US2935877A

US2935877A - Cam operated holding in switch

Info

Publication number: US2935877A
Application number: US703636A
Authority: US
Inventors: William C Sealey
Original assignee: Allis Chalmers Corp
Current assignee: Allis Chalmers Corp
Priority date: 1957-12-18
Filing date: 1957-12-18
Publication date: 1960-05-10
Anticipated expiration: 1977-05-10

Description

May), 1960 w. c SEALEY CAM OPERATED HOLDING IN SWITCh Filed Dec. 18, 1957 J/mambo i WWW .Q. fiaovtwg M KM United States Patent CAM OPERATED HOLDING IN SWITCH William C; S ealey, Wauwatosa, Wis., assignor to Allis- This invention relates in general to cam operated switches. More specifically this invention relates to a cam operated holding in switch for use with a tap changing transformer. i

A holding in switch is an auxiliary switch that is provided to maintain an electric motor energized duringa predeterminedcyele after a primary switch has started the motor to initiate the cycle. Holding switches are required for some tap changers for transformers. These tap changers include a plurality of stationary contacts which are connected to the taps of the transformer, a set of movable contacts which cooperate with the stationary contacts for changing the turns ratio of the trans former and a reversible motor for moving the movable contacts in a step motion between stationarycontact positions. The primary switches for energizing the motor may be actuated by a means sensitive to changes in electrical conditions such that the .tap changing mechanism tends to change the transformer turns ratio until the desired voltage output condition is reached. Since extraneous factors may produce the desired voltage output condition before a tap change is completed, the primary switches may open before the movable contacts have reached the next operating position. Therefore, it is often desirable to provide an auxiliary switch for keeping the motor energized until the tap change is completed.

In some tap changers of the prior art the holding in motor or to some other rotating part of the tap changer.

These cams may be directional or nondirectional. A nondirectional cam indicates only the period of motor operation during which an auxiliary switch should be held in. A,directional cam indicates thje period for holding in an auxiliary switch and it also indicates which of the two auxiliary switches corresponds to the direction of rotation of, the motor.

Some nondirectional cams comprise a circular disk which has a recessed or a raised portionfor operating a cam follower; As the cam rotates it moves the follower between a switch opening position and a switch closing position. The follower action in. such a cam 6T, rangement is substantially the same forcither direction the cam rotates. If a direction response is desired, it must be obtained by separate means suchas by the primary switches and cooperating relays.

A directional cam selectively operates one or the other of two auxiliary switches according to the direction of rotation of the cam and does not require a sepice follower in the recess. As the cam rotates during a tap change operation, the follower is moved out of the recess in the direction of the rotation of the cam and it closes one of the auxiliary switches. The follower rides on the outer surface of the cam and holds in the switch until the cam has rotated to a point where the follower again falls into the recessed portion and releases the switch.

These directional cams have some features which limit their use with tap changers. Usually it is desirable that the auxiliary switch be held in to energize the motor somewhat beyond a complete cycle to assure that the movable contacts of the tap changer are accurately positioned and to overcome mechanical losses between the movable contacts and the cam. Additional hold in time can be provided if the cam must travel past itsv neutral position to release the follower. Furthermore, it is usually desirable that the auxiliary switch close soon after the cam begins to rotate to assure that the motor will not stop shortly after the tap change has begun. Ad-

'ditional hold in time and quicker switch closing can be produce other undesirable results.

Most directional cams are easily adapted to provide some overtravel because the follower rides on the outer surface of .the cam at a point spaced in the direction of cam rotation away from the neutral position of the follower. If the contact point of the follower extends beyond the recess when the cam is in the neutral position, the cam must rotate somewhat past its neutral position before the follower can reenter the recess. [in directional cams of the prior art the overtravel of the cam has been increased by making the arc of the recessed portion of the cam very narrow so that the cam must travel farther beyond the neutral position before the recessed position reaches the point at which the follower will be released. Narrowing the arc of the recess provides only small increases in overtravel because the travel of the cam increases by only one half the are by which the open sector is reduced. Furthermore, if the open sector is very narrow, the recess. will not present a sufiicient area to allow the follower to return fully to its neutral positionwhen it is released. Since the distance the follower travels to release the switch is restricted in these designs, the adjustment of the switches is very critical. If the cam. and the switch are misaligned so that the follower does not release the switch, the motor will continue to operate the tap changing switch without control by the electrical responsive means and will produce an undesirable voltage output condition.

