Shielded transformer

A shielded transformer of the type particularly used as an isolation transformer, has a greatly reduced interwinding capacitance. Metallic overlap is provided, completely across a juncture of the metallic shield with faces of the windows in the core, and completely across a juncture of the metallic shield with the metallic case. This metallic overlap is tolerant to misalignments and variations in fit, completely eliminating gaps that cannot be economically made small with the butt joint of present art. The overlap comprises grooves in faces of the window or in the case. In a second embodiment, the overlap comprises grooves in channels on faces of the window and on the case. The shield fits into the grooves.

BACKGROUND OF THE INVENTION 
This invention relates to electrostatically shielded transformers of the 
type particularly used as isolation transformers to isolate sensitive 
electrical and electronic equipment from the voltage variations caused by 
electromagnetic and electrostatic interference, the interference signals 
being superimposed on the voltage signal supplied with power lines by 
public utilities. An isolation transformer, by itself, makes no attempt to 
regulate the amplitude of the supplied voltage signal, but does attenuate 
interference signals that generally are of a higher frequency and often of 
a transient nature. 
Interference can be caused by equipment belonging to other users of the 
power line, by electromagnetic or electrostatic fields from many kinds of 
equipment including electric welding machines, diathermy machines, 
automotive ignition systems, by lightning, and by various discharges in 
the power line equipment. 
The isolation transformer particularly attenuates common-mode interference. 
Variations in voltage caused by common-mode interference are equal in 
amplitude and phase with respect to ground on both lines of the power line 
pair. These variations are not transmitted from primary winding to 
secondary winding by normal inductive transformer action because there is 
no variation in voltage across the primary winding. They are, however, 
transmitted from primary to secondary in direct proportion to the 
capacitance between primary and secondary windings. The common-mode 
interference currents, being alternating currents, flow through this 
capacitance and eventually back to ground through the load when grounded; 
or through various capacitances between parts of the secondary winding and 
ground, and through the capacitance between the load and ground. This is, 
of course, objectionable. 
Isolation transformers using present art place a metallic shield between 
primary and secondary windings, and ground the shield. Common-mode 
interference currents will then flow through the primary-to-shield 
capacitance to ground, providing isolation for the secondary winding and 
its load from the common-mode interference on the primary. 
The primary and secondary windings are fabricated separately. They are then 
assembled with a multiplicity of ferromagnetic laminations that make up 
the core. The laminations may be stacked individually, some portions 
passing through the centers of the windings, or they may be preassembled 
in pieces that are placed through the centers of the windings and held in 
place by metallic bands. In any case the primary and secondary windings 
encircle some portion of the core, passing through at least one opening in 
the core. The opening is called a window. 
The metallic shield is then inserted between the primary and secondary 
windings, the shield extending both inside and outside the windows in the 
core. Often metallic end bells around the windings where they are not 
within the windows in the core. Typically four bolts passing through holes 
in the laminations and the end bells hold the transformer together. 
Generally the fit between the metallic shield and the faces of the windows 
in the core is poor. In the interest of economy, loose dimensional 
tolerances are used for the core, the windings, and the shield. The shield 
must be fairly rigid (typically 2.5 millimeters thick) so that it can be 
inserted without breaking up or being deformed. A thick shield is 
undesirable because as the spacing between primary and secondary windings 
increases, leakage inductance increases causing a degradation in no-load 
to full-load voltage regulation. 
In most core configurations, the faces of the windows comprise the edges of 
a multiplicity of stacked laminations. The shield butts up against what 
amounts to a saw-tooth surface. The poor fit between the metallic shield 
and the faces of the windows in the core causes gaps through which 
unintercepted electrostatic field lines extend between the primary and 
secondary windings. These gaps cause a capacitance, of small but important 
magnitude, to exist between primary and secondary windings. Furthermore 
the capacitance is highly variable between specimens assembled on the same 
production line. 
This residual capacitance directly between the primary and secondary 
windings has been called interwinding capacitance by manufacturers of 
isolation transformers. Although this term does not appear in standard 
electronics dictionaries, it is useful and descriptive and will be used 
here. 
Interwinding capacitance is determined by applying a measured common-mode, 
alternating current voltage between the shorted primary winding and 
ground. The voltage between the secondary and ground across a known 
impedance is measured, with the secondary winding shorted out, and the 
shield grounded. The capacitance is then calculated with elementary 
circuit theory, using the two voltage measurements, and the known values 
of applied frequency and load impedance. 
Isolation transformers using present art are rated according to 
interwinding capacitance. The lower the capacitance, the better the 
isolation, and the higher the price. Typical quality classes are 0.005, 
0.001, and 0.0005 picofarads. There is a need and a market for isolation 
transformers with much lower interwinding capacitance. 
SUMMARY OF THE INVENTION 
The major object of this invention is to provide shielded isolation 
transformers having greatly reduced interwinding capacitance without an 
appreciable increase in cost. 
