US3493698A

US3493698A - Tap-changing transformer including a selector switch and a transfer switch

Info

Publication number: US3493698A
Application number: US651284A
Authority: US
Inventors: Ulrich G E Schweitzer
Original assignee: Maschinenfabrik Reinhausen Gebrueder Scheubeck GmbH and Co KG
Current assignee: Maschinenfabrik Reinhausen Gebrueder Scheubeck GmbH and Co KG
Priority date: 1966-07-13
Filing date: 1967-07-05
Publication date: 1970-02-03
Anticipated expiration: 1987-02-03
Also published as: GB1156343A; SE308556B; DE1265292B; CH453494A; NL131699C; NL6709639A; BE701244A

Description

1970 u. e. E. SCHWEITZER 3,493,693

TAP-CHANGING TRANSFORMER INCLUDING A SELECTOR SWITCH. AND A TRANSFER SWITCH Filed July 5, 1967 2 Sheets-Sheet 1 l/vrgwrok Filed July 5, 1967 1970 u. e. E. SCHWEITZER 3,493,593 TAP-CHANGING TRANSFORMER INGLUDING A SELECTOR SWITCH AND A TRANSFER SWITCH 2 Sheets-Sheet I Fig 3 III United States Patent Int. Cl. H01h 19/58 US. Cl. 20011 Claims ABSTRACT OF THE DISCLOSURE Tap-changing transformers which include selector switches for selecting any desired tap of tapped transformer windings and which include transfer switches for switching a load to any particular tap preselected by the selector switches further include a pair of auxiliary switches each arranged in one of a pair of leads conductively connecting the selector switches to the transfer switches. Each of this pair of auxiliary switches has a closed position establishing a current path through one of the aforementioned pair of leads between selector switch and the transfer switch, and each of this pair of auxiliary switches has an open position wherein the current path through one of the aforementioned leads between selector switch and transfer switch is interrupted. The pair of auxiliary switches is interconnected by additional leads in such a way that whenever one of the pair of auxiliary switches is in the closed position thereof, a conductive connection is established between the closed auxiliary switch and a contact of the other auxiliary switch which is then in its open position. As a result of the presence of the aforementioned pair of auxiliary switches and the aforementioned conductive interconnection thereof both main contacts of the transfer switch are at the same potential at the end of every tap-changing operation performed by the transfer switch.

BACKGROUND OF INVENTION Tap-changing regulating transformers include, in addition to tapped transformer windings, selector switches and transfer switches. The former serve the purpose of selecting a particular tap of a tapped transformer winding intended to be connected into an electric circuit. Selector switches are not intended to open current-carrying circuits, nor to close on current-carrying circuits. These tasks are left to transfer switches. The latter perform all switching operations under load. Hence arcing is kept away from the cooperating contacts of the selector switches, but occurs at the cooperating contacts of the transfer switches.

This invention relates more particularly to regulating transformers including so-called Jansen type transfer switches. Such transfer switches are well known in the art and are described in many printed publications and patents as, for instance, in the text book by H. P. Young, Electric Power Control Systems, London, England, Chapman & Hall Ltd.; 1942 (see page 247), or in US. Patent 2,680,790 to B. Jansen: June 8, 1954; Load Changeover Switch for Tapped Transformers Using a Combination of Contact Movements, or in US. Patent 2,833,873 to B. Jansen; May 6, 1958, for Multi-Pole Tap Switch Construction for Changing Transformer Taps Under Load.

Transfer switches include a pair of fixed main contacts of which each is generally conductively connected to one of a pair of contiguous taps of a tapped transformer winding. As a result, the voltage that prevails between contiguous taps of a tapped transformer widing prevails also between the fixed main contacts of the transfer switch which is operatively related to the particular tapped transformer winding. In some instances the voltage between the fixed main contacts of a transfer switch may by far exceed the voltage normally prevailing between contiguous taps of a tapped transformer winding and amount to a dangerous voltage surge. This calls for snrge-voltage-proof designs of transfer switches in tap-changing transformers, greatly increasing the cost thereof.

