Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
Steel mesh heating system with reduced magnetic
field.
FIELD OF THE INVENTION
This invention relates to steel mesh heating
systems, such as used in reinforced concrete, having
reduced magnetic field effects and improved power factor.
BACRGROUND OF THE ll.v~ic.,lON
It is well known that a steel mesh, such as
that used to reinforce a concrete floor, can be made to
carry an electric current which causes the wires of the
steel mesh to heat up. The heat generated is transmitted
to the concrete floor which thereby reaches the desired
temperature. In outdoor areas where snow may accumulate,
the steel mesh is made to carry a sufficiently large
current, causing the snow to melt. R.S. Tice in Canadian
Patent Nos. 474,342, [1951], 474,343 [1951] and 490,741
[1953] describes such heating systems.
One aspect of steel mesh heating systems is
that the voltage is usually 30 volts or less, so as to
remain in the extra low-voltage category. Consequently,
steel mesh heating systems require large currents to
produce a given amount of heating power. For example, a
kilowatt installation operating on single phase
requires a current of 1000 amperes. The low voltage and
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large current are obtained by means of a step-down
transformer.
In addition to the inherent robustness of a
steel mesh heating system, it is well known that the
large heat capacity of the concrete floor or slab in
which it is embedded, enables the system to be used on an
off-peak basis, thereby reducing electricity costs (see
Canadian Patent No 841,164 [1970] to Voglesonger and
Canadian Patent 738,080 [1966] to Williams).
However, steel mesh heating systems of the
prior art have some drawbacks.
One drawback is that the power factor tends to
be low, typically around 80%. Most electric power
companies impose a surcharge when the overall power
factor of an industrial or commercial consumer is below
90%. Consequently, the cost of electricity for steel
mesh heating systems is potentially higher than that of
more conventional heating systems where the power factor
is close to 100%. The reason for the low power factor is
directly traceable to the strong magnetic field
associated with steel mesh heating systems. Any means
that reduces the magnetic field will automatically
improve the power factor.
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Another drawback is that the magnetic field
created by a steel mesh heating system may disturb the
image appearing on some computer and television and other
cathode ray tube screens. When this occurs, the heating
system may have to be disconnected during those periods
when the computers or TV sets are in use.
The use of highly permeable magnetic materials
to shield the magnetic field around a computer or TV set
is not feasible because the magnetic field may enter by
way of the very screen that has to be observed.
Furthermore, shielding is difficult because image
distortion and flickering is produced at very low
magnetic flux densities - typically 5 microtesla (~T)
(peak) in a heating system that operates on 60 Hz.
In this regard, it should be noted that an ac
current of 18 amperes flowing in a long, straight wire
produces a peak magnetic flux density of 5 ~T at distance
of one meter from its center. Consequently, steel mesh
heating systems that carry hundreds of amperes can
produce magnetic fields of considerable strength many
,
metres away.
Concern with possible biological effects of
magnetic fields is another factor that encourages
reducing the magnetic fields of conventional steel mesh
heating systems. In this regard, see Perlman: U.S.
_ 4 _ ~ ~ ~8~
Patent 4,998,006 [1991]; Wesseltoft Aquaterma: French
Patent Publication 2,611,106 [1988]; Rowe: Canadian
Patent Application 2,012,473 [1990]; Hjortsberg: U.S.
Patent 4,908,497 [1990], Shih: Canadian Patent
application 2,086,958 [1993]. These inventions are
mainly concerned with low-power devices, such as comfort
heaters and water beds, that are in particularly close
contact with the human body.
Opinions vary widely as to the acceptable
exposure limits to 50 Hz and 60 Hz magnetic fields. In
a publication by the American Conference of Governmental
Industrial Hygienists, entitled Sub-Radio Frequency
(3OkHz and below) Magnetic Fields, the International
Non-Ionizing Radiation Committee (INIRC) of the
International Radiation Protection Association (IRPA)
suggested the following occupational exposure limits:
500 ~T for the entire day;
5000 ~T for exposures of less than two hours
duration;
25000 ~T for exposure of limbs throughout the working
day.
The IRPA/INIRC exposure limits for members of
the general public were set at 100 ~T for continuous
exposures, and 1000 ~T for exposures of a "few hours per
day".
