US2004792A - Electric switching - Google Patents

Description

O. P. McczARTY ELECTMQ swITcHINQ Filed June 23, 1955 June ll, 1.935.

2 Sheets-Sheet 1 Inventor: Orin PmcCartB brg H 49M His Attorneg.

June 11, 1935. Q, P, Mec/mv y 2,004,792

ELECTRIC VSWI'lGI-IING Filed June 25, 1933 2 Sheets-Sheet 2 Fig. 5.

Inv'ehtror: Orn P. TTlcCaTtrg 7 Patented June 11, 1935 UNITED STATES PATENT OFFECE ELECTRIC SWITCHING Orin P. McCarty, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York Application June 23, 1933, Serial No. 677,200

11 Claims.

My invention relates to electric switching and more particularly to an improved method and apparatus for transformer tap changing under load.

It is often desirable to be able to transfer an electric current from one point to another, Asuch as from one tap to another of a voltage ing transformer, without interrupting rent. Heretofore, this has usually been regulatthe curdone by making use of a mid-tapped reactor-which is so arranged that one line connection is ma mid-tap and the other line connection de to the is made to a single transformer tap through two relatively movable contacts connected respectively to the terminals of the reactor. arrangement, l through the two halves of the reactor With such an the current normally dividesV whereby no net flux is produced and the impedance of the reactor is a minimum. When it is desired to change taps, one movable contact is rst transferred to an adjacent transformer tap and there after the other movable contact is so transferred. During both of these transfers the entire current passes through one-half of the reactor.

produces a considerable voltage drop due to the reactance of the reactor and at high current values this drop is often a sulliciently high per-u centage of the circuit voltage to produce lamp nickel. in additiomthe reactor, during the time the contacts are moving from tap to tap, con stitutes a 2 to 1 ratio auto-transformer which sometimes produces objectionably high voltages which have to be guarded against by relatively expensive design.

ampere characteristic impedances', that impedances in which the voltage drops is to say.7 increase and decrease, respectively, with increasesLin current therethrough.

The positive voltampere characteristic impedance may be of any suitable or well known type such, for example, as an ordinary reactor, but I prefer to use a simple resistor because of its much lower cost per unit rating. Broadly speaking, the negative volt-ampere chare acteristic impedance may be of any suitable or well-known type, but I prefer to utilize tric arc as this impedance.

an 816C- (Cl. 17h-119) Through the use of such impedances it is possible to limit definitely the maximum voltage attained during switching regardless of the magnitude of the current switched. This is because the volt-ampere characteristics of the impedances must intersect and the voltage represented by the point of intersection must be the maximum switching voltage attainable. This, in turn, is because the current to be switched will always choose the path having the lower voltage drop 10 and the voltage drop in one (or the other) impedance will always be lower than the voltage drop in the other, except at the current value corresponding to the intersection point. Specifically, the switching voltage will be limited by the voltage drop in the resistance at current values below the intersection point and it will be limited by the arc drop at currents above this point.

An object of my invention is to provide a new 20 and improved method and apparatus for electric switching.

Another object of my invention is to provide a new and improved system of transformer tap changing under load.

My invention will be better understood from the following description taken in connection with the accompanying drawings and its scope willbe pointed out in the appended claims.

In the drawings, Fig. i is a diagrammatic illustration of a system whereby my improved method may be carried out manually; Fig. 2 illustrates diagrammatically a modiiied apparatus for practicing my invention; Fig. 3 is a set oi? curves for illustrating the principles of operation of my invention; Fig. 4 is an automatic voltage regulating system utilizing the modification of my invention ,Y illustrated in Fig. 1 and Fig. 5 is a modiiied automatic voltage regulating system v Jtiiizing the modification shown in Fig. 2.

