U.S. patent number 3,693,053 [Application Number 05/193,963] was granted by the patent office on 1972-09-19 for metal oxide varistor polyphase transient voltage suppression.
This patent grant is currently assigned to General Electric Company. Invention is credited to Thomas E. Anderson.
United States Patent |
3,693,053 |
Anderson |
September 19, 1972 |
METAL OXIDE VARISTOR POLYPHASE TRANSIENT VOLTAGE SUPPRESSION
Abstract
A body of sintered metal oxide material exhibiting highly
nonlinear resistance characteristics includes a base and plurality
of members projecting therefrom. Electrodes in the form of
electrically conductive material are plated on some or all of the
major surfaces of the projecting members and base. The electrodes
provide connections to electrical conductors connected to the power
input or output terminals of a single or polyphase electrical
apparatus and the nonlinear resistance characteristics of the metal
oxide material provides desired line-to-line and line-to-neutral
transient voltage suppression in accordance with the connections of
the electrical conductors.
Inventors: |
Anderson; Thomas E.
(Schenectady, NY) |
Assignee: |
General Electric Company
(N/A)
|
Family
ID: |
22715757 |
Appl.
No.: |
05/193,963 |
Filed: |
October 29, 1971 |
Current U.S.
Class: |
361/434; 361/56;
361/118 |
Current CPC
Class: |
H02H
9/044 (20130101); H01C 7/102 (20130101) |
Current International
Class: |
H01C
7/102 (20060101); H02H 9/04 (20060101); H01c
007/12 (); H02h 003/26 () |
Field of
Search: |
;317/238,230,231,61,61.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kallam; James D.
Claims
What I claim as new and desire to secure by Letters Patent of the
United States is:
1. A polyphase transient voltage suppressor comprising
a body of sintered metal oxide material including a plurality of
members for protecting a plurality of polyphase conductors against
transient voltages, each said member provided with a pair of
parallel major surfaces, and
electrode means connected to said body along each of a plurality of
said major surfaces thereof, said electrode means being connectable
to one of the polyphase conductors of an electrical apparatus to be
protected against polyphase voltages, said sintered metal oxide
material having nonlinear resistance characteristics for limiting
the voltage appearing across selected portions of said body defined
by said electrode means and for providing a relatively low
resistance path for current therethrough during a transient voltage
state on the conductors and for providing a relatively high
resistance path during steady state operation thereof, the sintered
metal oxide material body having energy absorbing properties
whereby said suppressor controls transient voltages on the
conductors.
2. The polyphase transient voltage suppressor set forth in claim 1
wherein said body of sintered metal oxide material further
includes
a base member, said plurality of members projecting from a major
surface of said base member.
3. The polyphase transient voltage suppressor set forth in claim 2
wherein
said base member and said plurality of projecting members are
formed from a single body of the sintered metal oxide material to
form a unitary structure.
4. The polyphase transient voltage suppressor set forth in claim 2
wherein
said base member and said plurality of projecting members are each
separately formed from sintered metal oxide material, the members
being connected to form a unitary structure.
5. The polyphase transient voltage suppressor set forth in claim 2
wherein
said base member has a pair of parallel major surfaces, said
plurality of members projecting from a first of the major surfaces
of said base member.
6. The polyphase transient voltage suppressor set forth in claim 2
wherein
said plurality of projecting members are each of equal thickness
dimension.
7. The polyphase transient voltage suppressor set forth in claim 1
wherein
said plurality of projecting members are of unequal thickness
dimension.
8. The polyphase transient voltage suppressor set forth in claim 6
wherein
base member is of thickness dimension equal to 1/2 sin(.pi./n)
times the thickness dimension of one of said projecting members
where n is the number of phases.
9. The polyphase transient voltage suppressor set forth in claim 2
wherein
said plurality of members project radially outward along the major
surface of said base member.
10. The polyphase transient voltage suppressor set forth in claim 9
wherein
said plurality of projecting members have a common junction
projecting perpendicularly outward from the center of the major
surface of said base member.
11. The polyphase transient voltage suppressor set forth in claim
10 wherein
said base member is a right circular cylinder having a pair of
parallel major surfaces, and
the plurality of projecting members each having a curved side
surface which are continuous with the side surface of said base
member and perpendicular to the major surface of the base
member.
12. The polyphase transient voltage suppressor set forth in claim 2
wherein
said plurality of projecting members each having a side surface
projecting from a side surface of said base member in a direction
perpendicular thereto.
13. The polyphase transient voltage suppressor set forth in claim
10 wherein
said plurality of projecting members each have an end surface
remote from said base member, the end surfaces of said plurality of
projecting members each being parallel to the major surface of said
base member.
14. The polyphase transient voltage suppressor set forth in claim
13 wherein
the end surfaces of said projecting members being coplanar to form
a continuous flat surface parallel to the major surface of said
base member.
15. The polyphase transient voltage suppressor set forth in claim
14 wherein
said plurality of projecting members each being of rectangular
shape.
16. The polyphase transient voltage suppressor set forth in clam 5
wherein
said plurality of projecting members are three in number for
forming a three phase transient voltage suppressor.
17. The polyphase transient voltage suppressor set forth in claim
16 wherein
said three projecting members being oriented at 120.degree. angles
around the center of the major surface of said base member whereby
the end surfaces of said three projecting members form a
Y-shape.
18. The polyphase transient voltage suppressor set forth in claim 1
wherein
the sintered metal oxide material has an alpha exponent in excess
of 10.
19. The polyphase transient voltage suppressor set forth in claim 1
wherein
the sintered metal oxide material is comprised primarily of zinc
oxide.
20. The polyphase transient voltage suppressor set forth in claim 1
wherein
said electrode means comprise
an electrically conductive material formed on the major surfaces of
said plurality of projecting members.
