US3283211A - Device for reducing erosion in electric contacts - Google Patents

Description

Nov. 1, 1966 P. H. E. CLAESSON ETAL 3,283,211

DEVICE FOR REDUCING EROSION IN ELECTRIC CONTACTS Filed Sept. 20, 1965 2 SheetsSheet l INVENTOR9 5- 7- 5 PER Hf. CLAESSON KA RL E JARNBR/NK M7744 Mag ATTORNEYS Nov. 1, 1966 P. H. E. CLAESSON ETAL 3,283,211

DEVICE FOR REDUCING EROSION IN ELECTRIC CONTACTS Filed Sept- 20, 1965 2 Sheets-Sheet 2 L [I m L R! 04 K2. Ll r2 Ll C5 r2 7 rl m I L RI "Tz i 2 Cl C2 INVENTOR S ATTORNEYS United States Patent 3,283,211 DEVICE FOR REDUCING ERQSION IN ELECTRKI CONTACTS Per H. E. Claesson, 4 Sportstugevagen, Danderyd, Sweden, and Karl E. .larnbrink, 3 Grunisgatan, Farsta, Sweden Filed Sept. 20, 1965, Ser. No. 493,609 Claims priority, application Sweden, July 3, 1959,

12 Claims. (Cl. 317-11) The present application is a continuation-in-part of our application Serial No. 46,858 filed July 5, 1960, now abandoned.

The present invention relates to a protective device for reducing or eliminating the erosion to which electric contacts are subjected. Such erosion is caused by arcing between the contacts when the contacts are opened or closed.

The device is particularly intended for use in direct current circuits which contain inductive apparatus as for instance relay windings.

An object of the invention'is to provide a device for the said purpose which effectively reduces arcing be tween the contacts both on closure and opening of the contacts.

Another object of the invention is to provide a device of this kind which requires little space and can be made in a unit which can be easily mounted close to the contacts to be protected.

According to the invention the device comprises inductive means connected in series with the contacts and the power consuming apparatus in the circuit, and rectifier means connected in parallel with the series combination of the inductive means and the power consuming apparatus. The rectifier means is so poled in relation to the power source that it does not pass current when the contacts are closed.

In a preferred embodiment of the invention a capacitor is connected in series with the rectifier means and a resistor is connected in parallel with the rectifier means.

These and other objects and features of the invention will be more fully understood from the following detailed description in conjunction with the accompanying drawing in which:

FIGURE 1 is a schematic diagram showing an inductive circuit utilizing the invention;

FIGURE 2 is a graph showing current and voltages as functions of time;

FIGURE 3 shows a practical embodiment of the invention; FIGURE 4 is a schematic diagram of an alternative circuit where one terminal of the capacitor-rectifier circuit is connected to the positive pole of the power source; and

FIGURE 5 is a schematic diagram of a further embodiment where the capacitor-rectifier combination is connected across contacts K1K2.

Referring now to FIGURE 1, R1 designates the winding of a relay. The winding R1 is energized from a DC. power source (not shown) having its negative pole connected to a terminal of R1 and its positive pole connected to ground as indicated by the plus and minus signs in the right-hand part of the figure. The energizing circuit is controlled by the contacts K1 and K2. Contact K1 is connected to the winding R1 through an inductive element Z1 and a line L the resistance of which is represented by resistor r1. Contact K2 is connected to ground.

The capacitance of the line with respect to ground is represented by C1 and C2, the capacitance between contacts K1 and K2 by C3, and the parallel capacitance of winding R1 by C4.

3,283,211 Patented Nov. 1, 1966 When the energizing circuit is open, that is when contacts K1 and K2 are separated as shown in the figure, there is a voltage across the contact and the capacitances C1, C2 and C3 which is substantially equal to the voltage of the power source. When the circuit is to be closed, the contacts K1 and K2 are moved towards each other, and when during this movement the distance between them becomes sufficiently small, the said voltage will produce an are between the contacts, and the DC. energy stored in the capacitances will be discharged through the arc, thereby producing a considerable surge current which may cause damage to the contacts, unless precautions are taken to limit it.

The curve 1 in FIGURE 2 illustrates this surge current as a function of time in the case that no means are provided for limiting the current.