Because of the aforementioned circumstances special selector switch designs have been evolved making it possible to avoid differences in potential between the fixed main contacts of the transfer switch when the latter is in one of its two limit positions carrying current from a selected tap of a tapped transformer winding to a load circuit. While such special selector switches achieve the purpose for which they are intended, their use is subject to serious drawbacks. This invention, therefore, aims to avoid potential differences between the contacts of transfer switches when the latter are in one of the two limit positions thereof without resorting to any special selector switch design, and in spite of retaining the conventional desirable design of the selector switch.

SUMMARY OF THE INVENTION A tap-changing transformer embodying this invention comprises in combination a transformer winding including a plurality of taps and a selector switch including a plurality of fixed contacts each conductively connected to one of said plurality of taps. The aforementioned selector switch includes a pair of movable contact means each adapted to cooperatively engage different contacts of said plurality of contacts. The tap-changing transformer further includes a transfer switch having a pair of fixed main contacts, fixed auxiliary contacts arranged between said pair of fixed main contacts, resistor means conductively connecting each of said pair of fixed main contacts to fixed auxiliary contacts immediately adjacent thereto, a pair of movable main contacts and movable auxiliary contacts selectively engaging and disengaging said pair of fixed main contacts and said fixed auxiliary contacts in a predetermined sequence. The tap-changing transformer further includes a pair of leads each for conductively connecting one of said pair of movable contact means of said selector switch to one of said pair of fixed main contacts of said transfer switch. One of a pair of auxiliary switches is inserted into each of said pair of leads. Each of said pair of auxiliary switches has a closed position and an open position. The circuitry further includes lead means for conductively interconnecting said pair of main contacts of said transfer switch when one of said pair of auxiliary switches is in said open position thereof, and the other of said pair of auxiliary switches is in said closed position thereof.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a circuit diagram of a single phase tap-changing transformer embodying the present invention;

FIG. 2 is a diagrammatic representation of the sequence of steps involved in the operation of the structure of FIG. 1;

FIG. 3 is in part a side elevation and in part a vertical section of three pairs of auxiliary switches for a three phase transformer embodying the present invention, the aforementioned three pairs of auxiliary switches being integrated to form a unitary switch structure;

FIG. 4 is a section along IV--IV of FIG. 3; and

FIG. 5 is a section along VV of FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and more particularly to FIGS. 1 and 2 thereof, reference character Tr has been applied to generally indicate a tapped transformer,

reference character W has been applied to generally indicate the tapped winding of transformer Tr, reference character St has been applied to generally indicate the selector switch of the system, and reference character L has been applied to generally indicate the transfer switch of the system. Tapped winding W is provided with twelve taps indicated by

numerals

1, 2, 3, 4 9, 10, 11, 12. Selector switch St includes twelve fixed contacts which are circularly arranged. The even numbered

contacts

2, 4, 6 are arranged in a circular pattern in a first plane and the odd numbered contacts 1, 3, are arranged in a circular pattern situated in a plane spaced from the plane defined by the first mentioned circular pattern. Both circular patterns are coaxial. The fixed contacts of selector switch St may be supported by a squirrelcage-like structure formed by vertical insulating rods. Selector switch St is provided with a pair of contact arms 41, 42 pivotable about the common axis of the aforementioned circular patterns of

fixed contacts

1, 2, 3, 4 9, 10, 11, 12. The radially inner end of contact arm 41 engages and rests upon a slip ring 43 and its radially outer end is adapted to cooperatively engage the odd numbered fixed contacts of selector switch St. In like fashion the radially inner end of contact arm 42 engages and rests upon a slip ring 44 and its radially outer end is adapted to cooperatively engage the even numbered fixed contacts of selector switch St. Each

tap

1, 2, 3, 4 of tapped winding W is conductively connected with a corresponding fixed contact of selector switch St. Thus tap 1 is connected by lead 101 to fixed contact -1, tap 2 is connected by lead 102 to fixed contact 2, tap 3 is connected by lead 103 to fixed contact 3 and tap 4 is connected to fixed contact 4 by lead 104-. In order not to clutter FIG. 1 by a large number of tap and fixed selector switch contact interconnecting leads the leads between taps 5-12 and contacts 5-12 have been deleted in FIG. 1. Lead 51 is conductively connected to slip ring 43 and lead 52 is conductively connected to slip ring 44.

