US2248623A - Protective device - Google Patents

Description

yJuly 8, 1941. A. R. HAND 2,248,623

PROTECTIVE DEVICE Filed D80. 29, 1939 lYwvew/Ytof 1.

Afth U1 WQ. Heizel, by l/Mf H ie Attofw ey.

Patented July 8, 1941 PROTECTIVE DEVCE Arthur R. Hand, Schenectady, N. Y, assigner to General Electric Company, a, corporation of New York Application December 29, 1939, Serial No., 311,536

(Cl. 260-122l 3 Claims.

My invention relates to protective devices and concerns particularly apparatus for preventing the occurrence of excessive voltage in the secondary windings of current transformers.

It is an object of my invention to provide an improved protective arrangement which is light in weight, small in size, remains at relatively low temperature, and has means for adjusting the maximum temperature with the provision for definitely setting the adjustment, the tempera ture control being so arranged as to respond very quickly to tendencies for variation of temperaI ture. Other and further objects and advantages will become apparent as the description pron ceeds.

In carrying out my invention in its preferred form, I provide a protective resistor unit composed or" resistance material which is nonlinear in its voltage-current characteristicv which tends to decrease in resistance with an increase in. voltage or current.` The unit is connected across the terminals of the winding which is to be prom tected against occurrence of overvoltage, and to protect the unit against overheating a short-cirm cuiting mechanism is provided which responds directly to the temperature of the hot portion inside the unit and not to the temperature ci the outside surfaces which may he cooled by air currents or radiation.

The invention may be understood more readily from the following detailed description when considered in connection with the accompanying drawing and those features oi the invention which are believed to be novel and patentable will be pointed out in the claims appended heren to. In the drawing Figure l is a front elevation of one embodiment of my invention. Figure 2 is a vertical section cut by the plane 2Min shown in Figure i, as seen by looking in the direction of the arrows. Figure 3 is a perspective View ci a member supporting some of the elements of the apparatus. Figure 4 is a fragmentary hori` zontal section of the rightmhand end portion of the lower piece of the member shown in Figure 3 represented as being cut by the plane 4--4 indicated in Figure 2, the View being as seen by looking in the direction ci the arrows, and Figure 5 is a circuit diagram ci the apparatus illustrated in the preceding figures. Like referw ence characters are utilized throughout the drawing to designate like parts.

Although my invention is not limited to use for preventingovervoltages in the instrument windm ings of current transformers, the invention will be explained for the sake oi illustration in con nection with an arrangement for preventing the voltage in the secondary winding of a current measuring transformer from becoming excessive when the secondary burden is inadvertently removed. In Figure 5 there is schematically shown a current transformer I I, having a primary winding I2, which it will be understood is to be connected in series with the alternating current conductor in which current is to oe measured, and

a secondary winding I3 with output terminalsv i4 to which an ammeter or current coil of a wattmeter, relay or other device may be connected. For guarding against overvoltage, a resistor unit i5 oi the nonlinear type is also connected across the secondary winding of the transformer. To prevent the destruction of the resistor unit I5 by excessive heating, a temperature responsive shcrt=circuiting switch i6 is provided. Parts of the protective device are carried by a metallic member Il and the device as a whole in turn is secured to a supporting frame including a nom conducting plate I8.

The resistor unit l5 comprises a pair of disks i9 and 2o clamped together against either side of the vertical plate 2| of the member Il. The disks it and 2li are composed of a resistor material, the resistance of which decreases markedly with rise in applied voltage. For this pur pose I. may employ resistance material having a hyperbolic resistance-current characteristic. In mathematical terms the resistance-current characteristic may be expressed by the equation:

which may be reduced 4to the equation:

i i:icaa

I=KEb The equation symbols R, I and E represent resistance, current and voltage, respectively, and the symbols c, C, K and b represent constants the value of which depends upon the characteristics of the material. Such material is described in United States Patent 1,822,742 McEachron and may be produced in such a manner as `to malte the exponent a close to unity, for example about .9, thus making the exponent li very great, and permitting the current to rise Very greatly with a relatively slight rise in voltage. As explained more in detail in the Mc- Eachron patent, the material of which the disks i9 and 2li are composed is preferably a mixture of silicon carbide and carbon with a suitable binder, or of silicon carbide with other suitable materials such as tungsten and molybdenum and the like.. It will be understood that any suitable material having a nonlinear resistance-current characteristic may be employed. Preferably the surfaces of the disks I9 and 20 are sprayed with some conducting material, such as copper, for the sake of improving the surface conductivity with respect to heat and electric current.

