US2235769A - Electrical resistance - Google Patents

Description

March 18, 1941. R. PxMATTERN 2,235,769"

ELECTRICAL RESISTANCE Filed Nov. 21, 1938 Patented Mar. 18, 1941 UNITED STATES PATENT OFFICE ELECTRICAL RESISTANCE Delaware Application November 21, 1938, Serial No. 241,598

2 Claims.

The present invention pertains to variable resistances and particularly to a rheostat in which an arm moving through a limited range is capable of including any percentage of the resistance available in an electrical circuit.

a resistance 01' the above type in which varying the pressure on moulded resistance discs varies the resistance between successive taps.

20 Another object of the invention is to provide a control resistance for a positioning motor in which the overall resistance may be varied for varying the sensitivity 01' the control.

Another object is to provide a variable resist- 25 ance of the moulded disc type having taps along the length of the column of discs and provision for changing the number, thickness, and arrangement of discs between the various taps to give any arrangement of resistance values de 30 sired for the various taps.

Other objects will appear to those skilled in the art from the specification, appended claims, and from the drawing in which:

Figure 1 is a sectional view of an electrical 35 resistance, and

Figure 2 s a diagrammatic showing of the resistance of Figure 1 used as a control resistance for a motorized valve.

The resistance as a whole as shown in Figure 40 l is generally indicated at in. A container ll of insulating material is recessed to receive a pile of flat resistance elements l2. These resistance elements are in the form of thin discs and are formed of any material the resistance 5 of which changes when the pressure between the discs is changed. A suitable substance for this purpose is a mixture of Bakelite, gas black, stearic acid, and wood flour. Interspersed between these resistance discs are conducting metal plates i3 50 which serve as taps along the length of the resistance. An adjusting screw i8 extends through the left-hand end of container ii and is screwthreaded therein. A pressure plate 19 lies against the metal plate H which is at the left-hand end of the resistance pile. Adjusting screw i8 abuts pressure plate I! so that tightening screw it will exert a longitudinal force on the whole resistance pile, increasing the pressure on the discs II.

A conducting bar 20 having contact segments 5 2i insulated electrically from each other serves as a means to select the proportion of the resistance included in associated electrical circuits Wires 22 connect each of the metal plates H to a corresponding bar 2| oi the conducting bar 20. As illustrated, the resistance is in the form of a potentiometer with a lead wire 23 attached to the left-hand end of the resistance and a lead wire 24 attached to the right-hand end. A slider 28 movable across conducting bar 20 serves to connect lead wire 29 to the'various taps of the resistance through the various segments of the conducting bar.

The container Ii is so shaped that when adjusting screw I8 is backed away from pressure plate IS, the resistance discs I! are released but will remain in position. Conducting plates l3 may then be easily withdrawn and inserted in different locations with respect to the resistance elements or discs l2, or the discs changed for ones of different thickness or the number of discs may be changed. It will be apparent that by this arrangement of the resistance In, any arrangement of the resistance I0, any arrangement of resistance values between the various taps 30 may be obtained within a relatively large range. Thus the value 01 resistance between the various plates i3 may all be the same so thatequal steps of resistance may be had or a tapered resistance as shown may be provided if desirable. Turning adjusting screw i8 will change the overall resistance and at the same time change the resistance between the various taps proportionally no matter what arrangement of discs is chosen. It will also be apparent that any substance which 4 will change its resistance upon changes of pressure may be used instead of the resistance discs shown.

The resistance iii of Fig. 1 is shown diagrammatically in Figure 2 in control of a valve I2 which controls the flow of heating medium in pipe 33. The heating medium may be steam being transmitted to radiators or may be gas flowing to a burner. In either 0! these cases, it is well known that the flow of heating medium will not be in direct proportion to the valve opening but rather in an exponential relation to it. Thus when the valve is half open, flow will be much greater than half of the wide open The present invention provides means to control the opening of a valve in response to temperature in such a way that equal increments of temperature change will cause equal increments of change in flow of heating medium.

A motor 35 having two armatures 3B and 31 energized by windings 38 and 39, respectively, serves to operate the valve through motor shaft 42, gear train 43, shaft 45, valve wheel 46, connecting link 41, and valve stem 48. Motor armature 36 together with winding 38 serves to operate the valve toward closed position while armature 81 and winding 39 operate the valve toward the open position.

