FORM 2
THE PATENTS ACT, 1970
(39 of 1970) As amended by the Patents
(Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)


TITLE OF THE INVENTION
A dry transformer
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai - 400 030, Maharashtra, India, an Indian Company
INVENTORS
Joshi Kishor and Jain Prateek, both of Crompton Greaves Ltd, Analytics, R&D Centre, Kanjurmarg (East), Mumbai 400042, Maharashtra, India, both Indian Nationals
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:


FIELD OF THE INVENTION
This invention relates to a dry transformer. BACKGROUND OF THE INVENTION
A dry or resin cast transformer is generally air cooled and comprises a resin cast or resin impregnated core coil assembly including a low voltage winding (primary winding) and a high voltage winding (secondary winding) located over a magnetic material core in spaced apart relationship with each other and in spaced apart relationship with the core. Each of the low voltage winding and high voltage winding comprises a plurality of layers of conductors in spaced apart relationship with one another. Dielectric material insulation is provided in the spaces between the adjacent layers of the conductors and spaced air ducts are formed in the dielectric insulation extending along the height of the windings. The core coil assembly is mounted in a tank defining a space between the bottom of the tank and the bottom of the windings and the top of the tank and top of the windings. Atmospheric air enters the air ducts through the openings at the lower ends thereof and flow through the air ducts and exit the air ducts through the openings at the upper ends thereof thereby taking away the heat generated within the windings and keeping the windings cooled. The air ducts are generally rectangular, circular or elliptical shaped and the openings at the lower and upper ends of the air ducts have the same geometry as that of the air ducts. Air enters the air ducts through the openings at the lower ends thereof along a path defining a sharp angle almost about 90° with the bottom surface of the tank and exits through the openings at the upper ends of the air ducts whose geometry corresponds to and is restricted to the geometry of the air ducts. As a result, there is a pressure drop within the air ducts and the air flow velocity in the
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ducts is reduced. This in turn reduces the heat transfer coefficient and cooling efficiency of the transformer. The cooling efficiency of the transformer can be increased by increasing the spacing between the layers of the conductors so as to form larger air ducts. This will increase the outer diameter and size of the windings and the material cost and losses of the windings.
OBJECTS OF THE INVENTION
An object of the invention is to provide a dry transformer having increased heat transfer co-efficient and cooling efficiency without increasing the outer diameter and size and cost of the transformer and winding losses.
Another object of the invention is provide a dry transformer which is compact, economical and is simple in construction.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a dry transformer comprising a resin impregnated core coil assembly including low voltage winding and a high voltage winding located over a magnetic material core in spaced apart relationship with each other and in spaced apart relationship with the core, each of the low voltage winding and high voltage winding comprising a plurality of layers of conductors in spaced apart relationship with one another, the spaces between the adjacent layers of windings being provided with a dielectric material insulation and air ducts being formed in the dielectric material insulation along the height of the windings in spaced apart relationship with one
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another, wherein the openings of the air ducts at the lower end and upper ends thereof are bell mouthed.
The following is a detailed description of the invention with reference to the accompanying drawings, in which:
Fig 1 is an isometric view of a core coil assembly of a singie phase dry transformer according to an embodiment of the invention;
Fig 2 is a plan or bottom view of the core coil assembly of Fig 1; and
Fig 3 is an enlarged view of an air duct of the core coil assembly of Fig 1.
The core coil assembly 1 as illustrated in Figs 1 to 3 c,f tne accompanying drawings is resin impregnated or resin cast and comprises a low vo!tage winding 2 and a high voltage winding 3 located over a magnetic material core 4 in spaced relationship with each other and with the core. The spaces between the windings and between the low voltage winding and the core are marked 5 and 6 respectively. Each of the low voltage winding and high voltage winding comprises a plurality of layers of conductors marked 7 and 8 respectively in spaced apart relationship with one anotherDielectric material insulation 9a and 9b are provided in the spaces between the tuns of the conductors of the low voltage winding and high voltage winding, respectively and air ducts 10 are formed in the dielectric material insulation in spaced apart relationship with one another extending along the height of the windings. The air ducts are, for instance, circular, rectangular or elliptical shaped. Only a few air ducts have been shown in Figs 1 and 2 of the drawings
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for the sake of clarity. The openings at the lower and upper ends of each of the air ducts are bell mouthed or bell shaped or flared and are marked 11a and 1 lb, respectively. The bell shaped openings at the lower and upper ends of the air ducts have a venturi effect. Coolant air enters the air ducts through the bell mouthed openings at the lower ends of the air ducts which provide a smooth angular path for the air to enter the air ducts. The bell mouthed openings at the lower ends of the air ducts also allow a large volume of coolant air to enter the air ducts and converge into the air ducts and flow through the air ducts. The bell mouthed openings at the upper mouth of the air ducts allow the exiting air to smoothly diverge and expand to a larger area. As a result of all this and the venturi effect, pressure drop within the air ducts is reduced and the air flow velocity within the air ducts is considerably increased. Heat transfer co-efficient and cooling efficiency of the transformer are increased. According to the invention, the cooling efficiency of the transformer is increased without increasing the outer diameter and size of the windings. Therefore, there is also the benefit of saving of material cost and the transformer is economical and compact. Winding losses are also reduced. It is clearly understood that the air ducts configuration need not be circular, elliptical or rectangular and that the transformer can be multiphase also. Such variations of the invention are obvious to those skilled in the art and are to be construed and understood to be within the scope of the invention.
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We claim:
1. A dry transformer comprising a resin impregnated core coil assembly including low voltage winding and a high voltage winding located over a magnetic material core in spaced apart relationship with each other and in spaced apart relationship with the core, each of the low voltage winding and high voltage winding comprising a plurality of layers of conductors in spaced apart relationship with one another, the spaces between the adjacent layers of windings being provided with a dielectric material insulation and air ducts being formed in the dielectric material insulation along the height of the windings in spaced apart relationship with one another, wherein the openings of the air ducts at the to war one? and upper ends thereof are beh mouthed.
2. The transformer as claimed in claim 1, wherein the air ducts are rectangular, circular or elliptical shaped.
3. The transformer as claimed in claim 1 or 2, which is single phase or multiphase transformer.
Dated this 22nd day of December 2008
(Jose M A)
of Khaitan&Co
Agent for the Applicants
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