U.S. patent application number 11/519573 was filed with the patent office on 2007-01-25 for temperature detection method and apparatus.
Invention is credited to Jeffrey A. Hudson.
Application Number | 20070019706 11/519573 |
Document ID | / |
Family ID | 35060501 |
Filed Date | 2007-01-25 |
United States Patent
Application |
20070019706 |
Kind Code |
A1 |
Hudson; Jeffrey A. |
January 25, 2007 |
Temperature detection method and apparatus
Abstract
A resistance temperature detector device and system are provided
for measuring the temperature adjacent to windings within a stator
assembly. The resistance temperature detector comprises a resistive
element enclosed within a plurality of insulative layers, including
insulation panels and adhesive. These surrounding insulation layers
are sufficient to prevent partial discharge within any air gaps or
low dielectric materials adjacent to the resistive element when the
resistance temperature detector is disposed within a motor. A
method for detecting temperatures between windings of an electrical
machine is also provided.
Inventors: |
Hudson; Jeffrey A.; (Greer,
SC) |
Correspondence
Address: |
ROCKWELL AUTOMATION, INC./(FY)
ATTENTION: SUSAN M. DONAHUE
1201 SOUTH SECOND STREET
MILWAUKEE
WI
53204
US
|
Family ID: |
35060501 |
Appl. No.: |
11/519573 |
Filed: |
September 12, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10823178 |
Apr 13, 2004 |
7111983 |
|
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11519573 |
Sep 12, 2006 |
|
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Current U.S.
Class: |
374/152 ;
374/E1.011; 374/E1.018 |
Current CPC
Class: |
G01K 1/08 20130101; G01K
1/14 20130101 |
Class at
Publication: |
374/152 |
International
Class: |
G01K 1/08 20060101
G01K001/08 |
Claims
1. A resistance temperature detector system for detecting
temperatures between windings of an electrical machine, the system
comprising: a winding configured to receive an alternating current
voltage waveform during operation, the winding having a winding
insulating system disposed about a central conductor; and a
resistive temperature detector disposed adjacent to the winding for
detecting a temperature of the winding during operation, the
detector comprising a resistive element configured to receive a
measurement signal and to produce an output signal that is a
function of temperature, and a detector insulating system disposed
about and completely encasing the resistive element; wherein the
combination of the winding insulating system and the detector
insulating system have a capacitance sufficient that a voltage
stress level experienced by any air voids or low dielectric
materials adjacent to the resistive element resulting from voltage
applied to the winding during operation is below a stress level
that would cause partial discharge in such voids and materials.
2. A resistance temperature detector system for detecting
temperatures between windings of an electrical machine, the system
comprising: a stator having a plurality of winding slots; a
plurality of windings disposed in the winding slots and configured
to receive alternating current voltage waveforms during operation,
each winding having a winding insulating system disposed about a
central conductor; and a resistive temperature detector disposed
between adjacent windings in at least one of the slots for
detecting a temperature of the adjacent windings during operation,
the detector comprising a resistive element configured to receive a
measurement signal and to produce an output signal that is a
function of temperature, and a detector insulating system disposed
about and completely encasing the resistive element; wherein the
combination of the winding insulating system and the detector
insulating system have a capacitance sufficient that a voltage
stress level experienced by any air voids or low dielectric
materials adjacent to the resistive element resulting from voltage
applied to the winding during operation is below a stress level
that would cause partial discharge in such voids and materials.
3. A method for detecting temperatures between windings of an
electrical machine, the method comprising: providing a resistive
element configured to receive a measurement signal and to produce
an output signal that is a function of temperature; disposing the
resistive element within a detector insulating system to form a
detector, the detector insulating system having a desired
capacitance per unit area of sufficient magnitude that a voltage
stress level experienced by any air voids or low dielectric
materials adjacent to the resistive element resulting from voltage
applied to the windings during operation is below a stress level
that would cause partial discharge in such voids and materials.
4. The method of claim 3, further comprising coupling the resistive
element to connection plates for supply of the measurement signal
and for detection of the output signal.
5. The method of claim 4, further comprising coupling the
connection plates to a set of lead wires including a compensation
lead wire.
6. The method of claim 3, wherein disposing the resistive element
within the detector insulating system includes joining a plurality
of flexible insulative layers about the resistive element.
