U.S. patent number RE31,613 [Application Number 06/405,859] was granted by the patent office on 1984-06-26 for measuring transformer.
This patent grant is currently assigned to LGZ Landis & Gyr Zug AG. Invention is credited to Heinz Lienhard, Gernot Schneider.
United States Patent |
RE31,613 |
Lienhard , et al. |
June 26, 1984 |
Measuring transformer
Abstract
A measuring transformer comprises a measuring conductor carrying
the current I.sub.m to be measured, a pre-magnetizing winding which
carries a pre-magnetizing current I.sub.v, and a magnetic field
comparing means which is exposed to the magnetic field produced by
the current I.sub.m and the magnetic field produced by the
pre-magnetizing current I.sub.v and is alternately controlled in
both directions of saturation. The magnetic field comparing means
is a magnetic film which may be anisotropic, operated in the
magnetic preferential direction, and have a thickness of at most a
few microns. It may be secured to pole shoes of a magnetic core or
arranged between a flat measuring conductor and a pre-magnetizing
coil of flat cross-section. The measuring transformer can be used
as an input transformer in a static electricity meter. In a further
embodiment, the output pulses can be obtained directly from the
magnetic field comparing means.
Inventors: |
Lienhard; Heinz (Zug,
CH), Schneider; Gernot (Baar, CH) |
Assignee: |
LGZ Landis & Gyr Zug AG
(Zug, CH)
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Family
ID: |
25698130 |
Appl.
No.: |
06/405,859 |
Filed: |
August 6, 1982 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
918446 |
Jun 23, 1978 |
04309655 |
Jan 5, 1982 |
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Foreign Application Priority Data
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Jul 8, 1977 [CH] |
|
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8455/77 |
May 24, 1978 [CH] |
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5644/78 |
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Current U.S.
Class: |
324/117R;
324/127; 324/249; 336/212 |
Current CPC
Class: |
G01R
15/185 (20130101); H01F 38/30 (20130101); H01F
38/20 (20130101); G01R 19/20 (20130101) |
Current International
Class: |
G01R
15/14 (20060101); G01R 15/18 (20060101); G01R
19/18 (20060101); G01R 19/20 (20060101); H01F
38/30 (20060101); H01F 38/20 (20060101); H01F
38/28 (20060101); G01R 001/20 (); G01R 033/00 ();
H01F 001/00 () |
Field of
Search: |
;324/117R,117H,127,249
;336/178,212 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1392656 |
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Feb 1965 |
|
FR |
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1453011 |
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Nov 1965 |
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FR |
|
750109 |
|
Jun 1956 |
|
GB |
|
1007889 |
|
Oct 1965 |
|
GB |
|
1229154 |
|
Apr 1972 |
|
GB |
|
1335472 |
|
Oct 1973 |
|
GB |
|
Other References
Irons et al., "Magnetic Thin-Film . . . ", IEEE Trans. on
Magnetics, vol. Mag-8, No. 1, Mar. 1972, pp. 61-65. .
Schernus et al., "A New Digital Method", IEEE Trans. on Inst. &
Meas., vol. Im-21, No. 4, Nov. 1972, pp. 346-348..
|
Primary Examiner: Karlsen; Ernest F.
Attorney, Agent or Firm: Morgan, Finnegan, Pine, Foley &
Lee
Claims
What is claimed is:
1. A measuring transformer comprising:
a core of magnetizable material having an air gap;
an anisotropic thin film of magnetizable material positioned at
said gap, a surface of said film being parallel to a path of
magnetic flux bridging said gap;
a measuring conductor for carrying a current to be measured, said
conductor being disposed adjacent said core for magnetic coupling
therewith; and
a pre-magnetizing winding for carrying an alternating
pre-magnetizing current to induce an alternating varying magnetic
field which repetitively saturates the material of said film in
both directions of saturation, said winding being disposed adjacent
said core for magnetic coupling therewith, whereby the point of
time of the saturation of said film is dependent on the strength of
said current to be measured.
2. A measuring transfomer according to claim 1 wherein said film is
deposited on a substrate.
3. A measuring transformer according to claim 1 wherein said film
is a rolled foil.
4. A measuring transformer according to claim 1 wherein the
thickness of said film is at most a few microns.
5. A measuring transformer according to claim 1 wherein the product
of the gap length and the relative permeability of said core is
much greater than the length of the magnetic circuit of said
core.