21. The polyphase transient voltage suppressor set forth in claim
wherein
the electrically conductive material on adjoining major surfaces of
adjacent projecting members are interconnected to form a like
plurality of electrodes, and
a like plurality of electrical conductors having first ends
connected to the electrodes and second ends connected to the
polyphase power lines to thereby provide polyphase line-to-line
transient voltage suppression.
22. The polyphase transient voltage suppressor set forth in claim 5
wherein
said electrode means comprise
an electrically conductive material formed on the major surfaces of
said plurality of projecting members and on portions of said first
major surface of said base member, and on a second of the pair of
major surfaces of said base member, the electrically conductive
material on adjoining major surfaces of adjacent projecting members
and on adjoining portions of the first major surface of said base
member being interconnected to form a like plurality of electrodes,
and
a like plurality of electrical conductors having first ends
connected to the electrodes and second ends connected to the phase
lines of the polyphase power line, and additional electrical
conductor having a first end connected to the electrode on the
second major surface of the base member and a second end connected
to the power line neutral to thereby provide polyphase line-to-line
and line-to-neutral transient voltage suppression.
23. The polyphase transient voltage suppressor set forth in claim
21 wherein
said plurality of projecting members are each of equal thickness
dimension and are three in number for forming a three phase
transient voltage suppressor,
the electrically conductive material on adjoining major surfaces of
adjacent projecting members being interconnected to form three
electrodes, and
three electrical conductor having first ends connected to the
electrodes and second ends connected to a three phase power line or
three phase power input or output of an apparatus to be protected
to thereby provide three phase line-to-line transient voltage
suppression.
24. The polyphase transient voltage suppressor set forth in claim
22 wherein
said plurality of projecting members are each of equal thickness
dimension and are three in number for for phase three phase
transient voltage suppressor,
the electrically conductive material on adjoining portions of the
first major surface of said base member being interconnected with
the electrically conductive material on adjoining major surfaces of
adjacent projecting members, and
said plurality of electrical conductors are three in number, the
additional conductor being a fourth conductor having a first end
connected to the electrode formed on the second major surface of
said base member and a second end connected to the three phase
power line neutral to thereby provide three phase line-to-line and
line-to-neutral transient voltage suppression.
25. The polyphase transient voltage suppressor set forth in claim
24 wherein
the electrically conductive material on adjoining major surfaces of
at least one pair of adjacent projecting member are not being
interconnected and also separate from the electrically conductive
material on the adjoining portion of the first major surface of
said base member, the electrical conductor associated therewith
being connected to the electrically conductive material on the
adjoining portion of the first major surface of said base member,
whereby selected line-to-line transient voltage suppression is
obtained.
26. The polyphase transient voltage suppressor set forth in claim
25 and further comprising
at least one semiconductor device mounted on said suppressor and
having a first power electrode connected to the electrically
conductive material on at least one of said one pair of adjacent
projecting members and having a second power electrode connected to
the electrically conductive material on the separated adjacent
portion of the first major surface of said base member whereby the
phase of the power line associated therewith is polarity sensitive
to voltage transients on the two other phase conductors.
27. The polyphase transient voltage suppressor set forth in claim 5
wherein
said electrode means comprise
an electrically conductive material formed on the major surfaces of
said plurality of projecting members and base member, and further
comprising
cooling means associated with said plurality of projecting members
and base member for obtaining higher power operation of the
suppressor.
Description
My invention relates to a polyphase transient voltage suppressor
utilizing a varistor device, and in particular, to a varistor
device fabricated from a compact body of sintered metal oxide
material having highly nonlinear resistance characteristics which
provide exceptional voltage limiting characteristics.
Transient voltages resulting from any of a number of causes often
occur on polyphase electric power lines and may cause damage to any
electrical machine, appliance and the like connected to such power
line if the transient is of sufficient magnitude. In particular,
transient voltages occurring on the common three phase, four wire
power lines can cause damage to three phase and single phase
electrical apparatus such as motors and household appliances
connected across the three and single phase lines, respectively.
Thus, it is readily apparent that excessive peaks of voltage
transients must be reduced to levels which assure that the load
devices connected across the power lines are not damaged. A
recently developed material which exhibits highly nonlinear
resistance characteristics, and will be described in greater detail
hereinafter, has been found to have exceptional voltage limiting
characteristics as well as many other advantages such that it is
the basis of a new class of improved transient voltage suppressors.
This material, a sintered metal oxide, has a relatively high energy
handling capability and is capable of being fabricated into a
variety of shapes of various sizes.
Therefore, the principal object of my invention is to provide an
improved polyphase transient voltage suppressor.
Another object of my invention is to provide the structure of the
suppressor in a simple compact form.
A further object of my invention is to fabricate the suppressor
from a single body of metal oxide material.
In accordance with my invention, I provide an improved polyphase
transient voltage suppressor which comprises a single body of
sintered metal oxide material exhibiting highly nonlinear
resistance characteristics. The body includes a base and plurality
of members projecting therefrom or a plurality of members only.
Electrodes plated on some or all of the major surfaces of the
projecting members and base member provide connections to
electrical conductors connected at the input or output of an
electrical apparatus to be protected against transient voltages.
The particular interconnections of the electrical conductors
determine the particular line-to-line and, or line-to-neutral
protection obtained.
