rolls or so-called cooling drums 93 and 94 situated on opposite sides of the mill rolls 85. The strip 88 passes over these cooling drums and quite obviously the drum on the leading side rotates faster than the drum on the trailing side because the peripheral speed of the strip on the leading side is greater than the peripheral speed of the strip on the trailing side. In other words, the ratio of speeds of the drums 9 3 and 94 is a measure of the draft at any speed of the strip. These two pilot exciters 90 and 9| are preferably small, separately excited, direct current machines with their armatures connected in series relationship and diiferentially with respect to each other so that the difference of their voltages as measured at the conductors 95, 96 is a measure both of the mill speed and the percent draft of the strip. This differential voltage is utilized to energize the main field winding 91 of the bucking generator. It is interesting to note thatin this arrangement the direction of current through the field winding 9! does not change when the direction of the mill is reversed and since the auxiliary field 06 and the main field 9'! always act cumulatively it is unnecessary to reverse the auxiliary field as is done in the system of Fig. 1. Constant current regulators 98 and I00 are respectively connected with the dynamo-electric machines I0 and II. These regulators are in all respects identical with the regulators 30 and 3| illustrated and described in detail in connection with the system of Fig. 1. For this reason the details of these regulators are omitted from Fig. -2 for the purpose of simplification and for avoiding repetition.

The operation of the modification of Fig. 2 is similar to the operation of the system of Fig. 1 previously described with the exception that when the speed of the mill is increased by weakening the field of the mill motor 76 the voltages applied to the reel dynamo-electric machines I0 and II are likewise increased thereby limiting the speed range of the machines I0 and II that must be accomplished by field weakening under the control of regulators 98 and I00 to the range made necessary by build up or build down of the reels only. This increase in the voltage applied to the machines I0 and II in that part of the mill speed range eifected by weakening the field of the mill motor I6 is due to the fact that the contact arm of rheostat is mechanically connected to the contact arm of 111805081182 and these contact arms are so arranged with respect to the resistance sections which they control that when the rheostat 82 is weakening the field of the mill motor I6 to change its speed the rheostat, 00 is strengthening the field of the generator I2 to increase its voltage and consequently the voltages applied to the machines I0 and H. The remainder of the operation of this modification is the same as the operation of Fig. 1 and repetition of the description of the operation is unnecessary since it will readily be understood from the detailed description of the operation of the system of Fig. 1.

The modification shown in Fig. 2 has the very decided advantage that the voltage of the bucking generator I4 is adjusted automatically for any percent draft taken by the main rolls thus making it unnecessary for the operator to estimate the draft and to preset a rheostat as in the system of Fig. 1.

The arrangement in the modified system of Fig. 3 is very similar to the system of Fig. 2 and diflers from the system of Fig. 2 only in that the pair of pilot generators I 0| and I02 are me chanically driven by the reels I03 and I04. 'In certain mill installations there either are no cooling drums at all or if there are cooling drums such, for example, as drums I05 and I08, the friction between the strip I01 and these drums is not sufficient to provide a positive drive for the two pilot generators IOI and I02.

In order to overcome this difficulty in the case in which it is desired to use two pilot generators on opposite sides of the mill rolls I08, these two pilot generators are mechanically connected to the shafts of the reels I03 and I04. Due to build up and build down of the reels the speeds of the reels I03 and I04 are not measures of the linear speeds of the strip on oppositesides of the roll and obviously, therefore, the generated voltages of the pilot generators IN and I02 driven by these reels will not, in the absence of special provision, be measures of the linear speeds of the strips on opposite sides of the mill rolls. Thus it is necessary to take into account thel change of the active reel diameter,- and for this purpose field control rheostats H0 and III are respectively connected in the field circuit of the pilot I I03) decreases the roller H2 at the extremity of the rheostat arm will move nearer the center of the reel; the rheostat arm will therefore be moved in a counterclockwise direction and the field. of the pilot generator IOI will be suitably weakened in the same proportion as the diameter of the reel has been reduced so that the voltage of the pilot generator IOI will be a tolerable measure of the linear speed of the strip as it leaves the trailing reel.

At the same time the diameter of the leading reel I04 is building up and-the field of the pilot generator I02 is gradually strengthened through the action of the roller II3 on the rheostat III so that the voltage of pilot generator I02 will represent the linear speed of the strip on the leading side of the mill.

Thus with the relationship of the voltages of the two pilot generators IN and I02 to the peripheral speed of the strips on opposite sides of the mill rolls firmly established, it will readily be understood that these two pilot generators can be connected differentially and used to energize the main field windings of the bucking generator in the same manner as that already'described in connection with the system of Fig. 2.

The remaining parts of the arrangement is exactly thesame as that shown in Fig. 2 and the operation is also identical and consequently requires no repetition.

Although in accordance with the provisions of the patent statutes this invention is described What we claim as new and desire to secure by Letters Patent of the United States, is:

1. A control system for apparatus operating For example,

on a strip of material comprising a pair of dynamo-electric machines mechanically connected to the strip, an element for reducing the thickerator, means controlled by the amount of re--.

'the reduction in the thickness of said material efiected by said element for varying the excitation of said generatorin accordance with the amount of said reduction.