6. A measuring transformer according to claim 1 wherein said core
has two pole shoes and said film has two ends which are secured to
respective ones of said pole shoes.
7. A measuring transformer according to claim 1 wherein an
induction winding passes around said film.
8. An electric meter including an input measuring transformer for
the measurement of current, said transformer comprising:
a core of magnetizable material having an air gap;
a measuring conductor disposed adjacent said core for carrying the
current to be measured by the meter;
a pre-magnetizing winding disposed adjacent said core for magnetic
coupling therewith for carrying an alternating pre-magnetizing
current; and
an anisotropic thin film of magnetizing material positioned at the
air gap of said core and responsive to a magnetic flux of said
current to be measured and responsive to a magnetic flux of said
pre-magnetizing current to become saturated in alternate directions
of magnetization corresponding to alternating directions of said
pre-magnetizing current whereby the point of time of the saturation
of said film is dependent on the strength of the current to be
measured.
9. A measuring transformer for the measurement of current or
voltage comprising:
a magnetic core having an air gap;
a measuring conductor for carrying a current to be measured;
a pre-magnetizing winding for carrying an alternating
pre-magnetizing current, said measuring conductor and said
pre-magnetizing winding being positioned relative to said magnetic
core for linking magnetic fluxes of their respective currents with
said magnetic core; and
an anisotropic magnetic thin film comprising a ferromagnetic,
magnetoresistive material having contact means for the connection
of a current or voltage source, said air gap being bridged by said
magnetic film, a surface of said film being parallel to a path of
magnetic flux bridging said air gap, said film being linked by each
of said magnetic fluxes for providing states of magnetization in
alternate directions corresponding to alternate senses of said
pre-magnetizing current whereby the point of time of the saturation
of said film is dependent on the strength of the current to be
measured.
10. A measuring transformer according to claim 9 wherein said
magnetic film has a meander-line form.
11. A measuring transformer according to claim 9 wherein said
magnetic film is coated with diagonal strips of electrically
conductive material.
12. A measuring transformer according to claim 9 wherein said
magnetic film is magnetically coupled with a magnetic layer which
is thicker than said magnetic film.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a measuring transformer for the
potential-free measurement of currents or voltages, and to a static
electricity meter including such a transformer.
2. Description of the Prior Art
FIG. 1 shows a known measuring transformer which comprises an
annular magnetic core 1, a measuring conductor 2 for carrying the
current I.sub.m to be measured, a pre-magnetizing winding 3 and an
induction winding 4. The conductor 2 is passed through the closed
magnetic circuit of the magnetic core 1, but could also be wound in
a plurality of windings around the core 1, in the same manner as
the pre-magnetizing winding 3.
In operation of this transformer, a pre-magnetizing current
I.sub.v, which is preferably of a triangular waveform and which
flows through the pre-magnetizing winding 3, produces a magnetic
field which alternately drives the magnetic core 1 in both
saturation directions, the magnetic core 1 operating as a magnetic
field comparing means. If the current I.sub.m to be measured is of
zero value, a symmetrical induction voltage U.sub.a is induced in
the induction winding 4, and this induced voltage substantially
comprises positive and negative pulses which occur at the moment of
magnetism reversal of the magnetic core 1 and which follow at equal
spacings in time. If, on the other hand, the instantaneous value of
the current I.sub.m is greater than zero, then this current assists
the magnetizing effect of the pre-magnetizing current I.sub.v,
whereby there is a temporal displacement of the positive and
negative pulses of the induction voltage U.sub.a. This temporal
displacement can be evaluated as a measurement of the strength and
direction of the current I.sub.m to be measured. The induction
winding 4 is not absolutely necessary, as a voltage is also induced
in the premagnetizing winding 3, and the variation of that voltage
in time can be used in the same way as a measurement with respect
to the current I.sub.m.
A known measuring transformer supplies an induction voltage U.sub.a
whose pulses are relatively wide and have shallow flank angles of
inclination. Moreover, the addition of the magnetic fluxes or
magnetic fields which are involved, in the region of saturation of
the magnetic core 1, is difficult to control, and this results in a
complicated winding structure or costly compensation operations. In
addition, the temporal displacement of the pulses relative to the
passage through zero of the magnetic field is relatively great.
This situation is altered only to a minor extent if the magnetic
core 1 has a reduced portion, to reduce the saturation field
strength. This known measuring transformer is therefore not
suitable for the precision measurement of currents which vary
rapidly.