Referring now more particularly to` Fig. 1, I have shown therein a winding l connected in a circuit 2. Winding l is provided with a plurality of taps and alternate taps are connected to the. xed contacts of dial type switches 3 and, I, re- 45 spectively. Associated with dial switches 3 and t are a pair of switches or circuit breakers 5 and 6 which constitute essentially single pole, doublel throw switches having movable contacts or arms 'l and 8, respectively, connected to one of the cir- 50 cuit conductors 2. Contact arm l is connected to the circuit conductor 2 through a resistor 9 so that this contact may be said to be a high impedance contact with 'respect to contact 8 which may then be said to be a low impedance contact. 55

' With the circuit connections shown, the current flows through the entire winding I, the dial switch 3, contacts I I and 8, and out into the circuit 2, the resistor 9 being short circuited by the switch 5.

The operation of my invention, as illustrated in Fig. 1 is as follows: Assume that it is desired to transfer the current to the next adjacent tap on winding I. That is to say, to the tap to which connection is being made by the dial switch l. This transfer, or switching, operation is accomplished by successively snapping the switches 5 and 5, in either sequence desired, to their other contact making positions. Thus, assume that switch 6 is snapped from contact II to contact I3 and that thereafter switch 5 is snapped from contact I0 to contact I2. With this operation, as soon as contact 8 leaves.contact II the main current will be offered the choice of two paths, namely, a path through switch 5 and the resistor 9 or a path' through switch 6 and anl arc which may be drawn between the contacts 8 and Il thereof. As will be seen more clearly by reference to Fig. 3, the factor which determines which of the two paths the current will take is the magnitude of the current itself. Thus, in Fig. 3 there is plotted, with voltage as ordinates and current absciss, the volt-ampere characteristics of the resistor 9 and of an arc. As will be seen the resistor 9 has a positive volt-ampere characteristic, which is also a linear characteristic.' The arc has a negative non-linear volt-ampere characteristic. These characteristics must necessarily intersect and by proper selection of the value of resistor 9, this intersection point may be made to occur at any voltage desired, such for example, as at a voltage corresponding to the normal voltage difference between taps on winding I. Fig. 3 shows that at low current values, that is to say at current values below the value corresponding to the intersection point, the arc voltage drop is relatively highanti the resistance voltage drop is relatively low, whereas at high current values beyond the intersetion point, the arc voltage becomes less than the resistance voltage.

'Returning now to the operation of Fig. 1 and assuming that the current in circuit 2 is less than the current corresponding to the intersection point between the characteristics shown in Fig. 3, no arc will' form and all of the main circuit current will be transferred so as to flow through resistance 9. However, as soon as contact' 8 -engages contact I3 the current will transfer again through switch 6 and dial switch I to connect the adjacent tap. onv winding I because of the fact that the impedance of this new circuit is less than the impedance of the former circuit including the resistor 9. Switch 5 may now be snapped to its other contacting position without causing any circuit changes because of the fact that substanuauy no current win then be sowing through this switch.

,If, however, the current in circuit 2 had been high enough to be beyond the intersection point shown in Fig. 3, an arc would have been formed between contacts 8 and Il and most of the main circuit 'current would flow through this arc because the drop in this 'aire would be less than the drop in the resistance 9. However, as soon as contact 8 engages contact I3 the arc will be extinguished because the switches 5 and l6 are so constructed that the spacing between their contacts I9 and I2, and II and I3, respectively, is wide enough to prevent an arc being sustained between them by the tap voltage, which is the only voltage available for sustaining such an arc between these contacts. The switch 5 may now be snapped over to its other contacting position as before.

Assume now that with the switches vshown in position I, switch 5 is snapped to its other contact position before switch 6 is snapped to its other contacting position. As switch 5 snaps from the position shown to the other contacting position there is no change in the main current path because all the current has been flowing through switch 6, and it continues to do so after switch 5v completes its movement, due to the lower impedance path through switch 6 than through switch 5, which latter path will include the resistor 9. However, as soon as contact 8 of switch 6 starts to leave ycontact I I the current has a choice of flowingthrough an arc or transferring to switch 5 and flowing through resistor 9. As has previously been explained, this choice depends upon the magnitude of the current, and if the current is high it will flow through an arc until switch 8 completes its movement, whereupon the current will be transferred to the new tap through switch 6 and the arc will go out. However, if the current is low in value it will transfer to switch 5 and resistor 9 as soon as switch 8 leaves contact II and as soon as switch 8 completes its movement and engages contactY I3 the resistor 9 will be short circuited and substantially all of the main current will flow through switch 6.

