US2445088A

US2445088A - Current-limiting transformer

Info

Publication number: US2445088A
Application number: US519129A
Authority: US
Inventors: Schilling Lorell John
Original assignee: Babson Brothers Co
Current assignee: Babson Brothers Co
Priority date: 1944-01-21
Filing date: 1944-01-21
Publication date: 1948-07-13
Anticipated expiration: 1965-07-13

Description

July 13, 1948. 1.. J. SCHILLING 2,445,038

' CURRENT LIMITING TRANSFORMER Filed Jan. 21, 1944 Egg 1.

I I V/////AV//////I! Patented July 13, 1948 2,445,088 CURRENT-LIMITING TRANSFORMER Galcsville, Wis assignm- Lorcll John Schilling,

to Babson Bros. 00., a corporation of Illinois Application January 21, 1944, Serial No. 519,129

3 Claims. 1

This invention relates to a current limiting transformer, and more particularly to a transformer having means associated with the shunt portion thereof to adjust the reluctance of such portion.

One feature of this invention is that it provides an improved current limiting transformer; another feature of this invention is that it provides a current limiting transformer particularly adapted to supply energy to an electric fence; still another feature of this invention is that, as

1 a commercial manufacturing proposition, the

maximum current output of the transformer may be closely and accurately adjusted to the desired value; and yet another feature of this invention is that it enables current limiting transformers of the kind designed to energize electric fences to be manufactured more quickly and cheaply, with fewer rejects, while at the same time limiting the output current more closely to the desired value. Other features and advantages of this invention will be apparent from the following specification and the drawing, in which:

Figure 1 is a top plan view of a transformer embodying my invention; Figure 2 is a longitudihal section view along the line '2-2 of Figure 1: and Figure 3 is a transverse sectional view along the line 3-3 of Figure 2.

Electric fences for livestock are steadily increasing in favor; and such fences provide an electrical device which must be periodically energized briefly, with a high voltage, very low amperage electrical impulse. .While some few electric fences are continuously energized with an exceedingly low maximum current delivered thereto, the type of fence which has proved most effective and which has become most popular is energized about once each second for a very brief period. Experiments have proved that an impulse of about three milli-coulombs (three milliampere seconds) is very effective in stopping animals where the average voltage of the impulse is somewhere between 750 and 2,000 volts. Where the electric fence is energized from a conventional power supply (as 115 volt, 60 cycle current), it is desirable to limit the maximum current deliverable by the transformer to 25 milliamperes; and this is required by the laws of several states. On the other hand, a transformer which has its output current limited to much below this value fails to provide the desired "sting" necessary to stop animals and keep them from pushing through the electric fence, usually of only one or two wires.

While a transformer may be readily designed to deliver a desired output voltage (as for example 1500 volts) with a current limited to 25 milliamperes, manufacturing such transformers on a production basis without substantial variations in the maximum current deliverable proves to present considerable difficulty. The cores of such transformers usually comprise a stack of stamped laminations of sheets of iron or iron alloys having the desired magnetic permeability characteristics. There may be 20 or 30 such laminations of generally rectangular form four or five inches long and three or four inches wide, stacked up to make a core in the neighborhood of an inch thick. The primary winding or coil may have about 950 turns, and the secondary about 10,000 turns. With a transformer of these approximate dimensions, the central leg or shunt portion of the core should have an air gap therein provided by spacing the confronting lamination ends by about .020 inch, or slightly less.

All sorts of variations are possible in such a transformer, and most of these effect the maximum current deliverable. The generally rectangular core is generally formedby interleaving F-shaped laminations at opposite corners, and the amount of interleaving cannot be accurately reproduced. The operation of stamping out the laminations sometimes creates small burrs along certain of the edges, and these provide different spacings and relationships between the laminations in building up the core. The number of turns in the coils, particularly the secondary coil having in the neighborhood of 10,000 turns thereon, cannot be controlled with absolute precision without undue expense. All of these matters which have just been mentioned, and other factors entering into the manufacturing of such transformers, provide variations in the maximum current deliverable. If the production transformers are to have an output never exceeding 25 milliamperes (and this must be the case to meet the requirements of certain State laws), it has heretofore been thought necessary to design the transformer to have a maximum output in the neighborhood of 22 or 23 milliamperes,-so that there might be some variation on the higher side during production without an undue number of rejects. As a practical proposition, this resulted in a number of transformers being produced which varied on the low side and limited their current output to something around twenty milliamperes. This is undesirable for energizing an electric fence, since such a low maximum output, particularly if the interrupt-.

ing mechanism happens to be running fast and providing a still shorter operative period than desired, does not deliver suiiicient "sting" to provide an efficient electric fence.

I have devised a current limiting transformer, and a method of manufacturing such a transformer, which obviates these difficulties and enables electric fence energizing transformers to be manufactured, as a Production proposition, with their maximum current outputs held to within less than half of 1% variation. That is, an electric fence transformer embodying my invention can be designed to deliver 24.9 milliamperes with an open circuit secondary voltage of 1,500 volts; and such transformers can be manufactured with a production variation of less than 1 6 of a milliampere in their output, so that the transformers never deliver less than 24.8 milliamperes nor more than 25 milliamperes. I achieve this result by deliberately designing the transformers with considerably more reluctance in the shunt portion of the core (1. e., a considerably wider air gap) than would be necessary to effect the desired output; and then by adjusting the reluctance of the shunt portion of the core to the desired value by bridging the gap with a selectable number of bridging laminations, precision final adjustment being effected by movement of one of such laminations longitudinally of the shunt portion of the core.