A temperature responsive controller 58 includes a temperature responsive volatile fill bellows 52, which actuate a bell-crank member 53 through a lever arm 54 to actuate the slider 28 with relation to resistance l8 and make contact at various points on the resistance. A spring 55 biases the lever arm 54 against the bellows so that on increased temperatures the bellows will expand and move slider 28 to the left and on decreased temperatures the slider 28 will move to the right. In the position shown, half of the resistance is to the right and half to the left of slider 28.

A step-down transformer 68 having a primary winding GI and a secondary winding 62 is supplied with power from line wires 64 and 65. Lead wire 29 leads from one side of the secondary winding 62 to the slider 28 of controller 58, while a wire 68 leads from the other side of the secondary winding 62 to a slider 18 of a follow-up potentiometer ll having resistance wires 13. Slider 70 of the follow-up resistance is actuated by shait 45 which serves to open and close valve 32. When shaft 45 is turned in a direction to close valve 32, slider 10 will move toward the right-hand end of resistance 13 and when shaft 45 is turned to open the valve, slider 18 will move toward the left-hand end of the resistance.

A balanced relay 16 is connected in the control circuit and consists of windings l1 and 18 which.

cooperate in positioning an armature 18. A switch blade 8| moves with armature 19 to engage switch contact 82 or 83 depending on which winding T1 or I8 is more highly energized. If the two windings are equally energized, blade 8| is out of engagement-with either contact. Lead wire 23 from control resistance I8 is connected to one end of relay coil 11 while the other end of this coil is connected to follow-up resistance 53 through wire 86. The other relay coil '38 is connected to the control resistance I8 through lead wire 28 while its'opposite end is connected to the follow-up resistance I3 through wire 8!. Relay I6 is biased to open circuit or mid-position by any suitable means, not shown.

Also mounted on valve operating shaft 45 is an arm fill of insulating material which is adapted to operate limit switches 9| and 92 which are in the circuits of the two field windings 88 and 35, respectively, Each of these limit switches comprises a long and a short contact carrying blade, the blades being biased into contact making engagement with each other. In case of excessive movement of the valve, the arm 98 is adapted to engage the long blade of the respective limit switch and separate the contacts thereof.

When relay winding N is energized to a greater extent than winding 88, armature is will move switch blade 8! to the left to engage contact 82 establishing-a circuit through wire 95, switch blade 8i, contact $2, wire 8%, limit switch as, wire 91, field winding 38, and wire 88 which leads back to line wire 55. Energization of field winding 38 will cause armature 38 to rotate the valve towards closed position. Similarly, if relay coil 18 is energized to a greater extent than coil 11, switch blade 8| will engage contact 83 t0 estab- 5 lish a circuit from line wire 64 through wire 95, switch blade 8!, contact 83, wire I80, limit switch 92, wire I01, field winding 39, and wire 88 back to line wire 65 to operate the valve toward open position.

Operation of the control circuit is as follows: current from the left-hand end of secondary winding 82 of transformer 60 flows through wire 29 to slider 28 of control resistance l0. Here the flow of current is divided, one path being through 15 the left-hand portion of resistance to, wire 23, relay coil 11, wire 86, the left-hand portion of follow-up resistance 13, slider 18, and wire 68 back to secondary winding 62. A second current path is from slider 28 through the right-hand 20 portion of resistance id, wire 24, relay coil 18, wire 81, the right-hand portion of follow-up resistance l3, slider 18, and wire 68 to secondary winding 82. As long as the resistance value of the portion of resistance E8 to the left of slider 25 28 plus the resistance value of the portion of follow-up resistance 13 to the left of slider '18 is the same as the resistance value of the portion of resistance II) to the right of slider 28 plus the resistance value of the portion of resistance 13 30 tothe right of slider 18, the flow of current through the two relay coils I7 and 18 will be the same and armature 19 will remain in mid-position which will in turn hold switch blade 8| in open position.

Should the temperature surrounding controller 58 increase, slider 28 will be moved to the left, decreasing the resistance through the circuit including relay coil 11 and increasing the resistance in the circuit including relay coil 18. This will increase the flow of current in coil 11 and decrease the current flowing in coil 18 so that armature 19 will move switch blade 8| into engagement with contact 82 to energize the field winding 38 to move the valve towards closed position. This rotation of valve shaft 45 will move slider 18 of follow-up resistance II to the right to place more resistance in the circuit including relay coil ll. Thus the valve will move toward closed position until slider M has equalized the resistance in the two parallel control circuits to equalize the current flow in relay coils H and 18 and return armature l9 and switch blade 8! to mid-position.