7. The method of claim 6, wherein the flexible insulative layers
comprise a polyimide, polyester, polyamide-imide,
polyetheretherketone, polysulfone or polyphenylene sulfide.
8. The method of claim 6, wherein the flexible insulative layers
are joined by an adhesive.
9. The method of claim 8, wherein the adhesive and the flexible
insulative layers are selected to reduce the voltage stress across
and incidence of partial discharge within any air gaps or other low
dielectric materials disposed adjacent to the resistive
element.
10. The method of claim 8, wherein the flexible insulative layers
and the adhesive have dielectric constants between approximately 3
and 6.
11. The method of claim 3, further comprising disposing the
detector between adjacent windings of an electrical machine
stator.
12. A device comprising: a sensor configured to determine a
parameter of an electromechanical system; and an insulating system
disposed about the sensor, the insulating system comprising one or
more layers of a flexible insulating material and one or more
layers of an adhesive, wherein the sensor is disposed within the
one or more layers of adhesive, and the adhesive has a dielectric
constant substantially equal to or greater than that of the
flexible insulating material.
13. The device of claim 12, wherein the layers of adhesive and
flexible insulating material each have dielectric constants between
approximately 3 and 6.
14. The device of claim 12, wherein the flexible insulating
material comprises polyimide.
15. The device of claim 14, wherein the adhesive comprises an
acrylic adhesive.
16. The device of claim 12, wherein the flexible insulating
material comprises polyester, polyamide-imide,
polyetheretherketone, polysulfone or polyphenylene sulfide.
17. The device of claim 12, wherein the adhesive is selected from
the group consisting of epoxy, silicone, polyester, and
polyurethane adhesive systems.
18. The device of claim 12, wherein the parameter comprises
temperature.
19. The device of claim 18, wherein the sensor comprises a
resistive element configured to receive an input signal via a lead
and to produce an output signal that is a function of
temperature.
20. The device of claim 12, wherein electromechanical system is an
induction motor, and the device is configured to be embedded within
the induction motor adjacent to a winding of the induction motor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/823,178, which was filed on Apr. 13, 2004.
BACKGROUND
[0002] The present technique relates generally to the field of
electric motors and to methods and apparatus for detecting
temperatures within an electric motor. More particularly, the
technique relates to a novel approach to detecting the temperature
adjacent to a winding within an electric motor.
[0003] Electric motors, generators, and other similar devices, are
produced in a variety of mechanical and electrical configurations.
The configuration of these devices may depend upon the intended
application, the operating environment, the available power source,
or other similar factors. In general, these devices include a rotor
surrounded at least partially by a stator. For instance, one common
design of electrical motor is the induction motor, which is used in
numerous and diverse applications. Induction motors typically
employ a stator assembly including a slotted core in which groups
of coil windings are installed. By providing alternating current
power to certain windings at certain times, a dynamic magnetic
field is produced that causes the rotor to rotate within the
stator. The rotational speed of the rotor is a function of the
frequency of the alternating current power input and of the motor
design (i.e. the number of poles defined by the windings). This
rotation may be used to transmit a mechanical force to a driven
load via an output shaft coupled between the rotor and the driven
load.
[0004] Inverter drives are commonly used with such motors to vary
the frequency of the alternating current power driving the motor.
This, accordingly, allows the rotational speed of the rotor to be
varied as well. An inverter drive is configured to receive a direct
current input and to output a variable frequency waveform that
simulates alternating current power. A rectifying circuit may be
used with the inverter drive if alternating current power is being
provided to the inverter drive. In such a case, the rectifier
converts the incoming alternating current power to direct current
power for input into the inverter circuitry.
[0005] Electric motors and other similar devices are generally
configured to operate in a given temperature range. Heat is
generated within the motor from the passage of electrical current
through the coil windings, or from a variety of other sources.
Often, one or more resistance temperature detectors will be
disposed within one or more slots of a stator core, adjacent to the
coil windings. In this manner, the temperature of the coil windings
can be measured to determine if the temperature is within desired
operating parameters. However, these resistance temperature
detectors must remain operable in order to provide this
benefit.
[0006] In general, a resistance temperature detector includes a
resistance coil encased in an insulative cover. Current is applied
to leads of the coil that results in a measurable voltage drop
through the coil as a function of the resistance of the coil.