The features of my invention which I desire to protect herein are
pointed out with particularity in the appended claims. The
invention itself, however, both as to its organization and method
of operation together with further objects and advantages thereof
may best be understood by reference to the following description
taken in connection with the accompanying drawings wherein like
parts in each of the several figures are identified by the same
reference character and wherein:
FIG. 1 is a graphical representation of the nonlinear resistance
and resultant voltage limiting characteristics of metal oxide and
silicon carbide material for different values of the exponent alpha
plotted in terms of voltage versus amperes on a log-log scale;
FIG. 2a is a schematic diagram of a circuit application of the
metal oxide varistor connected across a load in a circuit supplied
by 600 volts with a 1,000 volt transient superimposed thereon;
FIG. 2b is a plot of volts versus amperes on a log-log scale for
depicting a graphical comparison of the steady state power
dissipation in metal oxide and silicon carbide varistors in the
FIG. 2a circuit when clamping the load voltage at 1,200 volts;
FIG. 2c is a plot of volts versus amperes on a log-log scale for
depicting a graphical comparison of the voltage clamped across the
load for the metal oxide and silicon carbide varistors for a
maximum steady state power dissipation of one watt in the
varistor;
FIG. 3a is an isometric view of a first embodiment of my polyphase
transient voltage suppressor constructed in accordance with my
invention;
FIG. 3b is a schematic diagram representation of the structure
illustrated in FIG. 3a;
FIG. 4a is an isometric view of a second embodiment of my
suppressor;
FIG. 4b is a schematic diagram representation of the structure
shown in FIG. 4a;
FIG. 5a is an isometric view of a third embodiment of my
suppressor;
FIG. 5b is a schematic diagram representation of the structure
shown in FIG. 5a;
FIG. 6a is an isometric view of a fourth embodiment of my
suppressor;
FIG. 6b is a schematic diagram representation of the structure
shown in FIG. 6a;
FIG. 7 is an isometric view of the structure shown in FIG. 3a with
cooling means for higher power applications; and
FIG. 8 is an isometric view of the structure of FIG. 3a with a
second embodiment of cooling means.
There are few known materials which exhibit nonlinear resistance
characteristics and which resort to the following equation to
relate quantitatively current and voltage by the power law:
where V is the voltage between two points separated by a body of
the material under consideration, I is the current flowing between
the two points, C is a constant and .alpha. is an exponent greater
than 1. Both C and .alpha. are functions of the geometry of the
body formed from the material and the composition thereof, and C is
primarily a function of the material grain size whereas .alpha. is
primarily a function of the grain boundary. Materials such as
silicon carbide exhibit nonlinear or exponential resistance
characteristics and have been utilized in commercial silicon
carbide varistors, however, such nonmetallic varistors typically
exhibit an alpha (.alpha.) exponent of no more than 6. This
relatively low value of alpha represents a nonlinear resistance
relationship wherein the resistance varies over only a moderate
range. Due to this moderate range of resistance variation, the
silicon carbide varistor is often connected in series with a gap
when used in a circuit for transient voltage suppression since
continuous connection of the varistor could exceed the power
dissipation capabilities thereof unless a relatively bulky body of
such material is used in which case the steady state power
dissipation is a rather severe limitation. An additional drawback
is the ineffectiveness of the voltage clamping action as a result
of the limited value of silicon carbide alpha exponent. The
moderate range of resistance variation results in voltage
limitation which may be satisfactory for some applications, but is
generally not satisfactory when the transient voltage has a high
peak value.
A new family of varistor materials having alphas in excess of 10
within the current density range of 10.sup.-.sup. 3 to 10.sup.2
amperes per square centimeter has recently been produced from metal
oxides although few applications have been disclosed for this new
metal oxide varistor material also referred to herein as MOV, a
trademark of the General Electric Co. Although the alpha of the MOV
materials, in which range the alpha remains substantially constant,
are identified by the current density range of 10.sup.-.sup. 3 to
10.sup.2 amperes per square centimeter, it is appreciated that the
alphas remain high also at higher and lower currents although some
deviation from maximum alpha values may occur. The MOV material is
a polycrystalline ceramic material formed of a particular metal
oxide with small quantities of one or more other metal oxides being
added. As one example, the predominant metal oxide is zinc oxide
with small quantities of bismuth oxide being added. Other additives
may be aluminum oxide, iron oxide, magnesium oxide, and calcium
oxide as other examples. The predominant metal oxide is sintered
with the additive oxide(s) to form a sintered ceramic metal oxide
body. Since the MOV is fabricated as a ceramic powder, the MOV
material can be pressed into a variety of shapes of various sizes.
Being polycrystalline, the characteristics of the MOV are
determined by the grain (crystal) size, grain composition, grain
boundary composition and grain boundary thickness, all of which can
be controlled in the ceramic fabrication process.
The nonlinear resistance relationship of the MOV is such that the
resistance is very high (10,000 megohms has been measured) at very
low current levels in the microampere range and progresses in a
nonlinear manner to an extremely low value (tenths of an ohm) at
high current levels. The resistance is also more nonlinear with
increasing values of alpha. These nonlinear resistance
characteristics result in voltage versus current characteristics
wherein the voltage is effectively limited, the voltage limiting or
clamping action being more enhanced at the higher values of the
alpha exponent as shown in FIG. 1. Thus, the voltage versus current
characteristics of the MOV is similar to that of the Zener diode
with the added characteristic of being bidirectional and over more
decades of current. The conduction mechanism of the MOV is not yet
clearly understood but it is completely unlike the avalanche
mechanism associated with the Zener diodes, a possible theoretical
explanation of its operation being that of space charged limited
current. The rated voltage and the voltage range over which the
varistor effect occurs are determined by the particular composition
of the MOV material and the thickness to which it is pressed in the
fabrication process. The MOV includes conduction changes at grain
boundaries resulting in the advantage of bulk phenomenon allowing
great flexibility in the design for specific applications simply by
changing the dimensions of the body of MOV material. That is, the
current conduction in the absence of closely spaced electrodes
along one surface of the MOV body is through the bulk thereof. The
bulk property of the MOV also permits a much higher energy handling
capability as compared to junction devices. Thus, since an MOV
device can be built to any desired thickness, it is operable at
much higher voltages than the Zener diode junction device and can
be used in a range from a few volts to several kilovolts. The
voltage changes across a silicon carbide varistor device are much
greater than across an MOV device for a given current change as
seen in FIG. 1 for the plot designated .alpha. = 4 and thus the
silicon carbide varistor has a much smaller voltage operating range
thereby limiting its applications as described hereinabove. The
thermal conductivity of MOV material is fairly high (approximately
one-half that of alumina) whereby it has a much higher power
handling capability than silicon carbide, and it exhibits a
negligible switching time in that its response time is in the
subnanosecond domain. Finally, the MOV material and devices made
thereof can be accurately machined, soldered, and operated at very
low voltages, capabilities not possible for the larger grained
silicon carbide.
