US2634386A

US2634386A - Control system for electric motors

Info

Publication number: US2634386A
Application number: US117655A
Authority: US
Inventors: Pell Eric
Original assignee: Cutler Hammer Inc
Current assignee: Cutler Hammer Inc
Priority date: 1949-09-24
Filing date: 1949-09-24
Publication date: 1953-04-07
Anticipated expiration: 1970-04-07

Description

April 7, 1953 E. PELL CONTROL SYSTEM FOR ELECTRIC MOTORS 3 Sheets-Sheet l Filed Sept. 24, 1949 April 7, 1953 E. PEL L 2,634,386

CONTROL SYSTEM FOR ELECTRIC MOTORS Filed Sept. 24, 1949 :5 Sheets-Sheet 2 EQUIVALENT RESISTANCE OF RHEOSTAT IN 0F Fuu, RHEOSTAT REs/s-rmvcs, 6 a 8 2'0 4'0 6'0 5'0 a6? 0F Fuu. RHEOSTAT SETTING.

CURRENT uv AMPs.

I00 0F FULL RHEOSTAT SETTING.

LINEAR SPEED OF MATERIAL.-

E. PELL CONTROL SYSTEM FOR ELECTRIC MOTORS April 7, 1 953 5 Sheets-Sheet I5 Filed Sept. 24, 1949 Patented Apr. 7, 1953 CONTROL SYSTEM FOR ELECTRIC MOTORS Eric Pell, Shorewood, Wis., assignor to Cutler- Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application September 24, 1949, Serial No. 117,655

10 Claims. 1

This invention relates to a control system for electric motors and more particularly to a tension control system for reel motors.

In the processing of steel strip in a cold reduction mill, it has been found that the gage of the finished strip is afiected by the speed of the strip. Experience has shown that in the cold reduction processing of steel strip in a so called dry mill, the gage of the strip increases with increase in linear speed thereof, and conversely, that in a so called wet mill the gage of the strip decreases with an increase in linear speed thereof.

Cold reduction processing of steel strip involves the feeding of strip stock through mill work rolls, which eifect the desired reduction, and coiling of the finished strip leaving the work rolls under tension on a wind-up reel. The mill work rolls and wind-up reel are usually driven. by electric motors individual thereto. After the mill roll driving motor has come up to speed, the strip advances to the wind-up reel at constant linear speed and the Wind-up reel driving motor is so controlled that the strip is wound at some constant tension. When the latter conditions obtain, the gage of the strip is maintained substantially constant. However, at the beginning of such an operation the motor driving the mill work rolls is accelerated from rest and a period of time elapses before the strip advances from the mill work rolls at a constant linear speed. During such period the gage of the strip may varyconsiderably. This also occurs during deceleration of the mill motor to rest, such as at or near the end of a processing operation. Thus a full coil of strip on the wind-up reel following such cold reduction process usually has portions adjacent to its inner and outer ends which varyfrom the desired gage. Ifsuch condition carries through successive processing steps, a finished coil of strip will have inner and outer end portions which are unusable for many purposes because of variation from the desired gage. As will be understood, it is desired that such por tions of varying gage be kept to a minimum.

It has been found that if the tension exerted on the strip advancing from the mill work rolls to the take-up reel is increased at any given linear speed of the strip, the gage of the strip will be reduced, and conversely, if the tension is decreased the gage of the strip will be increased. Therefore, if during, as well as after, the acceleration and deceleration periods of the mill motor, the torque of the wind-up reel motor can automatically be increased or decreased proportionately to the increase or decrease in speed of the mill motor, compensating increase or decrease in tension exerted on the strip can be afforded which will offset the effects of change in linear speed of the strip, which as aforeindicated, cause variations in its gage.

It is a primary object of the present invention to provide a reliable and accurate control system whereby a reel motor may be regulated so as to automatically vary the tension exerted on a strip of material in accordance with change in speed of movement of the material to the reel.

Another object is to adapt a known type of control regulator for a reel motor, which normally functions to eifect reeling of strip material at constant tension under constant speed advance of the material, to afford the aforementioned type of control.

Another object is to provide means affording compensating action of the aforementioned control regulator in proportion to the change in speed of the motor affording advance of the material.

Another object is to provide in conjunction with means of the aforementioned character for adjustment of the normal tension maintained by said control regulator, and for independent adjustment of the amount of tension compensating action afiorded by said control regulator for a given change in speed of the motor afiording advance of the strip material without such adjustments upsetting each other, and

Another object is to provide for effecting the aforementioned adjustments novel rheostat means affording linear relationship between output current and rheostat setting regardless of variations in the resistance of the load circuit.

