WO2008111093A2

WO2008111093A2 - Transformers

Info

Publication number: WO2008111093A2
Application number: PCT/IN2008/000138
Authority: WO
Inventors: Trimbak Sane Satish
Original assignee: Trimbak Sane Satish
Priority date: 2007-03-13
Filing date: 2008-03-11
Publication date: 2008-09-18
Also published as: WO2008111093A3

Abstract

A transformer comprising at least two coils (9,10), each of the coils defined by a body, at least a portion of which has a semi circular cross section and a window defined within the center of the body, said coils (9,10) operatively placed side by side in an abutting relationship with each other such that the abutting sides of the coils together form an element having a substantial circular cross sectional area; and a core (11 ) made of a magnetic steel strip wound around said circular element and enclosed within the windows of said coils (9,10).

Description

TRANSFORMERS

Field of the Invention

This invention relates to transformers.

Background of the Invention:

A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling, without any moving parts. It works on the Principle of Faraday's Law of Electromagnetic Induction, and other laws of electricity.

The basic functional parts of the transformer are:

A core made of magnetic material, a primary winding and a secondary winding. The primary winding is connected to a source of alternating current and the secondary winding is connected to an output load.

(i) A magnetic core concentrates the magnetic flux generated, and therefore is made with good permeability material. The types of cores used are metal, air, ferrites, metglass or a combination of magnetic materials in various forms.

(ii) Windings: The windings could be either two or more coupled windings viz. primary and secondary or a single tapped winding which functions both as the primary and the secondary. The windings are made of wire and wound together. The wire used is generally a good conductor, like a copper or aluminum wire with a coating of varnish or some other insulation. Windings basically act as electrical conductors. They are coupled with each other magnetically and hence have mutual inductance.

In constructing an isolation transformer, two coils are wound on the same core. The coil connected to the source of power is called the primary winding and the side from which the output is taken is called the secondary. A changing current in the primary winding creates a time-varying magnetic flux in the core, which induces a voltage in the secondary winding based on the principle of induction. The equation for the working of ideal transformer is: The instantaneous induced e.m.f.s in the various windings are proportional to their number of turns.

Vp _ Np

Vs ^~ Ns

Vp corresponds to Primary Voltage.

Vs corresponds to Secondary voltage.

Np corresponds to Number of turns on Primary side.

Ns corresponds to Number of turns on Secondary side.

If, the number of turns in both the windings equals each other and the ratio stands as 1, then voltages on either sides of the transformer are ideally the same. To increase the functionality of the transformer, the numbers of turns, number of windings are varied or a single winding may be tapped for an output. This categorizes transformers into step-up, step-down, multiple input/output or Auto mode type.

The preliminary uses of a transformer are:

High voltage power transmission for long distances made economically practical.

Step-up: The secondary has more turns than the primary.

Step-down: The secondary has fewer turns than the primary.

Transformers are made in (a) shell type construction, or (b) core type construction. In the shell type, the magnetic core surrounds the conductor coil and in the core type conductor coils surround the magnetic core. In both shell and core type transformer construction, magnetic material that is thin and insulated (to prevent eddy-current losses) in various strip or roll form or EIAJI laminations is used.

Fill factor is measured as area of enameled wire filled, divided by, area of bobbin that is actually available for winding.

In the prior art, the laminated shell-type is made from E-shaped and I-shaped pieces, leading to the name "EI transformer". In the EI transformer, the laminations are stacked in an interleaved fashion. The E and I pieces are staggered and alternately filled to overlap the gaps. If an air gap is needed, all the 'E's are stacked on one side, and all the Ts on the other creating a gap. Toroidal transformers are also known in the prior art. In a conventional toroidal transformer, a strip core is wound on a mandrel, then annealed and insulated to form a core. Conductor are then wound by hand or by toroidal winding machines, which first wind the conductors on a two part mandrel threaded through the toroid window and then the mandrel rotates and winds the conductor on the pre-formed toroidal core to form the windings. This configuration has its limitations enumerated below:

1) Conductor winding fill factors are very poor and the space for mandrel has to be left unoccupied.

