Note: Descriptions are shown in the official language in which they were submitted.
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~ ME-179
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AIR GAP ~ABI~IZATION I~ ~OL~GE gTA~OR
9AC~RO~D OF ~XE INVEN~IO~
The present invention relates to electric watthour
meters and, more particularly, to voltage stators for
watthour meters.
A conventional watthour meter employs a metallic
disk supported for rotation by the interaction of eddy
`; currents with magnetic fluxes produced by current and
voltage stators disposed adjacent the disk. The
rotation of the disk is retarded by a permanent magnet
; whose retarding torque is proportional to disk
rotational speed. The torque producing rotation is
proportional to the product of a load current times a
voltage; that is, the torque is proportional to the
power consumed by the load. The interaction of driving
and retarding tor~ues results in the disk rotational
speed also being proportional to the power consumed by
the load. Each rotation of the disk represents a
predetermined quantum of energy consumption. The turns
of the disk are accumulated to sum up the consumed
energy in units such as, for example, watthours or
kilowatt hours.
A conventional voltage stator, sometimes called a
pot stator, employs a core built up of a stack of
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mutually-insulated, generally E-shaped, lamellae of
magnetic material such as, for example steel. A coil
of many turns o~ fine wire is disposed on the center
- leg o~ the core. Line voltage is applied to the coil
to produce a magnetic ~lux in the core proportional
to the line voltage. A magnetic flux appears in the
two air gaps between the outside legs and the central
leg. This magnetic flux interacts with the disk to
induce eddy currents therein which, in turn, interact
with a magnetic flux produced by the current stator.
A watthour meter is a precision measurement
instrument re~uired to operate with high accuracy
over a large dynamic range of load power consumption.
For example, an electric meter may be required to
measure power consumption with an accuracy of batter
then one percent over a range of power consumption
Prom about 10 percent to about 600 percent of test
current with normal load voltage applied. Thus,
accuracy must be maintained over a power ratio of
about 60 or more.
Electric watthour meters provide for several
adjustments to compensate for inevitable errors
arising during manufacture. One adjustment
compensates for errors in torque produced at a light
load of 10 percent of test current and a power factor
of l.O. Such errors may arise from core nonlinearly;
that is, at 10 percent of test amperes, the iron in
the core may exhibit other than 10 percent of the
flux produced when it receives test amperes. Other
possible causes of light-load errors may arise due to
friction or to mechanical or magnetic dissymmetries.
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A light load adjustment conventionally consists
of a plate of conducting material, positioned in the
space betwe~n the poles of the core and the disk, and
means for adjusting its position in the space at
right angles to the disk radius.
The effect of the light load adjustment is
critically d~pendent on the magnitude of the main air
gap flux (between the center leg of the core) and the
disk. This magnitude, is, in turn, critically
dependent on the magnitudes of the magnetic fluxes in
the two auxiliary air gaps tbetween the two outside
legs and the center leg of the core~
Theoretically, at least, the causes of light
load error are constant for a given watthour meter.
~hus, in theory, once the light load adjustment is
properly completed, the compensatîon should be
permanent without requiring further adjustment. In
practice, however, changes in light load adjustmPnt,
once considered to be mysterious, do occur.
We have traced at least part of the change in
light load adjustment to disturbance in the positions
or spacing of the auxiliary air gaps. We observed
that a watthour meter, properly adjusted for light
load, frequently goes out of adjustment when it is
subjected to shock or high levels of vibration.
OBJEC~ ~ND B~MMaRY O~ ~HE INVB~ION
Accordingly, it is an object of the invention to
provide means for reducing a change in light load
adjustment.
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It is a further object of the invention to
provide means for stabilizing the auxiliary air gaps
in the voltage stator of an electric watthour meter.
It is a further object of the invention to
provide means for stabilizing the auxiliary air gaps
in the voltage stator of an electric watthour meter
thereby permitting a predetermined bias of an
intrinsic light load error.
Briefly stated, the present invention provides
non-magnetic metallic spacers welded into the
auxiliary air gaps of a voltage stator using welding
passes arranged in a predetermined sequence. The
presence of the spacers stabilizes the auxiliary air
gap against change in light load adjustment.
Sequences of welding passes are disclosed effective
for holding inherent light load error at nominal
values, or biasing it positively or negatively.
According to an embodiment of the invention,
there is provided a voltage stator comprisingo an
E-shaped core, a voltage stator coil on the E-shaped
core, first and second auxiliary air gaps in the
E-shaped core, a first non-magnetic metallic spacer
in the first auxiliary air gap, a second non-magnetic
metallic spacer in the second auxiliary air gap, and
means of securing the first and second non-magnetic
metallic spacers in their respective auxiliary air
gaps.
