Electric transformer for microwave ovens

The invention provides an electric transformer for microwave ovens including a conventional magnetic circuit with three columns, and electric circuits insulated by carcases and covers made from an insulating material. The secondary carcase forms a particular housing for the secondary heating winding so that the secondary high voltage winding and the secondary heating winding are partially superimposed. Thus an excellent electric insulation is provided without loss of space.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to electric transformers for supplying the 
power circuits of microwave ovens with electric power. These transformers 
include a primary electric winding, in the form of a coil, the two ends of 
the winding wire being connected electrically to tags for connection to 
the mains supply network. A secondary high voltage electric winding is 
formed of a coiled electric conductor a first end of which is connected 
electrically to the transformer magnetic circuit and to the ground of the 
microwave oven, and the other end of which is connected electrically to a 
high voltage output tag for connection to the high voltage electric 
elements of the microwave oven for supplying them with power. The 
transformer further includes a secondary electric heating winding formed 
of a coiled conductor, with a small number of turns, the two ends of the 
heating conductor being provided with tags for connection to the filament 
heating electric circuit of the magnetron of the microwave oven. The 
magnetic circuit of the transformer includes two lateral columns and a 
central column all three connected together by end cross pieces. An 
intermediate magnetic shunt partially loops the magnetic circuit between 
the lateral columns and the central column, deflecting a part of the 
magnetic flow between the primary electric winding and the secondary high 
voltage electric winding. 
The power usually delivered by such a transformer ranges between 200 and 
1500 W, so that it is a question of low power transformers. The primary 
supply voltage is generally 110-240 V. The high voltage output voltage is 
generally about 2300 V and must have a special waveform which the magnetic 
circuit with a special magnetic shunt confers thereon. 
It is thus indispensable to provide very efficient electric insulation, 
able to withstand a voltage of about 10 kV without breakdown between the 
different electric windings and between the electric windings and the 
magnetic circuit. 
2. Description of the Prior Art 
The electric insulation is generally provided by insulating foils folded 
and wrapped adequately about the windings and between the windings and the 
magnetic circuit. This insulating technique is described for example in 
the patent U.S. Pat. No. 2,858,514. Thus, all the transformers for 
microwave ovens at present on the market include such a type of electric 
insulation. The operations for providing such insulations are particularly 
time consuming and tedious, and substantially increase the production 
cost. These operations are necessarily manual and it is practically 
impossible to automate the production of such transformers. 
The German model of Utility DE-U No. 8 633 338 describes a transformer for 
microwave ovens in which the electric windings are formed on carcases made 
from an insulating material. Such a technique is not sufficient for 
providing complete insulation of the electric circuit with respect to the 
magnetic circuit and requires a complementary winding of a ribbon on the 
electric winding on the carcases. It can be further noted that the 
magnetic circuit described in this document has a complex shape, with cut 
outs which reduce the magnetic circuit section in certain zones and 
disturb the flow of the magnetic flux. 
SUMMARY OF THE INVENTION 
The purpose of the present invention is more particularly to avoid the 
drawbacks of known insulating techniques by insulating the magnetic 
circuits using rigid carcases and covers made from a molded insulating 
material, having particular shapes adapted more particularly for 
application to microwave oven transformers, and providing increased 
electric insulating between the electric circuits themselves and between 
the electric circuits and the magnetic circuit. 
Such an insulating structure in accordance with the invention further makes 
automatic assembly possible, by means of robust readily handled 
subassemblies which may themselves be manufactured automatically and be 
assembled together automatically. 
According to another object of the invention this structure reduces the 
number of parts to be assembled, and the number of parts to be stored. 
According to another object, the structure improves the mechanical 
protection of the windings and thus facilitates handling while avoiding, 
during such handling, partial damage of the windings and the production of 
insulating defects. The reliability is thus considerably increased. 
The main difficulty when attempting to provide transformer insulation for 
microwave ovens using rigid molded carcases and covers, is that such an 
insulating technique leads to substantially increasing the volume of the 
transformer. The carcase and cover walls must in fact have sufficient 
thickness to withstand the insulating voltage required and these walls 
occupy a not inconsiderable volume which becomes proportionally 
correspondingly higher when the transformer is provided for delivering 
relatively low power. The result is that this insulating technique using 
carcases and covers proves at first sight inapplicable to low power 
microwave oven transformers. The invention solves this problem by reducing 
the total volume of the electric circuits and their insulation by a 
partially superimposed arrangement of the windings. 