It should be understood that the illustration of selector switch Sr of FIG. 1 is diagrammatic. Selector switches of the kind under consideration are disclosed in considerable structural detail in U.S. Patent 3,176,089 to A. Bleibtreu et al., Mar. 30, 1965, for Load Tap Changers for Transformers; U.S. Patent 3,233,049 to A. Bleibtreu, Feb. 1, 1966, for Integral Selector Switch and Transfer Switch Unit for Tapped Regulating Transformers; and U.S. Patent 3,250,864 to A. Bleibtreu et al., May 10, 1966, for Regulating Transformers Having a Large Number of Taps.

The transfer switch L includes a cylindrical support 27 of insulating material for supporting the fixed contacts thereof.

Reference numerals

21 and 22 have been applied to indicate the fixed main contacts of transfer switch L and

reference numerals

23, 24, 25, 26 have been applied to indicate the fixed auxiliary contacts of transfer switch L. Fixed

auxiliary contacts

23, 24, 25, 26 are arranged between fixed

main contacts

21, 22. Fixed auxiliary contact 23 is conductively connected to a switch-over resistor R and fixed auxiliary contact 24 is conductively connected to a switch-over resistor R In a like fashion fixed auxiliary contact 26 is conductively connected to a switch-over resistor R and fixed auxiliary contact 25 is conductively connected to a switch-over resistor R The two resistors R R shown at the left side of FIG 1 are conductively connected by leads including lead 51 to fixed main contact 21, and the two resistors R R shown at the right side of FIG. 1 are conductively connected by leads including lead 52 to fixed main contact 22. Fixed main contact 21 may be engaged by linkage-supported movable main contact 31 and the latter may be separated from the former. In like fashion fixed main contact 22 may be engaged by linkagesupported movable main contact 32 and the latter may be separated from the former as shown to the right of FIG. 1. Each of linkage-operated movable

auxiliary contacts

33, 34, 35, 36 may engage one of the fixed

auxiliary contacts

23, 24, 25, 26 and be separated from the latter. FIG. 1 shows main contacts 21, 3-1 and auxiliary contacts 23, 33 in the engaged position thereof, all other contacts being in disengaged positions. The

linkages supporting contacts

31, 32, 33, 34, 35 and 36 may be operated in the required sequence by

cams

37 and 38 or other means as, for instance, those disclosed in U.S. Patent 3,174,097 to A. Bleibtreu, Mar. 16, 1965, for Transfer Switch for Tap Changers for Regulating Transformers or U.S. Patent 3,218,400 to A. Bleibtreu, Transfer Switches for Tap-Changing Regulating Transformers Having Squirrel-Cage-Shaped Support for the Fixed Contacts Thereof. It will be noted that the circuitry of FIG. 1 is in substance the same as that shown in FIG. 3 of U.S. Patent 3,174,097 and described in that patent. For the reasons stated in U.S. Patent 3,174,097 this circuitry is known as Flag-Pennant transfer switch circuitry, or briefly FP transfer switch circuitry. As shown in FIG. 1 the circuit under consideration includes a lead A connecting the movable contacts 31-36 of transfer switch L with a load (not shown).

The circuitry of FIG. 1 further includes a pair of auxiliary change-over

switches

53, 54 each having an open position and a closed position. In FIG. 1 auxiliary switch 53 is shown in its closed position indicated by solid lines, the open position of auxiliary switch 53 being indicated by dotted lines. In FIG. 1 auxiliary switch 54 is shown in its open position indicated by solid lines, the closed position of auxiliary switch 54 being indicated by dotted lines. It will be noted that auxiliary switch 53 is arranged in lead 51 for conductively connecting slip ring 43 with fixed main contact 21 and that auxiliary switch 54 is arranged in lead 52 for conductively connecting slip ring 44 with fixed main contact 22. Auxiliary Switch 53 includes a pivotable contact arm adapted to engage selectively fixed contact a and fixed contact b. In a like fashion auxiliary switch 54 includes a pivotable contact arm adapted to engage selectively fixed contact 0 and fixed contact d. Lead 56 connects conductively the pivotable contact arm of auxiliary switch 53, or the portion of lead 51 on the side of switch 53 adjacent transfer switch L, to fixed contact at of auxiliary switch 54. In like fashion lead 55 connects conductively the pivotable contact arm of auxiliary switch 54, or the portion of lead 52 on the side of switch 54 adjacent transfer switch L, to fixed contact b of auxiliary switch 53.