Conversely, if the temperature should fall, bellows 52 will move slider 28 to the right to decrease the resistance of resistance iii to the right of slider 28 and increase flow of current in relay coil 18 to move armature l8 and switch blade 8| to the right. This will establish a circuit through field winding 39, rotating armature 31 in such a direction as to open valve 32. Shaft 45 will then turn slider 18 of the follow-up resistance in a counter-clockwise direction to increase the amount of resistance to the right of slider 18 to compensate for the decrease in resistance of the portion of resistance id to the right of slider 28 This will again balance the current flowing in tapered type and so arranged that movements of slider 28 at the right-hand end oi resistance it! cut inlarger amounts of resistance than similar movements would cut in toward the left of 7 the resistance. As pointed out before, in a valve controlling the flow or steam, ior example, to an extended piping system, in order to obtain equal increments of iiow for equal increments of movement of slider 28 it is necessary that the valve move a smaller amount for a given slider movement when near its closed position than when'near its open position. Thus it will be seen from Figure 1 and from the diagrammatic showin or the resistance ill in Figure 2 that moving slider 28 from one segment oi contact bar 20 to another at the left-hand end of resistance will change the amount of resistance very slightly and cause a correspondingly small movement of valve 82, while moving slider 28 from one segment of the contact bar to another at the right-hand end of the resistance will change the amount of resistance in the two circuits by a large amount and change the setting of valve 32 a correspondingly greater amount.

As pointed out in connection with Figure 1, changing the pressure on the pile of resistance discs by means of adjusting screw it changes, the total resistance of resistance It. If the value oi follow-up resistance 13 remains fixed, it will be seen that changing the amount or resistance which controller 50 cuts in for a given temperature change will change the amount of valve movement. The greater the value or resistance ill with relation to follow-up resistance ll, the less movement or slider 28 will be necessary to move valve 32 from open to closed position. In this manner the sensitivity of control may be adjusted as desired.

It will be seen that a resistance has been provided which may be readily adjusted to obtain the desired sensitivity of control in such a system as is shown in Figure 2. Also by changing the arrangement of discs I: between plates I3 shown in Figure 1, any relation of valve position to controller position may be obtained. Numerous applications of a variable resistance of this type will occur to those skilled in the art, its application in the control circuit of Figure 2 being merely illustrative. .The scope or the invention is therefore to-be limited only by the appended claims.

I claim as my invention:

1. In a rheostat, in combination, a resistor unit comprising a series or resistance elements adapted to be placed in end to end relationship to form the complete resistor unit, said elements being removable from and insertable into said unit as desired, a contacting device in the form 65 of a series of fixed contacts electrically insulated from each other, means for tapping said contacts into said unit at various points between any desired resistance elements, said last mentioned means being readily movable so that the points at which said contacts are tapped into said unit may be easily changed, means for applying a variable endwise pressure to said elements to vary the overall resistance said unit, a slider cooperating with the contacts of said contacting device for making electrical contact selectively with the several contacts, and a condition responsive controller operatively connected to said slider for positioning said slider on said contacting device, movements of said condition responsive device being proportional to changes in value of the condition, whereby changes in condition occurring at the various parts of the range of said condition responsive device can be made to cause any desired change in resistance proportional to the total resistance and the total change in resistance ior full movement of said condition responsive device may be varied without changing the proportion of the total resistance which each step comprises.

2. In a rheostat, in combination, a resistor comprising a'stack o1 resistor discs with metal discs interspersed therebetween, supporting means for said resistor discs and said metal discs for maintaining all of said discs in stacked relation, retaining means for holding said resistor discs and said metal discs in said supporting means, said retaining means being releasable so that any one or any number of said resistance discs is removable and insertable from and into said supporting means, means for applying a variable endwise pressure to said discs to vary the overall resistance of said resistor, a series of contacts, a contact blade adapted to engage said contacts to make electrical contact therewith selectively, electrical connections between said contacts and said metal discs, and a condition responsive controller operatively connected to said blade for positioning said blade with respect to the several contacts, movements of said, condition responsive device being proportional to changes in the value of the condition, whereby the total change in resistance for full movement of said condition responsive device may be varied without changing the proportion of the total resistance which each step comprises, and changes in condition occurring at the various parts of the range of said condition responsive device can be made to cause any desired change in resistance proportional to the total resistance.

RAYMOND P. MA'I'IERN.

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