The voltage versus current characteristics plotted in FIG. 1 of the
drawings illustrate the nonlinear or exponential resistance
characteristics exhibited by varistor material, and in particular,
the increasing nonlinearity and enhanced voltage limiting obtained
with increased values of the exponent alpha (.alpha.) wherein the
top line .alpha. = 4 is typical for silicon carbide varistors and
the three lines .alpha. = 10, 25 and 40 apply to varistors
fabricated of MOV material. The VOLTS abscissa is in terms of the
voltage appearing across the terminals of a specific MOV device in
response to current flowing through the bulk of the MOV material
and represented along the CURRENT ordinate. Although the use of
linear scales on the graph would show the decreasing slopes
(decreasing resistance values) with increasing currents, such
curves can be readily manipulated by the choice of scales, and for
this reason, log-log scales are chosen to obtain a family of lines
each of which remains substantially straight within the indicated
current range. It can be seen from the FIG. 1 plots that the
resistance exhibited by the MOV material is quite high at low
current levels and becomes increasingly smaller in a nonlinear
exponential manner with increasing current levels. Extension of the
plots to lower and higher current levels would obviously indicate
correspondingly much higher and lower resistances, respectively,
and operation of the MOV device may transiently reach such levels
depending upon the particular circuit application of the device.
For purposes of comparison, each of the volts versus amperes plots
passes through the point identified by 100 volts and 1 milliampere.
It should be understood that metal oxide materials (zinc oxides)
are available having alpha exponents even greater than 40 which
thereby obtains even greater enhanced voltage clamping action than
that exhibited for the .alpha. = 40 line.
Referring now to FIG. 2a, there is shown a simple direct current
circuit utilizing a varistor connected across a load and serving
the basis for the graphs of FIGS. 2b and 2c to be described
hereinafter. In particular, the circuit includes a load 20
connected in series with a 600 volt d.c. source and a source 21
producing a 1,000 volt transient. The voltage transient is assumed
to be a square wave pulse of 10 millisecond duration. The internal
impedance of the 600 volt source is represented by series resistor
22 of 10 ohms. A varistor device 23 is connected across load 20 for
clamping or limiting the voltage thereacross to a desired value.
The direct current is used for convenience to simplify the example,
and is also valued for alternate current operation when properly
analyzed.
Referring now to FIG. 2b, there are drawn the resistance lines
which illustrate the operating characteristics of the circuit
illustrated in FIG. 2a. In particular, the upper 10 ohm source
impedance line indicates the voltage developed across load 20 with
increasing current flow therethrough, the maximum 1,600 volts being
the summation of the steady state 600 and transient 1,000 volts.
For a current flow of 40 amperes, the load voltage is 1,200 volts
due to the 400 volt drop across the 10 ohm resistor 22. In the case
wherein a silicon carbide varistor 23 is connected across load 20
for clamping the voltage at 1,200 volts for a current flow of 40
amperes, the load line for such varistor having an alpha exponent
equal to 6 crosses the steady state 600 volt line at 0.6 ampere
resulting in a steady state power dissipation of 360 watts in the
varistor device. In comparison, a metal oxide varistor of the same
clamping voltage at 40 amperes fabricated of zinc oxide material
having an alpha exponent of 25 has a steady state power dissipation
of a mere 90 milliwatts. Thus, the metal oxide varistor employed in
my invention has a steady state power dissipation which is in the
order of one four-thousandths of the dissipation in a silicon
carbide varistor. Obviously, a metal oxide varistor having an alpha
exponent in excess of 25 would have a steady state power
dissipation even less than the 90 milliwatts. It can be appreciated
that the 360 watt steady state power dissipation in a silicon
carbide varistor is virtually unbearable and would quickly result
in destruction of the varistor device unless such device was of
such massive volume that it would have the requisite energy
handling capability, a completely unpractical consideration. Also,
it should be noted that the more typical, commercially available
silicon carbide varistors have alpha exponents of 3 to 4 whereby
the problem associated therewith is even worse.
Another convenient manner in comparing the characteristics of the
metal oxide and silicon carbide varistor devices is illustrated in
FIG. 2c wherein a maximum allowable steady state power dissipation
of one watt is maintained for both varistor devices. In the case of
the silicon carbide varistor, (.alpha. = 6) the volt-ampere
characteristic line intersects the 10 ohm source impedance line at
the voltage level of 1,594 volts, that is, the silicon carbide
varistor is capable of clamping the load voltage at only 1,594
volts for a maximum applied voltage of 1,600. In comparison, the
metal oxide varistor having an alpha exponent of 25 and identical 1
watt steady state power dissipation is capable of clamping the
voltage across the load at 940 volts, an improvement of 654 volts
over the silicon carbide varistor. Thus, the silicon carbide
varistor has suppressed the applied 1,600 volts by a mere 6 volts
whereas the metal oxide varistor has suppressed it by 660 volts.
The FIGS. 2b and 2c graphs clearly indicate the superior steady
state power dissipation and voltage clamping characteristics
obtained by the metal oxide material as compared to a silicon
carbide varistor. These superior characteristics are utilized in
the polyphase transient voltage suppressor to be described
hereinafter in accordance with my invention.