Inasmuch as electric fences, their energizing apparatus, and current limiting transformers therefor, have been the subject .of previous patents, only the transformer is illustrated here and the remainder of this specification will be limited thereto. If fuller information is desired as to the electric fences and their energizing equipment and circuits, reference may be had to issued patents, as for example Pfanstiehl Patents 2,249,696 (July 15, 1941) and 2,304,954 (Dec. 15, 1942) previously issued to the assignee of the present application.

In the particular embodiment of my invention illustrated herewith, the transformer is shown as comprising primary and secondary coils l and l I encircling the legs lia and lib of a core li having a main flux circuit or path including the portions lia, lib, lie and lid and threading both the primary and secondary coils. The core also has a central shunt portion lie comprising the two parts lie and lie" separated by the air gap l3 best seen in Figure 3. Whereas, in a transformer 01' the kind given as a representative example above, the air gap should be in the neighborhood of .020 inch to give the desired maximum current output, the air gap ii is made .025 inch or even wider. This enables substantial manufacturing tolerances in the production and assembly of the transformer while always ensuring that the fixed shunt portion of the core will have a higher reluctance than that necessary to effect the desired maximum output of 25 milliamperes.

The theory of operation of current limiting transformers of the kind shown here is well known, but it is felt that it would be helpful to an understanding of this invention if a brief description thereof is made at this time. Magnetomotive force developed in the leg lia of the core by current flowing in the primary l0 causes magnetic flux to travel the main flux path and thread the secondary coil H; and this results in creation of an open circuit voltage across suchcoil II which bears the same proportion to the primary voltage as the turns ratio. As an appreciable load is thrownacross the secondary coil. however, current delivered to such load by the coil creates a countermagneto-motive force in the core leg lib which opposes the flux creating the voltage in the secondary coil; and it will be apparent that the heavier the load (the lower the resistance across the terminals of the coil) the greater this countermagneto-force will be and the more it will oppose or buck passage of magnetic flux through the leg lib. Under normal conditions, the air gap in the shunt portion lie of the core makes this shunt path of such high reluctance that substantially all ofthe flux passes through the main magnetic circuit including the leg lib. As countermagneto-motive force developed in the coil l l increases the effective or apparent reluctance of this leg, however, more and more flux passes through the shunt portion or shunt leg of the core. Since any flux passing through this shunt portion has no effect upon the secondary coil, variations in resistance across the secondary coil automaticall varies the amount of flux passing through that coil in such a way as to maintain the maximum current deliverable at the desired value.

By making the air gap ll considerably wider than would be appropriate to the maximum current desired, I am able to ensure the reluctance of the shunt portion always being greater than that necessary for the desired maximum current output. The reluctance interposed by the air gap may then be reduced, and the effective reluctance of the entire shunt leg of the core thus reduced, by bridging the gap with a selectable number of small bridging laminations here identified as lla-f. After the transformer has been assembled, with the primary and secondary coils in place and the corners of the F-shaped main laminations interleaved, the main core may be locked in place by bolting, clamping or any other appropriate means. Input and output connections may then be made to the transformer, the transformer energized with power of the same kind which is to be delivered to it in operation, and the maximum output on short circuit noted on a sensitive milliammeter. A selected number of bridging laminations may then be placed on the shunt portion of the core and this number varied, by putting on or taking off such bridging laminations, until a maximum current very close to that desired is obtained. Final adjustment of the maximum current output may be eflected by using a shorter bridging lamination, as the one here identified as Hf, and adjustably moving this longitudinally of the shunt leg to effect the final adjustment. That is, this smaller bridging lamination can be caused to extend only half way across the air gap, as shown; it may be pulled to the left, speaking with respect to the position shown in Figure 3, to reduce its effectiveness; or it may be moved to the right to increase its bridging effectiveness and reduce the reluctance of the shunt path. After the adjustment has been effected, the bridging laminations are locked in operative position on the shunt leg by any appropriate clamping means which does not effect the magnetic conditions. This is here shown as effected by a disk is of non-magnetic material, as Bakelite, here shown as operating not only as means to hold the bridging laminations in place, but also as means for mounting the whole transformer in a non-magnetic base plate ll, this being here shown as effected by the studs lid and I"). It will be understood that the bridgin laminations may extend the full width of the shunt leg,v

but preferably not its full length; and that the bridging means would have a thickness only a very small fraction of that of the transformer core, as for example only five or six laminations, or even fewer, as contrasted with about 30 laminations in the core. Under these conditions the bridging portion is very small compared to the fixed portions of the shunt leg of the core, and accidental loosening up or misadjustment of the bridging portion will not render the transformer output dangerous.

While I have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims.

I claim:

1..In a current limiting transformer having a ermeable core providing a main flux path and a shunt flux path across said main path, the shunt path having an air gap therein, apparatus of the character described, including: primary and secondary windings on the main flux path of said core; bridging means of permeable material adapted to bridge said gap and afiect the reluctance of the shunt portion of the core, this bridging means comprising a selectable number of laminations of said permeable material on the outside of the shunt portion; and means for locking the bridging means in desired relation to said shunt portion. 2. Apparatus of the character claimed in claim 1, wherein at least one of said laminations is adapted to be movably adjusted longitudinally of said shunt portion of the core.

3. Apparatus of the character claimed in claim 1, wherein the locking means comprises a nonmagnetic member adapted to overlie said bridging means.

LOREIL JOHN SCHILLING.

REFERENCES CITED The following references are ofrecord in the file of this patent;

UNITED STATES PATENTS Number Name 7 Date 1,859,115 Summers May 1'1, 1932 1,863,936 Schwager June 21, 1932 FOREIGN PATENTS Number Country Date 373,355 Great Britain May 26, 1932 606,024 France Feb. 27, 1926