~,,
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It is to be noted that the Earth's magnetic
field, albeit static, varies from 30 ~T at the equator to
70 ~T at the poles.
The SI unit of magnetic flux density is the
tesla. One microtesla (1 ~T) is equal to 10 milligauss
(10 mG).
OBJECTS AND 8TATEMENT OF THE lNV~c.~ION
It is an object of the present invention to
overcome the above described drawbacks with present steel
mesh heating systems, especially those used for concrete
floor heating and snow melting.
It is a further object of this invention to
reduce the magnitude of the magnetic fields created by
steel mesh heating systems.
It is a further object of the present invention
to improve the power factor of steel mesh heating
systems.
It is another object of the present invention
to eliminate the flicker and distortion of the image on
sensitive computer and television and other cathode ray
tube screens.
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It is still a further object of the present
invention to diminish possible biological effects of
magnetic fields on living organisms.
One known method of reducing the magnetic field
will now be described.
Consider an ac current Il flowing in a long,
straight wire. It is well known that the current will
produce an alternating magnetic field around the wire.
The magnetic field is constantly increasing, decreasing
10 and reversing. In a 60 Hz system, the flux density
reaches its peak value 120 times per second. The peak
flux density is given by the equation:
B~ = 0.282 Il/d
in which
B~ = peak flux density, in microtesla;
Il = effective or RMS current flowing in the wire,
in amperes;
d = perpendicular distance between the center of the
wire and the point of measurement, in metres.
The alternating magnetic field surrounding the
wire can be significantly changed by means of a second ac
current Ix having the same frequency, that flows in a
conductor positioned as close as possible to the wire.
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However, in order to essentially cancel the alternating
magnetic field produced by Il, instant by instant, the
current Ix flowing in the conductor must instantaneously
be equal and opposite to current Il flowing in the wire.
In other words, Il and Ix must be equal and 180 degrees
out of phase.
Bearing these principles in mind, the present
invention is concerned with reducing the magnetic field
surrounding a steel mesh and relates to a heating system
for use in a reinforced concrete floor using embedded
steel mesh or steel rods, which system includes the steel
mesh or steel rods which consist of a first plurality of
longitudinally extending and laterally spaced reinforcing
steel rods, opposite laterally extending busbars
connecting the opposite ends of the rods, and electrical
means allowing a total current to flow in and
electrically heat the rods; the improvement comprises a
multiconductor strip consisting of a second plurality of
longitudinally extending and laterally spaced rods and
opposite laterally spaced busbars connecting the opposite
ends of the second plurality of rods; the multiconductor
strip is disposed in superposed close proximity to the
first plurality of rods; the multiconductor strip is
electrically energized with a current which is
substantially equal and 180~ out of phase relative to the
total current flowing in the first plurality of rods
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whereby the magnetic field originally produced by said
total current is significantly reduced.
In one form of the invention, a current Ix is
caused to flow in a multiconductor strip, whose insulated
longitudinal conductors and insulated busbars are an
exact replica of the longitudinal wires and busbars in
the steel mesh, in the sense that the number, length and
spacing of the longitudinal conductors are the same.
This multiconductor strip carries the same current as the
total current in the steel mesh, except that the
respective currents are 180~ out of phase.
This multiconductor strip is laid on top of the
steel mesh in such a way that the respective insulated
longitudinal conductors and insulated busbars are
juxtaposed as closely as possible with the corresponding
longitudinal wires and busbars of the steel mesh.
In this way, the magnetic fields produced
respectively by the multiconductor strip and the steel
mesh will essentially cancel each other at every point in
space. The resultant magnetic field is thereby greatly
reduced, even in close proximity to the steel mesh.
However, since the multiconductor strip and the steel
mesh cannot both occupy the same place, it is recognized
that a weak magnetic field will subsist near the surface
of both the steel mesh and the multiconductor strip.
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The present invention is achieved using three
methods for causing the desired current to flow in the
multiconductor strip. They are respectively labelled
method S, method T, method U.
NETHOD 8
A multiconductor strip is connected in series
with the corresponding steel mesh so that the total
current in the multiconductor strip is essentially the
same as the total current in the steel mesh, with the
respective currents flowing in opposite directions.
