US2143357A - Motor control system - Google Patents

Description

Jan. 10, 1939. T B. MONTGOMERY 2,143,357

MOTOR CONTROL S YSTEM Filed Nov. 26, 1934 Patented Jan. 10, 1939 UNITED STATES PATENT OFFICE MOTOR CONTROL SYSTEM Application November 26, 1934, Serial No. 754,769

1'7 Claims.

This invention relates in general to control systems and particularly to systems for con- .trolling the dynamo electric machines used for tensioning materials being wound upon and un- 5 wound from reels or similar winding and unwinding mechanisms.

In systems of the above type, it is desirable to maintain a constant predetermined tension on the reeled material. Dynamo electric machines have been mechanically coupled to the reels and utilized to tension the material being reeled by energizing the machines so that one runs as a motor tending to wind the material on the winding reel and the other runs as a generator supplying a torque tending to prevent unwinding of the unwinding reel. Due to the change in effective reel diameters during winding and unwinding of the material, the tensioning torque supplied by the dynamo electric machines must also 20 change in proportion in order to maintain the tension on the material constant.

Various attempts to provide suitable tensioning systems have utilized mechanical mechanisms actuated by the slack or tension in the material to control the dynamo electric machines. Such mechanisms have been found to be either so delicate as to easily get out of order or if made sufiiciently rugged are not sufllciently accurate and stable to maintain the tension at the desired constant value. When devices such as rollers in contact with the material are used, these devices are physically in the way and are also undesirable due to the fact that they leave a mark on the strip of material. If the back tension is supplied by hydraulic brakes, the tension will only be constant if the reel diameter remains constant, however, it is difficult to secure constant friction with such mechanical devices. Furthermore, the energy required to produce this back tension is lost in friction.

To maintain a given setting of back tension automatically throughout a pass of the material is very desirable as it reduces the amount of supervision necessary. The ease of control and. smoothness in operation which can be obtained make the electrical method of tensioning especially adaptable and in addition a substantial power saving is efiected. A further advantage of the electrical power tension control is that constant tension may be maintained both at standstill and throughout any pass in either direction.

It is, therefore, an object of this invention to provide an improved method of and means for electrically and automatically maintaining a constant tension on material being reeled and unreeled.

It is also an object of the present invention to provide a tensioning system of the above type wherein the direction of the material is easily 5 reversed and tension is automatically maintained after such reversal.

It is also an object of this invention to provide a tensioning system of the above type wherein the same means for maintaining tension while 10 the material is being reeled and unreeled is utilized for maintaining tension while the material is stopped.

It is also an object of this invention to provide a tensioning system wherein a constant tension is maintained regardless of changes in lineal speed of the material.

It is also an object of the present invention to provide in a tensioning system of the type wherein dynamo electric machines are coupled to reels and energized to tension the material being reeled, a means responsive to breakage of the material for deenergizing the tensioning system and quickly stopping said machines.

It is a further object of the present invention to provide a strip rolling mill having a tensioning system of the above type with an easy and convenient means for utilizing the tensioning dynamo electric machines in setting up the material in the mill.

These and other objects and advantages are attained by this invention, various novel features of which will be apparent from the description and drawing herein and will be more particularly pointed out in the claims.

An illustrated example of an application of this invention is shown in the accompanying drawing in which the single figure is a diagrammatic showing of a control system embodying one form of the invention as applied to a strip rolling mill.

Referring to the drawing, a strip of material i2 is shown as being unwound from a reel l3, acted upon by rolls l0 and l l and then wound upon reel M. The strip I2 is pulled through the rolls by driving the forward reel, which is shown as M, by connecting it to the drive shaft of the main drive motor 1 by a clutch 29. The motor I is energized from a generator 6 driven by a synchronous motor 5, the control for the drive being 2. Ward Leonard control of the fields 20 and 9 of the generator and motor. Also driven by the motor 5 is an auxiliary generator 8 which supplies armature current to the back tension circuit.

Connected to the reels I3 and II respectively are the tensioning dynamo electric machines I and 2, the armatures of which are connected in series across the auxiliary generator 8. The field windings of the machines I and 2 are so connected that the machines tend to run in opposite directions, the machine 2 running as a motor tending to pull the strip l2 in the forward or arrow direction and the machine I tending to pull the strip in the opposite direction, thus supplying a torque to the reel l3 providing the desired back tension. The machine I has a main field winding l6 which is separately excited from a small exciter 3 and a compensating field winding l5. Field winding is of machine 2 is similarly excited from the exciter 4. The control of the excitation of machines I and 2 to maintain a constant tension is effected by controlling the energization of field windings 22 and 23 of the exciters 3 and 4 respectively. This is an advantage in that the currents involved are much smaller than they would be if the machines I and 2 were controlled directly.

