US2597873A - Electromagnetic switching device for controlling electric circuits - Google Patents

Description

May 27, 1952 F. KESSE NG ELECTROMAGNETIC S ITCH DEVICE FOR CONTROLLING C C CIRCUITS Filed Feb. ,1947

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lNVENTOR Fritz Kesselring.

ATTORNEY Patented May 27, 1952 ELECTROMAGNETIC SWITCHING DEVIGE, FOR CONTROLLING ELECTRIC'CIRCUITS Fritz Kesselring, Zollikon-Zurich, Switzerland, assignor to FKG- Fritz Kesselring Geratebau Aktiengesellschaft, a Swiss company Application. February 24, 1947, Serial No..730;330 In. Switzerland February 23, 1946 7 Claims. (Cl. 20087) (Granted under the provisions of sec. 14,. act of March 2, 1927; 357- 0. G.'5).

My invention relates. to electromagnetic, relays,

contactors, protective cut-outs, interrupters; andv other switching devices in, which the opening, or

armature.

It i's an. object. of my invention to provide electromagnetic switching devices capable of reliably operating. with switching, periods below 3 .l 0 seconds, i. e., that require less. than thisextremely short time from. the moment the operating current. or voltage is applied to the electromagnet untilv the. moment when. the armature completes the switching operation of. the. contacts. Another object; of the invention is to d'eviseelectromagnetic switching. devices. whose switching periods are as. short as 5 l0- seconds or. less. Still other objects of the invention, concern the provision of a switch design generally capable of faster performance than. those heretofore available. and willbe apparent from the following description in conjunction. with the drawing, in which:

Fig. I. is a schematic showing. of an electromagnetic switchi-ng, device according. to the invention with a lever type armature and two air gaps, while Fig... 2 shows. a view of. the same armature at a right. angle. to the. illustration in. Fig. 1

Fig. 3 is a diagrammatic illustration of the magnetic circuit.

Asv the following. explanations will be more readily understood with. referencev to. a concrete example, a general description. of. the device according to Figs.v I and 2-will' first be given before dealing with. thenovel features. embodied in that device.

The illustrated device. has an. armature I firmly mounted. on a pivot pin 2. which is torsionally elastic. to. permit the. armature to. be; attracted by the poles 8 of an; electromagnet when. the coil. 9 of. the magnet is. energized. Armature l' carries two contacts 3 and is normally biased by its pivot pin to; a position in which. the contacts 3; engage respective stationary members 6., An. air gap. is then formed between the respective. pole faces of magnet. poles and; armature. When the magnet lsenergi'zed', armature l is attracted so that contacts 3 engage respective stationary members. 7' while the air gap is. substantially closed. Members. 6. or members Tor all, these members consist. of stationary contacts. to cooperate with the movable contacts 3. in opening or closing a. circuit to be, controlled, Fig; 11 indicates the length l 01'' the armature and" the maximum length g of the air gap. both measured along the magnetic flux path. Aswill. be shownv below. the invention requires these and other magnitudes to have a given relation to one another and to the magnetic induction in the. air. gap under energized conditions of the. electromagnet.

While heretofore, electromagnetic. relays. for special purposes have been. constructed for. a switching period of about. 1.0, seconds, the. constructional principles employed in these specialrelays cannot be generally applied. and. are not suitable for power current switches. The switching periods heretofore actually attained for power current control are within the range from some hundredth down to a few thousands of a second. Now it can be shownand this is one of the recognitions thatv underlie the, present inventionthat even a switching, period of one thousandth of a second would offer no definite advantage in connection with the usual switch constructions. Let, for example, an alternating current of a frequency of C. P. S. be considered. The instantaneous value of this current at a moment which lies a thousandth of a second, that is, 18 electrically, before or after the current zero passage. still amounts to 44% of the effective current value or 31% of the maximum value. If the switching commences at this momentand this would be necessary in order to complete the switching stroke by the time the current passes through zero.-there is still. a very marked stressing of the contacts. Furthermore, in power, rectifier and converter current installations. the velocities of current increase due to disturbances in the nature of overloads and short-circuits may have the order of I to 1010 amperes per second. This means that within a thousandth of a secondthe' current may increase up to. 10,000 amperes, that is, generally speaking. up to many times the. rated current. This is of great importance when, for example, impedances must be so rapidly switched by such. a device that the short-circuit current assumes only a small proportion of what its value would otherwise be. Extensive tests have furthermore illustrated that in three-phase current installations during the period of a thousandth of a second immediately prior to the zero passage, doublepole arc short-circuits are converted to. a large ercentage into three-pole arc' shortv circuits.

whereas in a period of less than. 3-101- sec. a transition from two-phase to three-phase shortcircuits is practicallynolonger expectable. This is an important. recognition for the use of synchronous control or switching devices in multiphase installations. In multiple phase contact rectifiers, for instance, the commutation period, expressed in electric angular degrees, amounts to about 15, corresponding to 0.83 mili-sec. Consequently, only switching periods considerably below a thousandth of a second, and in particular less than 3-l0 sec., allow the initiation of the circuit interruption at a moment when the current has dropped to a small fraction of the rated current value. For mechanical rectifiers, in particular, the switching periods should be below 10 sec. For relays for synchronous control and devices for suppressing the short-circuit current by rapid switching-in of impedances, the switching periods should preferably be in the order of -10 see. or less.