Referring now to FIG. 3a, there is shown a first embodiment of my
polyphase transient voltage suppressor which provides three phase
line-to-line and line-to-neutral protection as indicated in the
schematic diagram representation of the device in FIG. 3b. My
suppressor consists of a body of sintered metal oxide material
exhibiting highly nonlinear resistance characteristics and the
structure includes a base member 30 and three members 31, 32 and 33
projecting therefrom. As one example, the sintered metal oxide
material may consist primarily of zinc oxide and a small percentage
of bismuth oxide. Base 30 and projecting members 31, 32 and 33 are
preferably formed as a single body by being molded in the desired
shape. Alternatively, the projecting members and base may be
fabricated as separate elements and then joined together to form
the unitary structure depicted in FIG. 3a although this latter
approach would appear to introduce many difficulties, would
probably be more costly and could have operating characteristics
inferior to that of the single formed body. Thus, it is to be
understood that the body of MOV material fabricated from separate
elements which are thence joined to form a unitary structure is
also considered to be in the scope of my invention although it is
not the preferred embodiment. The number of projecting members,
illustrated as three in FIG. 3a, is determined by the number of
phases to be protected against transient voltages. Thus, six
projecting members would be utilized in the case of a six phase
transient voltage suppressor.
For purposes of structural rigidity and simplicity of fabrication,
the projecting members 31, 32 and 33 have a common juncture coaxial
with the centerline axis of base 30 and project radially outward
from the juncture. Base 30 and projecting members 31, 32 and 33
each include a pair of opposed major surfaces which are generally
flat and parallel to each other. Thus, base 30 includes first major
surface 30a and a second major surface parallel thereto and forming
the back (unseen) end of the suppressor device depicted in FIG. 3a.
In like manner, projecting member 31 includes first major surface
31a and a second major surface parallel thereto but unseen in FIG.
3a. Projecting member 32 includes visible first major surface 32a
and an unseen second major surface parallel thereto, and a
projecting member 33 includes first major surface 33a and an unseen
second major surface parallel thereto. Base 30 may have any of a
number of forms and is depicted in the FIG. 3a embodiment as being
circular in cross section. Examples of other forms of the base
member are the triangular form illustrated in FIG. 5a and the
square or rectangular form in FIG. 6a. Projecting members 31, 32
and 33 may also be any of a number of shapes, but for convenience
of fabrication by the molding process and for purposes of forming a
compact body, these members are of rectangular or square shape. For
these reasons also, the side surfaces (31b and 33b are visible) of
the projecting members 31, 32 33 and the side surface 30b of base
member 30 are continuous, that is, are flush with each other, the
side surfaces of the projecting members being curved in conformance
with the circular curved surface of the particular base member
illustrated in FIG. 3a. And again for the above reasons, the end
surfaces 31c, 32c, 33c of projecting members 31, 32, 33 are
coplanar and parallel to the major surfaces of the base 30. The
projecting members may be arranged in any of a number of
orientations on the base member, but for ease of fabrication, they
are arranged in a Y-shaped pattern (120.degree. orientation) in
FIGS. 3a, 4a, 5a and in T-shaped pattern in FIG. 6a as typical
examples for a three phase suppressor.
Electrodes are provided on the major surfaces of the base and
projecting members for providing connections to electrical
conductors that are connected to a polyphase power line or to the
input or output electric power terminals of an electrical apparatus
being protected from voltage surges by my transient voltage
suppressor. The electrodes are in the form of metallized surfaces
which are plated on the major surfaces of the base and projecting
members for providing good electrical and mechanical contact
therewith. The metallized surfaces are obtained by a suitable
bonding process which may be accomplished by thick film techniques
or by pressure contacts, as two examples. The metallized surface
may be obtained by firing a thin layer of silver-glass frit (silver
and glass particles) on the MOV major surfaces. Ohmic contact is
utilized in order to take advantage of the bulk phenomenon
operation of the MOV material. As depicted in FIG. 3a, electrodes
are plated on each of the major surfaces of the metal oxide
varistor body. In particular, metallized surface or electrode 34 is
formed on the first major surface 31a of projecting member 31 and a
second such metallized surface of identical form is formed on the
opposite major surface (not seen) of member 31. In like manner, the
two major surfaces of projecting member 32 are provided with two
metallized surfaces, one of which 35 is visible in FIG. 3a and the
two major surfaces of projecting member 33 are provided with two
metallized surfaces, one of which 36 is visible. Finally, base
member 30 is provided with metallized surfaces along each of the
three 120.degree. sectors on major surface 30a, two of which 37 and
38 are visible in FIG. 3a. The other (unseen) major surface of base
member 30 which defines the back end of the device is provided with
a metallized surface along substantially all of its area except
along the edges. For purposes of assuring that current conduction
will be through the bulk of the MOV body (bulk operation), the
edges of the metallized surface electrodes terminate at some
predetermined distance from the side and end surfaces of the
projecting members 31, 32 and 33 and base member 30. In the case of
the FIG. 3a embodiment, the transient voltage suppressor device
provides three phase line-to-line and line-to-neutral protection
and for this reason the group of three metallized surfaces in each
of the 120.degree. portions of the device are interconnected as
shown, that is, the three adjoining electrodes are formed as a
single continuous metallized surface electrode. Thus, metallized
surfaces 34, 36 and 37 are formed as a first single metallized
surface electrode and thus are all at the same potential during
operation of the device. Alternatively, electrodes 34, 36 and 37
could be separate but interconnected by means of electrically
conducting leads. In like manner metallized surfaces 35, 38 and the
unseen metallized surface on the unseen major surface of member 33
are formed as a second single metallized surface electrode.
Suitable electrical conductors (leads) 39, 40, 41 have first ends
connected to the electrodes in corresponding 120.degree. portions
of the device and a fourth lead 42 is connected to the electrode on
the opposite (unseen) major surface of base member 30 to provide
the three phase, four wire input to the device. The leads may be
connected to the electrode surface along any point thereof, a
convenient connecting point for the three phase input being on the
120.degree. sectors of base member 30 as illustrated in FIG. 3a.
The remote ends of leads 39, 40 and 41 would be connected to a
three phase power line or to the three phase power input or output
terminals of the apparatus being protected against transient FIGS.
and the remote end of lead 42 connected to the neutral which may be
grounded.