In order to maintain a constant back tension, it is necessary to vary the excitation of the tensioning machines in direct proportion to the change in reel diameter while holding the armature current constant through the tensioning machines. The armature current is held constant bya regulator generally designated as 24 which is responsive to the current in the tension circuit-and acts upon the field excitation'of the leading machine. The flux of the trailing machine is varied in proportion to the change in reel diameter by a voltage balance regulator generally designated as 26.

The regulator 26 is controlled by balancing the generated voltage of the trailing machine against the voltage of a small generator 30 which is operated so that its voltage is a measure of the linealspeed of the strip between the trailing reel and the rolls ill. The generated voltage of the trailing machine is measured by adding to the voltage across the brushes, the voltage across one-half of the resistor 38 connected across the terminals of the auxiliary generator 8. The generator 8 supplies the losses in the machines I and 2 and the voltage of the generator 8 is a measure of the IR drop in the circuit. Neglecting the resistance of the leads, the IR drop in the circuit is substantially twice the IR drop in each machine and these voltage drops are equal. Hence the voltage across one-half of the resistor 33 is a measure of the 1R drop in one machine. As the terminal voltage of the trailing machine is the actual generated voltage less the IR drop through the machine, the addition of the voltage of one-half of the resistor 38 to the terminal voltage of the trailing machine I gives an accurate measure of the generated voltage of machine I. Since the actual generated voltage is directly proportional to the field flux, an accurate method of controlling the fiux is obtained, for the regulator 26 operates to raise or lower the field current of exciter 3 thereby changing the excitation of machine l proportionately to the change in lineal strip speed which is a measure of the change in reel diameter.

The control of the main drive motor I is effected by controlling the field 20 of the generator 6 from zero to full generator voltage and for higher speeds by weakening the field 9 of the motor I. This control is effected by a rheostat generally designated as 35 having a generator field resistor 39 and a motor field resistor 40.

A single operating member 41 controls both generator and motor fields and simultaneously controls a resistor 31 for a purpose that will be explained later. When the operating member I 41 is at the left as viewed in the drawing, the

mill is at its minimum speed, that is, the current in motor field 9 is a maximum and the current in the generator field 20 is a minimum. When the member 41 is in the position shown, full generator voltage is applied and the. motor field 9 is weakened. Further movement of the member 41 to the right further increases the mill speed.

An approximately constant torque, that is, a constant pulling tension is required to pull the strip through the mill. Therefore, at the beginning of any pass the main drive motor is connected to the empty leading reel and tostart the mill the voltage of the generator 6 is raised by the rheostat 35 and the current in the motor I is increased until a torque sumcient to start the strip is obtained. For any further increase in the voltage of the generator 6 the current in the motor 1 remains substantially constant for a given reel diameter as the motor field strength is constant up to full generator voltage. With this further increase in generator voltage after the strip starts, since the motor field strengthremains constant, the horsepower input is increased in direct ratio to the voltage and the speed of the motor increases in direct proportion. The drive is therefore a constant torque drive from zero speed to a given speed corresponding to generator voltage.

Above said given speed the field current 0 the motor I is weakened and since the mill requires substantially constant torque to provide a constant pulling tension on the strip, the motor armature current increases approximately in proportion to the decrease in field fiux and since the voltage of generator 6 is now constant the horsepower and consequently the speed of the motor increase in direct proportion to the increase in armature current. At any speed of the motor I (1) Horsepower=T R. P. M. K, and

(2) T=KI where T=torque at one foot radius, I=the armature current in amperes, =field flux and K=a constant,

From zero speed to the speed corresponding to full generator voltage, I and remain constant. From this given speed to full speed, I increases but 5 decreases a like amount so that their product remains constant. These conditions obtain only for the instant that the mill is started and brought up to speed, that is, these conditions are correct only for one given reel diameter.