While, as mentioned, the conventional electromagnetic switching devices have switching periods considerably higher than 3 10 sec., the invention affords the just-mentioned desirable reduction to much smaller time values and thus provides the technical and economical advantages apparent from the foregoing. This is achieved by the means and in the manner ex plained presently.

The switching period t of devices of the kind here dealt with can be expressed with sufficient accuracy by a magnitude A hereinafter referred to as the design magnitude and a second magnitude D which is hereinafter referred to as the dimensioning magnitude: t=A D. The design magnitude A is, to a large extent, independent of the dimensioning of the switch element but is greatly dependent upon the mass distribution, the ratio of the magnetic to the electrically conductive masses, and the angle between the resulting magnetic field in the air gap and the longitudinal axis of the armature. The dimensioning magnitude D which is given by the following equation:

In devices according to the invention, for

instance as shown in Fig. 1, the dimensioning magnitude D in order to attain switching periods of less than 3X sec., is smaller than l.5 l0- cm./gauss. In other words, the invention requires that the magnetic length Z of the armature and the magnetic length g of the air gap be rated relative to each other and to the mean magnetic induction B so that the value is below l.5 l0* cm./gauss. Since the induction B depends upon the energization or saturation conditions of the magnet as well as on the magnetizable cross-sectional area, it will be understood that the just-mentioned correlation, for any given magnetizing condition, places the magnetizable cross-sectional area of the armature in a given limit relation to the above-mentioned lengths. For still smaller switching periods of, for example, 10- sec. or less, the armature is preferably designed for a dimensioning magnitude D smaller than 0.5 l0 cm./gauss; in many cases, it is advisable if the length I of the armature is at the most 1 cm. It is also often advisable to provide a length 9 of the Working air gap of at most 0.01 cm. If it is desired to avoid rebound of the contacts the switching travel will preferably lie below that limiting value at which rebounding takes place at the existing inherent dampening effect of the material used. Finally, the mean induction B in the armature or working air gap is preferably given a value of at least 10,000 gauss. For example, with 1:1 cm., g:0.01 cm., and B:10,000 gauss, a dimensioning magnitude D:l0- cm./gauss is obtained.

As mentioned, the design magnitude A plays also a part with respect to the switching period t. However, this magnitude cannot be varied within such wide limits as the dimensioning magnitude D. Of particular influence relative to magnitude A is the positioning or type of movement of the switch element. The movement of the switch element or armature may be either translational or rotary. Other things being equal, the design magnitude is a maximum with a purely translating movement of the armature, since then the whole mass of the armature must be accelerated. Consequently, for short switching periods a positioning of the armature is often preferable that provides for rotary movements, in particular about the centre of gravity or about a pivot axis close to the centre of gravity. In many instances, devices are particularly suitable which require the fewest possible ampere turns for actuating the armature. This is obtained with advantage by means of an armature which rotates approximately about one end (see pivot 2 in Fig. 1), the air gap at this end amounting to at most of the working air gap. If the armature serves to actuate contacts disposed at both ends of the armature, the appertaining positioning or pivoting means are preferably designed to provide two or three freedoms of movement for the armature, as will be explained below with reference to the example shown in Figs. 2 and 3. It is also preferable to keep the entire movable mass of the armature structure as small as possible. To this end, the armature may also serve as a current conductor and carry the movable switch contacts at its respective ends. This may make it necessary, for improving the electric conductivity, to provide the armature structure with good conducting masses, for example, of copper or silver. Such masses should amount at most to of the mass necessary for magnetic purposes. The contacts are preferably made so light that their mass is at most of the mass necessary for magnetic purposes. In an extreme case, practically the entire current can be conducted by the magnetizable material of the armature and only thin coatings or platings need be provided on the contact surfaces, so that the additional masses for current conduction and contact formation are practically negligible. In order to obtain the most favorable magnetic attraction, the armature structure is so arranged with respect to the poles of the magnet that the angle between the magnetic attractive force and the longitudinal axis of the armature is greater than 70, that is, the attractive force, as far as possible, should be perpendicular to the longitudinal extension of the armature. With armaassume:

tures which rotate abouttheir centre of gravity and which have only negligible massin addition? to the mass necessary for magnetic purposes, ar-' rangementmagnitudesA of 1-2 gausssee/cm. areobtained.- If at the sametime'the dimensioning magnitude D has the value of about 0.4 1'0 cm./gauss, switching periods in the order of switching period: can be kept at aminimum-by a1- desi'gn whosev additionally movable elastic mass.

comes into action with at the most of the masses of the armature structure necessary for magnetic purposes..