Other objects and advantages of the invention will hereinafter appear.

The accompanying drawings illustrate certain preferred embodiments of the invention which will now be described, it being understood that the embodiments illustrated are susceptible of various modifications in respect of their details Without departing from the scope of the appended claims.

In the drawings:

Figure 1 is a diagrammatic showing of a reeling mechanism together with a control system therefor.

Fig. 2' graphically depicts an electrical relationship existing in certain parts of the control system shown in Fig. 1.

Fig. 3 graphically depicts other electrical rela- 3 tionships existing in the same part of the control system.

Fig. 4 is a diagrammatic showing of a reeling mechanism together with a modified form of control system therefor, and

Fig. 5 graphically depicts relationships of gauge of material to linear speed for various torque conditions.

In my patent application, Serial No. 115,792, filed September 15, 1949, I disclose a control system for controlling a reel motor so that constant tension is maintained on the strip material during coiling under varied accelerating and decelerating conditions of advance of the material to the reel as well as during constant speed advance of the material. The control system now to be described is built about the same regulator for the reel motor as disclosed in the aforementioned application. If desired the pres ent control system may be superimposed on the control system disclosed in the aforementioned application.

Referring to Fig. 1, it shows a length of material I0, such as a strip of steel, passing through mill work rolls II and I2 in the direction of the arrow and being coiled upon a wind-up reel I3. It may be assumed that the mill work rolls II and I2 are part of a dry type cold reduction mill.

The mill work rolls II and :2 are driven by a direct current motor I4 having a separately excited field winding I4 Field winding I4 is connected across opposite sides of a source of constant direct current voltage I5 in series with an adjustable resistor I6. Power is supplied to the armature of motor I4 by an adjustable voltage generator the armature of which is connected in a closed loop with the armature of motor [4. Generator I1 is provided with a separately excited field winding I I which is connected at one end to one side of source I5 and at its other end to the commutating arm of a potentiometer rheostat I8 which is connected across opposite sides of source [5. Generator I1 is preferably driven at a substantially constant speed by any suitable driving means.

The wind-up reel I3 is driven by a direct current motor I9 having a separately excited field winding l9 Field winding I9 is connected across the source I5 in series with a resistor 20. Power is supplied to the armature of motor i 9 by an adjustable voltage generator 2 I, the armature of which is connected in a closed loop with the armature of motor H) in series with a resistor 22. Generator 2I is provided with a separately excited field winding 2| which at one end is connected to one side of source I5 and at its other end to the commutating arm of a potentiometer rheostat 23. The commutating arm of rheostat 23 is preferably mechanically coupled with the commutating arm of rheostat I3 50 that movement of one will effect corresponding movement of the other. Generator 2! is preferably driven at substantially constant speed by any suitable driving means.

A bucking exciter 24 is connected across the resistor in a shunt relation in series with a rectifier 25. Exciter 24 is provided with a selfexcited field winding 26 connected across its terminals in series with an adjustable resistor 21, which winding supplies the necessary ampere turns for maintaining the field of the exciter at any point within its regulation range. Further,

exciter 24 is provided with a signal field winding 28 which is connected across the resistor 22 in and a commutating arm 33*.

series with an adjustable resistor 29. Also exciter 24 is provided with a reference field winding 30 which as will hereinafter be described in detail, is adapted to be energized in a variable manner. Exciter 24 is preferably driven at a substantially constant speed by any suitable driving means.

If the reference field winding 30 of exciter 24 were connected across opposite sides of source I5 in series with an adjustable resistor, such as in the manner disclosed in the aforementioned application, exciter 24 in such case would function to maintain the armature current of motor I9 at some predetermined value throughout the coiling of a length of material It: on take-up reel l3. Thus with field winding 30 energized at some constant level, the material [0 would be coiled at some constant value of tension under any constant linear speed advance of the ma terial ID from the mill work rolls II and I2 to takeup reel I3. Such arrangement and functioning of exciter 24 is well known.

As will now be described, the reference field winding 30 of exciter 24 in the present control system is so energized that under some given constant linear speed advance of material ID from the work rolls II and I2 to take-up reel I3, exciter 24 will function as above-indicated, but in addition, the energization of reference field winding 30 is automatically modified in proportion to the variation in speed of motor I4. The means affording such control of the energization of reference field winding 30 will now be described in detail.