Because the resistance varies with the temperature of the coil, by
measuring the voltage drop, compensating for certain errors, and
correlating the voltage drop to the known characteristics of the
coil, the temperature at the detector location can be calculated.
In motor applications this temperature signal may then be used for
various monitoring, control, preventative maintenance, and other
functions.
[0007] The environments in which conventional resistance
temperature detectors are placed are often severe, particularly in
high power motors. The detectors are subjected to elevated
temperature levels and, at times to significant voltage
differentials, particularly in inverter driven applications. The
corresponding voltage stress and electric field within these
environments may lead to premature failure of resistance
temperature detectors. Particularly, the electric field within the
motor may electrically break down small air gaps or other materials
or zones of reduced dielectric constant within a resistance
temperature detector, causing partial discharge and localized
heating within such zones. If any air voids or low dielectric
materials are disposed adjacent to the resistive element of the
detector, this localized heating may cause the detector to report
inaccurate temperatures or may cause the resistive element to
deteriorate, resulting in the failure of the detector. Moreover,
winding insulation systems are being further reduced in thickness,
leading to increasing voltage stresses, particularly across
temperature detector insulating layers. Such elevated stresses can
similarly lead to failure of the detectors.
[0008] There is, therefore, a need for a resistance temperature
detector that can better withstand the harsh electrical environment
present in many applications, and particularly within a slot of a
stator core. Such a device would allow for the monitoring of
temperatures of windings of electric motors or similar devices, and
thereby prevent damage and downtime, and improve reliability of the
entire motor system.
BRIEF DESCRIPTION
[0009] The present technique provides a novel technique for
measuring the temperature within an electrical device. The
technique is applicable to a wide range of equipment, but is
particularly well suited to measuring the temperature within the
stator assembly of an electric motor.
[0010] In accordance with a first aspect of the technique, a
resistance temperature detector suitable for detecting temperatures
between windings of an electrical machine is provided. This
detector includes a resistive element and a detector insulating
system disposed about and completely encasing the resistive
element. The resistive element is configured to receive an input
signal and to produce an output signal that is a function of
temperature. The provided detector insulating system disposed about
the resistive element may have a capacitance per unit area
approximately equal to or greater than the capacitance per unit
area of the winding insulating system.
[0011] In particular, a resistance temperature detector is provided
wherein a detector insulating system disposed about a resistive
element may have a capacitance per unit area of sufficient
magnitude that a voltage stress level experienced by any air voids
or low dielectric materials adjacent to the resistive element
resulting from voltage applied to the windings during operation is
below a stress level that would cause partial discharge in such
voids and materials.
[0012] In accordance with another aspect of the technique, a
resistance temperature detector system for detecting temperatures
between windings of an electrical machine is provided. The system
comprises a winding configured to receive an alternating current
voltage waveform during operation. The winding has a winding
insulating system disposed about a central conductor. A resistance
temperature detector, comprising a resistive element disposed
within an insulation system, is disposed adjacent to the winding
for detecting a temperature of the winding during operation. The
combination of the winding insulating system and the detector
insulating system have a capacitance sufficient that a voltage
stress level experienced by any air voids or low dielectric
materials adjacent to the resistive element resulting from voltage
applied to the winding during operation is below a stress level
that would cause partial discharge in such voids and materials.
[0013] The technique also offers a method for detecting
temperatures between windings of an electrical machine. The method
comprises steps for providing a resistive element configured to
receive a measurement signal and to produce an output signal that
is a function of temperature. The resistive element is disposed
within a detector insulating system, the detector insulating system
having a desired capacitance per unit area of sufficient magnitude
that a voltage stress level experienced by any air voids or low
dielectric materials adjacent to the resistive element resulting
from voltage applied to the windings during operation is below a
stress level that would cause partial discharge in such voids and
materials.