Considering actual conditions when the mill is started the power is applied to the empty leading reel which has no strip on it, but as the strip is pulled through the mill it winds up on the leading reel causing an increase in the efiective reel diameter of reel l4. With this increase in reel diameter the torque of the motor I must increase proportionally in order to effect the same pull on the strip, thus where P is the necessary forward pull on the strip, T is the motor torque at one foot radius and D is the reel diameter in feet.

Since P is constant, the motor torque must increase in direct proportion to the reel diameter and from zero to a given speed (full generator voltage), and since the motor field flux is constant, the armature current of the motor I will increase in direct proportion to increase in reel diameter. Above this given speed the motor armature current increases in proportion to the decrease in motor field strength as well as increase in reel diameter.

or the motor armature current increases directly as the reel diameter and inversely as the motor field strength.

After the strip is started through the rolls at any given speed setting, as the diameter of the leading reel increases, the motor armature current will increase proportionally to supply the necessary torque. Therefore, the strip speed will increase in proportion to the increase in effective reel diameter, the motor speed remaining the same.

It will be seen from Equation 4 that a constant back tension pull P will be exerted by the trailing back tension machine if the armature current I is held constant and the field flux 41 is changed in proportion to the change in effective reel diameter D.

To facilitate an understanding of the back tension system, it is first considered for the hypothetical case of constant reel diameter, and with the mill at standstill, the auxiliary generator 8 will have a constant excitation and will generate a given voltage and current in the back tension circuit. This current causes the machines I and 2 to exert a given torque and tension on the strip I2. Assuming constant and equal field excitations on the machines I and 2 for the hypothetical case of constant reel diameter, if the reel is started in the forward direction, machine I will generate a voltage which will add to the voltage of the generator 8 and machine 2 will generate a counter-E. M. F. which will oppose these voltages. The voltages of machine I and machine 2 will be directly proportional to the speed and therefore at any mill speed the counter- E. M. F. of machine 2 will for practical purposes counterbalance the generated E. M. F. of machine I and a constant current will be maintained in the back tension circuit, but the voltage will be proportional to the speed.

Under actual conditions of mill operation the conditions given above for constant reel diameter will be altered because as the strip unwinds from the trailing reel, the decreased efiective diameter raises the speed of machine I. This would cause an increase in voltage and current in the back tension circuit and a consequent increase in torque whereas according to Equation 4 the torque should be reduced in proportion to the increase in reel diameter.

To obtain the correct torque and diameter relationship the constant current regulator 24 which is responsive to the current in the back tension circuit operates on the field current of the leading machine 2. If the current tends to increase above the predetermined setting of this regulator, it acts to increase the excitation and therefore the counter-E. M. F. of the leading machine 2, which will reduce the current. Similarly, if the current falls below the regulator setting, the excitation of machine 2 will be decreased to cause an increase in the current in the circuit. This maintains the current constant regardless of resistance change in the circuit due to any heating of the conductors.

The voltage balance regulator 26 has two control coils I3 and 32 of equal strength balanced against each other. The coil 83 is connected across the armature of the machine I and onehalf of the resistance 38 shunted aeross the terminals of the generator 8 through resistor 31 adjusted by the position of rheostat 35, and the manually adjusted resistor 45. This coil 33, as previously disclosed, measures the generated voltage of the trailing machine I. The coil 32 is connected through the resistance 31 of the rheostat 35 to the pilot generator 30 which is driven by the strip I2 and generates a voltage proportional to the lineal speed of the strip independently of the reduction due to rolls I0.

For any given speed setting of the main motor I, the speed thereof will remain constant and the motor armature current will adjust itself to give the required torque and on any given pass with constant speed of the motor I, the strip speed will increase in proportion to the increase in diameter of the forward reel I4. The strip speed is measured by the pilot generator 30 and the increase in voltage due to increase in strip speed operates on the voltage balance regulator 26 to raise the voltage of the trailing machine I to increase its horsepower input in proportion to the increase in speed of the strip. Also, this R. P. M.=

where K is a constant and D is the effective diameter of the trailing reel, but from Equait is seen that to maintain constant tension, must vary directly as D, I remaining constant. Therefore since the generated voltage of the trailing machine I will tend to increase as D and is a measure of D and the voltage of the pilot generator increases as D and is a measure of D, the voltage balance regulator 26 will decrease the field excitation of machine I in direct proportion to the reel diameter D because it will automatically maintain the generated voltage of the trailing machine I equal or proportional to the voltage of pilot generator 30, thus maintaining constant back tension throughout the pass.