A direct current tachometer generator 3I driven by the shaft of motor I4 is connected at one of its terminals to one side of the source I5 and also to one end of a resistance element 32 of a potentiometer rheostat 32. Generator 3i is connected at its other terminal with the other end of resistance element 32* of rheostat 32.

Rheostat 32 is provided with a commutating arm 32*. Associated with rheostat 32 is a rheostat 33 provided with a resistance element 33

Commutating arms

32 and 33 of

rheostats

32 and 33 are mechani cally coupled and so arranged relative to their

respective resistance elements

32 and 33 that movement of one such arm to a certain setting: on its associated resistance will cause movement of the other arm to a corresponding setting on its associated resistance element. Commutating arm 32 of rheostat 32 is electrically connected to the center point of resistance element 33 of rheostat 33.

Commutating arm 33 of rheostat 33 is electrically connected to one end of a resistance element 34 of a potentiometer rheostat 34. Resistance element 34 at its other end is connected to the side of source I5 opposite the side with which the first mentioned terminal of generator 3| has connection. Rheostat 34 is also provided with a commutating arm 34 and said rheostat has associated therewith a rheostat 35 having a resistance element 35 and a commutating arm 35*. Commutating

arms

34 and 35 of

rheostats

34 and 35 are mechanically coupled and arranged in the same manner as aforedescribed for

rheostats

32 and 33. Commutating arm 34' is electrically connected to the center point of resistance element 35*- of rheostat 35.

Commutating arm 35 of rheostat 35 is electrically connected to one end of reference field winding 39 of exciter 24 in series with an adjustable resistor 33. The other end of reference field --winding 3i! connected to the same side oisource 15 with which the aforementioned end of resistance element 34 of rheostat .34 has connection.

As will now be explained, the resistance elements 32 and 33' of the associated set or rheostats 3233 are complementally formed so that the equivalent resistance of theset of rheostats in the circuit hereinbefore shown and described is constant, Preferably resistance element 32 of rheostat 32 is made so that its resistance per unit of length is constant. Referring to Fig, 2 curve A depicts how the equivalent resistance of rheo stat 32, considered alone, expressed in terms of per cent of its total resistance, would vary with the setting of its commutating arm 32 expressed in terms of per cent of full rheostat setting. From curve A .it will be seen that theequivalent resistance of rheostat 3.2 will be zero at either left or right hand extreme settings of arm 32 and will be a maximum of 25% when arm 32 is set on the center point of resistance element 33 However, the resistance element 33 of rheosta-t 33 is so tapered from its center point to its ends, that the total equivalent resistance of the

rheostats

32 and 33 considered together in the circuit hereinbefore shown and described, is constant preferably at a value of 25% of the total resistance of resistance element 32 of rheostat 32 for any setting of the commutating

arms

32 and 33 The constant equivalent resistance afforded by the set of rheostats 32--33 is depicted by the straight line B in Fig. '2.

The required taper for resistance element 33* of rheostat 33 can be readily determined by the ordinate distances between curve A and straight line B or C. Thus with arm 33 set at the left or right hand extreme positions, the resistance of element 33* then in circuit should preferably be 25% of the total resistance of resistance element 32 of rheostat 32 and would taper down to 0% when commutating arm 33" is set at the center point on resistance element .33, according to the ordinate distances between curve A and straight line B.

The resistance elements of 34 and 35 of the associated set of rheostats 34--'35 are likewise complementally formed, so that the equivalent resistance of that set of rheostats in the circuit hereinbefore shown and described is constant for any setting of the commutating arms. Resistance element 3 2 of rheostat 34 is preferably made so that its resistance per unit of length is constant, and, the resistance element 35 of rheostat 35 is preferably tapered similarly to resistance ele-- ment 33 of rheostat 33.

Whereas it is preferred that the resistance elemerits 32 and 34 of rheosta ts 3.2 and M be constant in resistance per unit of length, such is not necessary and such resistance elements may oi. necessity be tapered for certain applications. For example, with the resistance element 32 of rheostat 32 tapered in a certain manner assume that the equivalent resistance of rheostat '32 will vary according to the broken line .curve D of Fig. 2. Then in order that the rheostat set 32-43 will have constant total equivalent resistance, the resistance element 33 should be tapered between its ends according to the ordinate distances between curve I). and straight line B or C and the point of electrical connection of commutating arm 32 on resistance element 33 would be shifted to the right of the center position corresponding to the horizontal distance X depicted in Fig. 2. Thus to obtain constant equivalent resistance for the rheostat sets 32-33 or 34-35, it is only required that the. taper of the resistance elements of

rheostats

33 and 35 be complementally formed in accordance with the taper of resistance elements of rheostats 32 and .34 and that the electrical connection of thecommutating arms of rheostats 32 and .34 on the resistance element of .rheostats 33 and 35 be made at the point of minimum resistance of the latter.