It will thus be seen that regardless `of whether switch 5 or switch 5 is operated first, the current will always flow through the resistor 9, during the switching operation, if this current is below a predetermined value and it will always flow through an arc drawn from low impedance contact 8, during the switching operation, if this current is above a predetermined value. The particular current value below which the current ows through the resistor 9, during the switching operation, and above which it flows through the arc, during the switching operation, depends upon the ohmic value of the resistor and the. dimensions and spacing of the' contacts of switch 5. By properly correlating these factors, this particular current value may be made any value desired, and also the maximum switching voltage, which voltage appears at this particular current value, may therefore also be made any value desired. As stated above, one convenient value of the switching voltage is thenormal voltage between transformer taps. The critical current value may, for example, be of the order of magnitude of the rated full load current of the transformer, or-it may be twice the rated full load current and it will be seen from Fig. 3 that if the critical current'is of this order of' magnitude the switching voltage drop will progressively decrease as the current switched increases, so that very high currents, such as short circuit currents on the transformer circuit, may be handled with a lower voltage drop than at the critical current value.

Due to .the fact that all currents above the critical current value, corresponding tothe intersection point of Fig. 3, are carried by an arc, the resistor 9 may be of relatively small physical size and high ohmic value as compared with the relatively bulky and relatively low impedance or resistance midtap auto-transformers, reactors, or resistors utilized in conventional tapchanging switching arrangements. -In such arrangements the-reactor or resistor must be designed to carry heavy currents of the order of magnitude of short circuit currents, and consequently it must be a large device having a relatively low ohmic value so as to limit the switching voltage during high current values. Due to my invention, however, I may utilize merely a relatively small resistance Wire for the resistor 9.

After switches 5 and 6 have been moved dial switch 3 may be moved one tap in a clockwise direction and then switches 5 and 6 may be snapped back to the position shown in the drawings in either sequence. The operation, however, will be the same as that previously described in that for relatively low currents, the current will flow` through the resistor 9 during the transfer operation, whereas for relatively high currents it will flow through an arc drawn between moving contact 8 and stationary contact I3.

During the interval of time elapsing after one y of the switches has completed its movement and before the other one has started its movement there will lbe a circulating current which will flow through resistor 9 and which will be produced by the tap voltage which will then be .present across this resistor. However, this current is limited in magnitude by the resistor.

It was stated above that for currents below the critical value, no arc is formed and the current all flows to the resistor 9 during the transfer. This is true only in the sense that nofswitching arc is formed. However, at these low current values, there is always anv incipient arc formed, or drawn, by the contact B when it breaks direct contact with the stationary contact with which it has been in engagement. This is because substantially all the current normally flows through the contact 8 and when this contact breaks direct contact with a stationary contact, the current tends to continue to ow, thereby producing an arc. However, at low current values, this arc is lquickly drawn out bythe motion of the contact 8 because of the fact that the voltage necessary to maintain the arc. at these low current values, is higher than the voltage drop in the resistance 9, at these same current values, and consequently the resistance 9, in effect, short circuits the arc and limits the switching voltage at these low current values. The higher the current, below the critical value, the longer the incipient arc lasts, because the more nearly does the voltage drop in this arc correspond with the voltage drop in the resistor for the same current value. However, not until the critical current value is reached does the arc persist during the entire currrent transfer, or switching operation.

The presence of this incipient arc is, however, important and produces the following desirable effect which is due to the fact that the effective voltage drop in tap changing or switching apparatus, which voltage drop is technically known as regulation, depends not only yupon its numerical value but also upon its integrated value with respect to time, that is to say, it depends both upon the value of the voltage drop and also upon the time during which the voltage drop is present. During the existence of the incipient arc the voltage drop, or regulation, in the switching apparatus is less than the voltage drop in the re sistor 9, so that the incipient arc has the eifect of reducing the integrated regulation in the switching apparatus during low current values.

ready in engagement.