In the shown circuit the surge current occurring upon closure of the contacts is limited by the inductive element Z1 to such extent that it will have no harmful effect on the contacts. In order that the inductive element Z1 shall be as effective as possible to limit the current the connecting lead between Z1 and contact K1 should be as short as possible so that the discharge current from the shunt capacitances in the circuit will have to pass through Z1. It is not possible, however, to make the discharge current from the contact capacitance C3 pass through Z1, but this capacitance is small so the current produced thereby will not be harmful.

While the inductive element Z1 limits the current in the are appearing on the closure of the contacts, it will have the opopsite effect when the contacts are opened. When the current through the circuit is interrupted, the inductances of the element Z1 and the winding R1 generate a very high voltage which would cause considerable arcing and erosion of the contacts if means were not provided to suppress it. Curve 2 in FIGURE 2 illustrates this voltage as function of time in the case that no means are provided for suppressing it.

The voltage occurring across the contacts when they are opened can be limited by means of a circuit consisting of a resistor r2 in series with a capacitor C5 connected in shunt with the series combination Z1 and R1. By means of this circuit the voltage appearing across the contacts when opened is limited to a value not exceeding the current through the energizing circuit before the contact break multiplied by the resistance of resistor r2. In this way the voltage can be limited so that it does not exceeed -150 volts. Hereby the contacts are fairly well protected against so called glow arcing. However, the contacts are not protected against so called short arcs which will arise as soon as the voltage exceeds about 20 volts. Short arcs are particularly liable to arise when the contacts are of the so called activated type.

The resistance of resistor r2 may of course be made so small that the voltage cannot exceed 20 volts, but in such case it would cause an increase of the current in the are produced during closure of the contacts.

The above-mentioned drawbacks of the RC-circuit are eliminated by connecting a rectifier L1 in parallel with resistor r2 and in series with capacitor C5 as shown in FIGURE 1. Rectifier L1 is so poled that it does not pass current when the contacts are closed. The capacitor C5 is then charged by current through resistor 12 the resistance of which should be so large, for instance 2000 ohms if the supply voltage is 36 volts, that the current through it is of moderate strength. The rectifier L1 should also be so dimensioned that the voltage drop in its forward direction is below the voltage (about 20) required to produce a short arc.

Thus the rectifier L1 functions as a low resistance bypass circuit in parallel with resistor r2, to current flow when the switch contacts are opened thereby limiting the voltage appearing across the contacts when opened to a safe value; and on the other hand, the rectifier functions as a high resistance to current flow in the opposite direction which occurs during the closure of the contacts. In the claims, rectifier L1 is sometimes referred to as a resistance means. Where rectifier L1 is 'a semiconductor, the resistance provided by resistor r2 may be the reverse resistance of the rectifier. Where the reverse resistance of the rectifier is too high, a separate resistor r2 may be used. Moreover the capacitance of the rectifier should be small.

The capacitance of C should be large enough to keep the break voltage at a safe value until the distance between the contacts has been so large that an arc can no longer arise. This is illustrated by curve 3 in FIGURE 2. As soon as the contacts come out of engagement with each other a voltage V1 appears between the contacts. This voltage is equal to the voltage drop across rectifier L1. The voltage then rises at the same time as the contacts move from each other. It is assumed that after time t the distance between the contacts is so large that an arc cannot arise. The capacitance of C5 should be such that during the time t the sum of the voltage V1 and the voltage rise V2 across C5 does not exceed the voltage required to produce a short are. The time I, is of course dependent on the speed of movement of the contacts.

The circuit shown in FIGURE 4 contains the same elements as the circuit of FIGURE 1 and these elements have been given the same reference characters as in FIG- URE 1. It differs from that of FIGURE 1 in that one terminal of the capacitor-rectifier circuit C5-L1 is connected to the positive pole of the power source. As indicated by the ground symbols the positive pole of the power source is connected directly to contact K2, and thus in the circuit of FIGURE 4 the capacitor-rectifier cornbina tion CS-Ll is connected directly across the contacts K1K2. In this case the capacitor C5 is charged when contacts KI-KZ are opened and discharged when the contacts are closed. However, in FIGURE 4 as well as in FIGURE 1 the rectifier L1 is so poled that it presents its high back resistance to the current through capacitor C5 upon contact closing and its low forward resistance upon contact opening. Thus the effect as regards voltage limitation is exactly the same in the circuit of FIGURE 4 as in that of FIGURE 1, and for the purpose of the invention these two circuits are equivalent.