The thickness dimensions of projecting members 31, 32 and 33 are
generally equal, especially in the case of a three phase apparatus
being protected by the suppressor device, and in order to obtain
the proper voltage protection between phases and phase-to-neutral,
the thickness T.sub.o of base member 30 is related to the
thicknesses T.sub.1, T.sub.2 and T.sub.3 of projecting members 31,
32 and 33, respectively, as:
Obviously, the device depicted in FIG. 3a can also be used for
merely three phase line-to-line protection by disconnecting lead
42. In this latter application, the entire base member 30 could be
omitted and the device consist only of the projecting members.
Finally, the device may also be used for single and two phase
applications by utilizing the proper leads. Thus, for a single
phase application, two leads associated with opposite major
surfaces of one of projecting members 31, (leads 39, 41), 32,
(leads 40, 41), 33 (leads 39, 40) or base 30 (leads 42 and one of
39, 40, 41) are connected to the single phase input or output
terminals of the single phase apparatus being protected. The choice
of utilizing the MOV body of a projecting member or the thinner
base member in a single phase application is determined by the
voltage rating required.
In the case of a two phase transient voltage suppressor device, two
leads associated with opposite major surfaces of one of projecting
members 31 (leads 39, 41), 32 (leads 40, 41), or 33 (leads 39, 40)
and base 30 (lead 42) are connected to form the input or output
terminals of a two-phase voltage transient suppressor. The
thickness dimensions of the projecting members and base are related
as:
It should be noted that with proper design, considering apparatus
tolerances, a two phase suppressor can be made from a three phase
suppressor as shown in FIG. 3a but with optimum thickness T.sub.o.
Also, in an n polyphase system, the optimum thickness dimensions
are related as:
T.sub.1 = T.sub.2 = - - - - = T.sub.n = 2 T.sub.o sin .pi./n where
n is the number of phases.
The dimensions for a typical three phase, four wire transient
voltage suppressor constructed in accordance with my invention as
illustrated in FIG. 3a and adapted for voltage clamping at 240
volts RMS line-to-line is as follows: assuming the metal oxide
material has an alpha exponent of 25, the voltage rating of the
material is 80 volts RMS per millimeter. Thus, the thickness of
each of projecting members 31, 32 and 33 is 240/80 = 3 millimeters
for line-to-line protection and the thickness of base member 30 is
3/.sqroot.3 millimeters for line-to-neutral protection. The
metallized surfaces (electrodes) are each of approximately 0.001
inch thickness. The diameter of the base member 30 and length of
projecting members 31, 32 and 33 is determined primarily by the
maximum power dissipation anticipated. Thus, in the case of
anticipated long duration, high peak value transients, or highly
repetitive transients, the diameter and length dimensions are made
larger than for short duration and, or lower peak value or less
frequent transients. As a typical example, for the above described
240 volt suppressor, the diameter and length dimensions may each be
in the order of 1 inch.
FIG. 3b is a schematic representation of the three phase, four wire
suppressor device illustrated in FIG. 3a. In particular, each metal
oxide varistor portion of the device is illustrated schematically
as a separate varistor for indicating each phase-to-phase and
phase-to-neutral circuit. Thus, the particular varistor formed by
projecting member 31 and the pair of metallized surface electrodes
plated on opposite major surfaces thereof is depicted in the
schematic diagram as varistor 31. In like manner, projecting
members 32 and 33 and their metallized surfaces are indicated as
separate varistors 32 and 33 connected in a delta (.DELTA.)
configuration with varistor 31 for providing phase-to-phase
line-to-line protection. Finally, the varistors formed by the three
120.degree. sector portions of base member 30 and their associated
metallized surface electrodes are depicted as three separate
varistors 30 connected in a Y-configuration with a grounded
(neutral) center point for providing line-to-neutral protection,
with all members forming the full line-to-line, line-to-neutral
varistors.
Referring now to FIG. 4a there is shown a second embodiment of my
metal oxide varistor polyphase transient voltage suppressor which
is a structure similar to that illustrated in FIG. 3a with the
exception that each group of three adjoining metallized surfaces
(electrodes) are separated rather than forming a single continuous
surface as in FIG. 3a. In fact, the metallized surfaces along the
major surfaces of the projecting members 31, 32 and 33 may be
omitted since they serve no useful purpose in the FIG. 4a
embodiment which provides three phase line-to-neutral protection,
Y-connection, but does not provide the additional line-to-line
protection, delta (.DELTA.) connection, of the FIG. 3a embodiment.
Thus, in its most simple form, the three phase line-to-neutral
transient voltage suppressor would merely comprise a base member
30, a metallized surface along a first (the unseen) major surface
thereof and having lead 42 connected thereto, and three separated
120.degree. sector shaped metallized surfaces 37, 38 (and one
unseen) along the second major surface 30a and having leads 39, 40
and 41 respectively connected thereto. The projecting members 31,
32, 33 are illustrated in FIG. 4a for purposes of utilizing a body
of metal oxide material which is identical to that illustrated in
FIG. 3a, that is, for standardization purposes, it being recognized
that the simpler structure would be less costly if the fabrication
process easily permitted construction of the two different type
structures.
It should be evident that the FIG. 4a device can be utilized for
transient voltage protection for a delta (.DELTA.) connected
configuration utilizing only the three projecting members 31, 32,
33, and of one or more than one single phase apparatus, actually up
to six apparatus by providing leads on each of the electrodes and
thereby utilizing the three projecting members 31, 32, 33 and three
120.degree. sector portions of base member 30 as six separate
bodies of MOV materials which are operatively substantially
independent of each other.
Although the thickness dimensions of projecting members 31, 32 and
33 are generally equal, this is not always required, and may in
some cases not be desired. Thus, in the case wherein the device is
utilized for the protection of two to six single phase electrical
apparatus operating at two to four different voltage levels or
operating at the same voltage level but requiring two to four
different voltage protection levels, the thicknesses of the
projecting members would be different and determined by the voltage
ratings required.