Metallic overlap is provided at the juncture of the metallic shield and 
faces of the windows in the core. This metallic overlap is tolerant to 
misalignments and variations in fit, completely eliminating gaps that 
cannot be economically made small with the butt joint of present art. 
Providing this first overlap reduces interwinding capacitance by at least a 
factor of 10. Now another source of capacitance, due to fringing of the 
electrostatic field at the edges of the metallic shield outside of the 
windows in the core, becomes measurable. Therefore the metallic shield 
where not within the windows in the core is extended to the metallic case. 
Often this is only to the end bell portions of the case. Whatever the 
configuration of the case, metallic overlap is provided at the juncture of 
the metallic shield and the metallic case. Providing this second overlap 
reduces interwinding capacitance by at least another factor of 10. 
In a preferred implementation, the metallic overlap at the juncture of the 
metallic shield and the faces of the windows in the core is provided by 
extending the shield into grooves formed in the faces of the windows. 
These grooves comprise notches in the appropriate laminations of the core. 
The notches are rectangular and need be no larger than 3.0 millimeters on 
a side. Since laminations are normally stamped out of sheet stock, a small 
alteration of the stamping die provides the desired notched laminations 
and grooved core at essentially no increase in cost. 
With this implementation, the metallic overlap at the juncture of the 
metallic shield and the metallic case is provided by extending the shield 
into grooves formed in the metallic case. When the case comprises outer 
faces of the core and two end bells, this is economically provided by 
using grooved extrusions or castings as end bells. 
In another implementation, which may be preferred in certain constructions, 
either totally or in combination with the above implementation, the 
metallic overlap is provided by extending the metallic shield into grooves 
formed in channel pieces attached to faces of the windows in the core and 
to the metallic case. The channel pieces are attached to the faces of the 
windows with electrically conductive adhesive to preclude any gaps between 
the channels and the faces. The attachment of the channels to the metallic 
case may be aided or accomplished with screws. 
Another object of this invention is to provide a thinner metallic shield so 
that the spacing between primary and secondary windings can be reduced, 
with a resultant reduction in leakage inductance. With the grooves of this 
invention providing alignment, shields of between 0.1 millimeters and 0.3 
millimeters thick can be inserted between the windings without danger of 
breaking or deforming the shield.

DETAILED DESCRIPTION 
FIG. 1 illustrates an electrical schematic of a typical shielded 
transformer 22, used as an isolation transformer connected between the 
power line and the equipment to be protected. The transformer 22 comprises 
a primary winding 24, a secondary winding 26, a metallic shield 28, and a 
metallic case 30. 
Common-mode interference currents, being alternating currents, flow through 
the primary-to-shield capacitance 32 to ground. The currents also flow 
through the interwinding capacitance 34 and eventually back to ground; 
through the load when the grounded 36, or through the secondary-to-shield 
capacitance 38, the capacitance between load and ground 40, and the 
leakage resistance between load and ground 42. 
With a given common-mode noise voltage at the primary winding, the 
magnitude of the noise current through the interwinding capacitance 34, 
and thus the load 44, is directly proportional to the interwinding 
capacitance 34. This is because the impedance to ground in series with the 
interwinding capacitance is, in all cases, extremely low compared to the 
reactance of the interwinding capacitance 34. Clearly the lower the value 
of interwinding capacitance 34, the better the isolation. 
FIG. 2 shows a typical test configuration for measuring interwinding 
capacitance (34 of FIG. 1) by taking voltage measurements and calculating 
the capacitance using elementary circuit theory. A measured, common-mode, 
alternating current voltage from a voltage generator 46 is applied between 
the shorted primary winding 24 and ground. The voltage between secondary 
winding 26 and ground across a measurement load 48 is measured, with the 
secondary winding shorted out, and the shield grounded. 
Comparing FIG. 2 with FIG. 1, the following can be recognized: (1) the 
primary-to-shield capacitance 32 does not load the generator 46 and hence 
can be ignored because its reactance that shunts the generator is very 
large compared to the internal impedance of the generator; (2) the 
secondary-to-shield capacitance 38 can be ignored because its reactance is 
very large compared to the resistance of the measurement load 48; and (3) 
with the load left ungrounded 36, the leakage resistance between load and 
ground 42 and the capacitance between load and ground 40 can be ignored 
because their impedances are very large compared to the resistance of the 
measurement load 48. 
With these approximations, the equivalent circuit of FIG. 3 can be used for 
the test configuration of FIG. 2. Considering that the reactance of the 
interwinding capacitance 34 is very high compared to the resistance of the 
measurement load 48, the interwinding capacitance in farads, from 
elementary circuit theory, is equal to the voltage across the measurement 
load 48, divided by the product of the voltage across the generator 46, 
the resistance in ohms of the measurement load 48, and the alternating 
current frequency of the generator expressed in radians per second. In 
spite of all of these approximations, the error in measurement can readily 
be less than five percent. 