The features mentioned in the foregoing paragraph are embodied in the illustrated device whichwill now be described more indetail.

As mentioned above, the armature l is mounted'.

on a pivot pin 2. This pin consists of a thin torsion spring of such a slight bending strength that the armature cannot only rotate, but can also deflect at least in one, but generally in two directions. That is. the armature has at. least two freedoms of movement. The two contactpieces 3 of the armature are connected together by a strip or plate 4 of good conducting. material;

The laminations 5 of the armature consist. of iron or other good magnetizable materiaL. and

are called upon toassist as currentconductors.

The stationary members or contacts 6' limit the.

movement of the armature and this determines.

the length 9. of the air gap.- As mentioned in. order to ensure the smallest possible switchingperiods, the entire armature structure is rigidand the additional mass of the movable contacts 3 and of the good conducting plate 4 should amount to only asmall proportion of the mass. of the laminations 5. Furthermore, the angle between the magnetic axis of the armature and the direction of the magnetic field in the working air gap is greater than 70. The design magnitude A of this embodiment. independent of. the dimensions, amounts to about. gauss-see/cm. It. follows fromv the equation of the dimensioning. magnitude D, that the product of the length I of the armature l and the length g ofthe air gap mustbe as small as possible. Thesere'quirements are readily satisfied with the illustrated design. With a. length Z of 1 cm., an air gap length g of 0.0 1 cm. and amean induction B of 10.000 gauss, a device. according to the drawing has a switching period: of about 10" sec. When; the coi-lsof the electromagnet are excited, assuming that the time constant of the control: circuit is less than the: desired switching period, thearmature: commences to rotate counterclockwise. One of the two movable contacts 3 will first come into contact with: the corresponding stationary contact 1. Due to the low' bending stiffness of the torsion spring 2, or more. generally expressed,- since the movable system has at least two free-- doms of movement, the second contact 3 comes immediately thereafter in touch with the pertaining stationary contact. The working air gap is then fully closed on both sides and: the circuitis closed; If, on the contrarman interruption of the circuit is desired. the abutments 8 form stationery soonas; the magnetic field is excited; the? interruption process; come mences. An interruption, however,- can also be efi'ectedthe spring, force it the contacts I are supplied with; current; the interruption then takes place when: themagneticfielddisappears. If the conditions: aresuitable for an arc-free interruption the interruption can be terminated appreciablybefore the armature: reaches the end of its-travel. The attainable interruption periods may lie even below 10- sec.

While in the illustrated example, I have shown elastic biasing means for the armature, other resetting. forces. such. as a magnetic bias, gravity,

or centrifugal'forc'e may be used for this purpose.

Devicesaccording: to' thev present invention can be employed in multiple arrangements,v for instance, with relays: or; all kinds. protecting. cutouts; regulators ..switches for; low andhigh. voltage. reci'giilers converters; choppers and the like devices. in. multhphases circuits.

anexplanatory addendum. to the foregoing description of the invention, the following. elucidations of the above-given formulae may F be of interest to those desiring a further study.

As mentioned, the switching period t can be expressed with. suflicient accuracy as a function or? a; design; magnitude A. and adimensioning, magnitude; DzfzAi-D". It will now be shown how this equation can be developed. To this end, sec Fig. 3;. the f ollowing' values. will. he used:

lzlength'a of armature: k

gzlerrgth r of air gap; (switching travel) Aazarea'z of pole race Arz'cifossrs'ectional areaoif armature iridilctibn; in: the. gap

m:mass of armature related to the center of the air gap Fin:m agn'etic pull Flztotallr spring". bias-i force related-T to the center of the al gap :p'emeabiiity With an armature pivoted about its gravity axis and spring biased to open position, it may first'be'assumed',.in approximation, that the magnetic" i'iiductitui drops instantaneously to zero at the moment when. the io'r'c'e Fr. overcomes the? magnetic pull FM and the armature starts turning; azwayrrom. the magnet poles to the illustrated position. the armature movement is con'iirollecl.- by the spring force F'i which can be expressed; as:

By integrating twice; and" applying the limit condition: gzo for t":0, the gap distance or switching travel 9 can be? expressed as a function of time:

Its the ramming. the; term is called? design constant and the term B is called dimensioning constant," and the last-mentioned equation is expressed as t A-D.