2) Threading of conductor windings within the annular space of the core is difficult and time consuming.

There are other types, such as the C-core or "cut core" transformers. A magnetic steel strip is wound around a rectangular form of mandrel to make a C-core. After the required stack is wound, it is annealed in an appropriate inert gas atmosphere. After annealing, it is cut to form two C shapes. The faces of the cuts are then ground smooth if required so they fit very tight with a very small gap to improve permeability of the core. A conductor coil is wound and placed over a leg of one half of the core. The other half core is then pushed in bringing the two C halves together, and holding them close by a steel strap. There are shell type cores available in 'C form, which are similar to the EI cores.

Still other Shell type transformers are ones with a wound -iron core. Here, the core consists of two tight spirals of steel strip, each wound through the window of the preformed (circular cross-section) single coil of the conductor winding by machine operation. In addition to the advantage of having the flux everywhere directed along the grain, there are no joints to add to the reluctance of the magnetic circuit, thereby reducing the no-load current to a minimum; and the core structure is inherently rigid.

The General Electric Spirakore is an example of a wound core transformer. The GE Spirakore is constructed by winding steel strip cores on a conductor coil with circular cross section and calculated rectangular window.

Drawbacks of Spirakore Transformers: 1) A single conductor coil is wound in the full circular cross-section, or an approximation to a full circle without which this type of transformer is not practical to produce.

2) A full circular cross section conductor winding is very difficult to wind. The winding is time consuming, and any approximation to a circle increases the winding space factors, and the cost.

3) The requirement of circular winding economy also limits its size or rating and a large variety of windings, all in circular cross section is difficult to design & wind.

4) Only single Phase Transformers are practical.

5) For Three Phase Transformers, three Single Phase transformers have to be connected in three phase configuration.

6) Fill factor is also poor.

Still other types of transformers are R-core. A big roll of the steel core raw material is slit by means of a computer assisted device, so that as it is wound, the width of that slit continuously changes to form a net circular cross section of the core with required window dimension. The two conductor coils are wound by driving and rotating the flanges of the split bobbins placed over each leg of the core having circular cross section. It would be difficult to wind & produce high power or heavy winding transformers by this method.

Losses occurring in a transformer are summarized as follows:

1) Energy dissipation takes place due to resistance of the windings known as copper loss, and due to magnetic effects primarily attributable to the core, also known as iron loss.

2) Induced Eddy Currents circulating within the core cause resistive heating. These are minimized by laminating the core.

3) Hysteresis losses within the magnetic core occur due to periodic reversal of magnetic field.

4) A portion of the leakage flux, due to gaps in the magnetic circuit, may induce eddy currents within nearby conductive objects, such as the transformer's support structure, be converted to heat, and hence, contribute to stray losses. It may enter the core at an angle and produce extra watt-loss.

5) Part of the power used to operate the cooling system for large transformers is typically considered part of the losses of the transformer. 6) Magnetic fluxes in the core cause it to physically expand and contract slightly with the alternating magnetic field, an effect known as magnetostriction, this may produce audio noise.

7) Fluctuating electromagnetic forces may incite vibrations within nearby metalwork, creating a familiar humming or buzzing noise; and consuming a small amount of power.

Thus there is still a need for providing a transformer which is easy to manufacture and efficient. Also the fill factor for the conductor area should be better than that in the prior art, for toroidal transforers and nearly equivalent to the best.

Objects of the invention:

One object of the invention is to provide a transformer which is easy to manufacture, and versatile in its application.

Another object of the invention is to provide a transformer which has a good fill factor for conductor area.

One more object of this invention is to provide a transformer whose conductor and core areas are easily adjustable.

One more object of this invention is to provide a transformer which has minimum weight or volume or cost, and gives the same performance.

Still one more object of this invention is to provide reduced inventory for transformer core material.

Yet another object of this invention is to provide a transformer whose construction process is applicable to transformers of all VA ratings.