The use of the rheostat sets 32-33 and S t-35 in the present control system is advantageous as their use permit-s adjusting the level of normal energization of reference field winding 30 of exciter 24 and the degree of compensating energization afforded by generator 3! without one of such adjustments electrically upsetting the effect of the other adjustment.

Assume that material iii is advancing from mill work rolls H and H2 at a constant linear speed corresponding to .a predetermined preset speed being maintained by motor 5-: and that the commutating

arms

32 and 33 of

rheostats

32 and 33 are set in the left hand extreme position. Under such conditions the amount of current flowing in reference field winding 39 will be determined by the voltage of source I5 and the setting of the

commutating arm

34 and 35 of rheostats 34 and Thus for various settings of arms 34* and 35 between left and right hand extreme positions the value of the current flowing in reference field windin so will vary with the setting of said commutatin arms between said extreme positions in accordance with the straight line curve E depicted in Fig. 3. Now if the commutating

arms

32 and 33 of

rheostats

32 and 33 are set at some position other than the first assumed left hand extreme setting, then a certain portion of the output voltage of generator 3: will be superimposed on the voltage of source l5, and if the setting of commutating arms 34* and 33 of

rheostats

34 and 35 are varied s aforedescribed', the value of the current flowing in reference field winding 3!! will vary with such settings and according to another straight line, such as for example the straight line curve F depicted in Fig. 3. lhus it will be seen that for a minimum setting of rheostat set 32- E3the current flowing in referonce field winding 30 willvary directly as the settings of rheostat 3--3i2. Further, it will be seen that for a given change in setting of rheostat set 32-33 the change in value of current at each ofthe settings of rheostat set 5435 will be clirectly proportional to such change in setting of rheostat set 32--33. Similar relationship between change in reference field current and rheostat setting will hold true, if the setting of rheostat set '3435 is fixed at some given setting and instead the setting of rhecstat set 323 is varied between left and right hand extreme settings.

The operation of the control system will now be described.

Let ithere be assumed that rheostat sets 3233 and 34--35 have been adjusted appropriately; the procedure to be followed in arriving at such appropriate settings will be hereinafter explained. The control system will then function to insure the maintenance of the desired gage substantially throughout the length of the material in during a cold reduction processing operation in the following manner. At the start of such a processing operation, the motor i i driving the mill Work rolls H and 12 will accelerate from rest to some preset speed corresponding to the constant linear speed advance of material it! to be maintained throughout the greater part of the processing operation. During such acceleration of motor M, the output voltage of generator 3| increases proportionately to the change in speed of motor i4. Thus the value of the current flowing in reference field winding 30 of exciter 24 increases proportionately to the increase in output voltage of generator 3 I the increase in value of the current flowing in winding 30 for a given increase in speed depending upon the setting of rheostat set 32-33. As a result, the output voltage of exciter 24 increases proportionately to the increase in effective ampere turns of winding 30. Consequently the field strength of field winding l9 of motor 19 decreases proportionately to the increase in output voltage of exciter 23 and this causes a proportionate decrease in the counter voltage of motor 59. Therefore, the current and torque of motor I9 is caused to increase proportionately to the increase in speed of motor it. Such increase in the torque of motor is continues until motor I4 has reached its preset speed and then the torque of motor [9 is maintained constant-at least in so far as the operation of the present control system is concerned. The constant torque then maintained is such as to maintain the desired gage in material I!) for the particular constant linear speed of material it and setting of the mill rolls H and [2 then existing.

On deceleration of motor It from its preset operating speed to rest, such as would transpire at or near the end of a reduction processing of a length of material 10, the control system functions to gradually reduce the torque of motor 19 proportionately to the decrease in speed of motor M as the latter slows down.

Thus it will be seen that the control system affords torque compensation for motor l9 during accelerating and decelerating periods of motor it, which compensation oiisets the tendency of the material It to vary in gage with change in speed thereof.