For clamping the two disks against the plate 2 I, metallic contact disks 22 and 23 with threaded openings and a boltl 24 may be employed. The bolt 24 may also be used to secure the lentire unit to the insulating plate III. .Preferably an insulating sleeve 25 is provided to insulate the bolt 24, the plate 2| and the inner surfaces of the disks I9 and 20, and a washer 26 composed of insulating material may be interposed between the plate I8 and the contact disk 22.

As shown specifically in Figure 3, the plate 2| has a groove 21 cut therein adapted to receive a thermostatic strip 28 composed of bimetal. The bimetal consists of two intimately joined sheets of material having different temperature coeicients of expansion, such as brass and invar, for example, or nichrome and invar. As will be apparent from Figures 1 and 2, the greater portion of the length of the bimetallic strip 28 is within the resistor unit I5 between the disks I9 and 2|). However, the end portion 29 extends in order that a short-circuiting switch may be operated thereby.

At the lower end of the plate I1 is secured a horizontal piece or shelf 30 in front of which is secured a switch contained in a casing 3| and having an operating pin 32 protruding from the back wall of the casing and in operative relationship to the end portion 29 of the strip 29. The specific construction of the switch I6 is not a part of my invention. The switch is preferably one of the type in which fast snap action is obtained with complete opening or closing in response to very slight movement of the pin 32. If desired, a snap switch of the type disclosed in United States Patent 1,960,020-McGal1 may .1,

be employed. Insuch a switch there is an E-shaped piece of spring metal carrying a movable contact at the yoke end and having the arms of the E rigidly secured at their open ends to the stationary part of the apparatus. The two outer arms 33 (Fig. 2) are bowed and supported in a different plane from the center arm 34 so that very slight pressure and movement against the inner arm 34 near its supported end causes the yoke end or joined end of the arms of the E to snap from one position to another, the stationary contact being mounted opposite the movable contact carried by the yoke end of the E. These features need not be illustrated in detail since they are shown in the aforesaid McGall patent and do not constitute my invention. For the purpose of insulation there may be provided a button of molded insulating material 35 in which the pin 32 is mounted.

It will be apparent that with such a micrometric switch relatively minute movements of the end 29 of the bimetallic strip are suicient to actuate the switch. The position of the switch casing 3| with respect to the bimetallic strip end 29 in any given temperature determines the ternperature which must be attained by the resistor unit I5 to cause actuation of the switch I6. I provide an adjustable mounting for the switch casing I6 which permits making very slight variations in the position of the casing I6 while permitting the position of adjustment to be maintained very definitely. To this end I cut out the interior portion of the shelf 30 in the manner shown in Figure 3 to form a relatively stiff spring having two arms 36 and 31 which may be moved together or apart slightly but each relative motion is resiliently resisted. The switch casing I6 is fastened upon the front arm 36 in any suitable manner as by means of machine screws, and a pair of screws 39 and 40 are provided for adjustment. The screw 39 extends through a hole in the arm 31 and is threaded into the arm 36, whereas the screw 40 is threaded through the arm 31 and extends into an unthreaded hole in the arm 36. The screw 39 may be mounted for drawing the arm 36 toward the arm 31 to diminish the temperature of the resistor unit at which switch operation takes place, and a nut 4| on the screw 40 bearing against the arm 36 may be employed for forcing the arms 36 and 31 apart to move the switch casing I6 away from the bimetallic strip 28 and increase the temperature of the resistor unit at which switch closing takes place. It will be understood that in making adjustment by means of the two elements 39 and 4|, one of them must (be released sufficiently to permit operation of the other and when the adjustment has been made the second element may be tightened to retain the adjustment rigidly. For example, if the arms 36 and 31 have been drawn together to make adjustment by means of the screw 39, the nut 4| may then be run against the arm 36 so that parts cannot subsequently be jarred out of adjustment. Lock washers may be employed if desired.