DRAWINGS
[0014] The foregoing and other advantages and features of the
invention will become apparent upon reading the following detailed
description and upon reference to the drawings in which:
[0015] FIG. 1 is a perspective view of an exemplary electric motor
illustrating the various functional components of the motor in
accordance with certain aspects of the invention;
[0016] FIG. 2 is a front view of the frame and stator assembly of
the electric motor of FIG. 1;
[0017] FIG. 3 is a cross-sectional view of an individual slot in
the stator assembly of the exemplary electric motor with components
disposed therein;
[0018] FIG. 4 is an illustration of the various components of an
exemplary resistance temperature detector in accordance with
certain aspects of the invention;
[0019] FIG. 5 is a cross-sectional view of the assembled exemplary
resistance temperature detector of FIG. 4;
[0020] FIG. 6 is a model of an equivalent electrical circuit
representative of the capacitive characteristics of exemplary
layers between a winding and an element of a resistance temperature
detector disposed in a slot of a stator core in accordance with
certain aspects of the invention;
[0021] FIG. 7 is a model of an equivalent electrical circuit
representative of the reactive characteristics associated with the
capacitance of exemplary layers between a winding and an element of
a resistance temperature detector disposed in a slot of a stator
core in accordance with certain aspects of the invention; and
[0022] FIG. 8 is a graphical representation of Paschen's Law as
applicable to air, which may be used as a basis for anticipating a
point of partial discharge breakdown for the design of the present
detector system.
DETAILED DESCRIPTION
[0023] Turning now to the drawings, and referring first to FIG. 1,
an electric motor is shown and designated generally by the
reference numeral 10. In the embodiment illustrated in FIG. 1,
motor 10 is an induction motor housed in a conventional enclosure.
Accordingly, motor 10 includes a frame 12 open at front and rear
ends and capped by a front end cap 14 and a rear end cap 16. Frame
12, front end cap 14, and rear end cap 16 form a protective shell,
or housing, for a stator and a rotor. Stator windings are
electrically interconnected to form groups, and the groups are, in
turn, interconnected in a manner generally known in the art. The
windings are further coupled to terminal leads (not shown). The
terminal leads are used to electrically connect the stator windings
to an external power cable (not shown) coupled to a source of
electrical power. Energizing the stator windings produces a
magnetic field that induces rotation of the rotor and a rotary
shaft 18. The electrical connections between the terminal leads and
the power cable are housed within a conduit box 20.
[0024] In the embodiment illustrated, the rotor may include a cast
rotor supported on a rotary shaft 18 within the frame 12, front end
cap 14 and rear end cap 16. As will be appreciated by those skilled
in the art, the shaft 18 is configured for coupling to a driven
machine element (not shown), for transmitting torque to the machine
element. The rotor and the shaft 18 may be supported for rotation
within frame 12 by a front bearing set and a rear bearing set
carried by the front end cap 14 and the rear end cap 16,
respectively. The motor 10 may include a cooling fan within the
frame 12 or end caps 14 and 16 to promote convective heat transfer
through the frame 12. The frame 12 generally includes features
permitting it to be mounted in a desired application, such as
mounting bracket or integral mounting feet.
[0025] As will be appreciated by those skilled in the art, while
reference is made herein to a specific motor design, many different
types of machines and motors may employ detectors and temperature
detection systems based upon the present techniques. For example,
many different motor frame sizes, ratings and styles may benefit
from the improved temperature detector designs. Similarly, motors
operating on different electrical principles may receive the
detectors, including induction motors, synchronous motors, and so
forth. Also, application of the detectors is not limited to single
phase or three phase motors, but either type may require detection
of temperature of windings and, as such, may use detectors
constructed in accordance with the principles set forth herein.
[0026] Referring generally to FIG. 2, a cross-sectional view of a
partial motor is shown and designated generally by the reference
numeral 22. As depicted in FIG. 2, the partial motor 22 includes a
stator assembly 24. The stator assembly 24 may include a group of
laminations 26 which are stacked adjacent to each other, as shown,
or a group of segments that extend the entire length of the stator
core, or any other suitable configuration. The laminations 26 are
generally made of a ferromagnetic material, such as steel or
another iron alloy. Each of the laminations 26 has slots 28 that
align with the same features of the other laminations 26 to form
continuous slots through the entire length of the stator assembly
24. With regard to the slots 28, the general shape of the slots 28
may vary depending on the specific features and design
requirements. For instance, the slots 28 may include rectangular
shapes (e.g. with a notched tooth for retaining a top wedge) or may
be halved elliptical shapes, halved polygon shapes, or a generally
elliptical shape with a portion of one end removed for insertion of
the windings into the slots during manufacture of the stator.
