Three phase transformer with frame shaped winding assemblies

A three phase transformer, e.g. a distribution transformer comprising three, substantially identical, frame shaped winding assemblies each containing primary and secondary windings which together form half legs having substantially semicircular cross section. The winding assemblies are placed together to form a triangular structure, so called temple configuration, wherein the half legs of adjacent winding assemblies combine into three legs having substantially circular cross section. A magnetic circuit is formed around each leg by winding a continuous strip of a ferromagnetic sheet material around the leg.

The present invention relates to three phase electrical transformers in 
general and more specifically to three phase distribution transformers 
which are used for stepping down voltage at user locations along a power 
line. 
Transformers generally contain two or more electrical circuits, primary and 
secondary windings, consisting of multiturn coils of electrical conductors 
which are interlinked by means of one or more magnetic circuits or cores. 
Traditionally cores consist of a plurality of ferromagnetic laminations 
which are stacked together to form a closed loop, surrounding and coupling 
magnetically the primary and secondary windings. Cores may be manufactured 
either from mutually overlapping or abutting individual laminations or 
from a continuous strip of magnetic sheet material which is wound around a 
mandrel to form a closed circuit. The magnetic and electric circuits are 
combined either by assembling the cores around pre-wound primary and 
secondary coils, or by winding the conductor coils around one or more legs 
of the closed magnetic circuit. Another way of interlinking the circuits 
is to wind a continuous strip of magnetic material through the pre-wound 
electric coils to form one or more magnetic core segments which surround 
parts of the primary and secondary windings. 
Prior art describes numerous variations of the basic principle outlined in 
the above, all aiming towards producing and combining the circuits with a 
minimum of labor, and at the same time optimizing the usage of conductor 
and core material in order to improve economy and minimize magnetic and 
electrical losses. One way of shortening either the electric or the 
magnetic path is to use circular coils surrounding substantially 
circularly sectioned core legs, or a wound core which surrounds primary 
and secondary windings having a total, substantially circular cross 
section. 
Single phase transformers employing the latter principle are described in 
U.S. Pat. Nos. 2,160,589 2,314,912 and 4,906,960 which all relate to 
transformers wherein cores or core segments are formed by winding a 
continuous magnetic strip around the legs of pre-wound primary and 
secondary coil assemblies. 
U.S. Pat. No. 2,401,952 relates to a three phase transformer with a core 
structure consisting of three identical segments, each segment being 
manufactured from several continuous strips of core material with 
different width which are wound around a rectangular mandrel. To form a 
three phase core the segments are assembled in so called temple 
arrangement wherein one leg of each segment combines with that of another 
to form a stepped cross section with a substantially circular shape. A 
conductor winding assembly is wound onto a circular coil form surrounding 
each of the combined core legs. 
German Auslegeschrift No. 1 011 056 describes a related core structure 
wherein each of the identical core segments is manufactured by winding two 
continuous strips of core steel, one upon the other, around a rectangular 
mandrel. The combination of a constant width strip which is wound to form 
a rhombic part section and a tapered strip forming a triangular part 
section produces a combined trapezoidally shaped cross section of each 
frame leg. When set up in temple configuration each of the three legs will 
have cross section shaped substantially like a regular hexagon fitting 
into a circular coil assembly with good space factor. German 
Offenlegungsschrift No. 27 02 455 describes an even closer approximation 
to a circular cross section of the combined core legs by employing a 
plurality of tapered, continuous strips in each frame segment. 
It is a common drawback of the three temple cores described in the above 
that the winding of the frame sections and taper slitting of multiple 
strips is quite labor demanding. In addition the conductor winding 
assemblies must be wound around the core legs into the closed structure 
and this is usually done by rotating split bobbins around the legs. The 
winding is impeded by protruding terminals an taps and is a difficult and 
time consuming process which is not easily mechanized. Furthermore the 
core designs are poorly suited for amorphous core material which is 
difficult to slit and would require extensive clamping and supporting 
means in order to maintain the shape of the core segments during 
fabrication and later on in the completed transformer. 
The three phase transformer of the invention bears a certain resemblence 
with the described prior art by suggesting a perfectly symmetrical temple 
structure with an improved core-coil space factor. Contrary to the prior 
art, however, the 1 invention does not require slitting of multiple widths 
or taper slitting of core material, and the coil assemblies may be 
pre-wound and combined with the core sections in a simple, easily 
mechanized procedure. Furthermore, a transformer according to the 
invention is ideally suited for amorphous core material because each core 
section is wound from a constant width strip and shaped like a hollow 
cylinder which may be supported vertically resting on one end eliminating 
the need for clamping means. 
A preferred embodiment of the invention will be described in the following 
with reference to the drawings, in which