H. P. (7) Torque (trailing motor) KD (8) P (back tension)- -KD To consider the operation of the system shown the strip I2 will beconsidered'to be wound on the reel I3, the reel I4 being empty. To enter the end of the strip I2 into a slot in the reel I4, inching is obtained in the following manner: inching push button SI is momentarily closed, closing a circuit for the inching contactor ICI through contact I5 of I02 and contact 65 of the low voltage relay LVI and the normally closed contact of the overload relay 0L3. Relay ICI is therefore energized only if the relays 1C2, LVI and 0L3 are deenergized. Closing of contact 61 connects a short circuit across machine 2. Closing of contact 66 energizes the relay BF to close the circuit to the auxiliary generator field I9, thus' energizing machine I. A back contact 69 looks out relay 1C2, thus preventing inching in the opposite direction. Contact 10 closes a circuit for the dynamic breaking contactor TCI which disconnects the dynamic breaking resistor 43 from across the terminals of machine I by opening contacts 8| and completes the armature circuit of machine I by closing contacts 80. Another contact II opens a short circuit around the main coil 56-of the timing relay B'IT allowing this relay to close immediately energizing field contactors PF3 and PF4, closing the circuit of the field 22 of the exciter 3. The machine I will rotate only as long as the push buttonjl is held closed.

The rheostat 2| controlling the field current of the auxiliary generator 8 is of the potentiometer type and when in midposition (as shown) no current flows through the field winding I9. When the rheostat is in the extreme left hand position (as viewed in the drawing), the voltage is a maximum in one direction and when in the extreme right hand position, the voltage is a maximum in the opposite direction. Therefore both the direction and speed of the inching is easily controlled.

When push button 5| is released contactors ICI and TCI will immediately open, the latter connecting the dynamic breaking resistance 43 to machine I, thereby causing it to come to rest under dynamic breaking. .When ICI opens, a short circuit is established by contact ll around the main coil 56 of the timing relay B'DI so that after a predetermined time delay, if no further inching is done, field contactor PF3 will be opened. This feature prevents full field current being applied to the'machine I continuously for a very long period which might cause overheating at standstill.

When the desired inching has been eflected and the strip is entered in the reel I4 the main motor control push button 55 will be closed energizing the main control. The back tension master switch 53 will be closed which energizes the relay LVI which closes its holding circuit through contact 62. Contact 65 of LVI locks out inching contactors ICI and 102, thereby preventing further inching. Contact 64 closes the circuit for the relay BF, thus energizing the field I9 of the generator 8. Contact 63 closes the circuit for the relay LVX which by closure of contacts and 99 energizes relays TCI and T02 to disconnect dynamic breaking resistors 43 and 44 from the machines I and 2, respectively, and to,

complete the armature circuits of these machines at contacts 89 and 18. short circuit around the main coil 56 of timing relay BTT allowing this relay to close and energize field contactors PF3 and PF4 which establish field current in the machines I and 2 by energizing fields 22 and 23 of exciters 3 and 4 through resistor 46 and contacts 85, 39 and 38, I34. These machines exert a torque which supplies tension to the strip at standstill.

A control lever on the mill is now operated to close the switch FWD and engage the main motor drive to the reel I4 by clutch 29. The closing of the switch FWD energizes the relay FT3-4 and also the relay VT3. The relay VT3 by contact 9| connects the coil 33 of the voltage balance regulator 26 across the machine I and one-half of the resistance 38 through the resistance 31 of Contact 66 opens-the the rheostat 33. By contact 92 of VT3. the coil 32 of voltage balance regulator 23 is connected across the terminals of the pilot generator 36 through the resistance 31 of the rheostat 35. Energization of relay FT3-4 opens contacts 34 and 85, thereby deenergizing the fields of exciters 3 and 4 as above stated and energizing field winding 22 of exciter" 3 through resistance 36 of the motor operated rheostat 25 and contact 82. Contact 83 closes the circuit for the field of exciter 4 through constant current regulator 24 and the contact of relay PF4.