A second problem related to known transformer structures or microwave ovens 
is the fact that the magnetic shunts, positioned between the primary coil 
and the secondary coil fulfilling the function of current limiter required 
for a leak transformer, very often have a poorly defined position, and 
their electric insulation is difficult to provide. 
Another aim of the present invention is then to provide means for 
positioning the magnetic shunts accurately while repetitively providing an 
air gap whose dimension is well defined, so as to ensure the repetitivity 
of calibration of the transformer for mass production. The magnetic shunts 
are positioned by means of the carcases and covers which simultaneously 
provide 10 kV dielectric insulation between the windings and the electric 
shunts as well as the leak line distances required by the safety standards 
of different countries, by positioning on the central leg of the magnetic 
circuit and so not haphazardly. 
The structure of the invention makes it possible to assemble and fit the 
shunts and coils in the magnetic circuit automatically or 
semiautomatically. 
The shunts are grounded by contact on the internal part of the external leg 
of the magnetic circuit, or by an electric ground connection with the 
shunts and the magnetic circuit. In all cases, the metal plates forming 
the shunts are immobilized in all directions, so that they cannot vibrate 
and cause noise. 
Once impregnated with varnish, the mass formed by the varnish, the covers 
and the shunts substantially lowers the sound level of the transformer, 
which is an additional safety measure for mass production. 
A third problem related to known microwave oven structures is the space 
required by the electric connections of the transformer. Usually, the tags 
or coil terminals of the low voltage transformers are situated on the 
rigid carcases or on insulating elements made from bent metal sheets and 
adapted so as to receive tags. The arrangement of the transformers in 
microwave enclosures which are more and more compact makes it necessary to 
place the tags on the visible external face of the coils, that is to say 
on the periphery face part of the windings which is not covered by the 
magnetic circuit. In known structures, the tag can therefore no longer be 
disposed on the carcase and must be held in position by makeshift means 
such as parts added to the periphery of the windings with interpositioning 
of insulating elements, the parts for holding the tags in position being 
held on the windings by adhesive ribbons or other ribbon means. It will be 
readily understood that these structures cannot be produced by automatic 
means. Furthermore, they raise problems when soldering external conductors 
to the tags, such soldering causing local overheating likely to damage the 
insulating elements and reduce the electric insulation between the tags 
and the windings. Furthermore, positioning of the tags is not accurate, 
and the tags do not have a very great tear strength. 
The purpose of the present invention is therefore to avoid the drawbacks of 
known connection means by disposing the electric output tags on the 
insulating covers covering the carcases. A first advantage of this 
structure is that the tags disposed on the covers are held away from the 
windings, without direct contact with the means for electrically 
insulating the windings. The covers provide additional heat insulation 
between the tags and the windings so that the tags may be soldered without 
running the risk of damaging the insulation of the windings. The fact that 
the tags are held by rigid covers further facilitates the accurate 
positioning thereof, and makes possible automatic or semiautomatic fitting 
of the tag on the cover. 
The positioning accuracy of the tag and the fact that it is well insulated 
from the windings makes it possible to solder the tag-output wire by means 
of an automatic solder bath machine. The positioning accuracy of the tags 
further facilitates automatic quality control of the coils and 
transformers leaving the assembly lines. 
This arrangement further provides very high tear strength, without the risk 
of damage and without extra cost, even when the standards to be compiled 
with are very severe from the tear strength testing point of view, for the 
tag is adequately and rigidly implanted in the cover. 
All these arrangements result in a very substantial gain in productivity 
and reliability, and an improved appearance, which are not possible with 
the structure in which the tags are supported by bent and ribbon wound 
insulating materials. 
The cost of producing the transformers is also substantially reduced, since 
the cutting out and bending operations of a large number of insulating 
elements is avoided, doing away with all the adhesive ribbons which were 
necessary in known technologies. 
The invention further provides means facilitating the provisional fixing of 
the coil wires at the end of coiling, these means being formed by studs 
provided on the carcases and on which the outlet wire is provisionally 
wound. The wire thus temporarily attached at the end of coiling makes 
assembly line testing and handling possible without damage. 