The transformer illustrated in the drawings could represent a distribution 
transformer containing three, substantially identical winding assemblies, 
1-1, 2-2 and 3-3 each comprising a high voltage primary winding 4 and a 
low voltage secondary winding 5, the two windings being insulated between 
each other by means of a suitable dielectric 6. 
The assemblies are produced by winding the conductors around a rectangular 
mandrel with rounded corners on a coil winding machine containing suitable 
means for guiding and laying the conductors down in a precise and orderly 
manner. 
In the shown embodiment of the invention the first winding to be applied is 
the low voltage secondary 5, which consists of foil or strip of copper or 
aluminum and is laid down one layer to a turn interleaved with a suitable 
layer insulation such as e.g. dielectric paper or polyester film. During 
the winding process the conductor is gradually advanced paralell with the 
mandrel axis to form a coil with a rhombic cross section having 60 and 120 
degrees corner angles. 
After laying down the required number of turns the conductor is terminated 
and insulation 6 is placed on top of the winding, overlapping at both 
sides, the overlapping parts to be folded down later after the winding of 
the high voltage primary 4. 
In the illustrated transformer the primary 4 is wound with rectangular 
wire, and because of the requirement for additional interlayer insulation, 
its space factor is considerably poorer than that of the secondary 
winding. For this reason and because the conductor cross section has been 
adjusted to compensate for taking the longer track near the outside of the 
assembly, the high voltage winding occupies a considerably larger cross 
sectional area than the low voltage winding. 
During the winding of the primary the wire guide is advanced back and forth 
across the field to produce, as the layers build up, a cross section 
substantially as shown in FIG. 2, with the objective of filling, with good 
space factor the remainder of the semicurcular space available. 
After termination of the high voltage winding the main insulation 6 is 
folded up and down at the edges and the shape secured by means of 
bandaging and taping of legs and yokes whereafter the assembly may be 
removed from the winding mandrel. 
At this stage, if it is desired, the individual assemblies may be molded 
into a jacket of a suitable electrical resin to be used in a dry-type 
transformer or they may be used as is in an oil filled unit. As the next 
manufacturing step the assemblies are placed together in the temple 
configuration and secured by bandaging, and in the process flat insulation 
sheets 7 may be inserted between the half legs to insulate the assemblies 
from one another. 
In the case of an open, unmolded winding, at this point a tube 8 of a 
flexible dielectric like e.g. laminated glass fibre may be mounted around 
each leg and secured in place by means of e.g. electrical tape or resin, 
the tube serving the double purpose of providing additional winding 
insulation and a well defined cylindrical surface of each leg. 
After the completion of the temple shaped coil structure the next step is 
the winding of cylindrical cores 9 around each of the circular legs using 
a suitable ferromagnetic material like e.g. grain oriented silicon steel 
or amorphous steel. In both cases cores are pre-wound into the desired 
shape and stress relief annealed in order to restore the original magnetic 
properties of the core material. 
After annealing, the core cylinders are transferred to the temple structure 
by overwinding onto the cylindrical legs and during this process it is 
important to not exceed the elastic limits of the strip material which 
would re-introduce mechanical stress and again reduce the magnetic quality 
of the cores. The patent literature describes several ways of mechanizing 
this procedure using belts or rollers to rotate the core around the 
stationary leg as the diameter builds up. 
During the winding process the original configuration of the core is 
maintained, i.e. by taking the material up from the inside of the annealed 
core as it is wound onto the leg. This process is described in European 
Patent Application No. 83 300 004.5, Jan. 4 1983. An alternate way would 
be the overwinding of the core inside out onto an auxiliary mandrel and 
then transferring it to the leg while taking the material from the 
outside. 
Transferring the core cylinders to the legs can either be done one by one, 
or all three at the same time by means of three core winding machines 
placed at mutual 120 degree angles. After the securing of the last turn 
the only remaining step is hooking the transformer up and placing it in a 
tank or other enclosure with the core assembly resting on a sheet of 
insulating material and the cores resting on end upon bars or other 
structure suited to accept the weight without undue re-introduction of 
mechanical stress. 
Because the above mentioned support means are not part of the invention 
they are not shown on the drawings, and many variations hereof are 
possible without affecting the scope of the invention. 
Also the subject matter represented by the drawings and specification is 
only one version of a transformer according to and defined by the 
invention. Since many changes may be made in the above construction 
without departing from the scope of the invention, it is intended that all 
matter contained in the above description or shown in the accompanying 
drawings shall be interpreted as illustrative and not limiting. 
It is also understood that the following claims are intended to cover all 
of the generic and specific features of the invention herein described, 
and all statements of the scope of the invention which, as a matter of 
language, might be said to fall therebetween.