One practical effect or benet of this reduction in integrated regulation is the reduction of noticeable flicker in incandescent lamps energized from circuits in which my switching apparatus is applied to voltage regulators therein. Thus, flicker of incandescent electric lamps is produced by a dip in voltage, but due to the persistence of light effects in the human eye, it takes a dip in voltage, or reduction in illumination of the incandescent lamps, for an appreciable time to produce a noticeable flicker. Consequently,k the reduction in time duration or integrated effect of the regulation tends to reduce the noticeable flicker of incandescent lamps.

The modification shown in Fig. 2 differs from Fig. 1 in that the dial switches 3 and 4 and the snap switches, or contactors, 5 and 5 are all combined in a single unit I4. Unit I4 is provided with a two-pronged movable contact comprising a pair of contact prongs I5 and I6, which are insulated from each other, which are adapted to move as a unit and which are angularly displaced from each other with respect to their direction of movement. Contact I5 is connected directly in circuit 2 and contact I6 is connected therein through the resistor 9. 4Switch I4 has a series of circularly arranged stationary contacts I`I connected respectively to taps on winding I and which contacts are adapted to cooperate with the movable contacts I5 and I6.

In the operation of Fig. 2 assume that it is desired to transfer from the end tap to the next adjacent tap, that is to say, it is desired to trans fer the current from the right hand lowermost contact II to the next adjacent contact thereto. This is done by moving the contacts I5 and I6 as a unit in a counter-clockwise direction. The operation will correspond to that operation of Fig. 1 in which the movement of switch 6 precedes the movement of switch 5. Thus, switch I5 will first leave contact I1 thereby offering the current the choice of a path through an arc between contact I l and contact I1, or through the resistor 9 and contact I8. The current will choose the path having the lower voltage drop and this in turn will depend upon the current, as has been previously explained. The spacing of contacts I5 and I6 is such that contact I5 engages the next adjacent contact II before contact I6 leaves the contact II with which it is al- Consequently, there will be a momentary circulating current in resistor 9, after which contact I6 will leave contact I1 and all the current will be flowing through contact I5 which is engaging the next adjacent tap.

The operation is completed when both contacts I5 and I6 are in engagement with the next adjacent tap and all the current is flowing through y choice of two paths, through an arc or through F the resistor 9, respectively. This operation will be completed when Ythe contacts I5 and I6 return to the position shown in the drawings.

If the arrangements shown in Figs. l and 2 are operated in alternating current circuits, it is ad- LTI vantageous to make the tap changing operations, or current transfers, rapidly enough to complete the transfer beforethe current passes through zero, that is, during the half cycle in which the transfer is initiated, not only because this causes the minimum of burning at the contacts, but also because the extinction of the arc at zero current and its restriking at a possibly higher voltage is avoided. This latter condition, however, is not serious and periods of operation lasting several cycles of the alternating current are permissible, for the reason that the restriking voltage is higher only for small currents, and these may not cause a sufiiciently high drop inthe switching resistor to require the restriking n of the arc. In other words, the arc will always restrike after the current passes through zero unless the voltage required for restriking the arc exceeds the voltage drop/ in the resistor 9 for the same current value. This is because the voltages represented in Fig. 3 are only switching voltages and represent but a small fraction of the normal circuit voltage. Consequently, there is always ample voltage for restriking the arc and the only thing which limits the restriking voltage is the resistance drop in the resistance 9. At high currents, the restriking voltage is always less than the voltage drop in the resistance so that the arc will continue to restrike regardless of how slowly the contact 8 is moved. The only factor lmiting the slowness of motion of contact 8 is the burning ofthe contact due to heavy current arcs. Thus, when short circuit currents are switched by the arc, the duty on the contact is severe and consequently it isv desirable to move the contact 8 as rapidly as practicable.