Turning now to the mechanical structure of an isolation transformer in 
accordance with the present invention, FIG. 4a shows a layer of 
laminations comprising an E lamination 50 on the left and an I lamination 
52 on the right. Four holes 54 are provided through which mounting bolts 
will pass. Notches 56 are provided as shown. These notches are 
rectangular, generally less than 3.0 millimeters on a side, and are 
equidistant from the left and right sides of the layer. This layer is 
typical of layers to be stacked as alternate layers in forming a core. 
FIG. 4b shows a layer of laminations comprising an E lamination 50 on the 
right and an I lamination 52 on the left. The E and I laminations in this 
layer are identical to the E and I laminations in FIG. 4a. This layer is 
typical of layers to be stacked in between the layers of FIG. 4a in 
forming the core. 
FIG. 5 shows the core 58 stacked with the lamination layers of FIGS. 4a and 
4b. Outer faces 60 of the core form part of the metallic case (30 of FIG. 
1) of the transformer. Two windows 62 extend through the core 58. Each 
window 62 has four faces 64 within the core 58. The notches in laminations 
56 of FIGS. 4a and 4b become grooves 66 in the faces 64 of the windows 62 
in the core 58. 
FIG. 6 shows an isolation transformer comprising the core 58 of FIG. 5, the 
primary winding 24, the secondary winding 26, the metallic shield 28, and 
end bells 68, one of which is removed to show details of the windings and 
the shield. The outer faces 60 of the core and the end bells 68 compose 
the metallic case (30 of FIG. 1) that surrounds the primary and secondary 
windings. 
The primary winding 24 and the secondary winding 26 encircle a portion of 
the core passing through two windows (62 of FIG. 5) in the core 58. The 
metallic shield 28 is placed between the primary winding 24 and the 
secondary winding 26, including within the windows 62 in the core 58, the 
shield intercepting any possible electrostatic field line between any 
point on the primary winding and any point on the secondary winding. 
Considering the metallic shield 28 in more detail, and referring to FIGS. 
5, 6, and 7a, the shield extends into the grooves 66 in the faces 64 of 
the windows 62 in the core 58 to provide a metallic overlap at the 
juncture of the metallic shield 28 and faces 64 of the windows 62 in the 
core 58. 
FIGS. 8a and 9a show how the metallic shield 28 extends into grooves 70 in 
the end bells 68 to provide a metallic overlap at the juncture of the 
metallic shield 28 and the metallic case 30. 
FIG. 10 is included to further illustrate the transformer of FIGS. 6, 7a, 
8a, and 9a. 
FIGS. 7b, 8b, and 9b, modified portions of FIGS. 7a, 8a, and 9a, 
respectively, illustrate another implementation of the metallic overlap 
principle. The metallic shield 28 extends into grooves in channel pieces 
72 attached to faces 64 of the windows 62 in the core 58 with electrically 
conductive adhesive. The shield also extends into grooves in channel 
pieces 74 attached to the end bells 68. 
The metallic shield 28 normally comprises two overlapping members insulated 
from each other so as not to create a "shorted turn" around a portion of 
the core. The members are inserted between the windings after the 
laminations and the windings are assembled to become the core and 
windings. 
The shield members can be made of any high conductivity metal but usually 
of aluminum or copper with copper preferred due to its higher electrical 
conductivity, a safety consideration in regard to catastrophic shorting to 
ground such as experienced in a lightning strike. 
FIG. 11 shows conventional L-shaped members 76 composing the metallic 
shield 28. The narrow ends 78 of the members are rounded and tapered to 
make insertion easier. Edges 80 that butt up against the core are covered 
with metallic tape so as to avoid any gap between the shield 28 and the 
core 58. 
FIG. 12 shows an alternative implementation using two U-shaped members 82 
composing the metallic shield 28. All four long edges of each member are 
slightly over cut into the metal by the same amount. This facilitates an 
easy insertion. Each member is made snug in two of the grooves. By sliding 
the two members into the grooves in opposite directions a snug fit is 
obtained in all four grooves. 
With the grooves of this invention providing alignment and the shield 
members just described, shields of between 0.1 and 0.3 millimeters thick 
can be inserted between the windings without danger of breaking or 
deforming the shield. This can result in a reduced spacing between primary 
and secondary windings, and a resultant reduction in leakage inductance 
and hence better no-load to full-load voltage regulation. 
The preferred embodiment of FIGS. 4a through 12 shows a physical 
configuration highly influenced by the selection of the E-I laminations 
for the core. While this core configuration is often used in shielded 
isolation transformers, it is by no means the only configuration used. 
Similarly the innovations and novelty of this invention as expressed in 
the claims are not limited to transformers with E-I laminations. A person 
skilled in the art can readily extend the teachings here to other core 
geometries.