SUMMARY OF THE INVENTION
One object of the invention is to provide a measuring transformer
of the kind set out above, whose magnetic field comparing means is
virtually delay-free.
Another object of the invention is to provide a measuring
transformer of the kind set out above, whose output pulses mark the
moment of the passage through zero of the magnetic field, clearly
and with a high degree of accuracy.
According to the present invention there is provided a measuring
transformer for the potential-free measurement of one of current
and voltage, the transformer comprising a measuring conductor for
carrying a current to be measured, a pre-magnetizing winding for
carrying a pre-magnetizing current, and a magnetic field comparing
means which is exposed to the magnetic field produced by the
current to be measured and to the magnetic field produced by the
pre-magnetizing current and is alternately controlled in both
directions of saturation by the the magnetic field produced by the
pre-magnetizing current, said magnetic field comparing means being
a magnetic film of very small thickness in comparison with its
length and width.
According to the present invention there is also provided a static
electricity meter including an input measuring transformer for the
potential-free measurement of current, said transformer comprising
a measuring conductor for carrying the current to be measured by
the meter, a pre-magnetizing winding for carrying a pre-magnetizing
current, and a magnetic field comparing means which is exposed to
the magnetic field produced by the current to be measured and to
the magnetic field produced by the pre-magnetizing current and is
alternately controlled in both directions of saturation by the
magnetic field produced by the pre-magnetizing current, said
magnetic field comparing means being a magnetic film of very small
thickness in comparison with its length and width.
According to a further embodiment of the present invention, the
output pulses of the measuring transformer can be directly obtained
from the magnetic field comparing means.
The above, and other objects, features and advantages of this
invention will be apparent from the following detailed description
of illustrative embodiments which is to be read in connection with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a known measuring transformer, referred to above;
FIG. 2 shows a magnetic film applied to a substrate;
FIGS. 3 to 7 show respective different embodiments of measuring
transformers with magnetic cores;
FIG. 8 shows a pulse diagram;
FIGS. 9 and 10 show current dividers for the current to be
measured;
FIGS. 11 to 13 show respective different embodiments of measuring
transformers without magnetic cores;
FIG. 14 shows a further embodiment of a measuring transformer;
FIG. 15 shows parts of the measuring transformer of FIG. 14 in plan
view;
FIGS. 16 and 17 show variants of FIG. 15;
FIGS. 18 and 19 show further embodiments of a measuring
transformer; and
FIG. 20 shows part of the measuring transformer of FIG. 19 in plan
view.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 2, reference numeral 5 denotes a preferably anisotropic
magnetic film whose thickness d is very small in comparison with
the length h and the width b. This magnetic film 5 which serves as
a magnetic field comparing means, is preferably applied to a
non-magnetic substrate 6 which imparts thereto the necessary
mechanical strength and which comprises for example a glass or
plastic plate. The film 5 may be applied to the substrate 6 in
accordance with known methods by vapor deposition in vacuum or by
electrolytic coating. The film 5 can also be, for example, a foil
which is produced by rolling and which is secured to the substrate
6 by adhesive. Suitable materials for the film 5 are, for example,
known NiFe or NiFeCr magnetic alloys.
The anisotropic magnetic film 5 can be operated in the measuring
transformers described in greater detail hereinafter, in principle
in the magnetic preferential direction or in the non-preferential
direction. When operating in the preferential direction, the
coercive field strength of the magnetic film 5 should be as low as
possible and the wall speed should be high. As low as isotropic
field strength as possible is advantageous when the magnetic film 5
is operated in the non-preferential direction. The description of
the function of the embodiments described hereinafter relates to
operation in the magnetic preferential direction which has been
found particularly advantageous.
In FIG. 3, the same components as in FIGS. 1 and 2 are denoted by
the same reference numerals. A magnetic core 7 which comprises
ferromagnetic material of high permeability again carries the
pre-magnetizing winding 3 and engages in a tong-like configuration
around the measuring conductor 2, but differs from the magnetic
core 1 of FIG. 1 by an air gap 8 which is bridged by the magnetic
film 5. The two longitudinal ends of the film 5 are secured, for
example by adhesive, to the magnetic core 7, on the outer surface
thereof. The magnetic film 5 is advantageously substantially longer
than the air gap 8 so as to give the largest possible contact
surfaces 9 between the magnetic core 7 and the magnetic film 5. If
the magnetic film 5 is arranged on a substrate 6 (FIG. 2), the
substrate (which is not shown in FIG. 3 for the sake of improved
clarity of the drawing) is advantageously disposed on the outside
surface of the magnetic film 5, which is remote from the magnetic
core 7, so that there is no air gap at the contact surfaces 9
between the magnetic core 7 and the film 5. The width of the film 5
approximately corresponds to that of the magnetic core 7.
