US3560796A

US3560796A - Relay control system for prevention of contact erosion

Info

Publication number: US3560796A
Authority: US
Inventors: William R Landis
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1969-12-11
Filing date: 1969-12-11
Publication date: 1971-02-02
Anticipated expiration: 1988-02-02
Also published as: DE2060511A1; JPS5132343B1; GB1320358A; NL7018131A

Abstract

A control system for energizing an electrical relay and electrical load from a source of alternating current so that any relay contact bounce occurs during a portion of the applied alternating current source in a fashion to eliminate contact erosion. The control of the relay is accomplished by a synchronous switching means that is responsive to a voltage ripple superimposed on a direct current voltage from a bridge. The bridge contains an impedance which unbalances the voltage ripple in such a manner as to provide for a predictable mode of synchronous switching. The relay contacts are selected of dissimilar materials and operate with the predictable mode of synchronous switching so that there is little or no erosion of the contacts themselves.

Description

United States Patent RELAY CONTROL SYSTEM FOR PREVENTION OF CONTACT EROSION 10 Claims, 3 Drawing Figs.

U.S.Cl 317/11,

307/127 Int. Cl H01h 47/32 Field of Search ZOO/(Inquired);

335/(lnquired);317/11.1, 11, 9C;

[56] References Cited UNITED STATES PATENTS 3,430,063 2/1969 Webb 307/136 3.372.303 3/1968 Knott .t 307/136X Primary Examiner-J. D. Miller Assistant Examiner-Harvey F endelman Att0rneys Lamont B. Koontz and Alfred N. Feldman ABSTRACT: A control system for energizing an electrical relay and electrical load from a source of alternating current so that any relay contact bounce occurs during a portion of the applied alternating current source in a fashion to eliminate contact erosion. The control of the relay is accomplished by a synchronous switching means that is responsive to a voltage ripple superimposed on a direct current voltage from a bridge. The bridge contains an impedance which unbalances the voltage ripple in such a manner as to provide for a predictable mode of synchronous switching. The relay contacts are selected of dissimilar materials and operate with the predictable mode of synchronous switching so that there is little or no erosion of the contacts themselves.

MATERIAL RELAY SWITCHING usms s 2 LOAD MEANS PATENTED FEB 2|91| SHEET 1 OF 2 INVENTOR. WILLIAM R. LANDIS BWJWJM ATTORNEX 3.560.796 SHEEI 2 OF 2 AVERAGE DIRECT CURRENT VOLTAGE 52 VOLTAGE RIPPLE 53 TIME 5| PATENTEDFEH 2197:

FIG. 2

" VOLTAGE ATTORNEY.

TIME 5| R E E 0 E C GL T D N A -Ru N N O 6 LR V [-4 B O N 7 M V l 6 ME m M TM us I N CE R L O Y O H T c T T w N U G Wu W0 A AH P R 6 U WW llll A I w--- G W GR NE l WA um mm M R E m m R m I |.||l| |-.l|||| SI ||||||l.|| I l i I In a. E AW M9 V V 6 E A P m Wm M m L 2 A 1 R W EM TO R I E L JP N VFO U E AOV E G 0 3 I: T5 L I I BACKGROUND OF THE INVENTION In the use of relays, the contact life is a function of many variables. It has been long recognized that in switching a direct current energized load, it is possible to extend the contact life of relay contacts by selecting the contact materials so that there is little or no transfer of contact materials from one contact member to another in the are that is drawn during the bounce of the contacts or in the contacts separation. This same technique is not normally applied to contacts for relays used to control alternating current loads as the bounce of the contacts and the opening of the contacts normally are not related to the voltage phase applied. It has been found that when certain types of solid state switching circuits are used to control alternating current loads that contact .failures of relays occur prematurely and appear to be a function of transfer of materials between the contact members as is seen in the switching of direct current operated loads. An analysis of this problem has shown that under certain circumstances, the. use of solid state switches cause a form of synchronous switching so that the relay is operated at the same point in the applied alternating current wave form. This form of operation, if it occurs when the contact materials can readily transfer in an arc, causes premature failure of the contact structure.

Synchronous switching of relay operated loads, where solid state switches are utilized, normally is brought about by the alternating current ripple that is superimposed on the direct current average voltage of the power supply. This ripple can be removed by the use of exceedingly large capacitors in the filtering network but in most cases this is an impractical approach to the problem. This problem can also be solved by very carefully handpicking the diodes that are used to form the rectification means for the power supply, so that the ripple is very evenly balanced thereby eliminating the synchronous switching problem. This solution also is very undesirable as the cost of handpicking and matching diode characteristics is prohibitive. The present invention solves this problem very inexpensively and in a unique manner.