The proper setting of rheostat sets 32-33 and 34-35 to afford the required torque compensation for motor I9 under a given set of conditions can be determined by data obtained from test runs performed on such strip material. To facilitate the selection of proper settings of rheostat sets 32-33 and 34-35, it should be determined how the gage of material H3 varies for a number of different operating speeds of motor i l with a certain constant tension being maintained on material ID. This can be done by setting rheostat set 32-33 in a left hand extreme setting, making some intermediate setting of rheostat 34-35 between extreme settings to provide a constant torque operation of motor i 9, and then measuring the gage of material in for various operating speeds of motor 44. This data, when plotted graphically, will give an indication how the gage of material it varies with the speed of motor [4 with no torque compensation, such as for example, as depicted by the curve G in Fig. 5. Another such test run should be made while maintaining the setting of rheostat set 33-35 fixed and giving rheostat set 32-33 some intermediate setting between extreme settings, and determining how the gage of material it varies with changes in speed of motor M. The data thus obtained when plotted, may, for example, show that the gage varies with speed of material it according to the curve H. Such test runs should be repeated with different settings of rheostat set 32-33 until it is found that with some setting of the latter that the gage of material In is substantially constant for a wide range of speeds of motor 14, which condition is depicted o by the curve J in Fig. 5. It may be found that when the proper setting for rheostat set 32-33 has been determined, although the gage of material IB is substantially constant for various speeds of motor I 4 throughout a wide range, that the gage will not be that actually desired. In that event adjustment is made in the spacing of mill rolls H and 12 by some suitable screw down control (not shown) to bring the material 10 on gage.

Whereas the control system of Fig. 1 was discussed in conjunction with the processing of strip steel in an assumed dry mill, it can be readily adapted for use in conjunction with a wet mill process by merely interchanging the terminal connections of generator 3! from that shown in Fig. 1. As aforeindicated in a wet mill process, the gage of the strip material decreases with an increase in speed. With the aforementioned change in connections of generator 3|, the control system will function reversely from that indicated in conjunction with a dry type mill, both under accelerating and decelerating conditions of motor Hi, and provide the desired torque compensation for motor l9 as fits the conditions existing in a wet type mill with respect to variation in gage of strip with changes in speed.

Referring to Fig. 4, it shows a modified form of control system to afford the same type of control as hereinbefore described. In certain applications it is not feasible to connect the reference field winding 33 directly to one side of source i5 as shown in Fig. 1. Thus where another adjustment of the current flowing in winding 30 may be desired to be made, such as for example, to afford another compensating effect on exciter 24, the modified form of control system now to be described will be more satisfactory.

The control system depicted in Fig. 4 is in many respects similar to that shown in Fig. l and corresponding components bear the same reference numerals. The modified control system dispenses with the type of tachometer generator 3 rheostat sets 32-33 and 34-35, and adjustable resistor 33 as shown in Fig. l and is provided with other tachometer generator and rheostat apparatus which will now be described.

More particularly Fig. 4 shows tachometer generators i9 and M which are driven in synchronism by the shaft of motor [4. These two generators, if preferred, could also be combined into one generator or the B-Wire type. One terminal of generator lid and one terminal of generator 4 l, which is opposite in polarity to said one terminal of generator 433, are commonly connected to one side of source 5. The second terminal of generator 33 is connected to one end of a resistance element 42 of a potentiometer rheostat which rheostat is provided with a commutating arm 42*. The other end of resistance element 32 of said rheostat is connected to a point common to the connection between the commonly connected terminals of generators 40 and H and said one side of source l5. Rheostat 22 has associated therewith a rheostat 43 having a resistance element 43 and a commutating arm 33 said rheostats being constructed and arranged as a set like the sets of rheostat sets 32-33 and 34-35 hereinbefore described in conjunction with Fig. l. Commutating arm 32 of rheostat 42 is electrically connected to the center point of resistance element 33 of rheostat 43.

The second terminal of generator 36 is connected to one end of a resistance element 43 of a potentiometer rheostat 44 which is also provided with a commutating arm 44 The other end of resistance element 44 is connected to the end of resistance element 42 of rheostat 42 which has connection with a point common to the connection between the commonly connected terminals or" generators lil and 4'! and said one side of source I5. Rheostat has associated therewith a rheo stat 45 having a resistance element 45" and a commutating arm- 45", said rheostats-comprising a set like the rheostatset 42-43 The commutating arm 44 of rheostat 44 is electrically con nected to the center point of resistance element 4i": of rheostat 45.

The rheostat sets 42-43 and 44-45 preferably have their commutating arms mechanically cou pied so that they all move to corresponding positionson their associated resistance elements when thesetting or one such arm is changed.

