US2470825A - Electrical contact protection network - Google Patents

Description

May 24, 1949. g, MATHES 2,470,825

ELECTRICAL CONTACT PROTECTION NETWORK Filed April 10, 1945 4 Sheets-sheaf. 1

. INVENTOR R. C. MA 7HES ATTORNEY May 24, 1949.

R. c. MATHES 2,470,825

ELECTRICAL CONTACT PROTECTION NETWORK Filed April 10, 1945 4 Sheets-Sheet 2 Mil EN ron R. C. AM THE S ATTORNEY 1949- R. c. MATHES 2,470,825

ELECTRICAL CONTACT PROTECTION NETWORK Filed April 10, 1945 4 Sheets-Sheet :5

Fl 8 FIG 7 G C M R n 1:

Cl I" IN VE N TOR By RCZMATHES A TTORNE'V y 1949 R. c. MATHES 2,470,825

ELECTRICAL CONTACT PROTECTION NETWORK Filed April 10, 1945 4 Sheets-Sheet 4 FIG. [3

lNVE/VTOR R. C. MA THES ATTORNEY Patented May 24, 1949 ELECTRICAL CONTACT PROTECTION NETWORK Robert C. Mathes, Maplewood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 10, 1945, Serial No. 587,462

2 Claims.

This invention relates to contact makers and breakers and particularly to the protection of contacts from the ill eil'ects of starting and stopping the flow of comparatively heavy electrical currents.

An object of this invention is to provide an 7 improved type of circuit presenting a constant v resistance impedance to the contacts for the purpose of controlling both the breaking voltage and the closing current to the least damaging value corresponding to a given type of contact.

A further object of this invention is to provide a circuit arrangement which will particularly apply to a pair of contacts connected in parallel and intended to be operated in synchronism.

Contact protection becomes a problem where an attempt is made to provide a contact maker and breaker of a size and capacity to economically handle its ordinary duties but which must not be allowed to fail under emergency conditions which are liable to arise in service. For instance it is common knowledge that the resistance of the ordinary tungsten filament lamp rises greatly as the filament attains incandescence, whereby the current when the circuit for such apparatus if first closed is much greater than it is a short period thereafter. It will, therefore, be seen that if the circuit is closed and then almost immediately opened the contacts will be called upon to break a current many times heavier than usual. If the contacts through erosion have become other than perfect such an opening after a closing is not an uncommon occurrence and the result is further erosion and perhaps sticking contacts through welding or mechanical interlocking of the roughened surfaces.

It has been found expedient in certain circumstances in order to insure dependability in service to use a pair of contacts in parallel and while this goes along way toward solving the problem it does not provide a complete answer.

It has been discovered that the requirements for contact protection on the opening and on the closing of a circuit are mutually conflicting. By way of example, it is common practice to connect a condenser about a pair of contacts to protect them against the ill eflects of breaking a heavy current. Therefore, such a condenser becomes charged during the open periods of the contacts so that when they are closed the first rush of current therethrough is not only that due to the normal circuit functioning but also the discharge current of the condenser. The greater the capacity such a condenser has, the greater is the protection provided on the opening of the contacts, but unfortunately the greater also is the initial rush of current when the contacts are brought together.

In accordance with the present invention a type of circuit novel to the art of contact protection is provided for insertion between the contacts and the load circuits. This consists of a uniform line or one of that class of circuits known as constant resistance networks by means of which the breaking voltage and the closing curient can be held to constant controlled values for the short times of importance in opening or closing.

In accordance with the present invention a novel means is provided to protect parallel contacts consisting essentially of contact protecting networks for each contact pair arranged to mutually effect each other whereby cancellation of heavy transients may be brought about. A fea ture, then, of the present invention is a circuit network applied to each of two circuit makers and breakers arranged to operate simultaneously, connected in such manner that the interlinked magnetic circuits of the two branches of such circuit network are in mutual and opposing relation to each other.