The amount of overtravel of the cam can also be increased by providing more than one recess in the cam.

arate direction responsive means. "In some known di- 7 A cam with two equally spaced recesses must travel past its neutral position by the same angle as a similar cam with only one recess but it travels only half as far from neutral to neutral. When a cam with two recesses is suitably geared to turn at half the rate of a similar, cam with only one recess, it will provide twice as much overtravel as the cam with only one recess. Generally it is advantageous to provide a cam which may be geared to J operate over a half revolution since most tap changing switches move through a fraction of acircle in each operation. It is easier to adapt the cam to the switch if the ratio between the cam and the switch is decreased in this way. However, a cam of the known prior. art having these advantages may be unsatisfactory because I the lag in closing the switch at the beginning of the cycle access";

is also increased by increasing the number of recesses in the cam.

Another known method for obtaining additional overtravel is by allowing the follower to extend past the point at which the follower normally contacts the cam surface. The extended portion of the follower causm the cam to rotate past the point at which the normal contact point would be released. However, the follower must back ofi from the switch closing position after the releasing edge of the recess travels past the normal contact point. If the follower is extended very far beyond the contact point, the switches must be adjusted within narrow limits to prevent the follower from backing off from the closing position sufficiently to release the switch and stop the motor before the follower has actually entered the recess. The follower would then reclose this auxiliary switch if on the next cycle the opposite primary switch closed to operate the motor in the reverse direction.

It is also possible to obtain increased overtravel by increasing the distance between the follower pivot point and the cam surface since the follower-will then rest on the cam surface farther outside the neutral position of the recess. However, this method of providing overtravel increases the lag in closing the auxiliary switch at the beginning of a tap change.

The holding in switch of this invention overcomes the disadvantages of the prior art by calling for a cam mechanism in which the cam follower is shaped to ride on the outer surface of the cam at two spaced apart points. One point on the follower is positioned to ride on the cam surface near the normal position of the recess to provide a quick closing action of the switches, and the other point on the follower is spaced from the first point to provide overtravel. Since in this invention the hold in period depends in part on the shape of the follower, it is possible to increase the arc of the recess so that sufiicient space is provided for the follower to release the switches; and consequently, the switches are easy to adjust. Since the follower can be made to close the auxiliary switch after a slight rotation of the cam, this cam mechanism may use two or more recmses to provide the advantages of more than one tap changing cycle per revolution of the cam.

Therefore, it is an object of this invention to provide new and improved cam device for operating a holding in switch. Another object of this invention is to provide a new and improved holding in switch in which the closing and opening operations are independently adjustable.

Another object of this invention is to provide a new and improved holding in switch in which the switches are easily adjusted.

Other objects and advantages will be apparent from the following description when, read with the following drawings in which:

Fig. 1 is a front view of the holding in switch of this invention;

Fig. 2 is a side view of the holding in switch of this invention;

Fig. 3 illustrates the holding in switch of this invention at one point in a holding in operation;

Fig. 4 illustrates the holding in switch of this invention at another point in the holding in operation; and

Fig. 5 illustrates a motor control circuit using the cam operated holding in switch of this invention.

The cam operated holding in switch of this invention, as shown in Figs. 1 and 2, is particularly adapted for controlling a reversible motor 11 and comprises a rotatably mounted cam 12, a cam follower 14, a follower arm 15 and two switches i7, 17a. However, various modifications can be made in the switch of this invention to adapt it to mechanisms other than reversible motors.

The cam' may consist of a circular disk 18 which is provided with one or more recessed portions 20. Preferably, as shown, the cam consists of two

segments

21, 21a- 4 formed by a slot cut in the face of a circular disk. The cam is mounted on a suitable shaft 24, which is illustrated as the shaft of a motor 11 of a tap changer, not shown, and rotates in step with the motor.

The follower 14 is adapted to ride in the recess 20 between the

segments

21, 21a when the movable contacts of the tap changer are on an operating position and on the outer surface 26 of a segment during a tap change operation. As illustrated the follower 14 is attached to one end of the arm 15 and the arm is pivotally. mounted "at a! for swinging the follower 14 between positions.