To attain these objects as well as others, the transformer includes: 
a primary carcase made from an insulating material, comprising a central 
former for the passage of the central magnetic circuit column, and 
external flange and an internal flange, 
a secondary carcase made from an insulating material, including a central 
former for the passage of the central magnetic circuit column, an external 
flange and an internal flange, 
two primary insulating covers, each having three walls, covering 
respectively the external and internal flanges, and the electric winding 
in the zones facing the magnetic circuit, 
two secondary insulating covers, each having three walls, covering 
respectively the external and internal flanges and the electric winding in 
the zones facing the magnetic circuit, 
an external annular housing formed in one of the primary or secondary 
carcase flanges for containing the secondary electric heating winding, 
thus ensuring its electric insulating and its mechanical securing. 
The external annular housing may for example be disposed advantageously on 
the internal secondary carcase flange, but it will be readily understood 
that the advantage of a reduction in volume are obtained whatever the 
carcase flange on which the external annular housing is provided: internal 
or external secondary or primary carcase flange. 
In one embodiment, the external annular housing: 
is limited towards the center by an axial mechanical securing flange, 
has an axial depth only slightly greater than the diameter of the heating 
winding wire, 
is closed, on the parts of its periphery surrounded by the magnetic 
circuit, by corresponding insulating covers covering the carcase, which 
thus provide the electric insulation between the magnetic circuit and the 
heating winding. 
In a particular embodiment, the external annular housing occupies the part 
of the flange the closest to the winding axis, the part the furthest away 
being formed by a shoulder defining an axial cylindrical wall which 
radially limits the annular housing and which is connected to an axially 
offset radial wall. Thus, the main carcase winding covers the secondary 
heating winding externally. The coiling of the winding is thus 
facilitated. 
In all the embodiments, the main carcase winding and the secondary heating 
winding are partially superimposed and this arrangement substantially 
reduces the overall volume occupied by the windings. 
For positioning the shunts and holding them in position, the inner walls of 
one of the primary or secondary cover pairs include external housings 
defined by two lateral flanges in the external limit planes of the 
magnetic circuit and at least one transverse flange; the housings are 
intended to contain and to isolate a stack of metal plates forming 
magnetic shunts, the plates then being separated from the magnetic circuit 
by an air gap defined by the transverse housing flange. 
In a particular embodiment, the housings are provided on the covers of the 
secondary circuit. 
To solve the problem related to the positioning of the electric connection 
tags and holding them in position, one of the secondary covers has, in its 
zone not surrounded by the magnetic circuit, an electric connecting tag to 
which the first end of the secondary high voltage winding wire is 
connected; similarly, the primary carcase covers include, in their zone 
not surrounded by the magnetic circuit, tags for connecting the primary 
winding. 
It will be understood that the particular features concerning the 
construction and position of the housings for holding the magnetic shunts 
in position, the particular arrangements concerning the positioning of the 
electric connection tags and holding them in position, the particular 
arrangements concerning the external flange housings for insertion of the 
heating winding, may be used either separately or in combination.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIGS. 1 to 3 can be seen the general construction of a transformer in 
accordance with the invention. Conventionally, the transformer includes a 
magnetic circuit 1 with two lateral columns 2 and 3 and a central column 
4, the columns being connected together by a first end cross piece 5 and a 
second end cross piece 6. 
In practice, columns 2, 3 and 4 and cross piece 5 are formed by stacking 
metal sheet plates in the form of an E, and cross piece 6 is formed by a 
stack of rectangular metal plates fixed to the end of the legs of the E, 
to which they are welded. The particular structure of electric windings 
and insulating means of the present invention makes it possible to use 
magnetic circuits of completely traditional form, in which columns 2, 3 
and 4 and cross pieces 5 and 6 have constant appropriate widths for 
conducting the magnetic flux. 
The electric windings of the transformer are disposed about the central 
column 4 of the magnetic circuit and include a primary electric winding 7 
in coil form. The primary winding ends in two primary winding tags 8 and 9 
shown in FIG. 3, or two tags 80 and 90 shown in FIG. 6. A secondary high 
voltage winding 10, also in coil form, ends in two high voltage outlet 
tags 11 and 12, and a secondary heating winding 13 formed of a few 
electric conductor turns ending in two heating tags 14 and 15. 