In the :position of selector switch St,

auxiliary switches

53 and 54 and transfer switch L shown in FIG. 1 the following current path is established: tapped winding W, tap 3, lead .103, fixed contact 3 of selector switch St, contact arm or contact bridge 41, portion of lead 51 adjacent selector switch St, auxiliary switch 53, portion of lead 51 adjacent transfer switch L, fixed main contact 21, movable main contact 31, lead A. Contact bridge 42 is in engagement with slip ring 44 and fixed contact 2 of selector switch St, and thus the following current path is then established: tap 2 of winding W, lead 102, contact 2 of selector switch St, slip ring 44, the portion of lead 52 adjacent selector switch St. Since auxiliary switch 54 is in the open position thereof there is no conductive connection between tap 2 of winding W and fixed

contacts

22, 25, 26 of transfer switch L. Auxiliary switch 54 establishes, however, in its open position a conductive connection between fixed

contacts

22, 25, 26 of transfer switch L and engaging

contacts

21, 31 and 23, 33 of transfer switch L. This conductive connection includes the pivotable contact arm of auxiliary switch 54, its fixed contact a, lead 56 and the portion of lead 51 adjacent transfer switch L. As a result, all contacts of transfer switch L are at the same potential. This applies to the engaging pairs of

contacts

21, 31 and 23, 33 as well as to fixed

contacts

22, 24, 25 and 26, all having the same potential as tap 3 of winding W. Since all

movable contacts

31, 32, 33, 34, 35 and 36 are connected to the lead, or outgoing line A, the movable contacts not engaging any fixed contact are likewise at the po tential of tap 3. In other words, there are no differences in potential between any of the contacts of transfer switch L.

FIG. 2 illustrates diagrammatically the sequence of steps involved in a tap-changing operation frorn tap 3 to tap 4 of winding W. e indicates the polnt Off 11116 at which the tap changing operation begins and i lndicates the point of time at which the tap-changing operat1on ends. If it is intended to switch from tap 3 to tap of winding W the first tap-changing step consists in moving, or pivoting, contact bridge 42 from contact 2 to contact 4 of selector switch St. This step (illustrated by the first rectangle on the top of FIG. 2) is completed at the point of time g. Thereupon the transfer switch L connects the load to tap 4. This step (illustrated by the second rectangle from the top of FIG. 2) is completed at the point of time h. Auxiliary switch 53 remains closed up to the point of time 11 (illustrated by the third) rectangle from the top of FIG. 2), but auxiliary switch 54 closes already at the point of time 1 by pivoting 1n counterclockwise direction (as seen in FIG. 1) and 1s closed at the point of time i, at which time all the steps involved in the change from tap 3 to tap 4 are completed (the bottom rectangle of FIG. 2 refers to the timing of auxiliary changeover switch 54). The movable contacts of transfer switch L are operated in the proper sequence required for Flag-Pennant transfer switches or FP transfer switches. This is disclosed in considerable detail in the paper by Rudolf Klaus, Untersuchungen an Lastwiihlern and Lastumschaltern (System Dr. Jansen) fiir grosse SchalthaufigkeitInvestigations on selector switches and transfer switches (system Dr. Jansen) for high repetitive switching rates. At the point of time h movable main contact 32 engages fixed main contact 22 and movable auxiliary contact 36 engages fixed auxiliary contact 26, and all other contacts of transfer switch L are out of engagement. Sinceas mentioned aboveauxiliary switch 53 remains closed up to the point of time h and auxiliary switch 54 closes at the point of time f and remains closed thereafter, neither of both

auxiliary switches

53, 54 carries current when performing a switching operation. Hence arcing is no problem in designing auxiliary change-over

switches

53 and 54. The time during which there is a difference in potential between the contacts of transfer switch L is limited to the very short period when a tap-changing operation is being performed, and as explained above. Auxiliary switches 53 and 54 and the circuitry related to these switches maintain all contacts of transfer switches L at the same potential when that switch is in either of its two limit positions, applying the voltage of

taps

1, 2, 3, 4 9, 10, 11, 12 to the load of the transformer.