In the circuit shown in FIGURE 5 the capacitor-rectifier combination C5-L1 is connected in the same way as in FIGURE 4 that is directly across the contacts K1K2. In the circuit of FIGURE 5, however, the inductive element Z1 is connected between the positive pole of the power supply and contact K2. It will be realized that even in the circuit of FIGURE 5 the inductive element Z1 .is effectively in series with the contacts K1-K2 and the power consuming apparatus R1, and for the purpose of the invention this circuit is equivalent to the circuits shown in FIGURES l and 4.

The inductive element Z1 should have some resistance so as to damp the oscillations which are liable -to occur when the contacts are closed. The required inductance of element Z1 depends on the length of conductor L. If this conductor is short an inductance of 0.1 to mh. may be sufiicient. If the conductor is of considerable length (more than 10 meters) the inductance must be higher.

The inductive element Z1 is preferably a small coil wound on a ferrite core. I

The rectifier L1, capacitor C5, resistor r2 and the inductive element Z1 are preferably assembled in a selfcontained unit which is provided with means for fixing it mechanically to the contacts. FIGURE 3 shows an embodiment of this unit.

The components of the protective device are contained in a cylindrical cover 5 of suitable material. A strip of sheet metal 1 is bent so as to form two sleeves, one of which encloses the unit 5 and the other of which is pushed on a terminal lug of a contact carrying spring 4 belonging to a relay 3. A connecting lead 6 extends from one end of cover 5 and is soldered to the lug 4 at an opening 8. The other connecting leads 7 from the components extend from the other end of the cover 5 and may be bent or looped so that an external conductor can be easily soldered thereto.

The capacitor C5 can be in the form of a hollow cylinder so that it can serve as cover 5 for the other components of protective device. Thus the protective device requires little space and can be mounted very near to the contacts.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not re strictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is: I

1. A device for reducing the erosion of contacts in a circuit connecting an electric direct current source with a power consuming apparatus and serving to absorb current surges upon closing and opening of contacts, comprising in combination: a protective device comprising inductive means connected in series with said contacts and said power consuming apparatus and having one terminal connected to a first one of said contacts; and a series combination of a capacitor and resistance means which presents a very low resistance to current fiow in one direction and a comparatively high resistance to current flow in the other direction, said series combination having one terminal connected to said first contact and having its other terminal connected to said circuit at a point between said power consuming apparatus and a second one of said contacts, said resistance means being so poled that said comparatively high resistance is presented to current passing through said capacitor upon closure of said contacts.

2. A device for reducing the erosion of contacts as defined in claim ll wherein said resistance means comprises a rectifier and said capacitor and rectifier are assembled in a single unit which is provided with fixing means for fixing the unit mechanically to one of said contacts.

3. A device as claimed in claim 2 in which said capacitor is in the form of a'hollow cylinder, and the inductive means and rectifier are placed within said cylinder.

4. A device as claimed in claim 2, in which said fixing means is in the form of a sleeve adapted to be pushed over a terminal end of a spring carrying one of said contacts.

5. A device for reducing the erosion of contacts in a circuit connecting an electric direct current source with a power consuming apparatus where sufiicient distributed capacitance is present to cause a current surge upon closing of said contacts comprising in combinationz inductive means having sufficient inductance to reduce said current surge connected in series with said contacts and said power consuming apparatus and having one terminal connected to and being located physically at the position of a first one of said contacts; and a series combination of rectifier means and a capacitor, said series combination having one terminal connected to said first contact and having its other terminal connected to said circuit at a point between said power consuming apparatus and a second one of said contacts, said rectifier means being so poled that its reverse resistance is presented to current passing through said capacitor upon closure of said contacts.

6. A device for reducing erosion of switch contactsm a circuit connecting an electric direct current source wlth a power consuming apparatus and serving to absorb current surges upon closing and opening of contacts, comprising in combination: inductive means connected in series between said contacts and said power consuming apparatus and having one terminal connected close to a first one of said contacts; and a series combination of rectifier means and a capacitor, said series combination having one terminal connected to said first contact and the other terminal connected to a point between said power consuming apparatus and said direct current source in a line connecting said apparatus to one terminal of said source, said rectifier means being so poled that its reverse resistance is presented to current passing through said capacitor upon closure of said contacts.