The above-described transformer operates as follows:
In the rest-condition the magnetic film 5 is saturated and its
permeability corresponds to that of a vacuum. By virtue of the
current I.sub.m to be measured and the pre-magnetizing current
I.sub.v which flow through the magnetic core 7, a magnetic outer
field H.sub.a is built up in the air gap 8 where the magnetic film
is disposed, and the following equation applies to the field
H.sub.a, assuming ideal conditions: ##EQU1## where n.sub.v
represents the number of windings of the pre-magnetizing winding 3,
n.sub.m represents the number of windings of the measuring
conductor 2, and l represents the length of the air gap 8. As soon
as the outer field H.sub.a exceeds the wall movement field strength
of the magnetic film 5, a magnetism reversal process begins in the
magnetic film 5, and this process begins in the magnetic film 5,
and this process may be explained by the displacement of a Bloch
wall. This displacement occurs very rapidly so that the
magnetization zero in the magnetic film 5 in relation to the
condition H.sub.a =0 occurs, with only a very small delay. In this
time interval the permeability of the magnetic film 5 is very high,
the magnetic circuit is closed to the maximum extent by way of the
magnetic film and the magnetic flux in the magnetic circuit rises
steeply. This change in the magnetic flux is revealed in a steep
jump in voltage in the pre-magnetizing winding 3 and possibly in an
induction winding 4 (FIG. 1). Thereafter the magnetic film 5 is
saturated in the other direction, its permeability again
corresponds to that of a vacuum, and the Bloch wall has passed
through the entire width of the magnetic film 5.
The following relationship applies for the effective magnetic field
H.sub.eff which switches the magnetic film 5: ##EQU2## wherein L is
the length of the magnetic circuit in the magnetic core 7, A is its
cross-sectional area, a is the cross-sectional area of the magnetic
film 5, M.sub.s is the saturation magnetization of the magnetic
film 5, .mu..sub.o is the absolute permeability, and .mu..sub.r is
the relative permeability of the magnetic core 7. The
proportionality relationship:
is fulfilled if the second term in the above equation for H.sub.eff
disappears, that is to say, the relationship becomes:
and/or
Observing the second inequality also provides the greatest possible
proportionality factor k=1/l between H.sub.eff and .SIGMA.I,
because the following relationship then applies:
The current I.sub.m to be measured and the pre-magnetizing current
I.sub.v are therefore converted into precisely proportional
magnetic fields at the location of the magnetic film 5 which
operates as a magnetic field comparing means, this being effected
by way of the magnetic flux in the magnetic core 7. The moment of
the passage through zero of this magnetic field is marked clearly
and with a high degree of accuracy by an output pulse which is very
steep and which has a minimal delay relative to the occurrence
.SIGMA.I=0. In addition, the position in time of the output pulse
is substantially independent of the angle at which the currents
I.sub.m and I.sub.v cross in the current-time diagram.
The above-mentioned advantages are based on the particular magnetic
properties which can be achieved with a very thin magnetic film,
namely a small dynamic coercive field strength, a high switching
speed of the magnetic film, low eddy current losses, low saturation
field strength, low demagnetization (shift), low degree of
dispersion of the magnetic properties within the small and thin
magnetic film by virtue of high metallurgical purity and
homogeneity, and high uniaxial anisotropy.
The thickness d of the magnetic film 5 should be at most a few
microns, in order to keep the saturation field strength,
demagnetization, and eddy current losses as low as possible. A
greater film thickness does in fact result in a greater energy
content of the output pulses, but has its effect in particular in a
widening in time and not in an increase in voltage of the output
pulses. The film thickness d is, in a particularly advantageous
aspect, at most 2 microns; this gives negligible eddy current
losses in the magnetic film 5 and thus a switching speed which is
limited only by material parameters of the magnetic film, such as
wall mobility, purity, and the like.