It will be apparent from the foregoing that a tapchanging operation from a tap of winding W having a relatively high number to a tap of winding W having a relatively low number involves an inversion of the steps which have been described above. To be more specific, in such an instance auxiliary switch 53 is first closed and this is followed by opening of auxiliary switch 54.

The sequence of steps required of a transfer switch for performing a tap-changing operation depends upon the type of transfer switch under consideration. A number of important types of transfer switches, their operating characteristics, vector diagrams and sequences of contact operation have been outlined in the aforementioned US. Patent 3,174,097 to A. Bleibtreu. A tap-changing operation in the structure of FIG. 1 involvesparting of movable main contact 31 from fixed main contact 21 after movable auxiliary contact 34 has engaged fixed auxiliary contact 24. Upon completion of the aforementioned sequence of steps the resistors R R shown at the left of FIG. 1 are connected in parallel in the load circuit. Thereupon movable auxiliary contact engages its cooperating fixed auxiliary contact 25 and movable auxiliary contact 33 parts from its cooperating fixed auxiliary contact 23. Finally movable main contact 32 engages its cooperating fixed main contact 22 and movable auxiliary contact 35 parts from its cooperating fixed auxiliary contact 25.

Referring now to FIGS. 3 to 5, reference character S has been applied to indicate the housing of an operating mechanism for operating a transfer switch L arranged in a housing L below housing S. The housing L intended to receive transfer switch L houses also auxiliary switches corresponding to

auxiliary switches

53, 54 of FIG. 1. These auxiliary switches will be described below more in detail. Reference character G has been applied to generally indicate a gear mechanism for operating the aforementioned auxiliary switches. The gear mechanism comprises a pair of meshing gears 58, 59. The former is operated by a shaft 57 which, in turn, is operated by the operating mechanism (not shown) inside of housing S. The selector switch St is arranged below housing L for transfer switch L and the aforementioned auxiliary switches.

The upper part of shaft 57 and the lower part of shaft 57 are electrically insulated from each other and the aforementioned gear '58 is mounted on the lower end of shaft 57. Gear 59 operates shaft 62 by the intermediary of crank lever 61 mounted on shaft 62, tie rod 60 pivoted to crank lever 61 and pin 67 secured to gear 59. A substantially star-shaped cam 63 of electric insulating material having three cam arms enclosing angles of 120 degrees is mounted on and jointly pivotable with cam shaft 62. The radially outer ends of star-shaped cam 63 have transverse contact-operating projections of which one has been designated by the reference character 64. The transverse projections 64 of cam 63 are intended to engage pivotable contacts, and to be disengaged from these contacts. Reference numeral 65 has been applied to indicate one of these pivotable contacts. The structure of FIG. 4 includes a total of six such contacts. Each of these contacts is under the bias of a spring 66 tending to move the respective contact out of engagement with one of a pair of fixed contacts and into engagement with the other of said pair of fixed contacts. Reference character 53 has been applied in FIG. 4 to indicate an auxiliary switch comprising a pair of fixed contacts a and b and .a pivotable contact adapted to engage selectively fixed contacts a and b. In a similar fashion, reference character 54 has been applied to indicate an auxiliary switch comprising a pair of fixed contacts 0 and a and a pivotable contact 65 adapted to engage selectively fixed contact c with a selector switch. Leads 51 and 52 of FIG. 4 are the full equivalent of the

auxiliary switches

53, 54 of FIG. 1. In FIG. 4 reference character 51 has been applied to indicate a lead for connecting fixed contact a to a selector switch, and reference character 52 has been applied to indicate a lead for connecting fixed contact c with a selector switch. Leads 51 and 52 of FIG. 4 are the full equivalent to leads 51 and 52 of FIG. 1. In FIG. 4 reference character 55 has been applied to indicate a lead conductively interconnecting fixed contact b of switch 53 and the pivotable contact of switch 53, and reference character 56 has been applied to indicate a lead conductively interconnecting fixed contact d of switch 54 and the pivotable contact of switch 53. Leads 55, 56 of FIG. 4 -are equivalents of

leads

55, 56 of FIG. 1. FIG. 4 shows leads 55, 56 in dotted lines since these leads are situated at a lower level than star-shaped cam 63 and not exposed to view. For the same reason all the leads interconnecting the three pairs of auxiliary switches 53, 54-of which each pair is operatively related to one phase of a three phase circuithave been shown in FIG. 4, these connections being the same as shown in

switches

53, 54 to transfer switch L have been deleted in FIG. 4, these connections being the same as shown in 7 FIG. 1, and described in connection with this figure. FIG. 4 further shows but the

external connections

51, 52 of one pair ,of

switches

53, 54 or one phase, to the selector switch St, the corresponding connections of the two other phases of the three-phase circuit being deleted in FIG. 4,-