2. A control system for apparatus having an element for reducing the thickness of a length of material comprising a pair of dynamo-electric machines mechanically connected to said length of material, a variable voltage generator for supplying said machines, a second generator connected between said supply generator and one of said machines, and means responsive to the reduction of said material eiiected by said element for controlling the voltage of said second generator in proportion to the amount of said reduction.

3. A control system for apparatus having an element for reducing the gauge of alength of material comprising a pair of dynamo electric machines mechanically connected to said length of material, a supply source for said machines, control means providing operation of one of said machines as a generator and the other of said machines as a motor, means for introducing a voltage opposing the voltage of said source in the circuit of said machine operating as a geneach mechanically connected to said length of" material, control means providing operation of.-

one of said dynamo electric machines as a generator and the other of said machines as a motor,

' an auxiliary generator for introducing an opposfected by said element for controlling the voltage,

of said auxiliary generator in accordance with the amount of reduction of said strip.

5. A motor control system comprising a motor for driving a load, a pair of dynamo-electric machines mechanically connected to said load, a variable voltage generator for supplying said motor and dynamo-electricmachines, a second generator arranged in the connections between said supply'generator and one of said machines so that its voltage opposes the voltage of said supply generator, and an exciter driven by said motor for exciting said second generator so that its voltage is proportional to the voltage of said variable voltage generator. I l

6. A control system for reversible strip rolling mills and the likein which a reduction in the gauge of the strip is effected by mill rolls situated between winding and unwinding reels comprising incombination, a pair of dynamo-electric machines one connected to each of 'said-reels, a variable voltage generator for supplying said machines, control means providing operation of the unwinding reel machine as a'generator and the winding reel machine as a motor and for reversing the operation of said machines with reversal in direction of the strip, and means for extending the operating range of said machines beyond that obtainable with field control comprising a second generator connected in the circuit of the dynamo-electric machine operating as a generator, means for varying the voltage of said second generator in accordance with the difierence in the speeds of said dynamo-electric machines due to said reduction in gauge.

7. A control system for a strip rolling mill having a pair of mill rolls and winding and unwinding reels comprising a reversible motor for driving said rolls, a pair of dynamo-electric machines conne'cted'to said reels, a variable voltage generator for supplying said motor and said dynamo-electric machines, control means providing for operation of the machine connected to said unwinding reel as a generator, and means for extending the operating range of saiddynam-electric machines beyond that obtainable with field control comprising asecond generator connected in the circuit of the dynamo-electric machine operating as a generator so that its voltage adds to the voltage of said dynamo-electric machine, and means for controlling the excitation of said second generator so that its voltage is proportional to the voltage o'f said supply generator and to the difierence in the speeds of said dyna- "mo-electric machines due to the reductionin the gauge of said strip efiected by said rolls.

8. A control system for apparatus having'an element operating on a strip of material and a pair of reels for said strip comprising a pair of dynamo-electric machines, one connected to each of said reels, a generator connected in circuit between one of said machines and its source and means responsive to the speeds on opposite sides of said element for controlling the voltage of said generator.

" 9. A control system for apparatus having an element operating on a strip of material and a pair of reels for said strip comprising a pair of dynamo-electric machines one connected to each of said reels, a source of supply for said machines, a second generator connected in the circuit between one of said machines and said source, and a pair of generators driven by the strip on .opposite sides of said element for exciting said second generator in accordance with the speed on opposite sides of said strip.

10. A control system for reversible rolling mill apparatus and the like having a pair of reels serving alternately as winding and unwinding I reels and a pair of rolls for efiecting a reduction in the gauge of said strip comprising in combination a pair of dynamo-electric machines connected to said reels, a reversible motor for driving said rolls, variable voltage generating means for supplying said motor and said machines, control means for effecting operation bf the trailing reel machine as a generator and the leading reel machine as a motor and for reversing the operation of said machines with reversal in the directionof the strip, and means for extending the operating range of said dynamo-electric machines beyond that obtainable with field'control comprising a generator having its armature connected in the circuit of the machine operating as a generator sothat its voltage opposes the voltage of said generating means and a pair of generators driven by the strip on opposite sides of said rolls for exciting said generator so that its voltage is proportional to the difl'erence in the strip speeds on opposite sides of said element.

11. A control system for reversible strip mills and the like having a pair of reels and a pair of mill rolls for effecting a reduction in the gauge of the strip between said reels comprising a motor for driving said rolls, a pair of dynamo-electric machines respectively connected to said reels, control means providing for operation of the machine connected to the trailing reel as a generator and operation of the machine connected to the leading reel as a motor and for reversing the operation of said machines with reversal of the travel of the strip, a variable voltage supply generator for said motor, a second variable, voltage supply generator for said dynamo-electric machines, and means for extending the operating range of said dynamo-electric machines comprising a bucking generator connected in the circuit of the trailing reel dynamo-electric machine so as to oppose the voltage of said second supply generator and a pair of auxiliary generators arranged on opposite sides of said rolls and driven by said strip for exciting said bucking generator so that its voltage is proportional to the difference in strip speeds on opposite sides of said rolls due to the reduction in gauge of said strip.

FRANCIS MOHLER. LEONID A. UMANSKY.

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