Other advantages of the above-described measuring transformer lie
in the ease of production and the resistance of the mechanical
parameters in regard to mechanical loadings, in the freedom from
magnetostriction, the possibility of continuous manufacture, and
the way in which the magnetic film 5 can be secured without
problem, by adhesive and the like.
FIGS. 4 to 7 show advantageous forms of the pre-magnetizing winding
3, the induction winding 4 and the magnetic core, which can be
substantially combined together. In FIGS. 4 to 6, the
pre-magnetizing winding 3 and the induction winding 4 are in the
form of cylindrical coils which are arranged axially one beside the
other in FIGS. 4 and 5 and concentrically in FIG. 6. In FIG. 7, the
induction winding 4 passes around the magnetic film 5 in the region
of the air gap, whereby cross-talking of the pre-magnetizing
current I.sub.v to the induction winding 4 is substantially
prevented.
The magnetic core 10 shown in FIG. 4 comprises a U-shaped member
with inwardly bent pole shoes 11 and 12, the magnetic film 5 being
secured to the pole surfaces of the pole shows 11 and 12, which
surfaces lie in a common plane. The magnetic core 13 shown in FIG.
5 also comprises a U-shaped member with inwardly bent pole shoes 14
and 15, but in this case the inner surfaces 16 and 17 of the ends
of the pole shoes 14 and 15 again extent parallel to the limbs of
the U-shaped member. This makes it possible to avoid saturation
phenomena in the pole shoes 14 and 15.
The U-shaped magnetic core 18 shown in FIGS. 6 and 7 does not have
any pole shoes; the length of the air gap approximately corresponds
to the coil width of the pre-magnetizing winding 3.
In the above-described measuring transformers, the number of
windings n.sub.v of the pre-magnetizing winding 3, the number of
windings n.sub.s of the induction winding 4 and the length l of the
air gap 8 (FIG. 3) may be selected substantially independently of
each other. The pre-magnetizing current I.sub.v whose amplitude is
advantageously not greater than some ten milliamps, in order to
avoid expensive apparatus for the production thereof, is adapted to
the measuring current I.sub.m, with the number of windings n.sub.v.
The number of windings n.sub.s determines the magnitude of the
induced output voltage U.sub.a. The field strength H.sub.a produced
in the air gap 8 is determined by the selection of the air gap
length l.
FIG. 8 shows the form of the output voltage U.sub.a against time t,
which was determined in a measuring transformer as shown in FIG. 4,
with the following data:
Material of the magnetic core 10: Ferrite
Material of the magnetic film 5: NiFe
Length h of the magnetic film 5: 5 mm
Width b of the magnetic film 5: 1 mm
Thickness d of the magnetic film 5: 1.5 micron
Length l of the airgap: 1 mm
Number of windings n.sub.v of winding 3: 250
Number of windings n.sub.s of winding 4: 250
Amplitude of the current I.sub.v : 20 mA
Frequency of the current I.sub.v : 1 kHz
Measurements were effected with an amplitude of the output pulse of
30 mV, a rise time t.sub.r of 5 .mu.s, a decay time t.sub.f of 11
.mu.s and a pulse duration t.sub.p of 10 .mu.s.
In order to be able to measure very high currents with the
measuring transformer, while nonetheless keeping the number of
windings n.sub.v and the pre-magnetizing current I.sub.v within
acceptable limits, it may be advantageous for the current to be
measured to be divided by means of a current divider into the
current I.sub.m and a shunt current. FIGS. 9 and 10 show
advantageous examples of such a current divider.
The current divider 19 shown in FIG. 9 comprises a single metal
plate which has current connections 20 and 21, and a cut 22 which
extends longitudinally relative to the direction of current flow
and which divides the middle region of the metal plate into a
measuring current path 23 and a shunt current path 24. The two
current paths 23 and 24 are bulged outwardly in opposite directions
to a semicircular form and form an eye into which the magnetic core
7, 10, 13 or 18 can be inserted in such a way that the magnetic
circuit of the magnetic core encloses the measuring current path
23.
FIG. 10 shows a current divider 25 which also comprises a single
metal plate which however in this case is flat, provided with
current connections 26 and 27, a shunt current path 28 and a
measuring current path 30 which is separated from the current path
28 by a punched-out portion 29. In this embodiment the magnetic
core 7, 10, 13 or 18 is inserted into the portion 29 so that the
magnetic circuit of the magnetic core encloses the measuring
current path 30.