However, any suitable means for guiding the follower may be used. The follower 14 is constructed to define two spaced oppositely facing concave surfaces 28, 23a complementary to the outer surface 26 of a segment. The

surfaces

28, 28a are made concave to provide the follower 14 with two spaced apart points for contacting the cam. One

point

29 or 29a on each

surface

28, 28a is positioned in the normal posit-ion adjacent the leading edge of the cam surface 26 for initially actuating a

switch

17 or 17a. The

other point

30 or 30a is spaced from the actuating point 29., 29a in the direction of cam rotation and rides on the outer surface 26 of a

segment

21 or 21a until the cycle is completed at which time the follower 14 falls into the recess20 and releases the switch. The position of these two

points

29 or 29a and 30 or 30a determines the duration of the cycle. For example, either point may be positioned farther in the direction of cam rotation for increasing the amount of camrotation required to actuate or release a switch. On the other hand either of the points may be extended contrary to the direction of rotation'to decrease the amount of cam rotation required to actuate or to release a switch. Preferably the follower operating surfaces 28, 2811 are arcuate in shape so as to contact the circumference of the cam at points intermediate the

actuating point

29, 29a and the releasing point 39, 30a. To this end the shape of the follower surfaces 28, 28a intermediate the

points

29, 29a and 30, 30a is defined-by a circle of substantially the same radius as the cam 12 and having its center the same distance from the arm pivot 27 as the center of the cam. However, the operating surfaces 28, 28a of the follower 14 may be of any suitable configuration which provides an offset in the follower to allow two spaced apart points 29, 30 or 29a, 30a to contact the outer surface 26 of the cam 12. The concave surfaces 28, 28a of the follower may begin or terminate at the point on the cam surface 26at which'a flat follower would be tangent; and therefore, one of the points 29 or 34 may be substantially identical with

corresponding point

29a or 30a. Preferably, however, the concave surface extends to either side of the tangent point.

The

switches

17, 17a are positioned symmetrically about the follower 14 so that one or the other of the switches is actuated when the follower rides on the outer surface of the cam and is released when the follower enters the recess. Preferably the arm 15 is provided with

switch operating members

32, 32a at right angles to the arm near the pivoted end of the arm. However, any suitable arrangement of levers attached to the arm 15 or the follower 14 may be used or the switch contacts may be positioned so as to be closed directly by the arm or the follower.

Fig. 5 illustrates the holding in switch of this invention in a simplified control circuit for using the motor 11 with a tap changer for a transformer. The motor 11 has a raise winding 35 and a lower winding 36 corresponding to the direction of the voltage changes of the transformer. The control circuit includes a pair of voltage terminals 38, 38;: that are connectable to the voltage output of the transformer, a pair of power terminals 39, 39a that are connectable to a source of power for energizing the motor 11, anda voltage sensitive relay 41 that includes a raise primary contact 42 and a lower primary contact 42a for conmeeting one or the other of the

windings

35, 36 to a power terminal 39 to initiate'a tap change in response to the voltage condition n the"volta'ge tei'minals; The power terminals 39, 39amay beconnected to the voltage output of the transformer as is indicated in the drawing. The circuit of Fig. 5 further includes the cam operated holding in switch of this invention including the two auxiliary switches -17, 17a that maintain, the motor energized once a tap change has begun. The auxiliary lower switch 17 includes normally open auxiliary lower contacts44 that are in parallel with the relay primary lower-contact 42 and energize the motor independent oflthe relay when the switch 17 is actuated by the arm 32. The, auxiliary lower switch 17 also includes normally closed raise interlock contacts 45 that are in series with the raise primary contact 42 and are opened by the arm 32 during a lower'voltage. operation of the motor to isolate the primary raise contacts 424 of the relay 41 from the motor. The auxiliary raise switch 17a is analogous to the lower auxiliary switch 17 and includes auxiliary raise contacts 44a and lower interlock contacts 45a. The circuit just described is typical of motor control circuits that are well known in the transformer art, and the circuit is not a part of this invention.

When the holding in switch of this invention is used with a tap changer for a transformer, the follower is positioned in the recess 20 of the cam 12, as shown in Fig. 1, when the movable contacts of the tap changer are on an operating position. When, for example, the primary contact 42a closes to operate the motor 11 in a counterclockwise or lower voltage direction. The cam 12 rotates and the leading edge of the segment 21a of the cam bears against the follower near the actuating point 29a. As the cam 12 rotates, the follower is forced out of the recess, as is shown in Fig. 3, and then rides on the outer surface 26 of the cam. As the follower reaches the-position shown in Fig. 3, the left hand operating member 32 actuates its corresponding switch 17 thus closing the auxiliary lower contacts 44 and opening the raise interlock contacts 45. If the primary contact 42a of the tap changer motor 11 should now be opened by its electrical responsive relay 41, the tap changing motor 11 will remain energized by the auxiliary switch 17 until the tap change cycle is completed. The raise interlock contacts 45 are open and prevent the relay 41 from energizing the raise winding 35 of the motor 11 while the auxiliary switch 17 is maintaining the lower winding 36 energized. When the cam 12 has completed a cycle, the movable contacts of the tap changer should be on the next operating position. In

practice the movable contacts may lag the cam because of mechanical losses between the cam and the switch or for other reasons. Because of the overtravel feature of the cam and follower of this invention, the cam continues to rotate for a period suificient to assure that the switch is on position. When the edge of the recess 20 of the cam reaches the releasepoint Sim of the follower, as shown in Fig. 4, the follower 14 reenters the recess of the cam and the auxiliary switch 17 is released by the switch actuating member 32. Because the hold in period depends in part on the shape of the follower, the recess can be made sufliciently wide so that the follower will release fully even though the recess is not in the normal position. If the primary switch is still held closed by its controls, the motor 11 will continue for a second cycle.