Still another object of this invention is to provide a transformer where the core-building process can be mechanized. An additional object of this invention is to provide a method for assembling a transformer which is suitable for construction of single phase as well as three phase transformers. Another object of this invention is to provide a transformer which is robust.

The transformer envisaged in accordance with this invention reduces the limitations of the prior art.

Summary of the Invention:

According to this invention there is provided a transformer comprising,

at least two coils, each of the coils defined by a body, at least a portion of which has a semi circular cross section and a window defined within the center of the body, said coils operatively placed side by side in an abutting relationship with each other such that the abutting sides of the coils together form an element having a substantial circular cross sectional area; and

a core made of a magnetic steel strip wound around said circular element and enclosed within the windows of said coils.

Typically, said coils could be rectangular, circular or elliptical in shape.

Typically, the core is made of a single steel strip or a plurality of magnetic steel strips having different widths.

Typically, a stepped core assembly can be achieved by using different width and stacks of the magnetic steel strip.

Typically, a three phase transformer will typically have three coils and three core windings.

Typically, the clamps are welded to the core on one side and terminating wires on the other side. In accordance with another aspect of this invention, there is provided a method of making a transformer which comprises the following steps,

winding two coils around a mandrel having a semi circular cross section such that their bodies define a central window at the centre of the body;

abutting said coils in with each other such that the abutting sides of the coil together form an element having a substantial circular cross section;

partially unwinding a coil of magnetic steel strip to obtain the outer end of the strip, said outer end is passed through the window of one of the coils and looped around the circular element and passed back through the window of the other coil;

tac welding the end of the magnetic steel strip back to the steel strip at a predetermined point;

mounting a pair of unwinding rollers Rl and R2 on the steel strip and them rotating in opposite direction until the steel strip is unwound through said windows of the coils and the inner end of the steel strip is obtained;

fixing said inner end of the magnetic steel strip to the circular cross section element;

mounting a pair of winding rollers R3 and R4 on the steel strip and rotating them in the same direction so as to form a tightly wound core around the central circular cross section element;

spot welding the ends of the steel core to prevent it from unwinding or unrolling; and

annealing the core so that it retains its magnetic properties and shape, as a first step before core winding. Brief Description of Accompanying Drawings:

The invention will now be described with reference to the accompanying drawings, in which:

Figure 1 illustrates a conventional strip core transformer;

Figure 2 illustrates a conventional El-lamination transformer;

Figure 3 illustrates a conventional Spirakore shell type transformer;

Figure 4 illustrates a core-type transformer in accordance with this invention;

Figure 5 illustrates a conventional toroidal transformer;

Figure 6 illustrates the front view of the two conductor coil windings along with half circle cross-section of each coil and the circular cross section formed at their joint in accordance with this invention;

Figures 7 and 8 illustrate the schematic representation of the method of construction of the transformer in accordance with this invention;

Figure 9 illustrates a completed transformer in accordance with this invention and the cross section of the transformer;

Figure 10 illustrates a transformer in accordance with this invention for different Ratings i.e. more than one strip-widths wound;

Figure 11 illustrates the front, top and side view of the transformer in accordance with this invention;

Figure 12 illustrates .a conventional, core-type 3 -phase transformer; Figure 13 illustrates a method of construction of a 3-phase transformer in accordance with this invention;

Figure 14 illustrates an alternative method of construction of a 3-phase transformer in accordance with this invention; and

Figure 15 illustrates stepped core assembly in accordance with this invention.

Detailed Description of the Accompanying Drawings:

Figure 1 of the accompanying drawings shows a conventional transformer of core type assembly with a single core, dual-coil configuration. The core is made by stacking up cut strip laminations. Here, reference numerals (1, 2) refer to conductor winding coils and (3) refers to the strip core.

Figure 2 of the accompanying drawings shows a conventional EI transformer which has a core (4) made of E and I shaped laminations and coil (5) wound on the centre limb.

Figure 3 shows a conventional Spirakore, single-coil transformer where (7, 8) refer to the dual magnetic cores and (6) refers to the single conductor coil of circular cross section. The GE Spirakore transformer is constructed as under:

1) A conductor coil (6) with circular cross section and calculated rectangular window area is wound.