The drawings consist of four sheets having thirteen figures, as follows:

Fig. 1 is a schematic circuit diagram showing the relations of certain theoretical circuit elements in a contact protection network;

Fig. 2 is a graphical representation of transient voltage phenomena;

Fig. 3 is a similar graphical representation of transient current phenomena;

Fig. 4 is a graphical representation of voltage and current phenomena where a theoretically perfect section of ideal line is inserted between the contacts and the remainder of the circuit;

Fig. 5 is a schematic circuit diagram showing a constant resistance network of the form of Fig. 'l inserted between a pair of contacts and the load which it is to control;

Fig. 6 is a similar schematic circuit diagram showing a constant resistance network of the form of Fig. 8 inserted between a pair of contacts and the load which it is to control;

Fig. 7 is a fundamental schematic circuit diagram of one form of constant resistance network;

Fig. 8 is a similar fundamental schematic circuit diagram of another form of constant resistance network;

Fig. 9 is a compromise contact protection network which will give practically as good results as a theoretically good circuit;

Fig. 10 is a still further simplification in a compromise circuit;

Fig. 11 is a schematic circuit diagram showing an arrangement for the protection of two simultaneously operated contact pairs;

Fig. 12 is the same circuit redrawn to explain the conditions present when one of the contacts opens before the other; and

Fig. 13 is an equivalent circuit useful in explaining the theory of operation when the coil is so constructed that through leakage there is less than perfect mutual induction between the parts thereof.

In the study of erosion of relay contacts it has been found that phenomena at both the opening and the closing of the contacts are of great importance. Work has been done on the design of protection circuits designed in general to control the rate at which voltage across the contacts develops in the former case and the rate at which current can build up through the contacts in the other case. tion circuit is expected to take'care of both cases.

In the case of opening the contacts, the ill effects result from establishing and sustaining,

Usually a single invariable protecfor periods of a few microseconds to a few milliseconds, a destructive metallic are, a gaseous are or both types alternating. The existence and which mayv result in welding or in the transfer of material due to certain electrothermal effects. The severity of these effects depends chiefly on the contact materials, the battery voltage and the capacita'tive character of the load.

The palliative'measures required for the two cases are to a degree mutually conflicting and the final design must in general be a compromise with the relative seriousness of the two factors often remaining to be determined by experimental means. How this comes about can be readily seen by examining in a step-by-step fashion the building up of a typical protection circuit for an inductive load.

In Fig. 1 we have an inductive load which will first be assumed to be a pureinductance L with a resistance RL. The load current Ir. will be equal to the battery voltage En, divided by RL. We are interested in the voltage Eo when the contacts I and 2 are opened and the current when the contacts are closed, for several simple protection conditions provided by the conventional shunt circuit of Rc-C. If neither is present we know from common knowledge of transient current phenomena that the voltage E0 produced at the contacts I and 2 on opening the circuit thereat will be in the form of curve I of Fig. 2 and the current produced therethrough on the closing of these contacts will be in the form of curve 2 of Fig. 3. This is a very bad condition for voltage arcing and a very favorable condition for welding current control. If now we add the capacity C but omit Rc we will get curve 2 of Fig. 2 for Eoand curve I of Fig. 3 for Id. The relative severity for the-two conditions is now reversed and in most cases we would be little better off.

These curves have been oversimplified in order to bring out the general nature of the circuit reactions. This is particularlytrue of the number I curves designed to illustrate the ap roach to infinite voltage or current if a nearly perfect opening or closure were obtainable. Actually the dotted portions of the curves are extremely irregular in practice. In the case of breaking, different types of transients such as those described by A. M. Curtis in an article entitled Contact phenomena in telephone switching circuits published in the Bell System Technical Journal,

January 1940. P ges 40-82, may be set up, with intermittent low voltage metallic arcs or high voltage gaseous arcs. Similarly in the case of closure, the energy stored in the condenser may be dissipated in "preclosure arcs as well as in the rush of current dissipated as simple resistance heating.

If now we add the resistance Re into the network of Fig. 1 and satisfy the condition that Rc= RL=.\/CJ- we will get curve 3 in both Figs. 2 and 3. E0 is now equal to En for all time. after the contact opening and I0 is equal to later all time after contact closing. This comes about because the impedance looking into the network from the contact terminals is a constant resistance for all frequencies and cannot, therefore, bedistinguished from a single perfect'resistance.

Unfortunately Ea will generally be a higher voltage than can be permitted ta appe'ar across the contacts atv the momentof opening without serious arcing. As a first step in exploring a method for providing a controlled voltage, let us insert between the contacts and the restof the circuit of Fig. 1 a section of ideal-line'of characteristic impedance R0 and a time of transmission tx from one end to the other thereof. The rest of the circuit will still be assumed to meet the constant resistance relation stated'above. With the contacts closed there will flow the normal closed circuit current of IL. Upon opening the contacts the overall circuit will function as an in finite line of true resistance impedance until the first reflection arrives from the end of the line. That is, up to a time 2tx, there will be an open circuit voltage across the contacts.