Conimutating arm 43 of rheostat 43 is electrically connected to a resistance element-L 46 of a potentiometer rheostat 45 which is also proided with a commu'tati-ng arm 46 The other end of resistance element 46 is connected to the side of source which is opposite from the side with which the commonly connected terminals of generators 4i and ii have connection in series with a resistor 4i. Rhcostat 46 has associated therewith a rheostat 48 having a resistance element it and a commutating arm 48 The commutating arms itc and 48 of rheostats 45' and 48 are mechanicaliy coupled so that movement of one to a certain setting on its associated resistance element will afford a corresponding setting of the other com'mutating arm on its associated resistance element. Commutati-ng arm 46 is electrically connected to a point on resistance element 48* of rheostat 48 which is off center; said resistance element 48* having a taper different from the resistance elements of

rheostats

43 and 45, as will be hereinafter explained. The comm-utating arm 48 ofrheostat' 48 is electrically connected to one end of reference field winding 380i exciter 24 in series with a resistor 49. Commutating arm 45 of rheostat 45 iselectrically connected to a point common to the connection between resistance element-45 of rheostat- 48 and resistor 41 in- Series with'a resistor 50.

The modified control system is further provided with a potentiometer rheostat 5| having'a re sistance element 5 l and-a commute-ting arm 51; One end of resistance element 51 is connectedto the same side of source l5 with which the commonly connected terminals of

generators

40 and 31 have connection in series with a resistor 52. The other end of resistance" element; 5P is connected to the oppositeside of source I-5- in serics with a resistor 53;

Resistance element 5i, like resistance elements 52 44 and 46 of

rheostats

42, 44 and 46-, respectively, is formed so that its resistance is constant per unit of length. For the system depicted in Fig; 4i, the algebraic sum of the resistance of resistor 53 and the totalresistance of resistance element 51* of rheostat 5t preferably should not be greater than the resistance of re sister 52. I

Rhecstat- 5! has associated therewith a rheostat 54 having aresi'stance element 54 and a commutating arm 54* Comm-utating' arms 5t and 54 of rheostats 5'1 and 54 are mechanically coupled so that movement of one to acertain setting will afford movement of the other to acorrespcnding setting. Commutating arm Si is" electrically connected to oneend of resistance element 54 of rheostat 54 and commutating arm 10 54 of v the latter rheostat also has electrical connection with the same end of resistance element 54 The other end of resistance element 54 is connected to the opposite end of reference field winding 39 from that with which the commutating arm of rheostat 48 has connection.

As hereinbe'fore indicated, resistance element 48 ofrheostat 48 is tapered differently irom that of

resistance elements

43 and 45 of

rheostats

43 and 45. This is due to the fact that rheostat 48 associated with rheostat 48 has its resistance element it -connected to the source 15 in series with a resistor 41 which may be of substantial resistance value. Thus the maximum equivalent resistanceof rheostat 45 and the network associatedwith its resistance element 48 will not be at the center setting of commutator arm 46 Consequently the taper of resistance element '38 to be ccmplemental will be such that its point of minimum resistance is not midway between the ends of such element, and, since the electrical connection between the commutator arm 4t of rheostat 4'6 and resistance element 43 should connect with the point of minimum resistance of resistance element 48, this connection is oilset in respect to the midpoint of said element.

The resistance element 54 of rheostat 54 is provided with a taper which is different from that for any of the resistance elements 43 4-5 and 48 of

rheostats

43', 45, and 48, respectively. As the resistance values of

resistors

52 and 53 connected in circuit with the resistance element 5 l of rheostat 5! are preferably selected as hereinbefore indicated, resistance element 54 to be complementally formed will be tapered unidirectionall'y from one end to the other. Thus as the point of minimum resistance of element 54 occurs at one end, the commutator arm 5| of rheostat 5i will have its electrical connection with such end of resistanceelement 54 Adjusting rheostat set 51-54, which isincluded for additional and independent adjustment of the reference field current, to-some setting other than its right hand extreme setting, will cause some valueof current to flow in reference field winding 30. If rheostat set 46-48 is given a right hand extreme setting and thesettings of rheostat sets 42-43 and 44-45 are varied, there will be no change in the value 01 the current flowing in winding 30' from that determined by the setting of rheostat set' 51-541 This is due to the fact that under such condition of setting of rheostat set 46-48, the sum of the currents at the point Z in the modified control system of Fig. 4 will be constant regardless of the settings given to rheo-- stat sets 42-43 and 44-45 When rheostat set 46-48 is given some other setting toward its left hand extreme setting, the voltage at the pointZ will be influenced by the settings of all these rheostat sets in such a way asto properly modify the current in the reference field winding 30 in proportion to the settings of these rheostats. The value of current flowing in reference field winding 30' with variation in setting of rheostat sets 42-43 and 44-45, when the output voltages of generat'ors 40-41 are at" some constant value, is depicted by the straight line curve K of Fig. 3. If rheostat set 46-48 is given still another setting farther toward its left hand extreme setting, then the value of current flowing in reference field winding 31] may vary for various settings of rheostat sets 42-43 and 44-45 according to the straight line curve L of Fig. 3. If rheostat set 51-54 is then given another setting farther to-' ward its left hand extreme setting, the value of 11 the current flowing in winding 30 may then vary according to the straight line curve M of Fig. 3 when plotted against the settings of rheostat sets 42-43 and fi l45.