2) A steel sheet roll is slit to the widths required, and rewound on two mandrels of the same size and shape as the ultimate inner periphery of the completed cores.

3) The cores (7, 8) thus wound are annealed to restore the magnetic properties impaired by the bending of the sheet and for shape retention.

4) The mandrel is removed, and one of the wound cores is slipped over a roller, which is one of a pair of geared rollers, which revolve in opposite directions. The outer end of the wound core is passed around one leg of the circular coil and back through the window of the coil, between the coiled core and a second roller, and is then tack-welded to an adjacent inner turn at a predetermined point. On spinning the rollers, the steel strip is thus rewound through the window of the winding in such manner that the direction of its coiling is unchanged, but the coil is left in the loose form. 5) The core is made to form a tight spiral around one leg by means of another pair of rapidly spinning rollers, which are kept pressed against the outer layer. The core is prevented from unrolling by spot welding the ends.

6) The second core is wound around the other circular leg having circular cross section in similar fashion.

A conventional R core transformer will also have similar configuration as shown in Fig 3. In the R-core transformer the core (6) is wound first in circular cross section and the winding (7, 8) are threaded through it. A big roll of the steel core raw material is slit by means of a computer assisted device, so that as it is wound, the width that slit continuously changes to form a net circular cross section of the core (6) with required window dimension.

Figure 4 of the accompanying drawings illustrates a transformer in accordance with this invention having a wound core type construction. The transformer comprises two rectangular coils (9, 10) bodies of which have a semi circular cross section. The bodies of the coils define central windows. The two coils (9, 10) are placed side by side in an abutting relationship with each other such that the abutting sides together form an element (E) having a substantially circular cross section. The transformer includes a core (11) made of a magnetic strip material wound around said element (E) having circular cross section and enclosed within the annuli of the said coils (9, 10).

Figure 5 shows a conventional toroidal core transformer which is constructed by a method which is exactly opposite to the method of construction of the transformer in accordance with this invention. The strip core (12) is wound first and the conductor winding (13) is threaded through it later.

Figures 6, 7, 8 and 9 illustrate the method of construction of the transformer in accordance with this invention. This method can be described as follows:

1. Two coils (9, 10) are wound around an annular mandrel having a semi circular cross section. 2. The two coils (9, 10) are placed in a manner abutting to each other such that, the abutting sides together form an element (E) having substantial circular cross section.

3. The magnetic steel strip is partially unwound to obtain the outer end of the strip.

4. This outer end of a magnetic steel strip is passed through window of one of the coils and looped back and passed through the window of the other coil.

5. This end of the magnetic steel strip is welded back to the steel strip at a predetermined point.

6. Unwinding rollers Rl and R2 are mounted on the steel strip and rotated in opposite directions until the steel strip is unwound and the inner end of the strip is obtained.

7. The inner end of the magnetic steel strip is fixed to the circular cross section element (E).

8. A pair of winding rollers, R3 and R4 is mounted on the steel strip and rotated in the same direction so as to form a tightly wound core around the central circular cross section element (E).

9. The ends of the magnetic steel strip are spot welded to prevent it from unwinding or unrolling.

10. The core is annealed so that it retains its magnetic properties and shape, before it is wound through the coils.

As per the dimensions of coil and transformer, more rollers may be used at strategic locations to guide and /or revolve the steel coil.

All these core winding operations can be done manually as well as mechanized. Fig 6 of the accompanying drawings illustrates the cross-section of conductor coils (9, 10) in accordance with this invention. These coils have half circular cross section caused by the hemi-cylindrical windings on each side. These windings join at the centre of the assembled coils to form a full cylinder with a circular cross-section and required insulation, around which the core strip can be easily wound.

Fig 7 and Fig 8 of the accompanying drawings illustrate the method of winding the core (11) around the coils (9, 10) in accordance with this invention. Winding is achieved with two pairs of rollers; Rl and R2 rolling in opposite directions to perform the initial winding, and another pair, R.3 and R4 rolling in the same direction for the final tightening of the magnetic steel strip on the circular element formed by the coils.