METHOD T
A multiconductor strip is connected to an
auxiliary transformer so that the total current in the
strip has essentially the same magnitude as the total
current in the corresponding steel mesh, except for being
essentially 180~ out of phase.
NETHOD U
Multiconductor strips are laid on top of
respective steel meshes and the adjacent busbars of the
multiconductor strips are short-circuited.
Other objects and further scope of
applicability of the present invention will become
apparent from the detailed description given hereinafter.
It should be understood, however, that this detailed
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description, while indicating preferred embodiments of
the invention, is given by way of illustration only,
since various changes and modifications within the spirit
and scope of the invention will become apparent to those
skilled in the art.
BRIEF DE8CRIPTION OF THE DRAWING8
Figure 1 is a plan view of a typical steel mesh
heating system in which two meshes, connected in series,
are powered by a single-phase step-down transformer;
Figure 2 is a cross section view, in elevation,
of the heating system of figure 1, showing a portion of
the resultant magnetic field created by the currents
flowing in the two meshes;
Figure 3 is a plan view of a steel mesh heating
system in which three meshes, connected in Y, are powered
by a three-phase step-down transformer;
Figure 4 is a plan view of a multiconductor
strip composed of nine longitudinal conductors connected
in parallel;
Figure 5 is a schematic plan view of a
multiconductor strip composed of nine longitudinal
conductors juxtaposed on a steel mesh having nine
longitudinal wires, the steel mesh and the multiconductor
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strip being connected in series to a single-phase
transformer;
Figure 6 is a schematic plan view of a
multiconductor strip composed of three longitudinal
conductors symmetrically juxtaposed on a steel mesh
having nine longitudinal wires, the steel mesh and the
multiconductor strip being connected in series to a
single-phase transformer;
Figure 7A is a schematic plan view of two steel
meshes connected in series with two multiconductor
strips;
Figure 7B is a schematic plan view of the two
steel meshes of figure 7A, including two connecting
cables;
Figure 8A is a plan view of three steel meshes,
each having nine longitudinal wires, connected to a main
three-phase transformer;
Figure 8B is a plan view of three
multiconductor strips each composed of nine longitudinal
conductors connected to a three-phase auxiliary
transformer, the three strips to be juxtaposed over the
steel meshes of figure 8A;
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Figure 9A is a plan view of a steel mesh
heating system in which two steel meshes, connected in
series, are powered by a main single-phase transformer;
Figure 9B is a plan view of two short
multiconductor strips connected in series and powered by
an auxiliary single-phase transformer, the strips to be
juxtaposed anywhere along the length of the two meshes of
figure 9A;
Figure lOA is a schematic representation of a
steel mesh similar to that of figure 1 but including
leakage currents between the two meshes;
Figure lOB is a schematic representation of two
multiconductor strips to be juxtaposed over the two
meshes of figure lOA;
Figure 11 is a schematic representation of two
steel meshes connected in series, powered by a single
phase transformer, and two juxtaposed multiconductor
strips that are connected by one conducting link;
Figure 12 is a schematic representation of two
steel meshes in series, powered by a single phase
transformer, and two juxtaposed multiconductor strips
that are connected by two conducting links;
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Figure 13 is a schematic representation of
three steel meshes, powered by a three-phase transformer,
and three juxtaposed multiconductor strips connected by
four conducting links;
Figure 14 is a schematic diagram of a heating
system with two steel meshes side by side;
Figure 15 is a schematic diagram of a heating
system with two steel meshes superposed; and
Figure 16 is a schematic diagram of a heating
system with a multiconductor strip made of copper
superposed on a steel mesh.
For purposes of clarity and to simplify the
following description, the term "wire" will designate the
element that carries the current that produces the
unwanted magnetic field while the term "conductor" will
signify the element that carries a current that reduces
the unwanted magnetic field.
Also, the term "steel mesh" includes steel
rods.
DESCRIPTION OF PRESENT INSTALLATIONS
Figure 1 shows a typical steel mesh heating
system powered by a single-phase step-down transformer
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(1) for use in reinforced concrete floors, slabs or the
like (not shown). Two coplanar steel meshes (2) (each
typically six feet wide) are laid out side by side with
a given space (typically six-inch) between them. Each
mesh is composed of a lattice of longitudinal wires (3)
and transverse wires (4) that are welded together at
their points of intersection (5) (one typical lattice may
be 4" x 4" or 6" x 6"). Busbars Bl and B2 are disposed
at opposite ends of the longitudinal wires (3). The
large current Il supplied by the step-down transformer
(1) is fed to the steel meshes by means of cables (6),
typically made of copper or aluminum. The meshes are
joined in series by means of a conducting link (7) which
connects busbars B2 at the remote end of each mesh.
