US3768055A

US3768055A - Transformer providing half-turn secondary windings

Info

Publication number: US3768055A
Application number: US00265952A
Authority: US
Inventors: B Oliver
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1972-06-23
Filing date: 1972-06-23
Publication date: 1973-10-23
Anticipated expiration: 1990-10-23
Also published as: JPS4954819A

Abstract

A transformer is provided with a closed core having a center leg on which a primary winding is disposed and having two outer legs on which one or more secondary windings having an integer number of half turns are disposed. To insure flux equality in the two outer legs, balancing windings may be disposed on the outer legs or, alternatively, the secondary windings having the integer number of half turns may be wound in duplicate and crossconnected in parallel so that an extra half turn of one of these secondary windings passes between the center leg and one of the outer legs and an extra half turn of another of these secondary windings passes between the center leg and the other of the outer legs.

Description

United States Patent [191 [111 3,768,055 Oliver Oct. 23, 1973 TRANSFORMER PROVIDING HALF-TURN Primary Examiner-Thomas J. Kozma SECONDARY WINDINGS Att0rneyRoland I. Griffin [75] Inventor: Bernard M. Oliver, Los Altos Hills,

Calif- 57 ABSTRACT Assigneei uewleu'ltackard p y Pal) A transformer is provided with a closed core having a Alto, Callfcenter leg on which a primary winding is disposed and [22] Filed: June 23, 1972 having two outer legs on which one or more secondary windings having an integer number of half turns are PP 265,952 disposed. To insure flux equality in the two outer legs, balancing windings may be disposed on the outer legs 52 us. (:1 336/73 336/172 336/184 alternatively the secmdary windings having the 51 Int. Cl. .j Him 27/28 integer number half urns may be in dupli- [58] Field of Search 336/73 174 175 Cate and cmss-mnnected in Parallel that 336/182 half turn of one of these secondary windings passes between the center leg and one of the outer legs and [56] References Cited an extra half turn of another of these secondary wind- UNITED STATES PATENTS ings passes between the center leg and the other of the outer le s. 3,431,487 3/1969 Savage 336/73 g 5 Claims, 8 Drawing Figures Copper Srrops Folded 0nd Soldered 23\ Solder- Insulation Solder 12 PAIENIEBnm 23 ms sum 1 or 41 llllll l igure 2 PAIENIEnnn 23 ms SHEET 0F 4 2 m mm a: 2 2.6 uwtwws 960 EO V2235 35m 5300 TRANSFORMER PROVIDING HALF-TURN SECONDARY WINDINGS BACKGROUND OF THE INVENTION In the use of electrical transformers comprising a primary winding and a plurality of secondary windings on a core, it is often desirable, in order to provide voltages from the secondary windings that are very close to a desired ratio to each other, to be able to add the voltage corresponding to one half-turn of one secondary winding to another secondary winding. This is particularly true where the voltages desired are comparable to the voltage per turn; for example, in low voltage applications, where one-half volt increments are desired from secondary windings producing one volt per turn.

In a standard form of transformer utilizing EI laminations in the transformer core, a half-turn secondary winding is formed by a wire that threads one window of the core but not the other window. Therefore, a single turn of wire around the center leg of the transformer core, center-tapped to ground, forms two halfturn windings, one on each side of the center tap. So long as the two half-turn secondary windings are loaded symmetrically, the two secondary voltages and currents remain balanced. However, when the loading of the two half-turn secondary windings is asymmetrical, for example, a load on one half-turn secondary winding but not the other, the leakage reactance of the loaded halfturn secondary winding is much larger than the leakage reactance of one full-turn secondary winding, and the regulation of the transformer is very poor.

The added leakage reactance results from the fact that the current drawn through the loaded half-turn secondary winding sets up a counter flux in the core leg associated with the loaded half-turn secondary winding, and this flux travels around the core in a path including the other core leg associated with the other half-turn secondary winding and adds to the flux in that other core leg. The voltage across the loaded half-turn secondary winding tends todecrease while the voltage across the other half-turn secondary winding tends to increase, and the regulation is poor.