Eo=ILRo (1) This is shown in Fig. 4 for Ro Ro or RL. At longer times the voltage reaches the value Ea. Likewise when the contacts are open we have the line charged to a potential EB and for the same interval of time after closing we will derive a current through the closed contacts of Ic=Es/Ro (2) Thus by making the impedance of the uniform line Ro less than R0 or R1. we have been able to control matters for a short length of time so that the open circuit voltage conditions are made more lenient at the expense of making the short circuit condition more severe. However, the latter is not as severe as curve I of Fig. 3. In the case of very high current loads working fromlow voltage battery, the reverse result could have been secured by making R0 greater than Re. Depending on the characteristics of the load and/ or other protection equipment between the load and the assumed uniform line; the dotted parts of the curves in Fig. 4 may approach the final steady value either by slow approach or by E0 and I0 will need to be determined experimentally for every physically different pair of contacts at their normal operating speeds and pressures. Once these constants have been determined we know under ideal conditions what wattage load these contacts will handle to some degree independent of whether they are operated from a high or low voltage battery. The adjustment to different battery voltages is made simply by the selection of the characteristic resistance R of the uniform line.

Under practical conditions there are several factors which prevent reaching the full emciency of use indicated by the formula (3). 0n the break, relay chatter or "reclosures" will require that better protection than indicated may be required. The phenomena of reclosure" frequently occurs after the relay contacts have been somewhat roughened by the erosion process. On closure there is another phenomena which develops when the battery voltage is above 50 to 100 volts. This is known as "preclosure" and is presumed to be due to point discharges initiating an arc due to the high voltage gradients as the contacts come very close together and is discussed in the Curtis article hereinbefore noted.

For some limited fields of use small artificial lines, such as may be made by winding a solenoid to have a high capacity to ground, will be useful devices according to this invention. However such structures will in general be impractical for a great many situations, to meet which it is desired to provide other forms of constant resistance networks. Two of these are shown in Figs. and 6 derived respectively from the well-known forms of constant resistance networks shown in Figs. '7 and 8. In Figs. 5 and 6 the impedance looking to the right from the contacts will be a constant resistance R if the impedance looking to the right from points 3 and l is also a constant resistance R and if R'=Li/C1. This can be made approximately true down to some fre quency h if C: and La are made large enough. Then the constant resistance network proper which controls the initial voltage or current for an initial period t1 corresponding to the uniform control time 2t! of Fig. 4 is that part of the network between the contacts and the points 5 and 6.

The product L101 will be larger the longer the time over which it is desired to hold the initial i i= l The resonant frequency of this mesh would appear'to be a reasonable one to which to maintain the constant B characteristic, so

.and

1 fl "Ti;

At this frequency. 11, the reactance of the condenser C: should be small compared with that of the inductance L1. The resonant frequency 1: of La-C'n will thusbe considerably lower than {1 and the time constants of that mesh much longer. If it is desired to lengthen the time of flat voltage or current control without going to larger values of L1 and C1 (and hence also of La and C2) this can be done by adding additional sections identical with those included between the contacts and the points I and 4. Each of these if properly terminated to the right then automatically ro' vides the constant R terminating impedance required by the section to the left. Other more complicated types of constant resistance circuits may be employed, especially if it is desired to compensate for parasitic capacitance or inductance.

The next section of the iigures may be looked upon as a low-pass filter or might be designed as another constant resistance section designed to put another step in the curve of transition to the final current or voltage values. This would have an impedance between that of the first section and the final section. It will thus not be possible to matchthese sections and also have both of them constant resistance. An approximation' as shown in Fig. 9, however. may give very satisfactory results. Here R1 plusRa would be made equal to Re and R; would be equal to Cs would have its re-' actance small compared with that of Le at the frequency I:

In many cases encountered in telephone practice the simpler form of circuit shown 'in Fig. 10 may sufiice. In the case of a heavy telephone relay'load the values for Fig. 10 would be about as follows:

I1.==.4 ampere En=50 volts Eo== initially -8 volts whereupon R =-i20ohms and for ti=.0001 second.

C =%9- -5 microfarads C: should be about ten times C1. If t1 must be held for one millisecond, the above figures for C1 and Limust be multiplied by ten or if it must be held to only 10 microseconds they may be divided by ten. This shows the value of a high initial acceleration of opening if simple contact protection circuits are to be attained. Actually the lower figure would probably meet most telephone cases.