Thus it will be seen that for any level of constant output voltages of generators 4|! and 4|, the changes in value of current fiowing in reference field winding 30 will be directly proportional to the sum of the changes in the settings of rheostat sets 4848, 5l54 and 423 and 44- 45. Also it will be seen that changing the setting of rheostat set 5 l--54 changes the minimum value of the current flowing in reference field winding 30.

Assuming that proper settings of rheostat sets 42-43 and 44-45, in conjunction with rheo stat sets 46-48 and 5l54 have been made, the modified control system of Fig. 4 will function in the same manner hereinbefore described in conjunction with the control system of Fig. 1 to afford torque compensation for motor 19 which will ofiset the tendency of material [0 to vary in gage with change in linear speed. Proper setting for such rheostat sets can be determined in the same manner as hereinbefore described in conjunction with the rheostat sets of the control system of Fig. l.

The modified control system of Fig. 4 likewise may be readily adapted for use in conjunction with a wet mill process by merely interchanging the terminal connections of each of the generators 40- respectively, from that shown in Fig. 4.

I claim:

'1. The combination with an electric motor, a generator having its armature connected in a loop circuit with the armature of said motor, a regulator comprising an exciter having a field winding connected to be responsive to the current in said loop circuit to thereby maintain the current in said circuit at a predetermined value and a second electric motor, of a second field winding for said exciter, means connected to said second motor and affording a source of voltage varying with the speed of said second motor, a source of constant voltage, and means affording said exciter second field winding with supply of exciting current derived from the variable voltage and constant voltage sources and including first and second rheostat means, said first rheostat means according to its adjustment determining the nominal value of exciting current supplied to said exciter second field winding, said second rheostat means according to its adjustment determining the change in exciting current supplied to said exciter second field winding for a given change in speed of said second motor.

2. The combination with an electric motor, a generator supplying the armature of said motor, a source of power for the field winding of said motor, a regulator comprising an exciter having its armature connected in circuit with the field winding of said motor and having a field winding connected to be responsive to the armature current of said motor for maintaining said armature current at a predetermined value, a second electric motor, and means for controlling the speed of said second motor, of a second field winding for i said exciter, means connected to said second motor and afiording a source of voltage varying with the speed of said second motor, a source of constant voltage, and means afiording said exciter second field Winding with supply of exciting current derived from the variable voltage and constant'voltage sources and including first and second rheostat means, said first rheostat means according to its adjustment determining the nominal value of exciting current supplied to said exciter second field winding, said second rheostat means according to its adjustment determining the change in exciting current supplied to said exciter second field winding for a given change in speed of said second motor.

3. The combination with an electric motor, a generator supplying the armature of said motor, a source of power for the field winding of said motor, a regulator comprising an exciter having its armature connected in circuit with the field winding of said motor and having a field winding connected to be responsive to the armature current of said motor for maintaining said armature current at a predetermined value, a second electric motor, and means for controlling the speed of said second motor, of a second field winding for said exciter, a generator driven by said second motor and affording a voltage varying with the speed thereof, a source of constant voltage, and means affording said exciter second field winding with supply of exciting current from the last mentioned generator and said constant voltage source and including first and second rheostat means, said first rheostat means according to its adjustment determining the nominal value of the exciting current supplied to said exciter second field winding, said second rheostat means according to its adjustment determining the change in exciting current supplied to said exciter second field winding for a given change in speed of said second motor.