[0027] Where the stator assembly 24 is made up of laminations 26,
the laminations 26 may be kept in a compressed bundle under
pressure to maintain a specific form and to retain the alignment of
the slots 28. To maintain this lamination pressure, end plates 30
may be formed to the same general dimensions as the frame 12 (see
FIG. 1). The end plates 30 are used to hold the laminations 26 of
the stator assembly 24 in a generally fixed position. The end
plates 30 are able to maintain pressure on the laminations 26 by
being secured on opposite sides of the laminations 26. As will be
appreciated by those skilled in the art, the particular
construction of the stator assembly 24 described and shown herein
is intended as an example only. Many other designs, laminated and
otherwise, may be envisaged and may benefit from use of the
detectors described below.
[0028] Slot liners 42 (FIG. 3) may be disposed within slots 28. As
a means of supporting a portion of slot liners 42 that extend
beyond the slots 28, an insulating end piece or insulating end
lamination 32 may be positioned at each end of the laminations 26.
While such structures may not be common in some machine designs,
such as form-wound motors, they are discussed here for the sake of
completeness. Where used, the insulating end lamination 32 may
include any insulating material, such as a moldable plastic
material or composite material or structure, and may be a single
continuous component, such as a ring or other shape.
[0029] As shown in FIG. 2, the end plates 30 may be formed to the
same general dimensions as the frame 12 and typically have a
central opening. Located within the central opening of each end
plate 30 is the insulating end lamination 32. As discussed above,
the insulating end lamination 32 supports the portion of the slot
liners 42 that extends beyond the laminations 26. Within each of
the slots 28, two windings or coils 34 are placed, designated as
34A and 34B. In the illustrated embodiment, the stator coils are
single or stacks of conductive elements or formed rectangular wire
(e.g. form wound coils). It should be noted that the windings may,
alternatively include strands of conductive material, such as
copper wire, which are bundled together to form the coil 34. When
reference is made herein to the windings, or one or more winding
conductors, it should be thus understood that the conductor or
conductors may be a single element or a bundle of elements, such as
many strands of wire. The coils 34 pass thru the slots 28 and are
disposed over the slot liners 42. The coils 34 pass through end
plates 30 and the insulating end lamination 32 to extend beyond
these components. The method of inserting the coils 34 may vary as
will be appreciated by those skilled in the art. Each of the slots
28 may include a single coil 34 or a plurality of different coils
34 (typically two), depending on the specific design. For example,
in one embodiment, each of the slots 28 may have two coils 34A and
34B, with each coil 34A or 34B typically extending in opposite
directions along the insulating end lamination 32 to enter into
different slots of the stator. As will also be appreciated by those
skilled in the art, a wide range of winding patterns may be used,
including lapped patterns, concentric patterns, and hybrid
patterns.
[0030] A cross-section of an individual slot 28 is generally shown
in FIG. 3 with components disposed therein. As discussed above, the
slot 28 may include a slot liner 42 disposed within the slot 28 and
extending through the entire stator assembly 24. The slot liner 42
may include any insulating material generally known by those
skilled in the art, and may include a single insulating layer or
multiple layers of such material. The slot liner 42 may be as long
as the slots 28 or may extend beyond the slots 28. Each winding 34A
and 34B is shown enclosed within winding insulation 36. Winding
insulation 36 may be any insulating material known by those skilled
in the art and may consist of one or a plurality of insulating
layers. Resistance temperature detector 38 is shown positioned
within slot 28, between insulated windings 34A and 34B. Resistance
temperature detector 38 allows measurement of the operating
temperature within slot 28, with input and output signals being
communicated via lead wires 40. In a typical application, an
insulating "stick" is positioned between the windings of each slot
to separate the windings from one another and to aid in preventing
shorts. However, in those slots that receive a temperature
detector, no such sticks may be provided, or partial-length sticks
may be provided on one or both ends of the temperature detector to
separate the portions of the windings not separated from one
another by the detector. Also as shown in FIG. 3, in certain
designs a retaining element 39 or wedge may be placed at the
opening of each slot and retained by teeth or grooves.