SUMMARY OF THE INVENTION The present invention is directed to overcoming the detrimental effects of synchronous switching by in fact utilizing the synchronous switching phenomenon. In the present invention, a resistor or similar type of impedance is added to one leg of a bridge that supplies a rectified direct current for the switching current. The addition of the impedance unbalances the ripple voltage in a predetermined direction. The addition of an impedance, such as a resistor, adds very little cost and makes the ripple voltage from the rectifier bridge means consistently unbalanced in a defined or known manner. The bridge could also be unbalanced by using diodes of different materials such as two silicon-type diodes and two germanium-type diodes in opposite legs of the bridge. Also, a diode could be used in place of the resistor. This unbalance is then utilized to cause a solid state switching circuit to synchronously switch at a defined point in the phase of the applied alternating current voltage. This switching then allows for the pull in and bounce of associated relay contacts to occur during a portion of the applied alternating current wave form such that little or no transfer of the contact material occurs in the arc. In this case, one of the contacts is made of substantially pure tungsten. The other contact material can be any other material that normally works with tungsten. A typical example is silver cadmium oxide, Both tungsten and silver cadmium oxide contacts are well known and are given by way of example only.

As long as the switching occurs so that the transfer of material would tend to be from the tungsten contact, little or no transfer of material occurs. The are drawn on the bounce of the contacts upon closing current to a load is of insufficient temperature to melt the tungsten contact material and therefore no transfer of material occurs. With the present invention it is possible to manufacture a relay control system that is assured of having the contacts operate at a desired predetermined phase in the applied wave form of the alternating current supplied to the controlled load.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic representation of a simplified form of a bridge rectifier system and synchronous switching means to control a load by means of a relay;

FIG. 2 is a voltage-time graph of two of the voltages important to the present invention; and

FIG. 3 is a graph of the relationship of the applied alternating current source voltage to the full wave rectified voltage and the synchronous switching voltages in the system.

DESCRIPTION OF THE PREFERRED EMBODIMENT An alternating current voltage source 10 is connected by conductors 11 and 12 to a transformer primary 13. A control switch 14 is shown and can be any type of control means to supply energy to the primary winding 13 from the alternating voltage source 10.

A secondary winding 18 of the transformer is disclosed in the present application is a stepdown winding providing a low voltage on

conductors

15 and 16 that is supplied to a rectifier means 20. The rectifier means 20 includes a full wave bridge 21 including

diodes

22, 23, 24, and 25. Connected in series with diode 25 is an impedance means 26 disclosed as a resistor. The output of the full wave bridge 21 is on

conductors

27 and 28 with conductor 28 being grounded at 30. Also connected between

conductors

27 and 28 is a filter capacitor 31 to filter out the majority of the variation in the half waves that are combined by the two conventional current paths through the diodes in providing a direct current supply for the system.

Conductors

27 and 28 are connected to a relay switching means 32 that is disclosed as including a resistor 34, a unijunction transistor 33 having terminal 12 connected to a first terminal of relay means 35. The unijunction transistor 33 has its emitter 36 connected to one side 37 of a capacitor 38, the other side of the capacitor in turn is connected to conductor 28 that is common with the other terminal of relay means 35. A solid state switching circuit portion of the relay switching means 32 is completed by a charging resistor 40 from the emitter to terminal b of unijunction 33.

The relay means 35 includes a relay coil 41 and pair of

relay contacts

42 and 43. The relay contact 42 will be referred to as the anode contact while the relay contact 43 will be referred to as the cathode contact for reasons that will become apparent in the description of the operation of the present system. The relay coil 41 is electromagnetically linked to the

contacts

42 and 43 to operate these contacts when energy flows in the relay coil 41, as is well known in the relay art. A second set of

relay holding contacts

48 and 49 are also operated with

contacts

42 and 43 to hold the relay means 35 energized, once operated.

Relay contact 42 is connected by conductor 44 to the conductor l2 supplying energy to the system. Contact 43 is connected by conductor 45 to load means 46 and conductor 47 which in turn is connected to the switch 14 to receive energy whenever the primary winding 13 receives energy.

In the disclosure of FIG. 1, the anode contact 42 of the relay means 35 is made of tungsten which has a very high melting point. The relay contact 43, forming the cathode of the contact means, is made of silver cadmium oxide, a common contact material used with tungsten to provide a noneroding contact structure when energy is applied across the contacts of a polarity so that the contact 42 is the anode and the contact 43 is the cathode. In the presently disclosed system, the

contacts

42 and 43 operate as if the device was connected to a direct little or no transfer occurs and the contacts do not erode. The

manner in which the contacts are operated in a synchronous fashion to obtain the effect of always having the contact 42 the anode and the contact 43 the cathode is the basis of the present invention. This will first be described in connection with the circuit of FIG. 1 and then explained with reference to FIGS. 2and 3.