In Fig. 4 there is illustrated an automatic voltage regulating system embodying the switch features of Fig. 1. In this gure the winding I constitutes one of the windings of a two-winding transformer I8 whose voltage ratio is adjusted by means of the tap changing arrangement shown. The dial switches 3 and 4 and the contactors 5 and 6 are automatically operated in response to the voltage of one side of transformer I8 by means of a mechanism |9 operated by a reversible pilot motor 20 which is under the control of a contact making voltmeter 2| connected to be responsive to the voltage of one side of transformer |8 through a suitable supply, or potential, transformer 22. Contact making voltmeter 2| has a set of raise contacts 23 and lower contacts 24 for causing the motor 28 to operate in reverse directions depending respectively upon whether the voltage to be regulated is below or above a predetermined normal value. The motor circuits controlled by contacts 23 and 24 are energized in parallel with contact making voltmeter 2| from transformer 22.

Mechanism I9 consists essentially of two main parts, namely, a part for operating the dial type ratio adjusters 3 and 4 and a part for operating by a rapid successive snap action the contactors, or switches, 5 and 6. The first part comprises a pair of Geneva gears 25 and 26 connected respectively to turn the movable contacts of the dial type adjusters 3 and 4. `These Geneva gears are driven by a common operating'member 421 having a pin 28 for engaging their slots. Mem- 'ber 21 is arranged to be driven by motor 2|) in any suitable manner, such as by the worm gearing shown. y

The second part of the mechanism I9 comprises an interconnecting linkage for the arms 1 and 8 of the contactors 5 and 6, which linkage kis driven from another linkage 29, driven from a crank pin 38 on member 21, through a toggle like connection including' a compression spring- 3|. The interconnecting linkage between the arms 1 and 8 comprises a main link 32 which is connected to the arms 1 and 8 by means of secondary links 33 and 34, respectively, which latter links are freely pivoted at both ends to the link 32 and to the arms 1 and 8, respectively. Arms 1 and 8 are, of course, mounted on relatively xed pivots 35 and 36, respectively. Spring 3| is preferably surrounded by a sleeve 31 for preventing distortion of the spring when it is being compressed.

In the operation of the mechanism I9 if member 21 is rotated in a counter-clockwise direction ratio adjusters 3 and 4 will be moved successively clockwise one tap at a time, each ratio adjuster moving one tap for each complete revolution of member 21. Also, if member 21 is rotated clockwise the ratio adjusters will move counterclockwise one contact tap at a time, adjuster 4 leading adjuster 3 and each moving one tap for each complete revolution of the member 21.

As will now be described more in` detail, the mechanism I9 also causes the rapid successive snapping of the contactors 5 and 6 from one position to the other between each tap changing operation of the ratio adjusters 3 and 4. Thus for example, if member 21 is rotated in a clockwise direction, linkage 29 will move downwardly thereby compressing spring 3| and after the member 21 has rotated slightly more than one-quarter of a revolution clockwise, the left-hand end of spring 3| will become lower as viewed in the drawings, than its right-hand end, and consequently the following snap action will take place in an exceedingly rapid manner. Link 32 will pivot about its connection to link 34 instead of about its connection to link 33 because, although the force required to snap contactors 5 and 6 from one position to the other is approximately the same, there is a greater mechanical advantage applied to snapping contactor 5 by a pivoting action of the link at the connection between the links 32 and 34 than there is to snapping the contactor 6 about a pivot between the links 32 and 33. Consequently, link 32 rotates in a counter-clockwise direction about its pivotal connection to link v34 thereby causing arm 1 of contactor 5 to pivot about its xed pivot 35 `and snapping the arm 1 from contact I0 to contact |2. As soon as this action has been completed, the lpivotal connection between arm 1 and link 33 becomes a xed pivot thereby in turn stopping the `pivoting of arm 32 about its connection to link 34 and causing link 32 to pivot about its connection to link 33. This in turn causes link 32 to rotate in a clockwise manner about the latter pivot\thereby rapidly snapping the contact arm 8 of contactor 6 from its contact Il to its contact I3. It is, of course, understood that the motive power for this snap action is produced by the stored energy in spring 3|, which has been compressed during the rotation of member 21. It is obvious that the above sequential snap action of the contactors 5 and 6 would also take place if the member 21 were rotated slightly more than 90 degrees, or a quarter of a turn, in a counterclockwise direction. However, in that case the 'pin 28 would rst engage the Geneva gear 25 whereby the ratio adjuster 3 would be rotatedone tap in a clockwise direction. It should be noted that in the previously described quarter turn of member 21 in a clockwise direction the pin 28 will not quite engage the Geneva gear 26.