For illustrative purposes, figure 1 shows two
meshes, each having nine longitudinal wires laterally
spaced. In practice, the number of longitudinal wires
will depend upon the type of mesh. The longitudinal
wires may be considered to be rods.
The longitudinal wires are welded to busbars Bl
and B2 at the extremities of each mesh. The busbars are
of sufficient size to carry current Il and are intended
to distribute the current as uniformly as possible among
the longitudinal wires. The equal distribution of
current among the longitudinal wires over the length of
each mesh is further assisted by the presence of the
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transverse wires. In practice, the current is carried
mainly by the longitudinal wires.
When steel rods only are used instead of steel
mesh, the transverse wires are absent.
When the transformer (1) is excited, the
alternating current Il flows alternately down one mesh
and returns by the other as shown in figure 1. On a
60 Hz system, the current changes direction at the rate
of 120 times per second. The current is typically 800
amperes in a six-foot wide mesh having a 4"x 4" lattice
(19 longitudinal wires). The current carried by each
longitudinal wire is therefore about 42 amperes. The
current carried by the transverse wires is essentially
zero, except at the extreme end of each mesh, where the
currents in the longitudinal wires are progressively
funnelled into busbars Bl and B2.
The wires of the steel mesh are typically bare
and are therefore in direct contact with the concrete
slab. Consequently, owing to the difference of potential
that appears along and between the respective meshes,
some leakage current will flow through the concrete slab.
As a result, current I2 flowing in conducting link (7) is
somewhat less than current Il supplied by the
transformer. The leakage current is typically less than
4% of the total current Il. If the steel mesh is coated
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with an electrical insulation, the leakage current is
negligible.
The magnetic field extends over the entire
length of each steel mesh and even beyond and the
direction of the magnetic lines of force (8) depends
upon, and corresponds to that of current Il. These
magnetic lines of force shown are merely a schematic
representation of the complex magnetic field surrounding
the steel meshes.
Figure 2 shows a cross section view, in
elevation, of the steel meshes in figure 1. From the
observer's standpoint, the current is momentarily flowing
into the left-hand mesh (away from the observer and into
the paper) and out of the right-hand mesh (out of the
paper, towards the observer). Each mesh contributes to
the magnetic field and the dotted flux lines show a
portion of the resultant magnetic field created by both
meshes. The magnetic field is strongest near the surface
of the meshes and decreases progressively in value for
points that are farther and farther away.
When the ac current momentarily reaches its
peak value, the magnetic field attains its maximum
strength. It is well known that energy, measurable in
joules, is associated with such a magnetic field. On the
other hand, when the current is zero, the magnetic field
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is nil. Thus, the ac current produces an alternating
magnetic field whose energy constantly swings between
zero and maximum, at twice the frequency of the ac
current. This pulsating energy causes the meshes to draw
reactive power from the power source. It is this
reactive power which gives rise to an undesirable low
power factor.
The alternating magnetic field also produces
the flicker and distortion on some computer and
lo television and other cathode ray tube screens.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 3 shows, a typical steel mesh heating
system using three meshes energized by a three-phase
transformer. The secondary of the transformer is
connected in Y and the three meshes are connected by
means of two conducting links (7), creating a so-called
neutral. Busbars B1 and B2 are again used at each end of
the respective meshes.
The magnetic field created by a three-phase
heating system can extend over a broad region. For
example, in a typical snow-melting system that involved
a ramp 48 feet long by 28 feet wide, the alternating
magnetic field was strong enough to disturb a computer
screen located 40 feet above the ramp.
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To greatly reduce the magnetic field produced
by a current IA flowing in the mesh marked phase A, the
multiconductor strip (figure 4) carrying a current Ix,
equal to IA but 180- out of phase with it, would be
superposed in close proximity with the mesh. Similarly,
to reduce the magnetic field created by current IB
flowing in the mesh marked phase B, the corresponding
superposed multiconductor strip would carry a current Ix
equal to I~ but 180- out of phase with it. Similar
remarks apply to phase C. The multiconductor strip has
the same number of conductors as the mesh has
longitudinal wires.