FIG. 4b is a schematic representation of the three phase
line-to-neutral structure illustrated in FIG. 4a wherein varistor
devices 30 again comprise the three 120.degree. sector portions of
base member 30 and their associated metallized surfaces.
Referring now to FIG. 5a, there is shown a third embodiment of my
MOV polyphase transient voltage suppressor, and in particular, an
embodiment wherein line-to-ground protection is desired for all
three phases but only one line-to-line protection is required, or
desired, that is, suppression is obtained only between selected
lines. The FIG. 5a embodiment also illustrates the use of a
different shaped base member 30, such base member being of
generally triangular shape. The projecting members 31, 32, 33
extend outward to the corners of the triangle formed by the base.
The metallized surfaces 37, 38 (and unseen third metallized
surface) on the seen first major surface 30a of base member 30 may
conveniently be formed in triangular shapes as illustrated, in
120.degree. sector shapes as in FIG. 4a, or in any other desired
form. The group of three metallized surfaces 34, 36, 37 in the
first 120.degree. sector portion of the device in FIG. 5a are
spaced apart similarly to the same metallized surfaces in the FIG.
4a embodiment. Each of the two remaining groups of three metallized
surfaces in the other two 120.degree. sector portions are formed as
single continuous metallized surfaces as in the FIG. 3a embodiment,
or as separate surfaces as in the first 120.degree. sector portion
with leads 40 and 41 connected to their associated three separate
metallized surfaces. The three phase leads 39, 40, and 41 may be
connected to the three 120.degree. sector metallized surfaces on
base member 30 as in the FIGS 3a and 4a embodiments although leads
40 and 41 could be connected to the adjoining metallized surfaces
on the adjacent projecting members.
FIG. 5b is a schematic representation of the structure illustrated
in FIG. 5a wherein metal oxide varistor 32 provides the one
line-to-line protection.
All of the hereinabove described embodiments of my MOV polyphase
transient voltage suppressor have illustrated the three projecting
members oriented to form a Y-shape and thereby provide 120.degree.
sectors on the visible major surface 30a of the base member. Such
orientation of the projecting members is not a requirement, and the
fourth embodiment of my device in FIG. 6a illustrates another
orientation of such projecting members, this time forming a
T-shape. Also, the base member 30 is illustrated as being of square
or rectangular shape, it being appreciated that the three areas of
metallized surfaces on the visible major surface of base member 30
are preferably equal. The third portion 30c of the visible major
surface of base member 30, although not visible in the FIGS. 3a,
4a, 5a embodiments, is plated with metallized surface 60 on which
lead 41 is connected. Metallized surface 60 contacts metallized
surface 62 on major surface 32a of projecting member 32 and a
metallized surface on the unseen major surface of projecting member
31 which forms one-half of the top of the T. In like manner, the
three (unseen) metallized surfaces to which lead 40 is connected
form a continuous metallized surface. However, the continuous
coplanar metallized surface portions 34 and 36 are separated from
adjacent metallized surface 37 on base member 30a.
FIG. 6b is a schematic diagram of the structure illustrated in FIG.
6a and indicates that, with the exception of diodes 61, this
schematic diagram is the same as that depicted in FIG. 3b. The
insertion of diodes 61 in the two lower illustrated line-to-line
legs of the delta-connected varistors causes the two line-to-line
circuits common with lead 39 to be positive polarity sensitive,
that is, transient voltage suppression of both polarity transients
occurs on the line 40-to-line 41 and all three line-to-neutral
circuits, but negative polarity voltage transients are tolerated on
the line 39-line 40 and the line 39-line 41 circuits. The
polarity-sensitive phase protection is achieved on the structure
illustrated in FIG. 6a by mounting a single diode device 61 on the
metallized surface 34 plated on projecting member 31 or on the
coplanar metallized surface 36 on projecting member 33 wherein
members 31 and 33 form the top of the T formed by the three
projecting members. Alternatively, diode 61 could be mounted on
metallized surface 37 on the major surface portion 30a of base
member 30. The diode is preferably mounted on a metallized surface
rather than on a side surface of the device in order to provide
protection to the diode from accidental jarring of the side surface
against another object. The cathode electrode of diode 61 is
connected to the metallized surface portion 34 or 36, and the anode
electrode is connected to the metallized surface 37. Since the
metallized surfaces 34 and 36 are common on the common major
surface of projecting members 31 and 33, the single diode 61
appears operationally (and schematically) in both of the
line-to-line connections which are common with the line associated
with lead 39. Obviously, the sensitive phase may be made sensitive
to the opposite polarity voltage transients by reversing the
interconnection of the electrodes of diode 61 in the structure of
FIG. 6a.
Although FIG. 6a illustrates the use of only one semiconductor
device, it should be obvious that more than one diode, or one or
more other type semiconductor device may be mounted on my device to
obtain desired circuit functions.
The metal oxide bodies illustrated in FIGS. 3a-6a may be fabricated
by any number of methods, one suitable method being the use of a
double-acting press for simultaneously forming the base and
projecting members into a unitary structure.
Referring now to FIG. 7, there is shown the structure of FIG. 3a
with the addition of a plurality of 120.degree. sector cooling fins
70 connected in parallel relationship to the major surfaces of base
member 30 and a second set of cooling fins 71 associated with the
far (unseen) major surface of base member 30. The cooling fins 70
and 71 may be fabricated of a suitable electrically conductive,
high thermally conductive metal such as copper or aluminum. The
electrical conductivity of the cooling fins 70 and 71 permits the
use of the outermost fins as electrodes for connection to the leads
39, 40, 41 and 42 which have their remote ends connected to a power
line or apparatus being protected. The cooling fins 70 may be
suitably attached to or through the metallized surfaces on the
projecting members and the thinness of the metallized surface
permits good heat transfer from the body of metal oxide material to
the cooling fins. In the case of the base member cooling fins 71, a
core 72 which may be of the same material as fins 70, 71 acts as
the heat transfer agent from base member 30 to fins 71. This core
72 may also be utilized along fins 70 if desired, for improving the
heat transfer. In either case, core 72 has its side surfaces in
contact with side surfaces of adjacent cooling fins and its base in
contact with the metallized surface between adjacent cooling fins.