Making the current divider 19 or 25 in the form of an integral
metal plate ensures a constant current divider ratio which is
independent of ambient influences. The phase displacement caused by
the current divider 19 or 25 respectively: ##EQU3##
(.omega.=circuit frequency, L=inductance of the transformer,
R=resistance of the measuring current path 23 or 30) can be kept
small if the metal plate is of small cross-section, and thus the
measuring current path is of high resistance R, and the inductance
L is as small as possible, by suitable dimensioning of the
measuring transformer. A certain degree of compensation for the
phase displacement .phi. is already achieved by the finite
switching speed of the magnetic film 5; any additional compensation
which may be necessary can be achieved with simple phase displacer
members or by covering the shunt path 24 or 28 by a soft-magnetic
layer of suitable thickness.
As already mentioned, in the above-described measuring
transformers, conversion of the current I.sub.m and the
pre-magnetizing current I.sub.v into proportional magnetic fields
is effected by way of the magnetic flux in a magnetic core. Some
embodiments are described hereinafter, in which the current I.sub.m
and the pre-magnetizing current I.sub.v are converted directly to
proportional magnetic currents at the position of the magnetic film
5, so that no magnetic core is required.
In FIG. 11, a measuring conductor 31 for carrying the current
I.sub.m to be measured is in the form of a flat conductor. The
pre-magnetizing winding 3 is formed by a disc-shaped flat coil 32.
The magnetic film 5 is arranged between the conductor 31 and a
part, parallel thereto, of the flat coil 32, in a region in which
both the magnetic surface field of the conductor 31, which is
produced by the current I.sub.m, and also the magnetic surface
field of the flat coil 32, which is produced by the pre-magnetizing
current I.sub.v, are uniform. Such a region of uniform magnetic
field, which region is solely dependent on geometric factors, can
be provided if the conductor 31 and the part of the flat coil 32
which is parallel thereto are as closely adjacent as possible and
have the flattest possible cross-section, that is to say, a
thickness d.sub.1 and d.sub.2 which is small in comparison with the
width b.sub.1 or b.sub.2, respectively.
The flat coil 32 can be made in the form of a self-supporting coil
or strip or of wire, with one or more windings for each coil
winding layer. In addition, the flat coil 32 may comprise one or
more conductor plates which have a spiral copper layer on one or
both sides, in the manner of an etched printed circuit. The
magnetic film 5 can be secured for example directly to the
measuring conductor 31 by adhesive.
The transformer shown in FIG. 12 differs from that shown in FIG. 11
only insofar as the pre-magnetizing winding 3 is formed by a flat
cylindrical coil 33, while the magnetic film 5 is disposed between
the conductor 31 and one flat side of the coil 33. At the position
of the magnetic film 5, the magnetic outer field of the coil 33 and
the magnetic surface field of the conductor 31 are superimposed
upon each other.
In FIG. 13, a flat cylindrical coil 34 forms the pre-magnetizing
winding and passes around the magnetic film 5. A flat conductor 35
which carries the current I.sub.m passes in a loop around the flat
cylindrical coil 34. The magnetic inner field of the coil 34 and
that of the loop formed by the conductor 35 are superimposed on
each other at the location of the magnetic film 5. A magnetic short
circuit 36 comprising a material with a high degree of permeability
provides for a magnetic connection between the two opposite ends of
the magnetic film 5 which projects out of the coil 34, and thereby
reduces demagnetization of the magnetic film 5.
The above-described measuring transformers are used for the
potential-free measurement of direct or alternating currents. By
series connection of a high-value resistance with the measuring
conductor, by replacing the measuring conductor by a winding with a
suitably high number of windings, or by combining the two
possibilities just mentioned, they can also be used for measuring
direct or alternating current voltages. They provide very steep and
narrow output pulses whose displacement in time can be used as a
measurement of the instantaneous value of the magnitude and
direction of the electrical signal to be measured. The
above-described measuring transformers are advantageously used as
input transformers in static electricity meters.