7. A device for reducing the erosion of contacts in a circuit connecting a direct power source with a power consuming apparatus and serving to absorb current surges upon closing and opening of contacts, comprising in combination: inductive means connected in series with said contacts and said power consuming apparatus and having one terminal connected -to one of said contacts, and a series combination of a rectifier and a capacitor connected in parallel with the series combination of the inductive means and the power consuming apparatus, said rectifier being so poled in relation to the polarity of said power source that the reverse resistance is presented to the current from the power source.

8. A device as claimed in claim 7, in which the rectifier has a resistance in its forward direction which is sufficiently low to prevent the voltage arising across said cont-acts upon opening from exceeding 20 volts.

9. A device for reducing the erosion of contacts in a circuit connecting a direct current source with a power consuming apparatus and serving to absorb current surge upon closing and opening of contacts, comprising: an inductive element having terminals connected in series with said contacts and said power consuming apparatus; a capacitor and a rectifier each having terminals connected together in a series circuit and said series circuit being connected in parallel with the series combination of the inductive element and the power consuming apparatus; said rectifier being so poled in relation to the polarity of said power source that the reverse resistance of the rectifier is presented to the current from the power source when said contacts are closed; said inductive element, rectifier and capacitor being assembled in a single unit which is provided with fixing means for fixing the unit mechanically to one of said contacts.

10. The device as claimed in claim 9 wherein the ca pacitor is in the form of a hollow cylinder, and the inductive element and rectifier are placed within said cylinder.

11. The device as claimed in claim 10 wherein said fixing means is in the form of a sleeve adapted to be pushed over a terminal end of a spring carrying one of said contacts.

12. A device for reducing the erosion of contacts in a circuit connecting a direct current source with a power consuming apparatus and serving to absorb current surges upon closing and opening of contacts, comprising in combination: inductive means connected in series with said contacts and said power consuming apparatus and having one terminal connected close to one of said contacts; rectifier means connected in parallel with the series combination of the inductive means and the power consuming ap aratus, said rectifier means being so poled in relation to the polarity of said power source that the reverse resistance of the rectifier is presented to the current from the power source when the contacts are closed; a capacitor connected in series with said rectifier means.

References Cited by the Examiner UNITED STATES PATENTS 1,966,077 7/1934 Nyman 31711.4 X 2,011,395 8/1935 Cain 31711.4 X 2,199,909 5/1940 Burton et al. 317-11.4 X 2,531,220 11/1950 Kaplan 317-11.4 X 2,758,254 8/1956 Kramer 31711.4 X 2,802,149 8/1957 Germer et al. 31711.4 X

MILTON O. HIRSHFIELD, Primary Examiner. LEE T. HIX, Examiner.

R. V. LUPO, Assistant Examiner.

Claims (1)

1. A DEVICE FOR REDUCING THE EROSION OF CONTACTS IN A CIRCUIT CONNECTING AN ELECTRIC DIRECT CURRENT SOURCE WITH A POWER CONSUMING APPARATUS AND SERVING TO ABSORB CURRENT SURGES UPON CLOSING AND OPENING OF CONTACTS, COMPRISING IN COMBINATION: A PROTECTIVE DEVICE COMPRISING INDUCTIVE MEANS CONNECTED IN SERIES WITH SAID CONTACTS AND SAID POWER CONSUMING APPARATUS AND HAVING ONE TERMINAL CONNECTED TO A FIRST ONE OF SAID CONTACTS; AND A SERIES COMBINATION OF A CAPACITOR AND RESISTANCE MEANS WHICH PRESENTS A VERY LOW RESISTANCE TO CURRENT FLOW IN ONE DIRECTION AND A COMPARATIVELY HIGH RESISTANCE TO CURRENT FLOW IN THE OTHER DIRECTION, SAID SERIES COMBINATION HAVING ONE TERMINAL CONNECTED TO SAID FIRST CONTACT AND HAVING ITS OTHER TERMINAL CONNECTED TO SAID CIRCUIT AT A POINT BETWEEN SAID POWER CONSUMING APPARATUS AND A SECOND ONE OF SAID CONTACTS, SAID RESISTANCE MEANS BEING SO POLED THAT SAID COMPARATIVELY HIGH RESISTANCE IS PRESENTED TO CURRENT PASSING THROUGH SAID CAPACITOR UPON CLOSURE OF SAID CONTACTS.

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