US2312915A - Protective circuit - Google Patents

Description

March 2, 1943. F, KURTZ 2,312,915

PROTECTIVE CIRCUIT Filed Sept. 15, 1939 2 Sheets-Sheet 1 Z2 23 M-l March 2, 1943. E. F. KURTZ 2,312,915

PROTECTIVE CIRCUIT Filed Sept. 15, 1939 2 Sheets-Sheet 2 FIG.3.

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4 'Mdld Patented Mar. 2, 1943 PROTECTIVE CIRCUIT Edward F. Kurtz, Attleboro, Mass, assignor to Metals & Controls Corporation, Attleboro, Mass, a corporation of Massachusetts i Application September 15, 1939, Serial No. 295,039

6 Claims. (Cl. 175-294) This invention relates to protective circuits, and with regard to certain more specific features, to electrical overload protective circuits for energy translating devices such as motors, generators, transformers, electromagnets, and the like.

Among the several objects of the invention may be noted the provision of protective circuits provision of circuits of the class described in which only the indicated thermally responsive switches are required (supplem ntary relays, magnetic switches and the like being unnecessary); the provision of circuits of the class described wherein each 01. said thermally responsive switches performs a protective function govemed by the current in an individual, different supply wire, or phase, but in which the openings of any one such switch disconnects the entire energy translation device from its power supply; the provision of circuits of the class described requiring closing of all of the separate thermally responsive switches in order to energize the energy translating device; and the provision of circuits 01' the class described which are relatively simple and employ simple and economical devices therein. Other objects will be in part obvious and in part pointed out hereinafter.

The invention accordingly comprises the ele ments and combinations of elements, features of construction, and arrangements of parts which will be exemplified in the structures hereinafter described, and the scope of the application of which will be indicated in the following claims.

In the accompanyingv drawings, in which are illustrated several of various possible emhodh ments of the invention,

Fig. 1 is a diagram of a circuit embodying the For the sake of simplicity herein, the invention is described as it applies to a motor, but it will be obvious to anyone skilled in the art that the invention can be similarly and equally well applied to generators, transformers, electromagnets and other forms of energy-translating devices.

There is a growing tendency to use thermaloverload protectors in conjunction with electrical motors, which control the motor directly (without intervening relays or contactors) and are of such nature that they can be mounted directly in the motor by the motor manufacturer. In this way, the motor manufacturer can be assured that the overload-protection switches have the proper characteristics for the particular motor; and because the protector is responsive to the actual temperature of the motor, more accurate protective results can be obtained. One of the problems in this art has been the protection oi? polyphase motors in a simple and eillcient manner.

It has sometimes been the practice hereto fore, for example, on small three-phase motors, to use two protectors each comprising a singlepole switch, a thermally responsive element adapted to operate said switch, and a heater in series with said switch and thermally adjacent to said element; one such switch being connected in each of two terminal leads. Several forms of such protectors are described in detail in John D. Bolesky applications Serial Nos. 158,187 and 189,075, filed August 9, 1937, and February 7, 1938, respectively. But this method gives unsatisfactory performance for three-phase motors although it does serve to protect the motor. For example, unless the heating of the motor at the points of mounting of the two protectors is the same, unless the currents in the two terminal leads in which the protectors are mounted are exactly'the same for the two protectors, and unless the characteristics of the two protectors are exactly the same, 'one will precede the other in functioning. Since absolutely uniform conditions are practically unknown in actual practice, this nomsimultaneous functioning usually happens, and the motor consequently operates single phase until the other protector functions. ihe first protector to open will probably reclose first if it is of the self-resetting type and the motor will remain stalled as it cannot start on single phase. Thus it is likely that the heavy stalled current will again trip this protector before the second resets and the two will continue, on and ofi, until they accidentally fall into step long enough to restart the motor. This means indeterminable delay. To overcome this the manual-reset type of protector as shown in said patent application Serial No. 189,075-may be used, but unless mechanical arrangements are provided so thata single push button operates both protectors, two buttons will have to be pushed each time, and, furthermore, manual reset protectors cost considerably more than automatic reset types, and the majority of applications arebetter served by the automatic type. To meet these conditions it has heretofore sometimes been the practice to interlock mechanically two protectors so that they operate mechanically in unison, or at least so that one may not reset while the other remains open. But such mechanical interlocking, while giving the desired performance, is more costly than the type of protectors and method connecting herein described. Also, there are various other disadvantages in the mechanical interlocking, as such protectors must be specially made for polyphase use, require greater skill in design and manufacture, and are naturally more complicated.