Concornitant with a given tap-changing operation shaft 57 is operated and pivots star-shaped cam 63 by the intermediary of transmission means 58, 59, 67, 60, 61 and 62. In the position shown in FIG. 4 projection 64 of cam 63 engages movable contact 65 and maintains the latter against the action of a spring 66 in engagement with fixed contact d, the pivotable contact of switch 53 being in engagement with the fixed contact a thereof. The aforementioned clockwise motion of star-shaped cam 63 frees contact 65 of switch 54, and thus this contact is pivoted under the bias of its spring 66 from right to left into engagement with fixed contact 0. As star-shaped cam 63 continues its clock wise motion, the selector switch St and the transfer switch L are being operated, as fully described above in connection with FIGS. 1 and 2. Starshaped cam 63 has a pair of limit positions. As starshaped cam 63 reaches its left limit position, one of its contact-operating projections 64 engages the pivotable contact of auxiliary change-over switch 53 and moves said contact against the bias of its spring from fixed contact a to fixed contact b. This completes a tapchanging operation.

Shaft 57 performs pivotal motions of 180 degrees. The sense of these motions depends upon whether a change to a higher tap number or to a lower tap number is effected.

Switches

53, 54 change their respective positions irrespective of the sense of the pivotal motion of shaft 57.

Although I have shown and described spceific structures embodying my invention, it is to be clearly understood that the same was disclosed merely for the pur pose of illustration, and that changes and modifications may readily be made by those skilled in the art without departing from the spirit and scope of the invention.

I claim as my invention:

1. A tap-changing transformer comprising in combination:

(a) transformer winding have a plurality of taps;

(b) a selector switch having a plurality of fixed contacts each conductively connected to one of said plurality of taps, said selector switch further including a pair of movable contact means each adapted to cooperatively engage selectively different of said plurality of fixed contacts;

(a) a transfer switch including a pair of fixed main contacts, fixed auxiliary contacts arranged between said pair of fixed main contacts, resistor means conductively connecting each of said pair of fixed main contacts to said fixed auxiliary contacts immediately adjacent thereto, and said transfer switch further including a pair of movable main contacts and movable auxiliary contacts for engaging and disengaging said pair of fixed main contacts and said fixed auxiliary contacts in a predetermined sequence;

(d) a pair of leads each for conductively connecting one of said pair of movable contact means of said selector switch to one of said pair of fixed main contacts of said transfer switch;

(a) a pair of auxiliary change-over switches each inserted into one of said pair of leads and each having a first limit position establishing a conductive connection between one of said pair of movable contact means of said selector switch and one of said pair of fixed main contacts of said transfer switch, each of said pair of auxiliary change-over switches further having a second limit position;

(f) a pair of leads each under the control of one of said pair of auxiliary change-over switches for conductively interconnecting said pair of fixed main contacts of said transfer switch when one of said pair of auxiliary change-over switches is in said second limit position thereof.

2. A tap-changing transformer as specified in claim 1 including power driven operating means for operating said pair of auxiliary change-over switches, said operating means allowing one of said pair of auxiliary change-over switches to remain in said first limit position thereof, while causing the other of said pair of auxiliary changeover switches to be moved from said second limit position thereof to said first limit position thereof, and said operating means further allowing said other of said pair of auxiliary switches to remain in said first limit position thereof while causing said one of said pair of auxiliary change-over switches to be moved from said second limit position thereof to said first limit position thereof.

3. A tap-changing transformer as specified in claim 1 including a drive for said transfer switch arranged above said transfer switch, a vertical shaft operated by said drive for operating said selector switch, said selector switch being arranged below said transfer switch, and an operating mechanism under the control of said shaft for operating said pair of auxiliary switches, said pair of auxiliary switches being arranged between said transfer switch and said selector switch.