Figure 9 shows thus completed transformer in accordance with this invention in its front- view along with the cross section of its core (11) and coils (windings) (9, 10).

Figure 10 shows the transformer 4 in accordance with the invention, wherein it is shown that the core can be built by winding a variety of core strip widths (14,15) placed one above the other in order to obtain different ratings of transformer and thus, this fact can be utilized to reduce inventory of core materials.

Figure 1 1 shows clamps (16) are welded to the core (1 1) on one side and termination of wires (17) on other side. Figure 11 shows a completed transformer in accordance with this invention, to which, clamps (16) for mounting the transformer, can be welded on one side of the steel core and the wire terminations (17) fixed with insulating material, on the other side.

The invention can further be implemented to develop 3 -phase transformers.

Figure 12 shows a conventional 3-phase core type construction, which may have stepped circular/rectangular cross section for the cores (19) of large ratings.

Only two practical ways of constructing a 3-phase transformer based on the 1-ρhase transformer in accordance with this invention are shown in Figures 13 and Figure 14.These constructions use 3 -core-windings and 3-conductor windings. Fig 13 illustrates a method to build a 3~phase transformer where two cores (i & ii) are wound adjacent to each other and the third core (iii) is wound around them (Type 1). Cores (i) & (ii) are wound by the same method as described in the invention. Core (iii) is threaded through the two windows and over the two cores (i & ii). This core (iii) has oblong cross section, but the winding operation can be done (by the same procedure as described in the invention)as long as the outer diameters of core (i) & (ii) are sufficiently large enough to avoid the strip core material being mechanically damaged, distorted or stressed. There may be many ways of adjusting the three phase assemblies by positioning of the core and windings in three dimensions, referring to Figure 13, the window width of R & B conductor coils can be reduced and height increased to accommodate the core (iii) wound on top of cores (i) & (ii) reducing its LMT and weight, but marginally increasing winding heat dissipation, losses, reducing transformer mounting foot print, and the like.

Fig 14 illustrates an alternative method to build a 3-phase transformer wherein three conductor windings (R. Y, B) are wound, at 120 degrees with each other with half-circular cross section (Type 2). Their flat ends will face each other when assembled at 1_.20 degrees with respect to each other and the joints shall have circular cross sections. Truee cores (i, ii, iii) can now be wound around each circular section formed by the _. adjacent pair of coils Figure 14 (ii), as described in the invention.

In case of Type 1 the third core winding is oblong and for Type 2 the conductor winding is at 120 degrees with each other.

Alternately as mentioned before a stepped construction of core Figure 15, to approximate a circle is possible by using more than one strip width (23) wound one over other in required stacks (22). Stepped core circular cross section .winding of core assembly Figure. 15_. is possible, by using different widths and stacks of steel strips one over another, but practical only for high KVA transformers. This helps while winding conductor coil on a circular/oblong bobbin and one having the sharp corners removed.

The transformer in accordance with this invention was subjected to a number of tests and its .performance was compared with that of the conventional .transformer. The basic, design parameters for, both the .transformers were kept similar._. The parameters _.like weight of copper, weight of core, copper losses, iron losses, cost of transformer and the like were compared.

Test Results of single phase transformer made in Copper:

The basic common design parameters for the transformer construction were taken as follows:

VA Rating: 142

Flux Density: 1.095 T

Current density: 1620 A/ sq inch

Weight of the core: 2.46 kg

Enameled copper swg: 24

Cost of:- Copper Wire-Rs.500/- per Kg, Core-Rs.70/- per Kg, Aluminum wire-Rs.27O/- per

Kg.