As soon as the pass is started and the motors begin rotating, the constant current regulator 24 regulates the excitation of machine'2 and by controlling its counter-E. M. F. holds the current constant in the back tension armature loop circuit. The voltage balance regulator 26 operates to hold the voltage of the trailing machine I equal to the voltage of the pilot generator 30, thus as the speed of the pilot generator 30 increases (this machine has constant excitation) its voltage will increase in direct proportion. The voltage of the trailing machine I will increase accordingly, raising the horsepower to maintain a constant torque. However, as the trailing machine I increases in speed due to decrease in reel diameter of the reel I3, its speed will increase above the corresponding speed of generator 30 and its voltage will tend to increase above that of generator 30. The coil 33 of the regulator 26 will overbalance the coil 32 and close contacts 43, thereby energizing relay L.

Energization of relay L opens contact and closes contact 96 to connect the motor 21 of the motor operated rheostat 25 across the direct current bus bars I99 so that it will rotate in a direction to move the rheostat contacts to the left as viewed in the drawing. This will increase the amount of resistance 36 included in the field circuit, thereby decreasing the field current in the field 22 and decreasing the fiux in the machine I As this decrease in flux will be in direct proportion to the decrease in diameter of the reel I3 and as the armature current is maintained constant, it will be seen from Equation 4 that the back tension imparted to the strip by machine I will be held constant. The amount of tension is controlled by controlling resistor 45 in the circuit of coil 33.

'As the voltage of the back tension circuit varies directly as the speed of the main drive motor 1 varies, the coils 32 and 33 of the voltage balance regulator 26 must operate over a wide range of voltages from zero to a maximum which may be as high as 250 volts. It is not feasible to construct coils which will be sufficiently sensitive at low voltages and yet sufliciently rugged to withstand the maximum voltages without damage.

This problem is solved by the present invention in the following novel manner. The coils 32 and 33 are wound for a low voltage so as to give accurate regulation at low mill speeds. To prevent damage to the coils at high speeds and yet maintain accurateness of response, the resistance 31 of the rheostat 35 is connected in series with these coils. Thus as the rheostat 35 is operated to increase the mill speed and hence the voltages applied to coils 32 and 33, the amount of resistance 31 in series with the coils is proportionately increased. The voltage drop across the resistance 31 increases proportionally to the mill speed and hence the voltage applied to coils 32 and 33 remains substantially constant.

When the material has all passed through the rolls III the mill will be brought to a stop by control of the main motor. The back tension system will continue to exert its torque since current is still supplied by generator 8 and the excitation remains on the machines I and 2. To start the next pass the control lever is thrown to reverse position closing the switch REV and connecting the main drive to reel l3 by means of clutch 28. This deenergizes relays VT3 and FPS-4 and energizes relays VTI and Fri-2. The deenergization of relays VT3 and FT3-4 disconnects the connections as previously made and the energization of relay VTI connects the machine 2 and pilot generator 3| to the voltage balance regulator 26 through contacts 93 and 94 respectively. Energization of relay FII2 connects the fleld 22 of exciter 3 through the constant current regulator 24 through its contact 86 and through a contact 81 connects the field 23 of exciter 4 through the voltage balance regulator. Thus the machine I and reel l3 become the leading machine and reel and the machine 2 and reel l4 become the trailing machine and reel and a constant tension is maintained on the strip by themachine 2 in a manner similar to that described for the first pass.

The above description covers the normal operation of the mill. However, on occasion of strip breakage, the forward or leading machine will rotate. in the same direction as the main motor and the trailing machine will rotate in the opposite direction. It is, therefore, necessary to bring the back tension 'system to a standstill, as well as the main drive, as quickly as possible in order to prevent damage to the strip as well as to the equipment. Photoelectric relays 43 and 44 are so positioned relative to the strip l2 that the strip intercepts a beam of light necessary to energize these relays. Upon breakage of the strip or if for any other reason the beam of light is allowed to energize the relays 43 and 44, the contacts thereof are opened thereby deenergizing the relay PR which deenergizes relay LVI. This deenergizes the back tension system and causes the machines I and 2 to quickly come to a stop by means of dynamic braking. Further or alternative protection is obtained by means of friction relays 4| and 42. These relays are operated in dependence upon the direction of the strip, relay 4! being open as shown with the strip going in the forward or arrow direction. Upon breakage of the strip and rotation of the machine l in the reverse direction, relay 4| closes its contacts, closing a circuit to the relay SF which opens the circuit of LVI similarly deenergizing the back-tension system. The friction relay 42 operates similarly when the machine 2 is the trailing machine to energize the relay SR and deenergize the back tension system. A manual stop control is provided by switch 54 which deenergizes the holding circuit for relay LVI. Certain embodiments of the invention disclosed herein are claimed in my copending application Serial No. 208,539, filed May 18, 1938.

Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made on said reels, means for performing an operation on said strip while unwinding from one of said reels and winding upon the other of said reels, means for maintaining a constant tension on said strip during said operation comprising dynamo electric machines connected to each of said reels, said constant tension maintaining means also comprising means responsive-to the lineal speed of said strip for regulating the power supplied to one of said dynamo electric machines.

2. In combination, a pair of spaced reels adapted to be-joined by a strip of material wound on said reels, means for performing an operation on said strip while unwinding from one of said reels and winding upon the other of said reels, means for maintaining a constant tension on said strip during said operation comprising dynamo electric machines connected to each of said reels, said-constant tension maintaining means also comprising means responsive to the generated voltage of said dynamo electric machine connected to said unwinding reel for regu-. lating the power supplied to said last said dynamo electric machine.

3. In combination, a pair of spaced reels adapted to be joined by a strip of material wound on said reels, means for performing an operation on said strip while unwinding from one of said reels and winding upon the other of said reels, means for maintaining a constant tension on said strip during said operation comprising dynamo electric machines connected to each of said reels, said constant tension maintaining means also comprising means responsive to the armature current of one ofsaid dynamo electric machines for regulating the power supplied to the other of said dynamo electric machines.

4. In combination, a pair of spaced reels adapted tobe joined by astrip of material wound on said reels, means for performing an operation on said strip while unwinding from one of said reels and winding upon the other of said reels, means for maintaining a constant tension on said strip during said operation comprising dynamo electric machines connected to each of said reels, said constant tension maintaining means also comprising means responsive to the lineal speed of said strip, and to the generated voltage of said dynamo electric machine connected to said unwinding reel for regulating the power supplied .to said last said dynamo electric machine.

5. In combination, a pair of spaced reels adapted to be joined'by a strip of material wound on said reels, means for performing an operation on said strip while unwinding from one of said reels and winding upon the other of said reels, means for maintaining a constant tension on said strip during said operation comprising dynamo electric machines connected to each of said reels, said constant tension maintaining means also comprising means responsive to the generated voltage of said dynamo electric machine connected to said unwinding reel, and to the armature current of one of said dynamo electric machines for regulating the power supplied to the other of said dynamo electric machines.

6. In combination, a pair of spaced reels adapted to be joined by a strip of material wound on said reels, means for performing an operation on said strip while unwinding from one of said reels and winding upon the'other of said reels, means for maintaining a constant tension on said strip during said operation comprising dynamo electric machines connected to each of said reels, said constant tension maintaining means also comprising means responsive to the lineal speed of said strip, to the generated voltage of said dynamo electric machine connected to said unwinding reel, and to the armature current of one of said dynamo electric machines for regulating the power supplied to the other of said dynamo electric machines.

7. In combination, a pair of spaced reels adapted to be joined by a strip of material wound on said reels, means for performing an operation on said strip while unwinding from one of said reels and winding upon the other of said reels, 9. first dynamo electric machine connected to said winding reel, a second dynamo electric machine connected to said unwinding reel, means for supplying curr ent to the armatures of said dynamo electric machines, means operative on said first dynamo electric machine for maintaining said current constant, means operative on said second dynamo electric machine for regulating the field current thereof in accordance with the efiective diameter of said unwinding reel, means for reversing the direction of said strip of material through said operation performing means, means operative in response to actuation of said strip direction reversing means for shifting said constant current control means so as to be operative on said second dynamo electric machine and means for shifting said field current regulating means so as to be operative on said first dynamo electric means.

8. In combination, a pair of spaced reels adapted to be joined by a strip of material wound on said reels, means for performing an operation on said strip while unwinding from one of said reels and winding upon the other of said reels, a first dynamo electric machine connected to said winding reel, a second dynamo electric machine connected to said unwinding reel, means for sumplying current to the armatures of said dynamo electric machines, means operative on said first dynamo electric machine for maintaining said current constant, means operative on said second dynamo electric machine for regulating the field current thereof in accordance with the effective diameter of said unwinding reel, means for reversing the direction of said strip of material through said operation performing means, means for shifting said constant current control means so as to be operative on said second dynamo electric machine, means for shifting said field current regulating means so as to be operative on said first dynamo electric means, means for driving said operation performing means, means for regulating the speed of said driving means and means responsive to operation of said speed regulating means for maintaining accuracy of response of said field current regulating means over a varying speed range .of said driving means.

