FORM 2
THE PATENT ACT 1970
&
The Patents Rules, 2003
COMPLETE SPECIFICATION (See section 10 and rule 13)

1. TITLE OF THE INVENTION:
TOROIDAL CORE FOR A CURRENT TRANSFORMER
2. APPLICANT:
(a) NAME: Larsen & Toubro Limited
(b) NATIONALITY: Indian Company registered under the
provisions of the Companies Act-1956.
(c) ADDRESS: LARSEN & TOUBRO LIMITED,
L&T House, Ballard Estate, P. 0. Box: 278, Mumbai 400 001, India
3. PREAMBLE TO THE DESCRIPTION:
COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.

TOROIDAL CORE FOR A CURRENT TRANSFORMER
Field of invention
The present invention relates to electrical switches, and more particularly to a toroidal core for magnetic circuits of electric machines and facilities especially for transformers.
Background of the invention
Residual-current circuit breakers are designed to protect people and appliances from dangerous residual currents and should be tripped according to the respective, regulations in the event of residual current overload. Core balance current transformer (herein after referred as 'CBCT's' / 'Toroids') are used in Residual current circuit breaker (RCCB's). A residual-current device (RCD), or the residual-current circuit breaker (RCCB) or residual twin-direct current couplet (R2D2), are electrical wiring disc devices that disconnects a circuit upon detection of un balanced electric current between an energized conductor and a return neutral conductor. Such an imbalance leads to current leakage through a body of a grounded person who accidentally touches the energized part of the circuit. Hence, CBCT output during unbalanced condition become critical. Any malfunction in CBCT's output results to be hazardous.
Toroids are commonly used in the art as current transformers, couplers and the like in an earth fault circuit breaker. Over-magnetization leading to saturation of the core of the toroid is undesirable, as this may lead to improper operation and/or overheating. Toroidal cores are constituted from anisotropic oriented strips wound and firmly connected with one on other and being mutually insulated. The toroidal

cores are generally made from silicon steel sheets to a required width by cold rolling, thermal treating, pickling and longitudinal cutting. These oriented anisotropic strips deposited with two side insulation are after then wound to toroidal shape - toroid - and fitted with winding. Ferrite materials used as a core material in a current transformer's (CT) are susceptible to mechanical stress, both from winding the core and from encapsulation.
Accordingly, there exists a need for a toroidal core that reduces the mechanical stresses to minimum possible arising both from winding the core and from encapsulation. Also, there exists a need to at least alleviate the problem of over magnetization and ease of winding of the toroidal cores.
Object of the invention
An object of the present invention is to solve the problem of over magnetization and enable ease of winding.
Another object of the present invention is to increase the overall output of a current transformer.
Yet another object of the present invention is to reduce the mechanical stresses to minimum possible arising both from winding the core and from encapsulation.
Summary of the invention
Accordingly, present invention provides a toroidal core for a current transformer. The current transformer includes at least a pair of windings that wind on the toroidal core. Each winding of the pair of windings has a pair of terminals. The toroidal core

of the present invention includes at least two or more discs that are capable of being stacked coaxially. At least one disc of the at least two discs is configured with a gap extending radially forming a break on one side. Further the plurality of discs is wound by the pair of windings. Further at least one disc of the at least two or more discs is sandwiched therebetween.
Brief description of the drawings
Figure 1 shows a top view of a toroidal core of a current transformer, in accordance with the present invention;
Figure 2 shows a side view of the toroidal core of the current transformer of the figure 1;
Figure 3 represents a side view of a stack of a plurality of discs of the toroidal core, in accordance with the present invention;
Figure 4 shows a top view and a bottom view of the stack of the plurality of discs of the toroidal core of figure 3;
Figure 5 shows a cross sectional view of a disc of the toroidal core with a gap, in accordance with the present invention;
Figure 6 shows a cross sectional view of an alternate disc of the toroidal core with a gap of figure5;
Figure 7 shows a graph showing a hysteresis loop during operation of the toroidal core, in accordance with the present invention; and

Figure 8 shows a two part toroidal core of the current transformer, in accordance
with an alternate embodiment of the present invention.
Detailed description of the invention
The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiments.
Accordingly, the present invention provides a toroidal core for a current transformer having a plurality of discs stacked coaxially and at least a pair of windings. A disc of the plurality of discs configured with a gap extending radially forming a break on one side. The disc with the gap of the present invention enables the ease of winding thus increases an overall output of the toroidal core for the current transformer.
The present invention is illustrated with reference to the accompanying drawings, throughout which reference numbers indicate corresponding parts in the various figures. These reference numbers are shown in bracket in the following description.
Referring to figures 1-3, a toroidal core (2) incorporated in a core balance current transformer (1) (hereinafter 'current transformer (1)') is illustrated. The current transformer (1) includes at least a pair of windings (3 and 4) that winds on the toroidal core (2). Each winding of the pair of windings (3 and 4) includes a pair of terminals (5 and 6). The primary winding (3) has the pair of terminal 5 for the application of voltage Vp (Primary voltage) and the secondary winding has the pair of terminal 6 for the induced voltage Vs. The pair of windings (3 and 4) is wound on diametrically opposite sides of the toroidal core (2). The pair of windings (3 and