When the switch 14 is closed and energy is supplied through the transformer primary 13 to the transformer secondary 18, the rectifier means 20 has an average direct current output voltage developed across capacitor 31. Superimposed on the direct current output voltage is a ripple voltage that is normally present due to the fact that a sufficiently large capacitor 31 to eliminate all forms of ripple is impractical. Since the

diodes

22, 23, 24, and 25 normally would have slightly different forward voltage drop characteristics, it is obvious that the ripple that is generated would be slightly unbalanced. To insure that the unbalance exists and is always in the same direction, a resistor or impedance means 26 has been placed in series with the diode 25. The resistor 26 is selected to be large enough to always cause the unbalance in the ripple to be in the same direction or in a predetermined direction with respect to the phase of the alternating current voltage source. With this predictable unbalance, the relay switching means 32 can always be operated in a synchronous manner with respect to the phase of the applied alternating current voltage source 10.

The relay switching means 32 utilizes a conventional unijunction 33 along with a charging circuit made up of

resistors

34 and 40 for the capacitor 38. As soon as the voltage across capacitor 38 reaches a predetermined point or the emitter 36 peak point voltage, the capacitor 38 discharges through the emitter 36 of the unijunction transistor 33 and the coil 41 of the relay means 35. Since the relay switching means 32 utilizes a solid state switch of the unijunction type, the operation of a synchronous nature, that is,'it always occurs at the same point of ripple voltage from the power supply on

conductors

27 and 28. The unbalance created by the addition of the resistor 26 insures that the unijunction 33 discharges the capacitor 38 during the same applied phase or wave form of the alternating current voltage source 10 and therefore the

relay contacts

42 and 43 always come together during a predetermined phase of the alternating current voltage source. By selecting the

contact materials

42 and 43 properly, with respect to the instantaneous polarity at the time of contact closure, any bouncethat occurs generates an arc that tries to transfer material from the tungsten electrode to the silver cadmium oxide electrode. Since the tungsten does not melt at the arcing temperatures, little or no transfer occurs and the contact fails to erode. This mode of operation is insured from device to device by the addition of the resistor 26 to the rectifier bridge 2, and the exact means of carrying this out will be described in connection with FIGS. 2 and 3.

In FIG. 2, two wave forms of voltage 50 versus time 51 are disclosed. An average direct current voltage 52 is shown and is the voltage normally considered the output voltage across the capacitor 31 of the circuit of FIG. 1. An unbalanced superimposed ripple 53 is in reality present and has been exaggerated in size for clarity The ripple voltage 53 is presented as seen on an oscilloscope used to view the voltages in a model of the invention. The average direct current voltage 52 and its voltage ripple 53 are applied across a resistive system composed of resistor 34, the interbase resistance R of transistor 33, and the coil resistance of relay 41. The resulting current through resistor 34, transistor 33, and relay consists of an average current and a ripple current having a wave form similar to that shown in FIG. 2. This current causes a voltage to appear across the relay coil 41 and across the interbase resistance R of unijunction transistor 33 that is less in magnitude than the total voltage appearing across capacitor 31 butthat has an average voltage and ripple voltage wave form identical in shape but not size to that shown in FIG. 2. The transistor 33 bias current is not enough to operate relay means 41. The voltage appearing across relay 41 is added to a voltage within unijunction transistor 33 to establish the peak point voltage that capacitor 38 must charge to before conducting through emitter junction 36' of transistor 33. To repeat for clarity, the peak point voltage is not steady, but has the unbalanced ripple voltage superimposed on it.

In FIG. 3 an enlarged representation of parts of FIG. 2 have been disclosed along with reference voltages as to the phase of the applied alternating current voltage source and the output of the full wave rectifier bridge 21. In FIG. 3 a conventional alternating current voltage source wave form 60 is disclosed for reference. Aligned with the alternating current voltage source wave form 60 are the half wave cycle wave forms for the two halves of the rectifier bridge showing the one-half wave form cycle 61 with the resistor and the one-half wave cycle 62 without the resistor. It will be noted that the one-half of the cycle 61 with the resistor is slightly lower than the onehalf cycle wave form 62 without the resistor. This is the obvious result of unbalancingthe bridge 21 by the addition of the resistor 26. When the half wave cycles 62 and 61 are repetitively applied across the capacitor 31, a ripple voltage 53 occurs superimposed on the average direct current voltage 52. It will be noted in FIG. 3 that the peak of a voltage ripple 65 is smaller in magnitude in the portions that correspond'with'the half wave cycle 61 with the resistor than the larger portions disclosed as corresponding with the half-wave cycle 62 without the resistor. It will be further noted that the lower voltage valley 68 occurs during the half-wave voltage cycle 62. This wave form repeats itself and has voltage swings that are utilized for the synchronous switching of the unijunction transistor 33. Voltage 66 in FIG. 3 represents the average value of the peak point voltage and voltage ripple 65 is the variation in peak point voltage caused by ripple across filter capacitor 31. Voltage 55 is the timing capacitor 38 charge voltage and is virtually free of voltage ripple. When the timing capacitor charge voltage is equal to or greater than the instantaneous peak point voltage, emitter 36 of transistor 33 will conduct and charge stored in capacitor 38 will flow through emitter 36 and lead b of transistor 33 into relay coil 41 pulling in relay 35. If the peak point voltage ripple 65 is unbalanced the unijunction transistor firing point 69 will always occur where ripple voltage 65 is minimum. It will be noted that this firing occurs during the one-half of the wave cycle 62 without the resistor 26 present. Due to the mechanical mass of the relay contact structure, there is alwaysa delay in the pull in or of the closing of the