A magnetic shunt, formed of two stacks of rectangular plates 16 and 17 
shown in FIGS. 3 and 4, connects together the intermediate parts of the 
lateral columns and of the central column of the magnetic circuit, in the 
zone situated between the primary winding 7 and the secondary windings 10 
and 13. 
The electric windings are coiled on carcases made from an electrically 
insulating material, receiveing appropriately shaped covers also made from 
an electrically insulating material. 
As shown in FIG. 6, the primary carcase 18 includes a tubular shaped 
central mandrel 19 adapted for fitting about the central column 4 of the 
magnetic circuit. The central former ends in an external flange 20 and an 
internal flange 21, the assembly defining a peripheral annular groove 22 
in which the primary electric coil 7 is wound. The internal flange 20 is 
intended to be disposed outwardly of the transformer, namely on the side 
opposite the secondary carcase, whereas the internal flange 21 is intended 
to be facing the secondary carcase. 
The vertical parts 28 and 29 of the primary carcase shown in FIG. 6 are 
intended to be surrounded by the magnetic circuit of the transformer. The 
corresponding vertical internal walls of former 19 are smooth, as shown in 
the Fig., and receive with a loose fit the edge of the plates of the 
central column 4 of the magnetic circuit. The horizontal walls of the 
former are extended outwardly by flanges 23 and 24, flange 23 being shaped 
so as to receive tags 80 and 90 for the electric connection of the primary 
winding. Flange 23 further includes an external axial groove 81 
communicating with the annular peripheral groove 22 through an aperture 
82, groove 81 extending over the former 19. Groove 81 is intended to 
receive axially a safety member responsive to the temperature of the 
primary winding and the intensity of the current which flows therethrough. 
With this arrangement, the safety member is in the immediate vicinity of 
the winding, even in direct contact with the first turns, thus improving 
the tripping speed. The inner faces of flanges 23 and 24 as well as the 
inner horizontal faces of the former include longitudinal ribs such as rib 
25. Ribs 25 are chamfered at the ends, for example the chamfer 26, for 
facilitating fitting of the carcases on the magnetic circuit. Ribs 25 
define the bearing surface for the central column 4 of the magnetic 
circuit and make it possible to provide rounded corners on the former 19, 
for fitting to a column 4 with rectangular section and sharp edges. 
The external flange 20, in its horizontal upper portion, has a stud 27 for 
temporarily fixing the end of the electric winding during coiling, before 
the end of the conductor is fixed to tag disposed on flange 23, or 
preferably on one of the primary insulating covers. 
The vertical parts 28 and 29 of the primary carcase are intended to be 
covered by primary insulating covers such as cover 30 shown in FIG. 8. 
Cover 30 includes three main walls 31, 32 and 33, the parallel walls 31 
and 33 being provided for partially covering the flanges of the primary 
carcase, wall 32 being provided for partially covering the primary 
electric winding. It will be readily understood that the assembly formed 
by a cover 30, former 19 and the vertical carcase parts 28 defines an 
insulating case surrounded by the magnetic circuit and providing electric 
insulation between this magnetic circuit and the electric coil wound on 
the carcase. 
The ends of walls 31 and 33 are connected together by bands such as band 
34, having means for fixing an electric tag, and simultaneously giving 
rigidity to cover 30. An opening 340 is left between each band 34 and wall 
32 so that varnish may flow therein during impregnation of the coil. 
The primary carcase covers include, in their zone not surrounded by the 
magnetic circuit, tags 8 and 9 for primary winding connection. 
Similarly, the secondary carcase 41, shown in FIG. 5, has a central former 
35, an internal flange 36 and an external flange 37. The horizontal inner 
faces of former 35 are provided with ribs such as the longitudinal rib 38. 
Two flanges 39 and 40 extend the horizontal walls of the former 35 beyond 
the internal flange 36. These flanges 39 and 40 are shaped so as to have 
the same length as the dimension of the magnetic shunt and bear against 
the internal flange of the primary carcase. 
The base of the external flange 37 of the secondary carcase 41 has a hole 
42 for passing therethrough a first end of the secondary high voltage 
winding, this first end being connected to the high voltage tag 11 fixed 
to the magnetic circuit and connected to the electric ground of the 
microwave oven. 
In the embodiment shown in the Figs., the internal flange 36 of the 
secondary carcase 41 has a particular shape, as can be seen in FIGS. 5 and 
4, and includes an external annular housing 43 intended to contain the 
secondary electric heating winding 13. The external annular housing 43 is 
defined toward the center of the carcase by an axial peripheral flange 44. 