It will be understood that the strip 28 should have its upper end securely fastened in the groove 21. For this purpose a locating rivet 42 may be employed and the upper end of the bimetallic strip 28 is then brazed or welded to the metal forming the plate 2|.

It will also be understood that the representation of the switch I6 in Figure 5 is only schematic and that the thermostatic strip 29 is not used for making actual electrical contact to the stationary contact of the short-circuiting switch I6 when using a micrometer switch of the type illustrated in Figures 1 and 2.

The operation of the apparatus will be apparent from an examination of the drawing. In case the transformer I should become or remain energized after a burden has been disconnected from the output terminals I4, the voltage across the secondary winding I3 will tend to rise, increasing the current flowing through the resistor unit I5. A circuit may be traced from one end of the winding I3 through a conductor 43, the bolt 24, contact plates 22 and 23, the disks I9 and 20 in parallel, the plate 2|, a terminal bolt 44 and a conductor 45, back to the other end of the secondary winding I3 (Fig. 5). The passage of the current through the disks I9 and 20 produces a heating effect, which in turn deects the strip 28 against the pin 32 and causes the switch I6 to close (Fig, 2). Upon the closing of the switch I6 the resistor unit I5 is shunted when a parallel circuit is formed from the terminal bolt 24 through a conductor 46, the switch I6, a conductor 41, and back to the other terminal 44 (Fig. 1). Upon the shunting of the resistor unit it is permitted to cool and the contact I6 reopens, remaining open unless the voltage across the winding I3 is still excessive in which case it recloses to protect the resistor unit I5. If the burden remains disconnected from the terminals I4, the contacts I6 thus keep opening and closing, remaining closed long enough to prevent the temperature rise from becoming excessive in the material of which the disks I9 and 20 are composed.

In accordance with the provisions of the patent statutes, I have described the principle of operation of my invention together with the apparatus which I now consider to represent the best embodiment thereof but I desire to have it understood that the apparatus shown is only illustrative and that the invention may be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. Apparatus for protecting current transformers against overvoltage in their secondary windings, comprising a pair of disks of nonlinear resistance material, metalized surfaces on said disks, a supporting plate between the disks and extending beyond them, composed of a good electrical conductor, contact pieces on either side of the disks composed of electrical conducting material, means for clamping said disks together against said supporting plate, said plate having a groove therein between the nonlinear resistance disks, a bimetallic strip in said groove and projecting beyond the disks with the inner end of the strip secured to said plate, and a switch of the micrometer type supported at the extending end of said plate having a contact closing member in operative relation to the projecting end of the bimetallic strip, relatively movable contacts, one of which is electrically connected to said plate and the other of which is electrically connected to said contact disks, said switch contacts also being adapted to be connected across the secondary winding of the current transformer which is to be protected.

2. In combination a resistor unit comprising a pair of disks, a supporting plate between the disks and extending beyond them, composed of a good electrical conductor, said plate having a groove therein between the resistance disks, a bimetallic strip in said groove and projecting beyond the disks with the inner end of the strip secured to the said plate and a switch with which the thermostatic strip is in operative relation, whereby the switch is controlled by the temperature of the resistor.

3. An adjustable thermostatic protector switch comprising a mounting plate in thermal relationship to the inner portion of a resistor unit, a groove in said plate, a thermostatic strip secured therein extending beyond the end of said groove, a shelf secured to said plate beyond the resistor unit, a switch mounted on said shelf having a movable operating member in operative relationship to the end of said thermostatic strip, said shelf being split lengthwise along the intermediate end from one end nearly to the opposite end to form two legs, said switch being supported on one leg and the other leg being secured to said plate, and means for separating said legs or bringing them together to vary the spatial relationship between the operating member of the switch and the end of said deflectng strip.

ARTHUR R. HAT" J.

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