After one-half turn, in either direction, of the member 21 the linkage 29 will be in its lowermost position and link 32 will have its right-hand end down and its left-hand end up, as viewed in the drawings. If now the member 21 is rotated more than a quarter of a turn, in either direction, from a position with pin 30 lowermost the spring 3I will be compressed until a point is reached at which the left-hand end of the spring is above its right-hand end, at which time contactor will be rapidly snapped over whereby arm 1 will move from contact I2 to contact I0 very rapidly, whereupon oontactor 6 will be snapped over and its arm 8 will move from its fixed contact I3 to its other xed contact I I. It will be seen that contactor 5 always moves rst and that contactor 6 moves last, the reason for this being that the force applied Aby spring 3l is always first applied at the end of the long side of the lever comprising link 32. In other words, with respect to the pivot about the connection between links 32 and. 34 the application of the spring pressure is to the long end of the lever comprising the entire length of link 32 vas the long arm and the distance between the pivotal connection of links 33 and 34 on link 32 as the short arm of the lever. When the pivotal connection between links 32 and 33 is considered it will be seen `that the lever arm between the spring and the connection between links 32 and 33 is shorter than the portion of the link 32 between links 33 and 34, and consequently in that case the pressure is applied to the short end of the lever, and as the force required to snap each contactor is substantially the same, contactor 5 will always snap first as the spring force has the greatest mechanical advantage when applied to this contacter.

Turning now to the operation of Fig. 4 as a whole, assume that transformer I8 is so energized that its winding I is its primary winding and consequently its other winding will be its secondary winding, and assume further that transformer I8 is a step-down transformer. As is shown in the drawings the contact making voltmeter 2| is calling for an increase in voltage and 'consequently the motor 20 will be operated in such a direction as to cause member 21 to rotate in a counter-clockwise direction. As the contactors' are shown in the drawings, the electric circuit through the primary Winding I traverses the ratio adjuster 4. However, as member 21 rotates in a counter-clockwise direction in response to the action of the contact making voltmeter 2I, the pin 28 will engage the Geneva gear 25 thereby causing ratio adjuster 3 to move one tap in a clockwise direction. Shortly after this change in the position of ratio adjuster 3 the contactors 5 and 6 snap over successively, thereby transferring the current through ratio adjuster 3 to another tap on winding I thereby cutting out a few turns of winding I whereby the ratio of the transformer is so changed as to boost the voltage of the secondary winding. During the next half revolution of member 21 the pin 28 gill engage the Geneva gear 26 thereby rotating ratio adjuster 4 one contact in a clockwise direction and shortly thereafter the contactors 5 and 6 will be successively snapped over to the positions they now occupy in the drawings whereby the current is again transferred through the ratio adjuster 4 but througha new contact thereon which contact cuts out additional turns of the primary winding I, whereby the transformer ratio is so changed as to boost the voltage of the winding in the transformer in which the'contact making voltmeter responds. tinue, the ratio adjusters moving a tap at a time ju'st ahead of the action of the current transferring contactors until the voltage supplied the 'contact making voltmeter attains a normal value at which time the contacts 23 will separate. In a similar manner, if the voltage is too high the contacts 24 of the contact making voltmeter will close, thereby reversing the operation of motor 20. This in no way affects the successive snap action of the contactors 5 and 6 but it reverses the direction of rotation'of the ratio adjusters 3 and 4 whereby additional turns are successively cut in a tap at a time on the winding I until the voltage is restored to normal.