[0031] The individual components of an exemplary resistance
temperature detector 38 are illustrated in FIG. 4. In this
embodiment, lead wires 40 are electrically coupled to resistive
element 44 by means of connection plates 46. Lead wires 40 may be
used to perform a variety of functions, including supplying a
measurement input signal, detecting an output signal, and
permitting compensation for differences between the resistive
properties of the leads (i.e. due to their lengths). Resistive
element 44 typically comprises a very fine conductive wire, which
may be formed from platinum, copper, nickel, or any other suitable
conductive material. Further, a resistive element 44 may take any
number of forms. For instance, resistive element 44 may be wound
around a mandrel, may be disposed in various patterns, or may be
etched into another material. Any form may be used in accordance
with the present technique so long as the resistive element 44
provides the functionality described herein. Lead wires 40 and
connection plates 46 will often be made of copper, but like
resistive element 44, either or both may be formed of any other
suitable material, including platinum or nickel.
[0032] Resistive element 44 is enclosed within an insulating system
comprising insulation panels 48 and adhesive 50. Exemplary
insulation panels 48 comprise a flexible insulating material, such
as polyimide film, although other insulating materials or molding
processes (e.g. for molded insulating systems) may also be used
with the present technique. Such materials are commercially
available from a number of sources, such as from E. I. Du Pont De
Nemours & Co., under the commercial designation Kapton. Other
suitable materials may include polyester, polyamide-imide,
polyetheretherketone, polysulfone and polyphenylene sulfide. These
insulation panels 48 are bound to each other and to resistance
element 44 by adhesive 50, which may be an acrylic, an epoxy, or
any other suitable substance with suitable properties. The
resistive properties of resistive element 44 vary in a predictable
manner as a function of temperature. Thus, the potential difference
between the two ends of resistive element 44 may be measured as
current is applied to the element. The resistance of resistive
element 44 may be calculated from the voltage drop through the
element, and a corresponding temperature may be determined based
upon the known relationship between the temperature of the element
and its resistance.
[0033] A cross-section of an assembled exemplary resistance
temperature detector 38 is depicted in FIG. 5. As can be seen in
the illustration, the resistive element 44 of resistance
temperature detector 38 is enclosed within layers of insulation
panels 48 and adhesive 50. Specifically, resistive element 44 is
shown between insulation panels 52 and 54, within an adhesive layer
56. Adhesive layer 56 binds insulation panels 52 and 54 together.
Insulation panels 58 and 60, which are bonded to insulating panels
52 and 54 by adhesive layers 62 and 64 respectively, provide
further insulation for resistive element 44.
[0034] As described more fully below, it has been found that during
operation, high potential differences may be developed between the
windings of the stator and the resistive element of detector 38.
Essentially, the resistive element may appear as a virtual or
solidly fixed ground path for current applied to the stator, and
potential differences between the stator windings and the resistive
element may reach very high levels, particularly when the motor is
inverter-driven. The detector, and indeed the winding and detector
system are designed, however, to withstand such environments while
preventing partial discharge breakdown within the detector as
described below. The improved detector design and structure, then,
prevent premature failure of the detector resistive element, or at
least significantly prolong its life.
[0035] The permittivity of any dielectric material, relative to the
permittivity of a vacuum, may be expressed as a ratio known as its
dielectric constant. The collective layers of insulation panels 48
and adhesive 50 have a permittivity sufficiently high to reduce the
voltage stress level of any air gaps or voids, or low dielectric
materials (or other locations where partial discharge may occur)
within resistance temperature detector 38 to a level below that at
which partial discharge would occur under the operating conditions
of the machine. Exemplary winding insulation 36, insulation panels
48, and adhesive 50, have similar dielectric constants sufficiently
high to prevent partial discharge, such as between approximately 3
and 6, but other materials with dielectric constants outside this
range, or even widely varying dielectric constants, may be used if
the collective materials are sufficient to reduce partial
discharge. Such a construction extends the durable life of
resistance temperature detector 38 and allows it to be placed in
more severe electrical environments.