Accordiny to a feature of the invention, there
is provided a method for ætabilizing first and second
auxiliary air gaps in a core of a voltage stator,
comprising: forming first and second facing grooves
in facing surfaces of the first auxiliary air gap,
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ins~rting a first non-magnetic metallic spacer in the
first and second facing grooves, the first
non-metallic spacer forming first and second
interface lines with contiguous portions of the
voltage stator core, forming third and fourth facing
grooves in facing surfaces of the second auxiliary
air gap, inserting a second non-magnetic metallic
spacer in the third and fourth facing grooves, the
second non-metallic spacer forming third and fourth
interface lines with contiguous portions of the
sec~nd auxiliary air yap, forming a first weld in a
first direction at a first one of the first, second,
third and fourth interface lines, forming a second
weld in a second direction at a second one of the
first, second, third and fourth interface lines,
forming a third weld in a third direction at a third
one of the first, second, third and fourth interface
lines, forming a fourth weld in a fourth direction at
fourth one of the first, second, third and fourth
interface lines, and selecting a s~quence of the
first one, second one, third one and fourth one
effective for controlling an inherent light load
error in the voltage stator.
According to a further feature of the invention,
there is provided a method for stabilizing first and
second auxiliary air gaps in a voltage stator
comprising: affixing a first non-magnetic metallic
spacer in the first auxiliary air gap using first and
second welds, affixing a second non-magnetic metallic
spacer in the second auxiliary air gap using third
and fourth welds, and selecting a sequence of the
first, second, third and fourth welds giving a
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sel ctable one of a positive, negative and zero
inherent light load error.
The above, and other objects, features and
advantages of the present invention will ~ecome
apparent from the following descxiption read in
conjunction with the accompanying drawings, in which
like reference numerals designate the same elements.
BRIB~ D~SCaIP~ION OF ~ DRa~I~G~
Fig. 1 is side view of a voltage stator
according to the prior art.
Fig. 2 is a side view of a voltage stator
according to an embodi~ent of the invention.
Fig. 3 is a closeup view o~ a portion o~ the
voltage stator of Fig. 2 showing auxiliary air gaps
thereof.
Fig. 4 is a view corresponding to Fig. 3 rotated
90 degrees to show a further view of the auxiliary
air gaps.
Pigs. 5A-5D show the effect of welding pass
sequences on inherent light load error~
DETAILED DE8C~IPTION OF T~B P~EFERRED EMBODIMENT
Referring now to Fig. 1, there is shown,
generally at lO, a voltage stator according to the
prior art. For purposes of clarity of illustration
and description, voltage stator 10 is shown in a
state of partial assembly from which several elements
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found on a fully-assamblad voltage stator are
omitted. Such omitted elements may include, for
example, apparatus for light load adjustment, full
load adjustment, lag adjustment and elements for
voltage, overload and temperature compensation. Such
elements being conventional, their omission herefrom
will not interfere with an understanding of the
present invention by one skilled in the art.
Voltage stator 10 includes an E-shaped core 12
formed from a stack of thin lamellae (only the end
one of which is shown3 mutually insulated from each
other. E-shaped core 12 includes a center leg 14
joined to first and second side legs 16 and 18 by an
end bar 20. A voltage stator coil 22, containing a
large number of turns of fine wire, is disposed on
center leg 14. Magnetic circuit closure bars 24 and
26 extend inward from side legs 16 and 18,
respectively, toward center leg 14 to form auxiliarv
air gaps 28 and 30, respectively, with facing sides
of center leg 14.
The stability of auxiliary air gaps 28 and 30 is
critical to light load adjustment. Indeed, we have
found that mechanical shock to which electric
watthour meters are subjected prior to installation
is sometimes sufficient to produce a change in
auxiliary air gap 28 and/or 30 large enough to
degrade the light-load adjustment out of an
acceptable range.
Referring now to Fig. 2, there is shown,
generally at 32, a voltage stator according to an
embodiment of the invention. In most respects,
voltage stator ~2 is identical to voltage stator 10
of Fig. 1. For convenience in relating the elements
~il
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of voltage stator 32 (Fiy. 2) to those in voltage
stator 10 (Fig. 1), the same reference designators
are employed for unchanged elements and the samP
reference designators primed are employed for
corresponding elements which are modified in the
present invention.
In particular, auxiliary air gap~ 28' and 30'
are modified by the presence of non-magnetic spacers
34 and 36 therein. In the preferred embodiment,
non-magnetic spacers 34 and 38 are hollow cylinders
of a non-magnetic metal ~uch as, for example, a
~tainless steel such as L-605 from Atek Metal Center,
Inc. or any AISA Type 310 stainless st~el. Such
materials may be chosen to produce a flux in
auxiliary air gaps 28' and 30' corresponding
generally to that which would occur in the presence
of air.
A semi-cylindrical groove 38 in an end of
magnetic circuit closure bar 24' and a facing
semi-cylindrical groove 40 in center leg 14'
accommodate non-magnetic spacer 34. Similarly,
Pacing semi-cylindrical grooves 42 and 44 in center
leg 14' and magnetic circuit closure bar 26',
respectively accommodate non-magnetic spacer 36. A
pair of welds 46 secure each of non-magnetic spacers
34 and 36 in their respective auxiliary air gaps.
The light load error contributed hy a ~oltage
stator before setting its light load adjustment (no-t
shown) is herein called the inherent light load error
of voltage stator 32. An inherent light load error
may be positive, in which light load errors add to
disk speed, negative, in which they subtract from
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disk speed, or zero, in which they have no influence
on disk speed.