For heavy loads such as small motors with 5 to 10 amperes current the characteristic impedance becomes very low and R or R1 will be about 1 ohm. For these heavy currents it becomes especially important to hold the free wiring between the contacts and the protection network to a minimum. Such wiring is a short piece of uniform line of about 500 ohms characteristic resistance. Hence, at 5 amperes we would get an initial voltage of 2,500 volts lasting .002 microsecond per foot of wiring. Condensers should also be free of parasitic inductance.

In cases where the load conditions do not impose too stringent requirements, the circuit of Fig. 6 reduces to a form of protection which has often been used, simply the first series element and the first shunt condenser. In this situation the series element has been made by winding the inductance coil upon a .conductive permeable core. This provides the approximate equivalent of a coil shunted by a resistance and provides a small unit which can readily be mounted very close to the contacts to be protected, the importance of which has alread been indicated.

The design of contact protection circuits is thus seen to be one of many compromises which should always be thoroughly investigated experimentally. The importance or what happens in a i'ew microseconds depending on the very high frequency properties of connected circuit elements cannot be over-emphasized. A feature the constant resistance method of design is that the open circuit voltage can be held uniform alter the -initial jump for a selected short interval of time (corresponding to these critical first few microseconds) instead oi starting to rise immediately as is the case with the more conventional type oi protection circuit.

The principles of this invention may be further applied to the operation of two contacts inparallel. The use of double contacts is becoming increasingly more common so the use of this arrangement does not become unduly expensive in practical use.

The arrangement shown in Fig. 11 is one in which two contact pairs II and I 2 are intended to simultaneously open the circuits therethrough and to thus control a secondary circuit. When the device is normal a conducting path is closed between the conductors l3 and II and when the device is operated this path is opened.

Between the conductors l3 and I4 there is one path containing a condenser lli and another path containing in series a resistance It and a condenser I'I. Between the conductor l3 and the 8 with the condition maintained that !z.=Ii+l:. Then the energy stored in the coil ield which must be, dissipated in the resistances is proportional to 12-11 approachinglr. as a maximum if the difi'erentlal time oi opening is long.

Hence, at the worst, contact I! it it opens at a considerable time after contact I I (considerable in relation to the time constant of this circuit) will have to bear the full brunt oi the opening to the same degree thatthe single contact in Fig. 6 does. The more nearly the two contacts operate at the same time, the more benefit will be derived from the circuit.

However, it we look at the situation on closing,

made with considerable leakage between the two The situation here seems to be in a general way 1 similar to that in the case of a single contact; the circuit requirements for opening and closing are mutually conflicting and final design will be a matter of compromise. It is of interest in this connection that in certain cases the diilerence in time between the opening of two contacts may be quency impedance looking into the circuit from the contact ll (opened as indicated in Fig. 12) while the contact I! is still closed, we will get essentially a short circuit with high mutual inductance between the two halves of the coil l9 and 2|. Hence at the moment the circuit is opened at contact ii, if the other is still closed, we would expect a voltage build-up starting from zero similar to curve 2 of Fig. 2.

If we measure the impedance at the contacts Ii with contacts i2 open we will get a resistance at high frequencies. Hence, if the two contacts open together (or at the instant the contacts I 2 open after the contacts II have opened) we would get a controlled voltage jump corresponding to the differential division or the two currents in the upper and lower halves of the coil. If the currents divided equally as they normally would and the two contacts opened at the same instant there would be no voltage jump and we would systematically difierent from that corresponding to closing' For example, it has been observed that the difference in time of closing of two glass sealed contacts units whose magnetic circuits are operated in parallel may be as high as one millisecond. The difference in the time of opening, however, will not be more than or microseconds. This systematic diiference may be used to advantage in the design of the coil as it will dictate the amountof mutual impedance in the coil.

What is claimed is:

1. The combination of a pair of circuit makers and breakers connected in parallel and arranged to operate simultaneously, of a constant resistance protecting network in series with each consisting of an inductance shunted by a resistance, said inductances .being mutually coupled.

2. The combination of a pair of circuit makers and breakers connected in parallel and arranged to operate simultaneously, or a constant resistance protecting network in series with each consisting of an inductance shunted by a resistance, said inductances being mutually coupled and a condenser bridged about said combination.

ROBERT C. MATHES.

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

UNITED STATES PATENTS Number Name Date 1,929,057 Dellenbaugh Oct. 3, 1933 2,285,691 Wegener June 9, 1942 2,394,389 Lord Feb. 5, 1946

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