9. In combination, a reel for a strip to be wound thereon, means for unwinding said strip from said reel, a dynamo electric machine connected to said reel for retarding the motion thereof, a second means for varying the excitation of said dynamo electric machine to control its retarding action and a third means responsive to the generated voltage of said dynamoelectric machine and to the lineal speed of said strip for controlling said second means.

10. In combination, a reel for a strip to be wound thereon, means for unwinding said strip from said reel, a dynamo electric machine connected to said reel for retarding the motion there of, means including a balance type regulator comprising electroresponsive coils for varying the excitation of said dynamo electric machine to control its retarding action, a resistance connected in circuit with said coils, and means for simultaneously varying said resistance and the speed of said unwinding means.

11. A strip mill having in combination a pair of spaced reels adapted to be joined by a strip of flexible material wound on said reels, a plurality of operation performing rolls between which said strip moves, a separate dynamo electric machine mechanically coupled to each of said reels, a common source of electrical energy for said dynamo electric machines and means responsive to the current from said common source, the speed of said strip entering said rolls, and the generated voltage of said dynamo electric machine coupled to said unwinding reel for maintaining a constant tension on said strip.

12. A strip mill having in combination a pair of spaced reels adapted to be joined by a strip of flexible material wound on said reels, a plurality of operation performing rolls between which said strip moves, a separate dynamo electric machine mechanically coupled to each of said reels, a common source of electrical energy for said dynamo electric machines and means responsive to the current from said common source, the speed of said strip entering said rolls, and the generated voltage of said dynamo electric machine coupled to said unwinding reel for varying the excitation of said dynamo electric machines.

13. A strip mill comprising a pair of spaced reels adapted to be joined by a strip of flexible material wound on said reels, a plurality of operation performing rolls between which said strip moves, a separate dynamo electric machine coupled to each of said reels, means for energizing said dynamo electric machines whereby they tend to run in opposite directions, braking resistors, and means responsive to strip breakage for deenergizing said dynamo electric machines and for connecting one of said braking resistors across the terminals of each of said dynamo electric machines.

14. A strip mill comprising a pair of spaced reels adapted to be joined by a strip of flexible material wound on said reels, a plurality of operation performing rolls between which said strip moves, a separate dynamo electric machine coupled to'each of said reels, means for energizing said dynamo electric machines whereby they tend to run in opposite directions, braking resistors, and means responsive to the absence of said strip between said reels and said operation performing rolls for deenergizing said dynamo electric machines and for connecting said braking resistors across the terminals of said dynamo electric machines.

15. A strip mill comprising a pair of spaced reels adapted to be joined by a strip of flexible material wound on said reels, a plurality of operation performing rolls between which said strip moves, a separate dynamo electric machine coupled to each of said reels, means for energizing said dynamo electric machines whereby they tend to run in opposite directions, braking resistors, and means responsive to a sudden reversal of said strip for deenergizing said dynamo electric machines and for connecting said braking resistors across the terminals of said dynamo electric machines.

16. A strip mill having in combination, a pair of spaced reels adapted to be joined by a strip of material wound on said reels, a separate dynamo electric machine coupled to each of said reels, means for maintaining a constant armature current in said dynamo electric machines, means for varying the excitation of one of said dynamo electric machines in accordance with the effective diameter of said reel connected thereto, said excitation varying means comprising a balance type regulator having two electroresponsive coils and means responsive to the speed of said strip mill for compensating the voltages applied to said coils.

17. In a tensioning system for a strip mill, a pair of spaced reels adapted to be joined by a strip of material wound on said reels, dynamo electric machines coupled to said reels, means for performing an operation on said strip while unwinding from one of said reels and winding upon the other of said reels, means providing a voltage measuring the lineal speed of said strip entering said operation performing means, means for varying the energization of said dynamo electric machines to maintain a constant tension on said strip, said energization varying means comprising a balance type regulator having an electroresponsive coll responsive to said speed measuring voltage and an eiectroresponsive coil responsive to the voltage of one of said dynamo electric machines, a resistance connected in circuit with said coils, and means for varying said resistance in accordance with the speed of the other of said dynamo electric machines.

'I'ERRYL B. MONTGOMERY.

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