In the present invention, two thermal switches are used, each of which has a single-throw, double-pole switch actuated by a single thermal element, and the two switches are electrically connected in a circuit that accomplishes the above objects.

While it is not a part of the present invention, the construction of a switch which will satisfactorily accomplish the proper circuit operations, according to the invention, will first be described.

Referring to Figures 2, 3, and 4, numeral i indicates a base of electrical insulating material, upon which are mounted the various elements of the switch. A supporting plate 2 made of brass or steel or other electrical-conducting material is held on the base i by means oi a rivet 3 passing therethrough. Suitably attached to supporting plate 2 at one end thereof by welding, riveting or any other suitable fastening, is a thermostatic disc 4 such as, described and claimed in J. A. Spencer Patent 1,448,240, dated March 13, 1923. Disc 4 is shown electrically spot-welded to the supporting plate 2 by means of a tab 5 formed on the disc as an integral part thereof. A supporting and adjusting screw 5 is fastened to the center of the disc in a rotatable manner by means of proper shoulders and abut-= ments as indicated at numeral i, and, by en-=- gagement with a threaded hole 8 in the base I,

is used to adjust the operating temperatures of the element It. A lock nut 9 is used to hold screw 6 from turning after the adjustments are made. Welded or otherwise suitably fastened to the diametrically opposite point on the disc from the tab 5 is a contact button In. Button l0 makes good electrical connection with the disc 4. Cooperating with the button it! is a contact plate II which is held firmly on the base i by means of a rivet l2.

Extending through base i in a slldable manner in a hole therethrough' is a push rod it which is actuated by the motion of the thermal element 4, and which is made of an electrical in= sulating material such as Bakelite. Push rod l3 in turn moves a switch arm II on the other side of the base..' Switch arm i4 is made of spring steel or brass, or any other suitable electrical conducting material having the necessary resilient character. Switch arm it is mounted motor.

firmly at one end on the base by means of a rivet l5 passing therethrough. Fastened on the opposite or movable end of the switch arm it by welding or riveting so as to be in electrical con= nection therewith is an electrical contact button l6. Cooperating with movable contact button I6 is stationary contact plate H'- which is held securely to the base i by means of a rivet l8. Spring arm I is tensioned slightly to hold contacts l5 and i! normally engaged. In a recess l9 provided in the base i is mounted a heater wire 20 adapted to heat the thermal element 4. One end of heaterwire 20 is connected to contact plate i I and the other end to a terminal plate 2!. The purpose of the heater 28 is to offset the lag in temperature between the winding of the motor and the thermal element a when the winding temperature rises rapidly from ex cessive current as when the motor is stalled or very heavily overloaded.

Other details of the construction of the switch need not be recited herein. The operation of the switch is as follows:

If the disc 4 is heated either by increase in ambient temperature, or by the passage of electrical current through itand the electrical heater 20, or by a combination of these means, to a tern perature suiiicient to actuate it, it will snap its movable end and contact ID to the right from the Fig. 3 position, for reasons indicated in the abovementioned Spencer Patent 1,448,240. In doing so, the disc 4 breaks the electrical connection between movable contact it and stationary contact ii. At the same time, it also pushes to the right the insulating push rod is, which in turn pushes the switch member I 4 to the right, thus breaking the electrical connection between movable contact i5 and stationary contact ll. Thus, it is seen that the single thermally actuated element 4 aotuates what is in eiiect a single-throw, double-pole switch.

Any'other switch of suitable nature can be used in the circuits of the present invention.

Referring now to Fig. 1, there is shown in schematic form a circuit embodying the present invention. Numerals 22, 23, and 25 each indicate the power leads of a polyphase source of electrical current supply. Indicated at numeral 25 is a line switch controlling the motor and representing either a manual or an automatic switch of the type commonly used to start and stop the The two thermostatically actuated pro= tector switches of the type heretofore described are indicated at letters A and B and comprise the discs shown schematically as at la and ab, the switch arms shown schematically as at its and Nb with the contacts iiia, iia, ita and its in the motor winding 29; and terminal plate Zib is connected by a wire 32 to terminal 33 of the motor winding 29'.