It was found theoretically and by tests that for any given design the constant A can be varied only within very narrow limits while the dimensioning constant D is readily susceptible to change and hence represents the essential criterion for the duration of the switching period t. This explains the significance of the relation used in the foregoing description of the invention.

For the derivation just given, the assumption was made that the air gap induction disappears suddenly at the commencing moment of spring biased armature motion. In reality, the gap induction does not vanish immediately but retains some decaying impedient effect on the armature movement. Hence, if the switching period t is calculated for with B being the initial inductance (Ba), the,

resulting value is smaller than the time actually measured. It was found, however, that sufficiently accurate results are obtained if the value 13 in the foregoing equations is defined not as the initial inductance (Bn) but is given the approximate value of the mean magnetic induction in the air gap, this value B being smaller than the initial induction. Hence, the mean magnetic induction is referred to in the foregoing specifica tion.

I claim:

1. An electromagnetic switching device for switching periods below 3 l0- sec., comprising an electromagnet structure having two electrically conductive poles with respective pole faces mutually spaced and electrically insulated from each other to form stationary switch contact surfaces, a rigid and electrically conductive armature rotatable between two positions and biased toward one of them, said armature having substantially coincident gravity and pivot axes and being pivoted between said pole faces and provided with contact faces opposite said respective pole faces so as to form respective air gaps when said armature is in said one position, said armature having a magnetically effective crosssectional area approximately equal to that of each armature contact face and having a magnetically efiective length l correlated to the length g of each air gap and to the mean magnetic induction B of said gap so that the value is below 15x10 cm./gauss,

2. An electromagnetic switching device for switching periods below 3x10- sec., comprising an electric circuit, a stationary switch structure having an electromagnet and having rigid contact means connected with said circuit, a rigid magnet armature having a magnetic flux path in common with said magnet and being movable between two positions toward and away from said magnet, spring means connected with said armature and biasing it away from said magnet, said armature being electrically engageable with said contact means in one of said positions to close said circuit through said armature, said arma- 8 ture and said magnet forming together an air gap when said armature is in said one position away from said magnet, said armature and said gap having along said path respective lengths (Z and g) correlated to the mean magnetic induction B in said gap so that the value is below 1.5 10 cm./gauss.

3. An electromagnetic switching device for switching periods below 3X10- sec., comprising rigid stationary contact elements electrically insulated from each other, an electromagnet and a rigid armature structure having a common magnetic flux path, said armature structure being movable between open and closed positions relative to said contact elements to form a rigid contact bridge when in said closed position, said magnet and said armature structure forming together an air gap in one of said armature positions, said armature structure having along said path a length l of at most about 1 cm., and said gap having along said path a length 9 correlated to said armature length and to the mean magnetic induction B in said gap so that the value is below .5 l0 cm./gauss.

4. An electromagnetic switching device for switching periods below 3 10 sec., comprising rigid stationary contact elements electrically insulated from each other, an electromagnet and a rigid armature having a common magnetic flux path, said armature being movable between open and closed positions relative to said contact elements to form a rigid contact bridge when in said closed position, said magnet and said armature forming together an air gap when said armature is in said open position, said gap having along said path a length g of at most about .01 cm., and said armature having along said path a length l correlated to said gap length and to the mean magnetic induction B in said gap so that the value is below .5 l0" cm./gauss.

5. An electromagnetic switching device for switching periods below 3 l0 sec., comprising rigid stationary contact elements, an electromagnet having a pole face, a rigid armature having a common magnetic flux path with said magnet and having a pole face opposite that of said magnet, said armature being movable between contact closing and contact opening positions relative to said stationary contact elements and being biased to one of said positions, said pole faces being spaced from each other by a given air gap when said armature is in said one position, said armature having a magnetically effective cross-sectional area approximately equal to that of said armature pole face, and said armature and said gap having along said path respective lengths (l and g) correlated to the mean magnetic induction B in said gap so that the value 6. In an electromagnetic switching device according to claim 3, said armature having an electric resistance at most equal to the contact resistance obtaining between said armature and said stationary contact means when said armature is in said circuit closing position.

7. An electromagnetic switching device according to claim 3, comprising biasing means tending to move said armature to one of said positions in opposition to the attractive force of said magnet, said biasing means having a force of at least 20% of that of said magnet when said armature 10 is in attracted position.

FRITZ KESSELRING.

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

UNITED STATES PATENTS Number Name Date Egerton Jan. 18, 1921 Kardaetz Aug. 22, 1922 Bossart May 17, 1932 Bossart Nov. 8, 1932 Skrobisch June 24, 1947 Langer July 20, 1948 Coake Mar. '7, 1950

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