4) is spread annularly around the toroidal core (2) such that they overlap with each other.
A primary winding (3) of the pair of windings forms the part of an electrical circuit such that changing current induces a current in a neighboring circuit. The current through the primary winding (3) induces current in the secondary winding (4). An alternating electric current flowing through the primary winding (3) of current transformer (1) generates a varying electromagnetic field in its surroundings which causes a varying magnetic flux in the toroidal core (2) of the current transformer (1). The varying electromagnetic field in the vicinity of the secondary winding (4) induces an electromotive force in the secondary winding (4), which appears a voltage across the output terminals. If load impedance is connected across the secondary winding (4), current flows through the secondary winding (4) drawing power from the primary winding (3) and the power source. The two windings are placed around the periphery (circumference) at appropriate position, provided assembly part taken into the consideration.
The toroidal core (2) includes at least two discs or more. A stack of at least two or more discs are secured together by the pair of windings (3 and 4) to facilitate a stacked arrangement. In an embodiment, the toroidal core (2) includes at least three discs (7, 8 and 9) that are capable of being stacked coaxially one upon another. The stack of three discs (7, 8 and 9) is secured together by the pair of windings (3 and 4) to facilitate the stacked arrangement. The discs (7, 8 and 9) have identical shape and dimensions including thickness and in particular having same inner and outer diameters. The discs (7, 8 and 9) match with one another and together form a straight tubular structure. The cross-section of each disc (7, 8 and 9) on each side has a rectangular conformation with round and/or chamfered comers.

In the present embodiment, the discs (7 and 9) have substantially the same outer and inner diameters, as shown in figure. 4. Each disc (7 and 9) is configured with a surface (10 and 11) that is made of a ferromagnetic ferrite material. The ferromagnetic ferrite material of the preferred embodiment is selected any one from PL-3, PL-7, M50, SM100 or KB5. The surface (10 and 11) is fully covered by a protective coating (12 and 13). The protective coating (12 and 13) of the present embodiment is an epoxy or parylene material, that provides insulation between the disc (7 and 9) and the windings (3 and 4).
The toroidal core (2) includes at least one disc (8) of the at least two or more discs positioned such that, the disc (8) is sandwiched there between the other discs (7 and 9). The disc (8) is configured with a gap extending radially forming a break on one side. The gap present in the disc (8) enables the ease of winding of the current transformer.
The gap present in the disc (8) acts as an air gap to the magnet flux when the toroidal core (2) is magnetized, forming a break in the magnetic flux path. An extra magnetizing force is required to excite the air gap, in addition to the normal magnetizing force needed to excite the material of the toroidal core (2) itself. Accordingly, the hysteresis loop rotates clockwise about its origin, as shown in FIG.7., resulting in a relatively slow rise or fall of the otherwise steeply rising or falling sections of the hysteresis loop, compared with a typical toroidal core without an air gap in the art. The hysteresis loop is therefore markedly tilted, with its area extending to cover a relatively wider range of magnetizing force (H), whereby reducing occurrence of over-magnetization leading to saturation of the toroidal core (2).
The magnetic reluctance of the air gap is considerably larger than that of the material of the toroidal core (2). The magnetizing force required to create a certain

flux density within the toroidal core (2) is thus effectively determined by the reluctance of the air gap alone. For the same area within the hysteresis loop, the hysteresis losses and Eddy current losses are practically unaffected by the existence of the air gap. Thus, the real core losses (but not the apparent VA (volt-amperes) losses) and heating of the toroidal core will not change.
Referring to figure 5, a cross sectional view of an independent disc of the toroidal core (2) with a gap (14), in accordance with the present invention is illustrated. And referring to figure 6, a cross sectional view of an alternate disc of the toroidal core with a gap (14) of figure 5, in accordance with the present invention is shown. The gap (14) facilitates winding automation. Since, the ferrite materials are susceptible to mechanical stress, both from winding the core and from encapsulation, high permeability materials are particularly affected as illustrated in following table 1.
Table 1: Relation with gap;

Referring to figure 8, an alternate embodiment of the present invention is shown. Alternate embodiment includes a toroid with a gap (14) that enables the advantage

of ease of winding. After primary winding, stacking of the second part with ultrasonic welding in order to achieve the desired permeability is facilitated. The alternate embodiment includes a core having two independent parts of a toroid (solid enameled stacks). Once through with the winding process i.e. primary 3 and secondary 4 winding, the two part toroid is stacked together using ultrasonic welding. After winding the core is encapsulated with a bobbin. The arrangement facilitates additional mechanical strength to core. In general, the toroidal core of the present invention includes co-axial stack of at least two toroidal rings 7/9 and 8, at
least one of which 8 includes the gap 14 on one side, that may or may not be occupied by a filler. The gap 14 can be of any uniform width but is preferably as narrow as practically possible. The toroidal ring 8 including the gap 14 is preferably sandwiched by two toroidal rings 7 and 9 that are without a gap, or is at least an intermediate ring in the stack. The gap is filled up by non-ferromagnetic filler. Further, the filler has an outer surface which lies flush with that of the parts of the body of the first ring forming the gap. The body of the first ring and the filler are completely covered by an insulating coating.
In another embodiment, the toroidal core includes only two said toroidal discs that include one toroidal disc with the gap (14).
In yet another embodiment of the present invention, a toroid incorporates the toroidal core (2). The toroid core (2) of the present embodiment includes at least one winding wound on the toroidal core (2) securing at least two more discs together.
Advantages of the present invention

1. The toroidal core (2) resolves the problem of over magnetization and enables ease of winding.
2. The toroidal core (2) is cost effective and increases the overall output of the current transformer.
3. The toroidal core (2) is advantageously operable to reduce the mechanical stresses to minimum possible arising both from winding the core and from encapsulation.
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.

We claim:
1. A toroidal core for a current transformer, the current transformer having at least a pair of windings wound on the toroidal core, each winding of the pair of windings having a pair of terminals, the toroidal core comprising:
at least two or more discs capable of being stacked coaxially, at least one disc of the two or more discs configured with a gap extending radially forming a break on one side, the at least one disc of the at least two or more discs being sandwiched therebetween;
wherein the at least two or more discs are wound by the pair of windings.