The present invention identifies three methods
(S, T and U) in causing the desired current I~ to flow in
the multiconductor strip of figure 4.
DESCRIPTION OF METHOD S
Figure 5 shows how the magnetic field can be
reduced in the case of a single steel mesh (2) having
busbars B1 and B2. A multiconductor strip (10) is
connected in series with the steel mesh by connecting
busbar B4 to busbar B2 by means of a conducting link (7).
A single-phase transformer (1) is connected to busbars B1
and B3. The steel mesh and the multiconductor strip
respectively have the same number of longitudinal wires
and conductors. Because of the symmetrical arrangement
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of the wires (3) and conductors (9), the total current I~
supplied by the transformer will divide substantially
equally among the longitudinal wires and conductors.
Consequently, the currents flowing in the individual
conductors are substantially equal and opposite to those
flowing in the individual juxtaposed longitudinal wires.
Owing to its greater resistivity and also due
to skin effect, the resistance of the wires in a steel
mesh is typically 10 to 15 times greater than that of a
copper conductor of equal length and cross section. It
follows that, when the conductors of the strip are made
of copper having the same cross section as that of the
steel wires, the voltage drop along the conductors is
much less than that along the longitudinal wires.
For example, referring to figure 5, if the
voltage drop along the steel mesh between busbars B1 and
B2 is 10 volts, the drop along the multiconductor strip
between busbars B2 and B3 is typically 1 volt. Thus, to
produce the same current, the transformer may have to
provide only a slightly higher voltage for the new
heating system as compared to a conventional system.
Indeed, it may happen that the reduced magnetic field so
diminishes the reactance of the steel mesh that the
required transformer voltage remains essentially the
same.
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Consequently, in reducing the magnetic field by
means of multiconductor strips made of copper or
aluminum, the kVA capacity of the transformer, and its
secondary voltage, remain essentially unchanged.
It is understood that the multiconductor strip
can also be made of a ferrous material.
Referring to figure 5, it is seen that, if
busbars B2 and B4 are combined to form a single busbar,
the latter will no longer carry a large current because
the current in the steel wires is immediately "captured"
by the conductors of the multiconductor strip.
Consequently, the cross section of said single busbar can
be considerably less than that of busbars B2 and B4.
Referring now to superposed busbars B1 and B3,
it is seen that the currents which they carry also flow
in opposite directions. As a result, the strong magnetic
fields produced individually by these busbars essentially
cancel each other when the busbars are in close
proximity.
The overall result in figure 5 reveals that the
magnetic field surrounding the steel mesh is greatly
reduced, as well as that around the busbars. The
resulting magnetic field is therefore much less than
before, which reduces the reactive power absorbed by the
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heating system. The power factor is therefore improved.
Typically, the power factor is 88% for a heating system
such as illustrated in figure 5, compared to 80% for a
conventional steel mesh heating system without a
S juxtaposed multiconductor strip.
In some cases, there is no need to reduce the
magnetic field to an absolute minimum at every point in
the space surrounding the steel mesh. Thus, one may wish
to reduce the magnetic field only in a region that is
three or four meters away from the mesh. Under these
conditions, it is possible to use a lesser number of
conductors in the multiconductor strip, as compared to
the number of longitudinal wires in the steel mesh. For
example, in figure 6, three conductors are evenly spaced
so as to symmetrically straddle the longitudinal wires of
the steel mesh. Each conductor carries one third of the
total current I1 flowing in the mesh. With this
arrangement, the magnetic field can be reduced to an
acceptable level (less than 5 ~T) at a typical distance
of four meters from any point aIong the wire mesh.
The three insulated conductors in figure 6 are
made of copper of appropriate cross section. If the
total cross section of the conductors is equal to that of
all the longitudinal bare wires, the voltage drop in the
insulated conductors will again be typically one tenth of
the voltage drop in the steel mesh. Thus, in figure 6,
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if the voltage drop in the steel mesh between busbars B1
and B2 is, say, 10 volts, that in the conductors between
busbars B2 and B3 is about 1 volt.