relay contacts

42 and 43. This is normally referred to as the relay transfer time and has been disclosed as a portion of the curve 65 occurring between

points

63 and 64. If there is any bounce, it occurs subsequent to point 64 and has been shown as a contact bounce time 67. The contact bounce time is short, and will be confined within the desired load voltage phase when

contacts

42 and 43 close. Since the

contacts

42 and 43 close during a predetermined phase of the applied alternating current voltage source if any bounce occurs, the arc can be made to exist with contact 42 being the anodemade of tungsten while the cathode is of a lower melting material such as silver cadmium oxide. As such, there is little or no transfer of material between thecontacts, and the contacts are subject only to the mechanical wear without the deterioration caused by the normal bounce. of conventional relay contacts.

In the present invention the simple addition of a resistor to the conventional diode bridge circuit makes the operation of the relay contacts predictable from unit to unit and contact materials can be selected so that the application of a potential to a load does not create an arc which causes a transfer of the herent differences between the

diodes

22, 23, 24, and 25, unless these diodes are handpicked and matched to have exactly the same characteristics. In a production type device the handpicking of conventional diodes is a very costly process. The addition of a resistor 26 in a typical embodiment to which the present invention is applicable adds a cost of approximately $0.03 to the overall circuitry of the device. The present invention takes advantage of the normal synchronous switching action of a relay switching means utilizing solid state devices. This overcomes the random application of the ripple voltage that is superimposed on the direct current voltage and converts it from a detrimental or questionable form of operation to a consistent and predictable type of operation where the predictable nature can be utilized to advantage. The applicant has disclosed a preferred embodiment and has set forth one pair of preferred contact materials. The applicant does not wish to be limited in scope to these specific parameters but wishes to be limited in scope solely by the appended claims.

lclaim:

1. A relay control system for energizing electrical load means during a predetermined phase of an alternating current voltage source to prevent erosion of relay contact means during the closure and subsequent bounce of the contact means, by the contact means being of dissimilar materials that resist erosion by the load current arc that is drawn at said predetermined phase of the alternating current voltage source, including: rectifier means adapted to be connected to said alternating current voltage source and having a direct current output voltage with a superimposed ripple voltage; said rectifier means including means to unbalance said ripple voltage; relay switching means including relay means energized by said output voltage and synchronously switched by a voltage swing of said unbalanced ripple voltage; and said relay means having contact means of dissimilar materials to inhibit contact erosion upon said contact means bouncing during said predetermined phase of said alternating current voltage source; said load means adapted to be energized from said source by the operation of said relay contact means, with said contact means being synchronously operated with said alternating current voltage source by said unbalanced ripple voltage so that said relay contact means bounce during said phase of said source when said contact means resist erosion caused by any load current arc.

2. A relay control system as described in claim 1 wherein said rectifier means includes a full wave rectifier bridge.

3. A relay control system as described in claim 2 wherein said bridge unbalancing means is a resistor.

4. A relay control system as described in claim 3 wherein said relay switching means is a solid state switching circuit.

5. A relay control system as described in claim 4 wherein said solid state switching circuit includes a unijunction transistor and a capacitor connected to said relay means to energize said relay means when said ripple voltage swing causes said capacitor to discharge through said unijunction transistor.

6. A relay control system as described in claim 5 wherein said relay contact means has at least one contact generally made of tungsten.

7. A relay control system as described in claim 1 said bridge unbalancing means is a resistor.

8. A relay control system as describedin claim 7 wherein said relay switching means is a solid state switching circuit.

9. A relay control system as described in claim 8 wherein said solid state switching circuit includes a unijunction transistor and a capacitor connected to said relay means to energize said relay means when said ripple voltage swing causes said capacitor to discharge through said unijunction transistor.

10. A relay control system as described in claim 9 wherein said relay control means has at least one contact generally made of tungsten.

wherein

Claims

7. A relay control system as described in claim 1 wherein said bridge unbalancing means is a resistor.

8. A relay control system as described in claim 7 wherein said relay switching means is a solid state switching circuit.