In the measuring transformers described above, with the moment of
passage through zero of the magnetic field, produced by the
pre-magnetizing current and the current to be measured, an output
impulse is induced in the pre-magnetizing winding or in a separate
induction winding, thus marking the passage through zero of the
magnetic field with great accuracy. However, said transformer has a
disadvantage in that the premagnetizing winding or the induction
winding, from which the output impulse is obtained, is inductively
coupled with the measuring conductor. High frequency interference
signals, flowing in the measuring conductor, are therefore
inductively transferred to the pre-magnetizing winding, serving as
the output winding, where they are superimposed on the output
impulses. In an evaluating circuit, connected to the measuring
transformer, such interference signals cannot be readily
differentiated from the output signals marking the passage through
zero of the magnetic field. A suppression of the interference
signals in the evaluating circuit is not possible if the spectrum
of said signals is the same as that of the output impulses, or is
in the proximity thereof.
In a further embodiment of the instant invention the output pulses
can be directly obtained from the magnetic field comparing means.
As great as insensitivity as possible to interference signals is
achieved with the measuring transformer in accordance with this
further embodiment. A change in resistance of the magneto-resistive
magnetic film occurs only at the moment of passage through zero of
the magnetic field. To have any effect at all, the interference
signals must either occur in the proximity of this moment, or must
be very strong in order to bring about a change in the magnetizing
direction of the magnetic film, which is statistically less
probable than the above-mentioned case of interference. A further
advantage is that the pre-magnetizing current in the
magneto-resistive magnetic film does not give rise to an
interference signal which superimposes itself on the output
impulses obtained at the contacts of said magnetic film. This also
affords the advantage that the premagnetization current does not
have to obey a continuous function, but can be a step-shaped
signal, for example.
Referring to FIG. 14, reference numeral 41 indicates a magnetic
core of ferromagnetic material whose magnetic circuit encloses the
air gap 43 between two pole shoes 42, whereby the pole surfaces 44
of the pole shoes 42 lie in a common plane. The magnetic core 41
engages in a tong-like configuration around the measuring conductor
45 which carries the current I.sub.m to be measured. Moreover, the
magnetic core 41 carries a pre-magnetizing winding 46 through
which, for example, a triangular-shaped pre-magnetizing current
I.sub.v flows.
The following described parts of the measuring transformer are
drawn in an exploded view in FIG. 14 to give better lucidity. A
magnetic film 48 of ferromagnetic, magneto-resistive material,
whose thickness is very small in comparison with its length and
width, is arranged on a non-magnetic, electrically insulating
substrate 47. Said film is advantageously magnetic, anisotropic or
uniaxial. The magnetic film 48 may be applied to the substrate 47
in accordance with known methods by vapor deposition in vacuum or
by electrolytic coating. Photo-lithographic methods can be used,
for example, for design purposes. NiFe alloys and triples derived
therefrom (e.g., NiFeCr or NiFeCo) or higher alloys are preferably
suited as magneto-resistive materials. The active length and width
of the magnetic film 48 corresponds to the dimensions of the air
gap 43, e.g., each 1 mm. The typical thickness of the magnetic film
48 lies in the order of 40 nm. In order to avoid extremely low
values for the thickness of the magnetic film 48 and nevertheless
to achieve suitably high resistance values for detection of the
change in resistance, the magnetic film can be formed in a
meander-like design. The longitudinal ends of the magnetic film 48
are coated, for example, with a 100 nm thick conducting layer 49 of
gold, copper or the like, the outer ends of which each carry a
contact 50 made of a good conducting material. The pole surfaces 44
of magnetic core 41 lie above the conducting layer 49 so that the
magnetic film 48, which practically lies on a plane with the pole
surfaces, bridges the air gap 43. The preferential magnetic
direction (easy axis) of the magnetic film 48 can lie parallel to,
vertical to or, for example, at an angle of 45.degree. to, the
direction of the magnetic field in the air gap 43. The direction of
the current flowing in the magnetic film 48, and produced by the
current or voltage source connected to the contacts 50, is parallel
to the direction of the magnetic field in the depicted example.