One possible solution to this problem is to create an air gap between the two core halves, each including one outer leg, such that flux generated in one outer leg cannot be added to the flux in the other outer leg. This essentially separates the transformer into two transformers with a common primary, either of which can be loaded. Such a configuration has merit'if the transformer core consists of two pairs of C-cores, but is not practical when utilizing EI lamination type cores.

SUMMARY OF THE INVENTION In the present invention one or more short-circuited secondary windings are associated with the two outer legs of the core in such a manner that the normal magnetizing flux in the center leg of the core, which flux divides equally between the two outer legs, induces no emfs around these windings. However, any flux produced by the mmf of a current drawn from a half-turn winding does induce an emf in these windings, causing current to flow therein such as to oppose the flux. This forces a flux balance in the two outer legs, and a substantial reduction of the large leakage reactance normally produced by the aforementioned flux imbalance.

In one embodiment of the present invention, the short-circuited secondary winding comprises a first III winding associated with one outer leg of the transformer core and series connected with a second winding associated with the other outer leg of the core. Each of these secondary windings passes through its associated window in the core in the same direction and the same number of times. The two secondary windings are connected in parallel so that the emfs produced by the normal balanced magnetizing flux oppose each other, and no circulating current flows.

In another embodiment of this invention the shorted secondary winding structure consists of a single turn associated with each core leg, the single turns being connected together as before.

DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are front and top views, respectively, of a transformer core having a pair of half-turn windings thereon.

FIG. 3 is an equivalent circuit diagram of the two half-turn windings of FIG. 1.

FIG. 4 is a view of the transformer core of FIG. 1 showing a balancing secondary winding wound thereon in accordance with one embodiment of the present invention.

FIG. 5 is an equivalent circuit diagram similar to the circuit of FIG. 3 with the secondary of FIG. 4 added thereto.

FIG. 6 is a view ofa transformer core including a balancing secondary winding structure in accordance with a second embodiment of the invention.

FIGS. 7(a) and 7(b) are two views of a transformer showing the formation of the secondary windings of FIG. 6 on the core.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2, there is shown a typical transformer core utilizing standard El laminations and having a center leg portion 11 on which it is assumed the primary winding is wound and two outer leg portions, the right leg 12 and the left leg 13. A half-turn winding is formed by a wire that threads one window of the core but not the other window. For example, wire AB passing through window 14 forms a half-turn winding while wire B"C passing through window 15 forms a second half-turn winding. If the wire ends B and b" are connected together to form a junction B, then a full turn winding ABC is formed. If A is then connected to C, the circuit will enclose (link) the flux in the center leg of the core. However if A is connected to B, only the flux in the left leg 13 is linked. If point B is grounded, the open circuit voltages to ground at points A and C will be -E/2 and E/2, respectively, where E is the voltage induced in one full-turn winding ABC. Thus, so far as open-circuit voltages are concerned, both AB and BC are half-turn windings, and these half-turn winding are added to associated secondary windings on the core to provide secondary windings each consisting of an integer number of turns plus one half-turn.

So long as these two half-turn winding AB and BC are loaded symmetrically, equal currents will be drawn through them, and the flux produced in the core by the current in one secondary winding will be counterbalanced by the flux produced in the core by the current in the other secondary winding. However, if the two half-turn windings are loaded asymmetrically, then one of the half-turn windings will draw a greater current than the other, and the flux in one outer core leg will decrease while that in the other outer core leg will increase. As a result, voltage will be substracted from the loaded secondary winding and added to the unloaded secondary winding and regulation is poor.

This may be more readily understood by referring to the equivalent circuit of the secondary winding ABC of FIGS. 1 and 2 shown in FIG. 3. When a load is connected from C to ground, the current flowing from B to C and back to B via ground causes an upward mmf in the right leg 12 of the core. The flux in center leg 11 cannot be altered because of the voltage driven primary winding around the center leg. However, the flux produced by the upward mmf can return down the left leg 13. Thus, the half-turn winding BC is linked by a permeable ring comprising the top and bottom of the core (a length of 4a) and the two side'legs (a length of 2b).