Considering the circuit connections for each of the motor winding terminals:

For terminal 28, current passes from the power supply through wire 22, thermostatic disc heater 20a, and wire 2?. Forterminai 33, cur

pisses through power line 24, thermostatic eater and wire 32. For terminal 3!, asses through power line 23, switch arm Ell, switch arm Mo, and wire 30.

or the protective circuit is as otectors are made to match the motor iteintics properly, and are properly mount-= or in "the motor, the currents traversing .ters and discs plus the ambient temture within the motor where the protectors will cause the discs to open when maximum safe temperature for the motor line is reached under any operating condi- It is not likely that the discs will function simultaneously, but either one on functioning interrupts two of the three wires 22, 23, and 24 suppl ing power, and. hence either deenergiaes the motor. Similarly, if both switches should cut out, ,he motor will not be re energized until the last does so, either automatically or manually depending upon the particular type of switch emplayed.

Fig. 7. is shown a delta connected threewinding 29 for the motor or device prod. Eiowever, the circuit shown serves equally if a three-phase, Y-connectecl winding 34 c) or a two-phase, three-wire winding 35 is substituted. ..e principle of this invention can b equally well applied in other types of polyphase systems as the two-phase, four-wire system shown in 5, which one protector switch (A and B) is comiected in series with each of two windings "-d with the extra pole of each switch (A in series with the other winding 31 or 36,

c.iown in Fig. 5, numerals 38, 39, 40, and 4| icating the four power lines. n referring to Fig. l, the circuit is likewise operative if the switch pole represented by conand i "la were connected in series in wire or if the pole represented by contacts 16b were connected in series in wire 2?. In act, it is immaterial in what relationship the two tones are connected except that obviously the eaters Ella Elli; should not be in the same e a heater Zea (or 2012) is desirable for types or" illlDbDlS, it is not needed forsome oi energy-translating devices, particularly ose which only gradual heating can occur.

.law or the above, it will be seen that the veral objects of the invention are achieved and r advantageous results attained.

many changes could be made in the above tructions without departing from the scope of e invention, it is intended that all matter con- "ive and not in a limiting sense.

I I in combination with an electrical polyphase translating device having at least three t minals, a protective circuit therefor double pole switches each having ly responsive means for actuating it, and

at least three power supply lines for the device, one of said lines being connected to one of said power terminals with one pole of one switch in series therewith, a second line being connected. to a second power terminal with one pole of the other switch in series therewith, and the second pole of each of said switches being connected in one of the lines other than the line in which its respective first pole is connected.

2. A circuit as set forth in claim 1, including electrical heating means in each of the said switches adapted to afiect their respective thermally responsive means in accordance with the electrical current passing through said heating means, the respective heating means for each switch being connected in series in one of the power lines connected through the switch it affects but not in the same power line with the other heating means.

3. A circuit as set forth in claim 1, including electrical heating means in each of the said switches adapted to afiect their respective thermally responsive means in accordance with the electrical current passing through said heating means, the respective heating means for each switch being connected in series in one of the power lines controlled by that switch.

4. In combination with. an electrical polyphase energy translating device having two windings with two power terminals each, a protective circuit therefor including two double pole single throw switches each having thermally responsive means for actuating it, and four power supply lines for the device, one of said power lines being connected to one terminal of the first winding with one pole of the first switch in series therewith, a second nected to the other terminal of said first winding with one pole of the second switch in series therewith, a third power line being connected to one terminal of the second winding with the other pole of the first switch in series therewith, and the fourth power line being connected to the other terminal of the second winding with the other pole of the second switch in series therewith.

5. A circuit as set forth in claim 4, including electrical heating means in each of the said switches adapted to affect their respective thermally responsive means in accordance with the electrical current passing through said heating means, the respective heating means for each switch being connected in series inone of the power lines controlled by that switch.

6. In combination with an electrical polyphase energy-translating device having at least three power tedminals, a protective circuit therefor including two double-pole switches each having thermally responsive means for actuating it, and at least three power supply lines for the device, one of said lines being connected to one of said power terminals with one pole of at least one switch in series therewith, a, second line being connected to a second power terminal with one pole of the other switch in series therewith, and the second pole of each of said switches being connected in one of the lines other than the line in which its respective first pole is connected.

EDWARD F. KURTZ.

power line being con"

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