In FIG. 15 can be seen the arrangement of the magnetic film 48, the
pole surfaces 44, the conducting layer 49 and the contacts 50
viewed from the side of the magnetic core 41. The above-described
measuring transformer operates as follows: In the rest condition,
the magnetic film 48 has a constant ohmic resistance in the order
of, for example, 100.OMEGA.. A magnetic field is built up in the
air gap 43 of magnetic core 41 by the current to be measured
I.sub.m and the pre-magnetizing current I.sub.v. The magnetic film
48 changes its resistance discontinuously at each passage through
zero of said magnetic field. If a current or voltage source is
connected to the contacts 50, then said change in resistance
manifests itself in the form of a needle-shaped voltage or current
impulse which marks the moment of passage through zero of the
magnetic field unequivocally and with great accuracy. Since the
magnetic film 48 is controlled up to saturation, the height of the
output impulse is independent of the strength of the magnetic
field. If the instantaneous value of the current to be measured
I.sub.m differs from zero, then its magnetizing effect superimposes
itself on that of the pre-magnetizing current I.sub.v, whereby the
output impulses are displaced with respect to time. This
displacement with respect to time can be interpreted, in an
evaluation circuit connected to contacts 50, as a measure of the
strength and direction of the current to be measured I.sub.m. The
coupling in of the magnetic field into the magnetic film 48 is most
effective when carried out in the preferential magnetic direction
of the magnetic film. The change in resistance achieved thereby is
at its lowest. It can be increased if, in accordance with FIG. 16,
strips 51 of gold, or another material with good electrical
conducting properties, are applied diagonally at 45.degree. to the
active surface of the magnetic film 48. Such a "barber's pole"
arrangement of the magnetic film 48 causes the current direction in
the said magnetic field to be rotated by 45.degree..
If the preferential magnetic direction of the magnetic film 48 does
not run parallel to the direction of the magnetic field, but forms
an angle of, for example, 90.degree. or 45.degree. to the magnetic
field, then a crossed configuration in accordance with FIG. 17 is
advantageous. In comparison with FIG. 15, in FIG. 17 the magnetic
film 48, together with the conducting layers 49 and the contacts
50, are so rotated in the drawing plane that the current in the
magnetic film 48 flows in a vertical direction to the magnetic
field.
The configuration according to FIG. 18 differs from that according
to FIG. 14 by virtue of a magnetic layer 52 arranged between the
magnetic film 48 and the substrate 47, and insulated from said
magnetic film 48 by means of a very thin insulating layer 53. The
magnetic layer 52 likewise comprises a ferromagnetic NiFe alloy,
but is essentially thicker than the magnetic film 48. The typical
thickness of the magnetic layer 52 is 1 to 2 microns. The magnetic
layer 52 also enables a good coupling in of the magnetic field when
said magnetic field does not run in the preferential axial
direction of the magnetic field 48. On the basis of the magnetic
coupling between the magnetic layer 52 and the magnetic film 48, a
larger change in resistance of said magnetic film 48 results during
the passage through zero of the magnetic field.
The magnetic layer 52 can be preferably applied with a crossed
configuration according to FIGS. 19 and 20. With this arrangement,
the magnetic film 48 is positioned in the same manner as with the
crossed configuration according to FIG. 17, namely directly on the
substrate 47, and said arrangement also boasts the conducting
layers 49 and the contacts 50, whereby the direction of the current
in the magnetic film 48 is vertical to the direction of the
magnetic field between the pole surfaces 44. The preferential
magnetic direction of the magnetic film 48 is likewise vertical to
the direction of the magnetic field. The magnetic layer 52 rests on
the pole surfaces 44, bridges the air gap 43 and crosses the
magnetic film 48 at right angles. The preferential magnetic
direction of the magnetic layer 52 is parallel to the direction of
the magnetic field. The magnetic film 48 lies below the magnetic
layer 52, whereby magnetic film 48 and magnetic layer 52 are
electrically insulated from each other by means of a very thin
insulating layer not shown in the drawing. As a result of the
magnetic coupling between the magnetic layer 52 and the magnetic
film 48, the magnetizing of the magnetic film 48 in the passage
through zero of the magnetic field is rotated so that a strong
change in resistance in ascertainable at the contacts 50.
Preferably the magnetic film 48 forms, together with one or three
resistors, a voltage divider or a bridge circuit respectively.
These resistors are advantageously magneto-resistive magnetic films
of the same type as magnetic film 48, so that temperature
influences are compensated. Furthermore, these resistors can
likewise be exposed to the magnetic field of the magnetic core 41
so that their output signals superimpose themselves in a manner as
to be advantageous for the evaluation. Of course, the
above-described magneto-resistive magnetic film 48, provided with
contacts, can also be applied with measuring transformers in
accordance with FIGS. 11 to 13 above, which have no magnetic
cores.
Although illustrative embodiments of the invention have been
described in detail herein with reference to the accompanying
drawings, it is to be understood that the invention is not limited
to those precise embodiments, and that various changes and
modifications can be effected therein by one skilled in the art
without departing from the scope and spirit of the invention as
defined by the appended claims.
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