In Fig. 5 the winding I is connected as a bu'ck and boost winding for an autotransformer, the rest of whose winding is indicated at 38. Winding 38 may be considered as the secondary winding of the autotransformer and this Winding is shown connected to an output circuit 39. Contact making voltmeter 2I is energized in response to the voltage of circuit 39 by means of a winding 40 inductively related with the winding 38. Also, winding I is provided with two tap changers I4 which are so arranged that they can reverse the connections of winding I in the circuit 2. Thus, as shown, the tap changers are in an extreme position in which the circuit current traverses all of Winding I. If the left-hand tap changer I4 is moved two contacts in a clockwise direction and if the right-hand tap changer I4 is moved two contacts in a clockwise direction, the current will flow directly from the upper side of circuit 2 through the left-hand tap changer I4 through a straight connection to the right -hand tap changer I4 and back to the other side of the circuit 2 'through winding 38. Consequently, with the tap changers in that position no buck or boost is produced and the transformer is the equivalent of a I to I ratio autotransformer. Further operation of the tap changers in a clockwise direction serves to reverse the connection of winding I, thereby serving to produce a voltage buck instead of a voltage boost, or vice versa.

The ratio adjusters I4 are adapted to be operated simultaneously by the following mechanism. Each of the adjusters I4 is connected to be driven by motor 20 through a common driving gear 42 engaging two driven gears 43. Interposed in the drive between gears 43 and the ratio adjusters I4 are spring drives 44 which are so arranged that a given angular rotation of gears 43 stores up' energy in a spring which Yenergy is suddenly released when cams 45 withdraw latches 46 from their respective stops. The springs then impart a sudden rapid motion to the rotatable/contacts of the ratio adjusters I4 whereby these adjusters are driven one tap. The particular spring drive represented by 44 forms no part of the present invention and may be of any type, such for example, as the type which is shown and described in Patent No. 1,891,101 granted on an application of George E. Le Count and assigned to the assignee of the present application.

In the operation of Fig. 5, if the voltage is too low, as is indicated by the position of the contact making voltmeter 2l the motor 20 is rotated in such a direction as to drive the movable contact members of the ratio adjusters I4 in a given ,direction, for example, clockwise, whereas over voltage conditions will cause the contact making voltmeter to close the contacts 24 there- This action will conby reversing the operation of motor 20 whereby the direction of operation of the movable contacts of the ratio adjusters I4 is also reversed. In this manner taps are changed on the winding I whereby the voltage is changed until a normal voltage is reached. l

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention, and I therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

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

1. In apparatus for transferring a variable electric current having relatively high and low values from one point to another without interrupting the current, the combination of alternative electrical conducting paths for the current i during the transfer, one of said paths having a nating electric current having relatively high and low values from one point to another, the combination of alternative electrical' conducting paths for the current, one of said paths containing an arc and the other of said paths containing a non-arcing impedance, said arc and said impedance having volt-ampere characteristics which intersect at a current value intermediate said highand low values.

3. In apparatus for transformer tap changing under load, the combination of alternative electrical conducting paths for the transformer load current, one of said paths being an electric arc path, and the other of said4 paths being a relatively high resistance, said paths having volt ampere characteristics which intersect at a current value which is intermediate the no load and short circuit current values of the transformer.

4. In combination, a current carrying electric circuit, means for switching the current in said circuit from one point thereon. to another by means of an arc drawn between relatively movable contacts in said circuit, and means for limiting the switching voltage drop at relatively low current values comprising an impedance connected in parallel with said arc, said arc having a higher ohmic value at said low currents than said impedance. Y

5. In combination,` a Widely variablecurrent electric circuit having apair of unequal potential points, and means for switching the current in said circuit from one point to the other at a definite maximum switching voltage drop in said circuit regardless of the value of current iiow therein comprising relatively high andv low irnpedance contacts, meansv for successively moving said contacts from electrical connection with one of said points to the other with an intermediate position of each contact when it is out of direct electrical contact connection with either point whereby an arc is always drawn when the low impedance contact leaves direct contact connection with one of said points, saidv arc path having a maximum length such that its voltampere characteristic intersects the volt-ampere characteristic of said impedance at an intermediate circuit current value.