The leakage reactance between A and C is L (lk where L is the inductance of the secondary with the primary open and k is the coupling coefficient. If k is, for example 0.995, then the leakage reactance of a full turn is L (lk )-L /100. The leakage inductance, L, added by the half-turn winding BC as a result of the ring core is L L (reluctance to normal flux/reluctance to flux around outside ring) or z L (a b) /(4a 2b) Assuming b 2a, then L z 3/8L2,

other side and the regulation is poor.

One possible manner by which the inductance L can be greatly reduced is by separating the core into two halves by introducing an air gap in the plane P of FIG. l.'This separates the transformer into two transformers with a common primary, and either of the two transformers can be loaded. Although this arrangement has merit if the core consists of two pairs of C-cores, it is not practical when using EI laminations.

In accordance with one embodiment of the present invention, and referring to FIG. 4, the leakage inductance L is greatly reduced by providing a shortcircuited secondary winding configuration comprising a first winding section 16 about the left leg 13 and a second winding section 17 about the right leg 12. Each secondary winding section 16 or 17 passes through its associated window 14 or in the same direction the same number of times. As a result, the normal magnetizing flux in the center leg 11, dam, which divides equally into the two

outer legs

12 and 13 induces no emfs around these windings. Any leakage flux, produced by the mini of a current drawn from a half-turn winding does induce emfs causing currents to flow in these windings which oppose the leakage flux, and the effect is to force a flux balance in the two outer legs. As a result, half-turns may be added to any secondary winding on the centerleg without incurring a large leakage reactance.

The equivalent circuit diagram of FIG. 5, which is similar to that of FIG. 3, shows the short-circuited secondary coupled to L; in FIG. 5, R is the resistance of the balancing winding of FIG. 4 while N is the total number of turns in the balancing winding. In this equivalent circuit, L has been effectively replaced by R/N' The windings of FIG. 4 can be prewound single layer solenoids that are soldered together at X and Y after core assembly. It should be noted that the balancing windings themselves can be used to give half-turn voltage steps as for example, between terminals X and Y.

Another embodiment of the present invention is shown in FIG. 6 wherein a pair of short-circuited secondaries 21 and 22 are employed. As with the embodiment of FIG. 4, each secondary circuit passes through each window in the same direction the same number of times (in this illustration once). In the equivalent circuit of FIG. 5, R is the half the resistance of either winding of FIG. 6 while N is the total of turns.

The secondary windings of FIG. 6 may be formed on the core structure as shown in FIGS. 7(a) and 7(b) by first inserting four short strips 23 of copper through the two

windows

14 and 15, the strips 23 being insulated from the core, the ends of the strips being folded against the sides of the core as shown. Thereafter, the shorted secondary windings may be formed-by soldering the ends of the two suitably insulated crossed copper strips 24 to the associated pair of strap inserts 23 to form the figure eight secondary windings 21 and 22 on the top and bottom sides of the core.

I claim:

1. A transformer comprising a core structure including a central leg portion and two outer leg portions with window openings in the core structure between the central leg portion and the outer leg portions, a primary winding disposed on the central leg portion, a pair of secondary windings disposed on the core structure, at least one of these secondary windings including a halfturn winding comprising a wire passing in one direction through the window opening between the central leg portion and one of the outer leg portions so that current passing through the half-turn winding produces a flux in said one of the outer leg portions, and a shortcircuited winding structure disposed on both outer leg portions so that a current is induced in the shortcircuited winding structure by the flux in said one of the outer leg portions to produce a counterbalancing flux in the other of the outer leg portions, said shortcircuited winding structure comprising a first plural turn winding disposed on one of the outer leg portions and a second plural turn winding'disposed on the other of the outer leg portions, said first and second plural turn windings passing through their associated window openings in the same direction and the same number of times and having their ends interconnected in series.

2. A transformer comprising a core structure including a central leg portion and two outer leg portions with window openings in the core structure between the central leg portion and the outer leg portions, a primary winding disposed on the central leg portion, a pair of secondary windings disposed on the core structures, at least one of these secondary windings including a halftum winding comprising a wire passing in one direction through the window opening between the central leg portion and one of the outer leg portions so that current passing through the half-turn winding produces a flux in said one of the outer leg portions, and a shortcircuited winding structure disposed on both outer leg portions so that a current is induced in the shortcircuited winding structure by the flux in said one of the outer leg portions to produce a counterbalancing flux in the other of the outer leg portions, said shortcircuited winding structure comprising a pair of shortcircuited winding circuits each including a pair of windings with each winding of the pair being coupled to a different one of the outer leg portions and passing through its associated window opening in the same direction and the same number of times.