The cooling fins would be utilized in large steady state power
dissipation applications, that is, when the clamping voltage is
very close to the steady state operating voltage, resulting in
large steady state power dissipation in the MOV material, and in
the case of anticipated long duration and, or high peak value
voltage transients or the case wherein transients are repetitive in
rather quick succession. My suppressor without the use of cooling
fins is generally adequate to cope with the normally isolated
transient voltages that occur on the three phase power line. The
radius of each cooling fin may be less than, equal to, or greater
than the radius of the base member 30, as desired or required by
the particular application. Forced air can be provided over the
cooling fins to increase the heat transfer, if necessary.
FIG. 8 illustrates a second embodiment of cooling fins for the
structure illustrated in FIG. 3a. In the FIG. 8 embodiment, the
cooling fins 70 are oriented radially with the juncture of
projecting members 31, 32 and 33 and may each conveniently be in
rectangular form as illustrated. Each group of fins and the
120.degree. core portion 80 from which they radiate may be a single
body fabricated by extrusion. Obviously, the outer edge of each
cooling fin in the FIG. 7 embodiment and the three outer edges of
each fin in the FIG. 8 embodiment may have shapes other than that
illustrated. In FIG. 8, as well as in FIG. 7, the leads 39, 40 and
41 connecting the device to a three phase power line or the three
phase terminals of an apparatus being protected against transient
voltages may be connected to the metallized surfaces on projecting
members 31, 32 and 33 or to like surfaces on the 120.degree. sector
portions on base member 30, although they may also be connected to
the cooling fins which are in electrical and mechanical contact
with the metallized surfaces. The lead connection on the cooling
fins may be preferred in some cases, especially high power
applications, wherein the cooling fins would generally be at a
lower operating temperature than the body of metal oxide material
and the metallized surfaces plated thereon. The radial cooling fins
in the FIG. 8 embodiment, as in the case of the FIG. 7 embodiment,
may also extend beyond, be equal to, or less than the radius of
base member 30. Cooling fins may also be associated with base
member 70 as in the case of FIG. 7, and may be of the form
illustrated therein, or radial as illustrated for fins 70 in FIG.
8.
The thickness dimensions of the base and projecting members of the
body of metal oxide material are generally selected in accordance
with the above equation for a voltage rating at a desired level
above the circuit rated voltage at which the voltage clamping or
suppression action will occur. Thus, the thickness of such members
may be selected for a voltage rating (line-to-line for projecting
members 31, 32 and 33 and voltage-to-neutral for the base member)
which is in the order of 10 percent above the circuit rated
voltage, as one example. The principal advantage of my invention
is, of course, the superior transient voltage suppression obtained
with the use of a metal oxide varistor. This superior voltage
suppression is obtained primarily due to the following three
exceptional properties of MOV material: (1) the resistance
characteristics are highly nonlinear (alpha greater than 10) over a
wide range of current and result in a high degree of voltage
limiting, (2) the response time is negligible, and (3) the
relatively high thermal conductivity permits rapid dissipation of
heat developed in the MOV material due to the voltage transient. As
can be seen in the FIG. 1 graph, during the transient (high)
voltage condition, the MOV material provides a relatively low
resistance path for the current which thence decays at a rate
determined primarily by the L/R or RC time constant of the circuit,
the resistance of the MOV increasing substantially as the voltage,
and current, are decreasing. During steady state circuit operation,
the MOV exhibits a relatively high resistance and low power
dissipation and has a negligible effect on the circuit operation.
The very compact nature of my transient voltage suppressor permits
it to be mounted at the input or output power terminals of the
apparatus being protected in any convenient manner which may even
include merely having the device hang from the power terminals of
the apparatus by means of the connecting leads 39, 40, 41 and 42 in
the case wherein the apparatus is not subjected to severe
mechanical vibration. In the case of such vibration, or for other
reasons, a suitable mounting means can be connected on base member
30 or one of the projecting members for providing a rigid support
for the device on or adjacent to the apparatus being protected.
Another major advantage of my invention is that since all the
devices can have the same shape for three phase systems, single
type devices, but of various sizes (especially in the thickness of
the MOV) can easily be molded to accommodate various voltage
ranges.
Having described several embodiments of a metal oxide varistor
polyphase transient voltage suppressor, it should be obvious that
the base member (if employed) and projecting members of the body of
MOV material can assume any of a number of shapes and the
metallized surfaces can be coordinated to obtain desired
line-to-line and line-to-neutral transient voltage suppressions.
The high resistance of the MOV body during circuit steady state
operation permits only a small current flow therethrough and
therefore results in very low steady state power dissipation as
indicated in FIGS. 2b and 2c. In view of this low steady state
power dissipation, the MOV body is physically small for the energy
handled and thereby permits the connection or mounting of the
device very close to the apparatus being protected from the
transient voltages. The use of the metal oxide varistor for
transient voltage suppression provides a device having many
advantages over one wherein other components such as selinium
barrier layer rectifiers, Zener diode junction devices or silicon
carbide devices are employed since each of such components have at
least one of the following limitations which is not present in the
metal oxide varistor: the nonlinear resistance relationship varies
only a moderate range, the power dissipation in the steady state
operation of the circuit is excessive, the voltage clamping action
is not adequate, the component does not have bulk properties and
therefore is not applicable for power circuit applications, the
component has polarity restrictions which are not usually favorable
for mounting in conjunction with a semiconductor device, the
component is not available for high voltage applications. Thus,
while my invention has been particularly shown and described with
reference to specific embodiments thereof, it should be obvious by
those skilled in the art that obvious changes in form and detail
may be made without departing from the scope of the invention as
defined by the following claims.
* * * * *