Although but one embodiment of this invention has been illustrated, it will be apparent to those skilled in the art that variations are possible within the scope of the appended claims.

What is claimed is: v

1. An actuator for a switch controlling a motor through predetermined cycles comprising a rotatably mounted cam having an arcuately circumscribed outer surface inter rupted by an inwardly extending recessed portion, a

follower positioned to ride in said recess and on said outer surface according to, the position of said cam, and an arm having one end attached to said follower and the other end adapted to close said switch when said follower 6 V r is on said arcuate surface and to open said switch when said follower. is in said recess, said follower being concave to define two spaced apart points for contacting said arcuately circumscribed surface.

2. An actuator for a switch controlling a motor through predetermined cycles comprising an element adapted to move in a circular path and having an arcuate outer surface complementary to said path, a follower positioned to ride within said circular path and on said outer surface according to the position of said element, and an arm having one end attached to said follower and the other end adapted to actuate said switch when said follower is on said arcuate surface and to release said switch when said follower rides within said path, said follower being concave to define two spaced apart points for contacting said arcuate surface.

3. An actuator for a switch controlling a motor through predetermined cycles comprising a rotatably mounted cam having an arcuate outer surface interrupted by an inwardly extending recessed portion, a follower positioned to ride in said recess and on said outer surface according to the position of said cam, means for guiding said follower between said position in said recess and said position on said arcuate surface, said follower being adapted to actuate said switch when said follower is on said arcuate surface, said follower being concave to define two spaced apart points for contacting said arcuate surface.

4. An actuator for a switch controlling a motor through predetermined cycles comprising a rotatably mounted cam having an arcuate outer surface interrupted by an inwardly extending recessed portion, a follower positioned to ride in said recess and on said outer surface according to the position of said cam, and an arm having one end attached to said follower and the other end adapted to actuate said switch when said follower is on said arcuate surface and to release said switch when said follower enters said recess, said follower being concave to contact said cam along its said arcuate surface.

5. An actuator for a switch controlling a reversible motor through predetermined cycles comprising an arm adapted at one end for moving between two switch actuating positions, a follower attached to the other end of said arm, and a rotatably mounted cam having an arcuate outer surface interrupted by an inwardly extending recessed portion, said follower being positioned to ride in said recess when said switch is intermediate said actuating positions and on said arcuate surface. when said switch is in one said actuating position, said follower adapted to be moved from said recess in two; directions in accordance with the direction of rotation of said cam to move said arm into one of said actuating positions, said follower having opposed concave surfaces for contacting said arcuate surface at two spaced apart points during rotation of said cam.

6. An actuator for a switch controlling a reversible motor through predetermined cycles comprising an arm pivotally mounted at one end, a switch actuating member attached to said arm and positioned to move between two switch actuating positions, a follower attached to the one end of said arm, and a rotatably mounted cam having an arcuate outer surface interrupted by an inwardly ex-,

tending recessed portion, said follower being positioned to ride in said recess when said member is intermediate said actuating positions and on said arcuate surface when said member is in said actuating positions, said follower adapted to be moved from said recess in two directions in accordance with the direction of rotation of said cam to pivot said armand move said switch actuating member into one of said actuating positions, said follower having opposed concave surfaces for contacting said arcuate sur face during rotation of said cam.

7. In a transformer having a motor energizable in response to electrical conditions for operating a tap changing switch to vary the transformer voltage output, and having switch means for energizing said motor independthe position of said cam, and an arm having one end attached to said follower and the other end beingvadapted to close said switch when said follower is on said arcuate surface and to open said switch when said '-r is in said recess, said follower being concave to define spaced apart points forcontacting said arcuate surface.

References Cited in the file of this patent UNITED STATES PATENTS Marinier Oct. 28, 1919 Haskins May 30, 1933 Linde Apr. 20, 1937 Chappell Aug. 28, 1951 Sears Nov. 22, 1955 Goldberg Apr. 10, 1956 FOREIGN PATENTS Great Britain Feb. 12, 1958