4. A tap-changing transformer as specified in claim 1 wherein said plurality of fixed contacts of said selector switch includes even contacts arranged in a circular pattern at a predetermined level and odd contacts arranged in a circular pattern at another level than said predetermined level, wherein said transfer switch is arranged at a level above said selector switch and wherein said pair of auxiliary switches is arranged at a level intermediate said selector switch and said transfer switch.

5. A tap-changing transformer as specified in claim 1 comprising:

(a) a drive for said transfer switch arranged at a level above said transfer switch;

(b) a housing for said transfer switch, said pair of auxiliary change-over switches being also located inside said housing at a lower level than said transfer switch;

(0) an insulating shaft driven by said drive for said transfer switch and extending in downward direction; and

(d) a gear box arranged at a level below said pair of auxiliary change-over switches, said gear box housing a mechanism driven by said shaft for operating said selector switch and said gear box further housing gear means and linkage means driven by said shaft for operating said pair of auxiliary changeover switches.

6. A tap-changing transformer as specified in claim 1 wherein each of said pair of auxiliary change-over switches includes a pivotable contact and spring means for biasing said pivotable contact to a first limit position, said pair of auxiliary change-over switches further including a pivotable cam selectively engaging said pivotable contact of each of said pair of auxiliary changeover switches to move said pivotable contact against the action of said spring means to a second limit position thereof.

7. A tap-changing transformer as specified in claim 1 including:

(a) a power driven selector-switch shaft;

(b) an auxiliary change-over switch operating shaft operated by said selector switch operating shaft by the intermediary of a transmission;

(c) a substantially star-shaped cam having three arms enclosing angles of degrees mounted for joint pivotal motion on said auxiliary change-over switch operating shaft; and

(d) three pairs of auxiliary change-over switches, each of said pairs of auxiliary change-over switches including a pair of spring biased pivotable contacts selectively engageable by one of the arms of said cam to be moved against spring bias from a first limit position to a second limit position.

8. A tap-changing switching system for tapped transformer windings comprising in combination:

(a) a selector switch having a plurality of fixed contacts, a pair of fixed slip-rings, and a pair of contact bridges adapted to selectively interconnect said plurality of contacts and said pair of slip-rings;

(b) a transfer switch including a pair of fixed main contacts, fixed auxiliary contacts arranged between said pair of fixed main contacts, resistor means conductively connecting each of said pair of fixed main contacts to said fixed auxiliary contacts immediately adjacent thereto, and said transfer switch further including a pair of movable main contacts and movable auxiliary contacts for engaging and disengaging said pair of fixed main contacts and fixed auxiliary contacts in a predetermined sequence;

(c) a pair of leads each conductively connecting one of said pair of slip-rings of said selector switch to one of said pair of fixed main contacts of said transfer switch;

(d) a pair of auxiliary change-over switches each inserted into one of said pair of leads and each having a first limit position establishing a conductive connection between one of said pair of slip-rings of said selector switch and one of said pair of fixed main contacts of said transfer switch, each of said pair of auxiliary change-over switches further having a second limit position; and

(e) means for shunting said pair of fixed main contacts of said transfer switch when one of said auxiliary switches is in said second limit position thereof.

9. A switching system as specified in claim 8 including common drive means for said selector switch, said transfer switch and said pair of auxiliary change-over switches, and means for positively controlling the sequence of operation of said selector switch and said pair of auxiliary change-over switches.

10. A switching system as specified in claim 8 including:

(a) a selector-switch operating shaft;

(b) an auxiliary change-over switch operating shaft operated by said selector switch operating shaft by the intermediary of transmission means;

(c) a cam mounted for joint pivotal motion on said auxiliary change-over switch operating shaft; and

(d) a pair of pivotable change-over switch contacts each spring biased to a first limit position thereof and each engageable by said cam and movable by said cam against spring bias to a second limit position thereof.

References Cited UNITED STATES PATENTS 3/1965 Bleibtreu et al. 200-18 1/1941 St. Palley 32343.5

OTHER REFERENCES ROBERT S. MACON, Primary Examiner M. GINSBURG, Assistant Examiner