3. A transformer comprising a core structure includin g a central leg portion and two outer leg portions with window openings in the core structure between the central leg portion and the outer leg portions, a primary winding disposed on the central leg portion, a pair of secondary windings disposed on the core structure, said secondary windings including a wire passing through each window opening and having a tap at its midpoint to provide two half-turn secondary windings, current passing through one of these half-turn secondary windings producing a flux in the core structure, and a shortcircuited winding structure disposed on both outer leg portions so that a current is induced in the shortcircuited winding structure by said flux to produce a flux in the core structure counter balancing the flux produced therein by the current in said one of the halfturn secondary windings.

4. A transformer as in claim 3 wherein said shortcircuited winding structure comprises a pair of windings each coupled to a different one of the outer leg portions, both of these windings being connected in series and passing through their associated window openings in the same direction and the same number of times.

5. A transformer as in claim 3 wherein said shortcircuited winding structure comprises a pair of shortcircuited winding circuits, each including a pair of windings with each winding of the pair being coupled to a different one of the outer leg portions and passing through its associated window opening in the same direction and the same number of times.

Claims

1. A transformer comprising a core structure including a central leg portion and two outer leg portions with window openings in the core structure between the central leg portion and the outer leg portions, a primary winding disposed on the central leg portion, a pair of secondary windings disposed on the core structure, at least one of these secondary windings including a half-turn winding comprising a wire passing in one direction through the window opening between the central leg portion and one of the outer leg portions so that current passing through the half-turn winding produces a flux in said one of the outer leg portions, and a short-circuited winding structure disposed on both outer leg portions so that a current is induced in the short-circuited winding structure by the flux in said one of the outer leg portions to produce a counterbalancing flux in the other of the outer leg portions, said short-circuited winding structure comprising a first plural turn winding disposed on one of the outer leg portions and a second plural turn winding disposed on the other of the outer leg portions, said first and second plural turn windings passing through their associated window openings in the same direction and the same number of times and having their ends interconnected in series.

2. A transformer comprising a core structure including a central leg portion and two outer leg portions with window openings in the core structure between the central leg portion and the outer leg portions, a primary winding disposed on the central leg portion, a pair of secondary windings disposed on the core structure, at least one of these secondary windings including a half-turn winding comprising a wire passing in one direction through the window opening between the central leg portion and one of the outer leg portions so that current passing through the half-turn winding produces a flux in said one of the outer leg portions, and a short-circuited winding structure disposed on both outer leg portions so that a current is induced in the short-circuited winding structure by the flux in said one of the outer leg portions to produce a counterbalancing flux in the other of the outer leg portions, said short-circuited winding structure comprising a pair of short-circuited winding circuits each including a pair of windings with each winding of the pair being coupled to a different one of the outer leg portions and passing through its associated window opening in the same direction and the same number of times.

3. A transformer comprising a core structure including a central leg portion and two outer leg portions with window openings in the core structure between the central leg portion and the outer leg portions, a primary winding disposed on the central leg portion, a pair of secondary windings disposed on the core structure, said secondary windings including a wire passing through each window opening and having a tap at its midpoint to provide two half-turn secondary windings, current passing through one of these half-turn secondary windings producing a flux in The core structure, and a short-circuited winding structure disposed on both outer leg portions so that a current is induced in the short-circuited winding structure by said flux to produce a flux in the core structure counter balancing the flux produced therein by the current in said one of the half-turn secondary windings.

4. A transformer as in claim 3 wherein said short-circuited winding structure comprises a pair of windings each coupled to a different one of the outer leg portions, both of these windings being connected in series and passing through their associated window openings in the same direction and the same number of times.

5. A transformer as in claim 3 wherein said short-circuited winding structure comprises a pair of short-circuited winding circuits, each including a pair of windings with each winding of the pair being coupled to a different one of the outer leg portions and passing through its associated window opening in the same direction and the same number of times.