We have discovered that the manner of making
welds ~ and, in particular, the sequence in which
such welds are produced can have a substantial
influence on the magnitude and direct.ion of inherent
light load error in final voltage stator 32. For
purposes of subsequent description, the two welds 46
securing non-magnetic spacer 34 are identified with
the letters A and B, and those securing non-magnetic
spacer 36 are identified with the l~tters C and D.
Referring now to Figs. 3 and 4 showing closeups
of the area of voltage stator 32 containing welds 46,
it will be seen that each pair of welds 46 securing a
non-magnetic ~pacer are parallel and non-touching.
Each weld 46 is made in a single pass~ preferably
employing an industrial laser, industrial
electron-beam, or other suitable heating device. In
addition, a metal, inert-gas (MIG) welding technique
may be employed. Electron-beam apparatus suffers
from shielding and throughput problems. Metal,
inert-gas welding techniques conventionally require
.~ post-welding cleaning of the weld to remove
contaminant artifacts. We have discovered that an
industrial laser such as, for example, an S-48 laser
commercially available from Coherent ~eneral, avoids
the drawbacks of competing devices and produces
satisfactory welds 46. The S-48 laser is a carbon
dioxide laser having an output power adjustable from
about 200 to about 800 watts and an enhanced pulse
mode of about 2000 watts produces satisfactory welds.
In all of the abova types of welding done in the
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atmosphere, a shielding gas at the point of welding
is desirable to prevent oxidation o~ the weld. One
satisfactory shielding gas is argon.
Using an industrial laser, the laser beam is
aimed at the interface between the magnetic circuit
closure bars 24 and the non-magnetic spacers 34,
whereby the line of material heating is about equally
spread between the two materials.
Although other techniques may be used to control
the path of the welding apparatus for achieving a
desired sequence for forming welds 46, we have
discovered that a conventional X-Y positioning table
(not shown) is convenient. Voltage stator 32 is
mounted on the positioning table which i~ then driven
along X and Y axes normal to the beam direction under
manual or computer control to attain the desired
speeds and weld sequences. The penetration of welds
46 into the material is controlled laryely by the
power of the laser beam. We have found that, for the
specified material~, a laser power output of from
about 400 to about 600 watts produces suitable welds
with outputs of from about 450 to about 550 watts
being preferred. At these power ranges, a transport
speed on each welding pass of from about 25 to about
60 inchas per second produces satisfactory results
with a transport speed of from about 27 to about 40
inches per secGnd being preferred. Although some
interaction between laser power and transport speed
may be demonstrated, the minimum useful laser power
is that below which satisfactory material penetration
is not attained. The maximum laser power is that
above which excessive loss of magnetic material
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substantially changes auxiliary air gaps 28' and 30'.
Other combinations of laser power and workpiece
transport speed may be selected giving corresponding
results without departing from the spirit and scope
of the invention. Different types of material in
magnetic circuit disclosures 24' and 26, as well as
in non-magnetic ~pacers 34 and 36 may require changes
in one or both of laser power and workpiece transport
speed. One skilled in the art with the present
disclosure for reference would be fully enabled to
select such changed parameters without requiring
experimentation.
Referring now to Figs. 5A-5D, four acceptable
weld sequences are shown, together with their effect
on inherent light load error. The path during which
; welding i5 performed is shown in solid line and the
non-welding return paths are shown in dashed line.
The sequence in Fig. 5A, for example, employs
rsversing directions passing from welds D, C, B, and
A. The resultant effect is a positive inherent light
load error. In contrast, the pattern in Fig. 5B
employs welding passes all in the same direction,
using the se~uence At B, D, and C to produce zero
inherent light load error. The welding sequences in
Figs. SC and 5D produce inherent light load errors in
the negative direction and zero respectively. The
abo~e sequences are illustrative only and are not
intended to be exhaustive. Other sequences with
predictable effects on inherent light load error may
be discovered and all such sequences should be
considered part o~ the present invention.
The ability to bias inherent light load error
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may be useful in the design of a light load
adjustment. For example, it may be useful to know
beforehand the amount and direction of inherent light
load error expected in a voltage stator 32 since this
is the direction and an indication of the amount of
subsequent adjustment which will be re~lired to
overcome it. If a predictable negative bias in
inherent light load error can be produced by
performing the welding sequence in Fig. 5B, an
apparatus for light load adjustment may be required
only to increase disk speed at light load and not be
expected to retard such disk speed.
Although patentability is not conditioned on a
particular theory to explain the effect we disclose
herein, and we do not intend to be bound to any
particular theory to explain the effect of welding
sequences on inherent light load adjustment, we
believe that the expansion and contraction of the
weld nugget in welds 46, and/or of non-magnetic
spacers 34 and 36, as they are heated and cooled
during the welding process, may have an effect on the
dimensions and other parameters of auxiliary air gaps
28' and 30' (Figs. 2 and 3).
Having described preferred embodiments of the
invention 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 may be effected
therein by one skilled in the art without departing
from the scope or spirit of the invention as defined
in the appended claims.