[0036] Certain electrical properties of the various exemplary
insulation and adhesive layers shown between a winding 34B and
resistive element 44, in FIGS. 3 and 5, are modeled in the form of
equivalent circuits in FIGS. 6 and 7. The equivalent capacitance of
each layer disposed between resistive element 44 and winding 34B
within a slot 28 is represented in FIG. 6, while the corresponding
capacitive reactance of each layer is represented in FIG. 7. The
capacitance and reactance of the entire system of insulation and
adhesive layers is modeled between an outer surface 66 of a winding
34B and resistive element 44, which is depicted as virtual or
solidly fixed ground 68. Moving from this outer surface 66 to
ground 68, any electrical signal would have to pass through the
following layers: winding insulation 36, with an equivalent
capacitance 70 and reactance 82; outer temperature detector
insulation panel 58, with an equivalent capacitance 72 and
reactance 84; outer temperature detector adhesive layer 62, with an
equivalent capacitance 74 and reactance 86; inner temperature
detector insulation panel 52, with an equivalent capacitance 76 and
reactance 88; and inner temperature detector adhesive layer 56,
with an equivalent capacitance 78 and reactance 90. Also modeled is
a capacitance 80 and reactance 92 corresponding to an air void
within adhesive layer 56. Note that where no void 80 is present,
the equivalent circuit would, of course, be different due to the
elimination of the air void from the series circuit (i.e. from
between capacitance 78 and ground 68), resulting in a different
capacitance 78.
[0037] Each layer of the insulation system and its corresponding
reactance impedes the flow of current through the system. The
reactance of each layer of the insulating system opposes the flow
of current through each layer in an amount inversely proportional
to the capacitance of each layer. Thus, as the capacitance of each
layer increases, its capacitive reactance decreases. The voltage
stress across each layer is proportional to its reactance.
Particularly, the ratio of the voltage stress across one layer to
the voltage stress across the entire system is equal to the ratio
of the reactances of the one layer and the entire system.
Accordingly, increasing the capacitance of any particular layer
decreases the corresponding reactance of the layer as well as the
voltage stress across the layer. Because the potential difference
across the entire circuit does not change, the decrease of voltage
stress across one layer is redistributed to the other layers in an
amount proportional to their respective reactances. In accordance
with the present technique, the layers of the winding and
resistance temperature detector insulation systems comprising
winding insulation 36, insulation panels 48, and adhesive 50, are
selected to reduce the voltage stress across any air gaps or other
low dielectric materials disposed adjacent to resistive element
44.
[0038] Paschen's Law essentially states that the breakdown voltage
of any gap between two materials is a function of the product of
the pressure of the gas inside the gap and the distance between the
two materials. The breakdown voltage of air, as predicted by
Paschen's Law, is graphically represented in FIG. 8. The graph
represents the minimum voltage required at each pressure-distance
product necessary for breakdown, and consequent partial discharge,
to occur. This pressure-distance product is shown along the
horizontal axis of the graph, while the corresponding breakdown
voltage is shown along the vertical axis. The minimum breakdown
voltage for air is 327 volts, corresponding to a pressure-distance
product of approximately 5.7 Torr-mm. At this minimum point, a
higher pressure-distance product results in too many electron
collisions within the gas for breakdown to occur, while a lower
pressure-distance product presents too few. As shown by the graph,
the voltage required to breakdown the air in the gap increases as
the pressure-distance product deviates from this minimum.
Accordingly, the layers of resistance temperature detector 38 have
dielectric constants such that the voltage stress across any air
gaps remains below the breakdown voltage of the gap corresponding
to the pressure and size of the gap. As would be understood by
those skilled in the art, temperature may affect the minimum
breakdown voltage in several ways, such as by altering the pressure
within the gap or changing the electrical properties of each layer
between coil winding 34 and resistive element 44.
[0039] While the invention may be susceptible to various
modifications and alternative forms, specific embodiments have been
shown in the drawings and have been described in detail herein by
way of example only. However, it should be understood that the
invention is not intended to be limited to the particular forms
disclosed. Rather, the invention is to cover all modifications,
equivalents, and alternatives falling within the spirit and scope
of the invention as defined by the following appended claims. For
example, while multiple layers of insulating material are
illustrated in the embodiment presented above, a single, or even
more layers, or molded systems may be provided where desired. Such
considerations will be based upon such factors as the insulating
properties of the materials and the potential differences likely to
be encountered in operation. Similarly, additional elements may be
provided in the detector, such as partial insulating pieces
adjacent to ends of the detector (lengthwise) to establish adequate
insulation paths at those locations that are at least as robust as
those offered by the layered materials on either side of the
resistive element.
* * * * *