4. The combination with an electric motor, a generator supplying the armature of said motor, a source of power for the field winding of said motor, a regulator comprising an exciter having its armature connected in circuit with the field winding of said motor and having a field winding connected to be responsive to the armature current of said motor for maintaining said armature current at a predetermined v value, a second electric motor, and means for controlling the speed of said second motor, of a second field winding for said exciter, a pair of generators driven in synchronism by said second motor and afiording voltages varying with the speed thereof, a source of constant voltage, and means afiording said exciter second field winding supply of exciting current derived from said generators and said constant voltage source and including first and second rheostat means, said first mentioned rheostat means according to its adjustment determining the nominal value of the exciting current supplied to said exciter second field winding, said second rheostat means ac cording to its adjustment determining the change in exciting current supplied to said exciter second field winding for a given change in speed of said second motor.

.5. The combination with an electric motor, a generator supplying the armature of said motor, a source of power for the field winding of said motor, a regulator comprising an exciter having its armature connected in circuit with the field winding of said motor and having a field winding connected to be responsive to the armature current of said motor for maintaining said armature current at a predetermined value, a

of, a source of constant voltage, and means affording said exciter second field winding supply of exciting current derived from said generators and said constant voltage source and including first, second and third rheostat means, said first rheostat means according to its adjustment being determinative of the nominal value of eX- citing current supplied to said exciter second field winding, said second rheostat means according to its adjustment determining the change in exciting current supplied to said exciter second field winding for a given change in speed of said second motor, said third rheostat means according to its adjustment also being determinative of the nominal value of exciting current supplied to said exciter second field winding and independently of the adjustment of said first rheostat means determining the minimum value of such exciting current.

6. In combination, a source of electrical energy,

a load circuit and adjustable rheostats having their resistor adjusting elements interconnected for adjustment of said rheostats together, a first of said rheostats having its resistor connected across said source and having its adjusting element connected to said load circuit through a second of said rheostats, the control element of said second rheostat affording inclusion of its respective resistor in varying amounts in series with said load circuit and the last mentioned resistor comprising tapered steps which render said rheostats efiective jointly to adjust the load current at different load resistance values with proportionality to the rheostat adjustments.

'7. In combination, a source of electrical energy, a load circuit and adjustable rheostats having their resistor adjusting elements interconnected for adjustment of said rheostats together, a first of said rheostats having its resistor connected across said source and having its adjusting element connected to said load circuit through a second of said rheostats, the control element of said second rheostat affording inclusion of its respective resistor in varying amounts in series with said load circuit and the last mentioned resistor comprising tapered steps which render said rheostats effective jointly regardless of value of load resistance to adjust the load current always establishing for different settings at a given load resistance value current values bearing like ratios to the current values obtained by corresponding settings at any other different load resistance value.

8. For a network, a compound type rheostat comprising separate resistors and individual adjusting elements for said resistors interconnected for effecting adjustments of their respective resistors simultaneously, one of said resistors and its adjusting element affording a potentiometer type regulator and the other resistor being series related to the first resistor selectable by the adjusting element thereof and the second mentioned resistor being appropriately tapered for network use of said compound rheostat and for constant equivalent resistance for all settings thereof when so used.

9. In combination, a network supplied with electrical energy and having a circuit to be adjusted in respect to the flow of electrical energy, a resistor connected in said network across a source of supply, an element affording said resistor an adjustable tap common to said circuit to be adjusted, a second resistor, and an element to include said second resistor in varying degrees in said circuit to be adjusted and in series with said tap of the first mentioned resistor, said elements being interconnected for adjustment of their respective resistors in unison and said sec ond resistor being so tapered that for all settings of said elements the equivalent resistance of the network is constant.

10. In combination, a plurality of diiiierent sources of electrical supply, and a network including a load circuit to be supplied with energy from each of said sources and a plurality of compound rheostats for regulating the energy supplied to said load circuit from each of said sources, each of said rheostats comprising a resistor in circuit with an associated source, an adjustable tap for said resistor, a second resistor and an adjustable tap for said second resistor to include said second resistor in series in varying degrees with said load circuit and the adjustable tap of the first mentioned resistor, said adjustable taps of each rheostat being interconnected to vary the effective portions of their respective resistors in unison and said second resistor of each rheostat being so tapered that for all settings of the rheostat the equivalent resistance of the network is constant.

ERIC PELL.

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

UNITED STATES PATENTS Number Name Date 1,883,624 Doran Oct. 18, 1932 2,257,731 Coe Oct. 7, 1941 FOREIGN PATENTS Number Country Date 476,831 Great Britain Dec. 16, 1937 552,092 Great Britain Mar. 23, 1943