Figure 7A shows a heating system in which two
steel meshes (2) are connected in series with two
multiconductor strips (10) by means of three conducting
links (7) whereby the current I2 flowing in said
multiconductor strips is the same as, but 180~ out of
phase with, the total current Il flowing in the steel
meshes. Consequently, the magnetic field originally
produced by said total current is significantly reduced.
It sometimes occurs that the multiconductor
strips lie at some distance (4" to 6") above the steel
meshes. To obtain the desired magnetic field at a given
location above the steel meshes, it may then be necessary
to reduce the total current flowing in the strips,
compared to that flowing in the meshes. Referring to
figure 7B, this can be accomplished by connecting two
identical cables 14, 15 of appropriate resistance, at the
points where the strips and the meshes are connected,
namely at the junction between respective busbars B2, B4,
so as to create an electrical by-pass.
The current I2 flowing in the strips is less
than current Il in the meshes by the amount of current I3
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that is bypassed by the cables. By adjusting the
resistance of the cables, and hence the value of I3, the
resultant magnetic field created by the meshes and strips
can be minimized over a specific region in the space
S surrounding the heating system. The cables 14, 15 are
laid side by side, in close proximity, so that their
magnetic fields cancel each other.
DE8CRIPTION OF METHOD T
Figures 8A and 8B show how an auxiliary
transformer and three multiconductor strips can be used
to reduce the magnetic field surrounding a three-phase
steel mesh heating system (figure 8A) powered by a main
transformer T1.
In figure 8A, the three-phase steel mesh system
is made according to prior art, but three multiconductor
strips, connected to a three-phase auxiliary transformer
TX, (figure 8B) are laid on top of the steel mesh and in
close proximity thereto. The magnitude and phase of the
three currents IXA~ IXB~ IXC delivered by TX are arranged
so as to essentially cancel the magnetic field due to
currents IA~ IB~ Ic flowing in the steel mesh. The three
multiconductor strips constitute a three-phase system (in
figure 8B, to simplify the diagram, they are shown
alongside the steel mesh system instead of in their
actual position on top of and in close proximity
thereto). If the total cross section of the copper
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conductors in each strip is equal to that of the wires in
each steel mesh, the voltage drop in each phase of the
multiconductor strip is again typically one tenth that in
the steel mesh. Consequently, the power of the auxiliary
transformer TX is about one tenth that of the main
transformer Tl. The power factor of the overall system
is again improved because the resultant magnetic field is
much less than before. It should be noted that by
appropriate design, the windings of the auxiliary
transformer could be incorporated into the main
transformer Tl. In such a case, only one 3-phase,
three-winding transformer would be required.
In the event that the currents in the steel
meshes and the multiconductor strips are not essentially
lS 180~ out of phase, it is possible to modify the design of
the auxiliary transformer or the main transformer so that
this objective is achieved. The modification involves
changing the magnitude of the respective leakage
reactances of the transformers, a procedure well known in
the art.
The multiconductor strip may sometimes have be
placed at some considerable distance (four to six inches)
above or below the steel mesh. Under these
circumstances, the respective magnetic fields of the
multiconductor strip and the steel mesh do not cancel
each other as effectively as when the mesh and strip are
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in close proximity. By an appropriate choice of the
voltage ratio of the auxiliary transformer, the current
flowing in the multiconductor strip can then be increased
or decreased, so as to minimize the magnetic flux density
at a given location. Thus, one of the advantages of
Method T is that the current flowing in the
multiconductor strip can be set at a level such that the
magnetic field at a given location in space can be
minimized.
lo Another advantage of Method T is that the
multiconductor strip can be installed without altering an
existing steel mesh heating system. This is particularly
useful when the magnetic field must be reduced in an
already existing steel mesh heating system.
Still another embodiment of Method T permits
reducing the magnetic field over a specific area of the
steel mesh heating system. For example, the short
multiconductor strip (10') of figure 9B is laid like a
mat over only a portion of the steel mesh of figure 9A.
Current Ix is again equal to and 180 out of phase with
current Il. In figures 9A and 9B, it is understood that
the multiconductor strip is actually superposed on any
part of the steel mesh heating system. For example, it
may be superposed in the middle or the far end of the
steel mesh. The magnetic effect will be localized
because the two cables (11) feeding the strip are laid
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side by side, thus annulling their respective magnetic
fields.