Output Results:

1) For conventional transformer:

Total number of turns = (Primary + Secondary): 1 173

Core area in square inch: 2.5

Copper weight in Kg: 0.581

Total transformer weight in Kg: 3.04

Total conductor resistance in ohms: 18.8

Total copper loss in Watts: 7.1

Total cost of the transformer in rupees: 462.00

2) For transformer in accordance with this invention: Total number of turns = (Primary + Secondary): 782 Core area in square inch: 2.93

Copper weight in Kg: 0.54

Total transformer weight in Kg: 3.00

Total conductor resistance in ohms: 17.53

Total copper loss in Watts: 6.65

Total cost of the transformer in rupees: 444.00 It can be seen from the test results that even after keeping the basic design parameters similar to that of the conventional transformer, substantial weight reduction of core is achieved. Though there is a slight increase in the copper weight and hence slightly higher copper losses for the transformer in accordance with this invention, the total losses and the price remains unaffected.

Test Results of the equivalent single phase transformer made using aluminum winding:

The basic design parameters for the transformer construction were:

VA Rating: 142

Flux Density: 1.095 T

Current density: 1000 A/ sq inch

Weight of the core: 2.65 kg

Enameled copper swg: 22

Output Results:

For transformer in accordance with this invention:

Total number of turns = (Primary + Secondary): 1029

Core area in square inch: 2.85

Aluminum weight in Kg: 0.28

Total transformer weight in Kg: 2.93

Total conductor resistance in ohms: 18.7

Total Aluminum loss in Watts: 7.1

Total cost of the transformer in rupees: 255.00

By keeping the conductor resistance and losses equal or less than that of conventional transformer with copper winding, and still reduce price, the transformer is made using aluminum conductor winding. An impractical increase in weight or next size of El- Laminations would be required for making a conventional transformer using aluminum winding. But constructing a transformer in accordance with this invention shows good reduction in price, keeping the transformer weight minimum, conductor resistance and losses low.

Test results of three phase transformer of conventional type & in accordance with this invention made in copper: The basic common design parameters for the transformer construction:

KVA Rating: 1.30

Flux Density: 1.095 T

Current density: 1770 A/ sq inch

Core Area per phase in Square Inch: 3.168

Total number of turns = (Primary + Secondary): 996

Enameled copper swg: 20

Enameled conductor area in sq mm per phase: 744

Cost of:- Copper Wire-Rs.500/- per Kg, Core-Rs.70/- per Kg

Output Results:

1) For conventional transformer: Core weight in Kg: 10.70 Copper weight in Kg: 4.3317 Total transformer weight in Kg: 15.0 Total conductor resistance in ohms: 21 Total copper loss in Watts: 64

Total transformer losses in Watts: 85.4

Total iron losses in Watts: 21.4

Total cost of the transformer in rupees: 2914.00

2) For transformer of Type 1 in accordance with this invention: Core weight in Kg: 9.44

Copper weight in Kg: 4.29

Total transformer weight in Kg: 13.7

Total conductor resistance in ohms: 21.30

Total copper loss in Watts: 69

Total transformer losses in Watts: 83.86

Total iron losses in Watts: 14.86

Total cost of the transformer in rupees: 2805.80

3) For transformer of Type 2 in accordance with this invention: Core weight in Kg: 6.91

Copper weight in Kg: 4.89 Total transformer weight in Kg: 11.8

Total conductor resistance in ohms: 22

Total copper loss in Watts: 71

Total transformer losses in Watts: 84.8

Total iron losses in Watts: 13.8

Total cost of the transformer in rupees: 2928.70

It is clear from the results that, even after keeping all basic design parameters equal, substantial weight reduction of core was achieved by constructing the transformer in accordance with this invention. Although there was a very slight increase in copper weight, the total losses and prices were nearly equal.

Test results of three phase transformer of Type 1 in accordance with this invention made in copper and optimized:

The basic design parameters for construction of Type 1 optimized transformer:

KVA Rating: 1.30

Flux Density: 1.095 T

Current density: 1770 A/ sq inch

Core Area per phase in Square Inch: 4.737

Total number of turns = (Primary + Secondary): 646

Enameled copper swg: 20

Enameled conductor area in sq mm per phase: 497

Output Results for transformer of Type 1 in accordance with this invention: Core weight in Kg: 10.70 Copper weight in Kg: 3.38 Total transformer weight in Kg: 14.1 Total conductor resistance in ohms: 15.2 Total copper loss in Watts: 50 Total transformer losses in Watts: 71.40 Total iron losses in Watts: 21.4 Total cost of the transformer in rupees: 2439.00 Test results of three phase transformer of Type 2 in accordance with this invention made in copper and optimized:

The basic design parameters for construction of Type 2 optimized transformer:

KVA Rating: 1.30

Flux Density: 1.095 T

Current density: 1770 A/ sq inch

Core Area per phase in Square Inch: 5.072

Total number of turns = (Primary + Secondary): 604

Enameled copper swg: 20

Enameled conductor area in sq mm per phase: 466

Output results for transformer of Type 2 in accordance with this invention: Core weight in Kg: 10.70 Copper weight in Kg: 3.19 Total transformer weight in Kg: 13.9 Total conductor resistance in ohms: 14.36 Total copper loss in Watts: 46.60 Total transformer losses in Watts: 68.06 Total iron losses in Watts: 21.5 Total cost of the transformer in rupees: 2344.00

Since the cost of copper is very high as compared with iron core, the area of iron core was increased to achieve price optimization. The weight of a transformer in accordance with this invention and that of the conventional transformer was matched. A price reduction of 18 to 20 % was achieved and the performance of the transformer in terms of net losses was improved.

Tests were carried to verify the claim that the half round cross-sectional winding of conductors improves the fill factor as compared to toroidal core windings and is nearly equal to other conventional square or rectangular cross-sectional windings. A rectangular cross- section winding of conventional and half round of invention is filled with medium covering enameled wire gauges of different diameters. The fill factor is measured as: - Area of enameled wire filled, divided by, Area of bobbin that is actually available for winding.

The test results show improvement in the fill factor of the coil windings.

While considerable emphasis has been placed herein on the particular features of a transformer, the improvisation with regards to it, and the specific steps of the preferred process, it will be appreciated that various modifications can be made, many steps can be taken and that many changes can be made in the preferred steps without departing from the principles of the invention. These and other modifications in the nature of the invention or the preferred steps of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

Claims

Claims:

1) A transformer comprising,

at least two coils, each of the coils defined by a Body, at least a portion of which has a semi circular cross section and a window defined within the center of the body, said coils operative Iy placed side by side in an abutting relationship with each other such that the abutting sides of the coils together form an element having a substantial circular cross sectional area; and

2) A transformer as claimed in claim (1), wherein said coils could be rectangular in shape.

3) A transformer as claimed in claim (1), wherein said coils could be circular in shape.

4) A transformer as claimed in claim (1), wherein said coils could be elliptical in shape.

5) A transformer as claimed in claim (1), wherein said core is made of a single steel strip or a plurality of magnetic steel strips having different widths.

6) A transformer as claimed in claim (1), wherein a stepped core assembly can be achieved by using different width and stacks of the magnetic steel strip.

7) A transformer as claimed in claim (1), wherein a three phase transformer will typically have three coils and three core windings.

8) A transformer as claimed in claim (1), wherein clamps are welded to the core on one side and terminating wires on the other side. ) A method for constructing a transformer as claimed in claim (1), said method comprising following steps, winding two coils around a mandrel having a semi circular cross section such that their bodies define a central window at the centre of the body; abutting said coils with each other such that the abutting sides of the coil together form an element having a substantial circular vncross section; partially unwinding a coil of magnetic steel strip to obtain the outer end of the strip, said outer end is passed through the window of one of the coils and looped around the circular element and passed back through the window of the other coil; tac welding the end of the magnetic steel strip back to the steel strip at a predetermined point; mounting a pair of unwinding rollers Rl and R2 on the steel strip and them rotating in opposite direction until the steel strip is unwound through said windows of the coils and the inner end of the steel strip is obtained; fixing said inner end of the magnetic steel strip to the circular cross section element; mounting a pair of winding rollers R3 and R4 on the steel strip and rotating them in the same direction so as to form a tightly wound core around the central circular cross section element; spot welding the ends of the steel core to prevent it from unwinding or unrolling; and annealing the core before winding through the coils, so that it retains its magnetic properties and shape.