6. In combination, a transformer winding, a pair of taps thereon, a pair of movable contacts electrically connected to one of said taps, a variable current circuit having high and low current values and including said Winding and said movable contacts, means for first moving one of said contacts from electrical connection with said rst speciiled tap to electrical connection -with the otherof said taps and then doing likewise with the other of said contacts, an impedance connected in series with one of said contacts wherebyv substantially all of the circuit current normally flows through the other of .said contacts so that an arc is drawn only when the other contact changes connection from 'tap to tap, the path of said arc having a volt-ampere characteristic which intersects the volt-ampere characteristic of said impedance at a current value therethrough which is intermediate said high and low circuit current values. A

7. In combination, a transformer winding, a pair of 'taps thereon, a pair of electrically parallel switching means each having a movable contact and a pairof stationary contacts connected respectively to said taps, a resistance connected in series with one of said movable contacts, said resistance having an ohmic value such that at a predetermined current flow therethrough of the order of magnitude of rated full load current of lsaid transformer winding the voltage drop in said resistance is equal to the rated tap voltage of said transformer winding, the width of sai`d other movable contact and the spacing of its associated stationary contacts being so dimensioned with respect to the rated voltage of said winding and the rated tap voltage of said winding that the arc drawn by any current which is less than said predetermined value will be drawn out during the movement of said other movable contact between its associated stationary contacts, but that the arc drawn by any current switched by said other movable contact which is greater than said predetermined value of current will be carried by said movable contact completely across between its associated stationary contacts, said tap voltage being insuilicient to maintain the arcbetween said stationary contacts.

8. In combination, a variable current alternating current circuit, a transformer connected therein, a pair of taps on said transformer, a pair of parallel connected tap changing switches in said circuit each having two stationary contacts connected respectively to said tapsA and each having a movable contact adapted to move between its stationary contacts without short circuiting them while doing so, means for successively relatively rapidly moving said movable contacts from their respectively .associated stationary contacts which are connected to one tap to their other associated stationary contacts, and a resistance in series with one of said movable contacts, the spacing of the contacts of the switch having the other movable contact being so correlated to the ohmic value of said resistance that the arc drawn by said other movable contact has a voltampere characteristic which intersects the voltampere characteristic of said resistance at an intermediate value of current in said circuit.

9. In combination, a variable current alternating current circuit, a transformer connected therein, a pair of taps on said transformer, a pair of parallel connected `tap changing switches in said circuit each having two stationary contacts connected respectively to said taps and each having a movable contact adapted to move between its stationary contactstwithout short circuiting them while doing so, means for successively relatively rapidly moving said movable contacts from their respectively associated stationary contacts which are connected to one tap to their other associated stationary contacts, and a resistance in series with one of said movable contacts, the spacing ofthe contacts of the switch having the other movable contact being so correlated to the ohmic value of said resistance that the arc drawn by said other movable contact has a volt-ampere characteristic which intersects the volt-ampere characteristic of said resistance at an intermediate value of current in said circuit, the normal voltage between said taps being insuflicient to maintain said arc.

10. In combination, a variable current alternating current circuit, a transformer connected therein, a pair of taps on said transformer, a pair of spaced stationary contacts connected respectively to said taps, a pair of electrically parallel movable contacts adapted to move as a unit between positions wherein they both engage the same stationary contact and having an intermediate position wherein each movable contact engages a different stationary contact, a resistance in series with one contact, said resistance having an ohmic value which is so correlated to the dimensions and spacing of said contacts that an arc drawn by the other movable contact will have a volt-ampere characteristic which intersects the volt-ampere characteristic of said resistance at a predetermined intermediate circuit current value.

1l. In combination, a variable current alternating current circuit, a transformer connected therein, a pair of taps on said transformer, a pair of spaced stationary contacts connected respectively to said taps, a pair of electrically parallel movable contacts adapted to move as a unit between positions wherein they both engage the same stationary contact and having an intermediate position wherein each movable contact engages a different stationary contact, a resistance in series with one contact, said resistance having an ohmic value which is so correlated to the dimensions and spacing of said contacts that an arc drawn by the other movable contact will have a volt-ampere characteristic which intersects the volt-ampere characteristic of said resistance at a predetermined intermediate circuit current value, the normal voltage between said taps being insuicient to maintain an arc between said stationary contacts.

ORIN P. MCCARTY.

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