Figure lOA shows a steel mesh heating system
similar to that of figure 1 except that leakage currents
I4, Is flow between the two meshes. These currents
represent only a portion of the stream of leakage
currents flowing between the two meshes. In effect,
owing to the changing difference of potential over the
length of the meshes, the leakage currents are largest
between busbars B1, B1 and zero between busbars B2, B2.
As a result of these leakage currents, the
total currents flowing in the meshes decrease
progressively over the length of the meshes. Thus, the
total currents are successively Il, Il-I4, and Il-I4-15, as
shown in figure lOA. Consequently, for similar points in
space along the length of the meshes, the magnetic flux
density varies slightly. In order to adequately minimize
the resultant magnetic field when multiconductor strips
are used, the latter must likewise carry currents that
vary over their length.
This feature can be realized by interconnecting
the strips with suitable resistances R4 as shown in
figure lOB. These resistances are adjusted so that they
respectively carry currents I6, I7 which, in their overall
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effect, produce currents similar to leakage currents I4
and I5, but flowing in the opposite direction. These
resistances could be constituted of lengths of wire of
suitable composition (copper or iron). To ensure an
equal distribution of current among the longitudinal
conductors, busbars B5, B6 must be welded to the
longitudinal conductors. The number of sets of busbars
and resistors depends upon how finely the magnetic field
has to be controlled, but two sets are considered to be
sufficient, as shown in figure lOB.
When retrofitting an existing heating system,
the multiconductor strips are always laid at some
distance (2" to 4") above the existing steel meshes,
because the latter are embedded in concrete. Under these
circumstances, and provided that the resulting magnetic
field conditions are acceptable, the conductors of the
multiconductor strips do not have to be insulated, but
may be bare. The reason is that the concrete slab offers
a sufficiently high resistance to leakage current flow
between the steel meshes and the multiconductor strips so
as to essentially provide an "insulation" between them.
As regards this "insulation", it is useful to
note that the resistivity of concrete lies usually
between 50 n.m and 1000 n.m, while that of copper is
typically 20 x 10-9 n.m and that of iron is
about 300 x 10-9 n.m. Thus, in this context, as compared
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to copper and iron, concrete can be considered to be an
"insulator".
The fact that bare conductors can sometimes be
used when retrofitting is required, constitutes another
potentially useful feature of the invention.
It is understood that the steel mesh can have
an insulating covering instead of being bare. The
insulation can be as thin as considerations of mechanical
abuse will allow, because the voltages involved are under
30 volts. An insulated steel mesh has the advantage of
being easier to install because it eliminates the problem
of contact with other conducting bodies such as steel
structures, water pipes, electrical conduits, etc.
Furthermore, an insulated mesh prevents leakage currents
from flowing in the concrete.
DESCRIPTION OF NETHOD U
Method U reduces the magnetic field surrounding
a steel mesh by means of electromagnetic induction.
Referring to figure 11, two steel meshes (2) are
connected in series by means of link (7) to the terminals
of a step-down transformer. The arrangement is identical
to that shown in figure 1. Two multiconductor strips
(10), superposed on the two meshes, are connected in
series by virtue of electrical link (12). Busbars B3 are
not connected. The alternating magnetic field created by
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.,_
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current Il links with the multiconductor strips and
thereby, by Faraday's law, induces a voltage in them.
The total induced voltage E2 appears between busbars B3.
If these busbars are connected by an electrical
link 13, as shown in figure 12, the induced voltage
causes a current I2 to flow in the multiconductor strips.
According to Lenz's law of electromagnetic induction,
this induced current I2 tends to oppose the magnetic
field that produces said induced current. Thus, by
connecting the multiconductor strips this way, the
resulting magnetic field surrounding the steel mesh will
be less than before. Consequently, the power factor will
improve, and the magnetic field will be reduced
everywhere in the space surrounding the steel mesh
heating system.
The two multiconductor strips in figure 12 are
effectively short-circuited together. The closer the
strips are to the meshes, the better is the
short-circuiting action, and the lower is the resulting
magnetic field. In practice, the total current I2 in
figure 12 is less than Il and the phase angle between the
two currents is less than 180~. Nevertheless, the
magnetic field is significantly reduced and therefore the
power factor is raised.