The housing has an axial depth only slightly greater than the diameter of 
the heating coil wire 13. Its periphery is defined by a shoulder 45 
forming a cylindrical wall defining the annular housing radially and 
connected to an external part of the flange 36. In other words, flange 36 
is formed of an external radial wall 47, connected by shoulder 45 to an 
internal part 48, the internal part 48 being offset axially inwardly of 
the carcase with respect to the external radial wall 47, so as to leave 
room for housing 43. In the end zone of the winding, namely in the left 
hand part of the secondary carcase shown in FIG. 4 it can be seen that the 
secondary high voltage winding 10 covers the secondary heating winding 13. 
The secondary carcase 41 is adapted for receiving secondary insulating 
covers such as cover 50 shown in FIG. 7. Similarly to the primary winding 
carcase, the secondary carcase 41 receives two covers such as cover 50, 
covering its vertical parts shown in FIG. 5. Cover 50 has a shape similar 
to that of cover 30 and has three main walls 51, 52 and 53. A band 54, 
connecting together the ends of walls 51 and 53 in the zone not surrounded 
by the magnetic circuit, receives an electric connection tag 12 to which 
is connected the second end of the electric conductor of the secondary 
high voltage winding 10. 
The internal flange 36 further has on its external face studs 49 provided 
for cooperating with a facet 510 of the corresponding wall 51 of the 
insulating cover 50 so as to define means for holding the outputs of the 
secondary electric heating winding in position. 
Wall 51, or internal wall of cover 52, comprises an external housing 55 
defined by two lateral flanges 56 and 57 in the external limit planes of 
the magnetic circuit and a transverse flange 58 in the plane of a vertical 
wall of the central carcase former 35. Housing 55 is provided for 
containing and insulating a stack of metal plates forming the magnetic 
shunt 16 or 17. The plates are in contact with one of the lateral columns 
of the magnetic circuit and are separated from the central column 4 by an 
air gap defined by the transverse flange 58 of the housing. 
In the preceding embodiment, since the metal plates are in contact with one 
of the lateral columns of the magnetic circuit, the shunts are grounded by 
contact with the main magnetic circuit. In another embodiment, external 
housings 55 may be provided also defined by two lateral flanges 56 and 57 
in the external limit planes of the magnetic circuit, a first transverse 
flange 58 in the plane of a vertical wall of the central carcase former 35 
and a further second transverse flange, not shown in the Figs., along the 
lateral leg of the magnetic circuit. In this case, the external housing 55 
is a bath closed on four sides, and the air gap is defined by the sum of 
the thicknesses of the two transverse flanges. It is then necessary to 
provide an electric ground connection between the shunts and the magnetic 
circuit. 
In the embodiment shown, housings 55 are disposed on the secondary 
insulating covers. In another embodiment, housings 55 are disposed on the 
primary insulating covers. The arrangements shown in the Figs. may however 
be preferred since it facilitates automatic assembly of the unit: it is in 
fact preferable to assemble first of all, on the magnetic circuit, the 
secondary windings then the primary winding; it will be readily understood 
then that, using this method of assembly, the stack of shunt plates may be 
simply laid in housings 55, in which the plates are held in position by 
gravity. 
Covers 50, when they are fitted to the secondary carcase 41, partially 
close housing 43 and provide the mechanical fixing and electric insulation 
of the secondary heating winding 13. 
The secondary carcase has, on the external edge of one of its flanges, a 
radial stud 360 on which the first end of the secondary high voltage 
winding wire may be temporarily wound before connection to the outlet tag, 
the other end of the winding wire leaving the carcase through the radial 
hole 42 close to the central tunnel and connected to the magnetic circuit. 
With such a structure, it will be readily understood that it is possible to 
form the primary windings and the secondary windings separately each on 
their respective carcases, then the insulating covers may be fitted for 
defining independent subassemblies which may be handled without damage to 
the electric windings. The secondary winding is stacked on the magnetic 
circuit, then the magnetic shunts are placed in their housings 55 and 
finally the primary winding, then cross piece 6. 
The present invention is not limited to the embodiments which have been 
more explicitly described, but includes, the different variants and 
generalizations thereof contained within the scope of the following 
claims.