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""~1.'~ ~
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In a three-phase heating system (figure 13),
the three steel meshes are covered by three
multiconductor strips that are short-circuited together
by means of links (12) and (13). Referring to phase A in
this figure, current IXA is inevitably less than IA and
the two currents are less than 180 out of phase.
Similar remarks apply to phases B and C, respectively.
The result is similar to that of figures 8A and 8B,
except that no auxiliary transformer is required. As
well, the resultant magnetic field in figure 13 is not
reduced to the same extent as is possible with the use of
an auxiliary transformer, as in figures 8A and 8B.
EXANPLES
The following examples illustrate the
application of the present invention to a few practical
situations. It is revealed that the power factor
increases and the magnetic field is greatly reduced by
using the invention herein described.
EXAMPLE 1
A steel mesh heating system was laid out
according to figure 14. The steel meshes were each 6
feet wide, 48 feet long with a 4"x 4" lattice and 19
longitudinal wires. The longitudinal and transverse
wires were made of a ferromagnetic material having a
diameter of 7/32 ". The two meshes were spaced 6 inches
apart.
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The transformer was excited from a 600 V,
60 Hz, single-phase source and the following readings
were taken:
voltage between busbars B1 = 17 V;
current Il = 803 A;
active power = 11.1 kW;
apparent power = 13.7 kVA;
reactive power = 8 kvar;
power factor = 81%;
impedance between busbars Bl = 21.17 mn.
Table 1 shows the magnetic flux density at 9 points
at a level 3 feet above the steel mesh.
TABLB 1
MAGNETIC FLUX DENSITY AT A LEVEL 3 FEET ABOVE THE
STEEL MESH
Position on meshCorresponding flux density
(~T)
1 2 3 203 157 84
4 5 6 220 203 119
7 8 9 233 163 80
average flux density = 162 ~T
EXAMPLB 2
The two steel meshes were then superposed (figure
15) with a 2.5" vertical space between them. The
following readings were taken.
voltage between busbars Bl = 15.8 V;
current Il = 782 A;-
2 ~ g
~bl~ 32
active power = 10.9 kW;
apparent power = 12.4 kVA;
reactive power = 5.9 kvar;
power factor = 88~;
impedance between busbars B1 = 20.20 mn.
Table 2 shows the magnetic flux density at 9 points
at a level 3 feet above the steel mesh.
TABLE 2
MAGNETIC FLUX DENSITY AT A LEVEL 3 FEET ABOVE THE
STEEL MESH
Position on mesh Corresponding flux density
(~T)
1 2 3 39 27 12
4 5 6 23 26 22
7 8 9 12 22 13
average flux density = 22 ~T
These observations show that the power factor
increased from 81% (steel meshes side by side) to 88%
(meshes superposed).
The observations also show the corresponding
decrease in the magnetic flux density. Thus, at a height
of 3 feet from the ground, the flux density dropped from
an average of 162 ~T to an average of 22 ~T.
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."
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EXANPLE 3
A multiconductor strip composed of 6 insulated
copper cables size 1/0 was superposed on the steel mesh,
with a 1/2" vertical space between them (figure 16). The
following readings were taken.
voltage between busbars Bl, B3 = 9.37 V;
current Il = 8 4 8 A;
active power = 7 kW;
apparent power = 8 kVA;
reactive power = 3.9 kvar;
power factor = 88%;
impedance between busbars Bl, B3 = 11. 05 mn.
Table 3 shows the magnetic flux density at 9 points
at a level 3 feet above the steel mesh.
TABLE 3
MAGNETIC FLUX DENSITY AT A LEVEL 3 FEET ABOVE THE
STEEL MESH
Position on meshCorresponding flux density
(~T)
1 2 3 39 18 10.4
4 5 6 14.5 15.5 13.2
7 8 9 8.8 8.2 9.2
average flux density = 15.2 ~T
Although the invention has been described
above with respect with various specific forms, it will
be evident to a person skilled in the art that it may be
modified and refined in various ways. It is therefore
wished to have it understood that the present invention
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"
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should not be limited in scope, except by the terms of
the following claims.
