Note: Descriptions are shown in the official language in which they were submitted.
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21457 PCT/EP99/02020 Transl. of WO 99/60588
The invention relates to a load selector for tapped
transformers according to the introductory clause of the first
claim. Such load selectors are known from German 3,833,126.
Load selectors are used on transformers to switch the
taps of the control windings of these transformers under load and
thus to compensate accurately for voltage variations. Load selec-
tors are less expensive to make and use since they do not separate
the functions of selector and load switch. During the switching
process the various contacts draw arcs.
In order to avoid this, German 3,833,126 proposes a load
selector wherein each phase being switched is provided with two
jointly moved mechanical switch contacts on a common contact
support and a respective vacuum switch is provided on the contact
support in series with each of the movable mechanical switch
contacts. The stationary tap contacts engaged by the swingable
mechanical switch contacts of the load switch are arranged on a
concentric circle on the surface of an insulating cylinder. The
vacuum switches are mounted horizontally on the contact support and
are operated by a cam lying on a further concentric circle that is
provided axially between the ring of fixed tap contacts and an
additional commutating ring.
Both of these load selectors have several disadvantages.
First, as a result of the horizontal orientation of the vacuum
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switches on the common contact support the insulating cylinder and
thus the load switch must be of excessively large diameter. Vacuum
switches have certain minimum dimensions depending on their ratings
and in particular are fairly long: Furthermore the actuating stem
of the vacuum switch projects longitudinally from it. It is
therefore clear that the common contact support has a radial
dimension determined by the dimensions of the vacuum switch and its
actuating means, and this in turn determines the diameter of the
load selector.
In addition to these general problems of size there is
the considerable disadvantage with horizontal installation of the
vacuum switches according to the state of the art that when the
load selector is filled with oil, air is trapped in the top of the
folds of the vacuum-switch cuff and the oil cannot fill this
region. This results when the vacuum switch is actuated in an
uneven loading of the vacuum-switch cuff, creating the danger that
it ruptures. Such trapping of air in the vacuum-switch cuff can
only be avoided with horizontal vacuum switches by filling the load
selector in a vacuum; such vacuum filling is however not possible
or is excessively expensive at the site, that is during service or
repair.
It is furthermore important that load selectors normally
also have a considerable length, that is height. The fixed tap
contacts of the individual phases to be switched are arranged in
circles one above the other, the means for actuating the rotatable
switching shaft are provided inside the load selector which carries
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the contact support of each phase, and at the top and bottom of the
load selector are means for holding its switching shaft, so that in
general, as stated, the load selector is fairly long. The result
is that the switching shaft inside it is also fairly long.
Such switching shafts are normally made of GFK or another
insulating material and metal switching shafts have also been
suggested. The contact supports are secured with the known load
selectors by means of screws to the switching shaft. German
4,414,941 describes fixing the contact support on the switching
shaft by means of a clamping flange.
Tolerance errors, different coefficients of expansion of
the oil-filled housing and the switching column, and bending can
create problems in the known load selectors with the interaction of
the elements carried on the individual contact supports, in partic-
ular the vacuum switches which work with the annular actuating
means, because the actuating stems of the vacuum switches normally
move through a relatively short linear stroke.
It is an object of this invention to avoid these disad-
vantages and to provide a load selector of the known type that
efficiently positions the vacuum switches and ensures their accu-
rate and sure actuation under all operating conditions.
This object is attained by the load selector of the first
patent claim. The dependent claims relate to particularly advanta-
geous features of the invention.
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With the load selector according to the invention the
vacuum switches are mounted upright on the contact support. The
contact supports have slide contacts that are exactly guided by
rollers in an output ring. Bumps in this output ring control the
vacuum switches. Slide contact blades that are provided in the
contact support conduct the current to the output ring. Further
slide contact blades on the contact support assist conducting the
current.
According to a further feature of the invention the
contact supports are each movable on the switching shaft. The
particular advantage of the load selector according to the inven-
tion is, in addition to its small size, that the overall forces for
actuating the vacuum switches as well as the contact force are
axially transmitted via the output ring and the fixed contacts to
the stable oil-holding housing. In this manner radial stressing of
the switching column, in particular enough to make it bend, and the
change in switching times of the vacuum switches this causes are
avoided with a reduction of the contact forces. With the rotatable
and axially displaceably mounted contact supports that are guided
axially by the output ring, the switching sequence is ensured even
with considerable axial shift caused by differential thermal
expansion of the switching column and the oil-filled housing.
The invention is more closely described in the following
by way of the exemplary drawing. Therein:
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21457 PCT/EP99/02020 Transl, of WO 99/60588
FIG. 1 is a first embodiment of a load selector according
to the invention seen in section from the side;
FIG. 2 is a portion of the contact support of this load
selector in section seen from above;
FIG. 3 is a second embodiment of the load selector
according to the invention seen in section from the side;
FIG. 4 is a schematic diagram of the circuit of the load
selector according to the invention;
FIG. 5 is a modified circuit;
FIG. 6 is a typical switching sequence of a load selector
according to the invention.
The load selector shown in FIG. 1 is an embodiment of the
invention with a contact support 3 rotatably mounted on a switching
shaft 2 and vacuum switches 27 and 28 mounted upright on it. In
particular with regard to structure:
The load selector is comprised of an insulating cylinder
1 provided centrally with a longitudinally extending switching
shaft 2 also of insulating material. As is known, the switching
shaft 2 is rotatable, to whihc end there is normally a maltese
drive which is not shown. Also not shown is the bearing in the
bottom of the insulating cylinder 1 for the switching shaft 2.
The switching shaft 2 carries at each plane of the fixed
contacts 16 to be engaged and which are described more closely
below a contact support 3 that is mounted on a pivot block 4 on the
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switching shaft 2. The pivot block 4 is fixed by screws 4.1 and
4.2 to the switching shaft 2.
The contact support 3 is formed of a bearing part 5, a
support housing 6, and a contact housing 7. The individual parts
of the contact support 3 are connected together by screws 8 and 9.
The contact housing 7 itself is formed in the illustrated embodi-
ment of an upper contact-housing part 10 and a lower contact-
housing part 11; both parts are connected together via further
screws 12. The contact housing 7 can also be made in one piece.
The bearing part 5 has a bore 13 holding a bolt 14 that forms the
connection with the bearing block 4 and thus permits pivoting
movement. The bolt 14 is secured in place by a cross pin 15. The
entire contact support 3 is thus pivotal as a unit about the bolt
14 relative to the axially nonmovable switching shaft 2.
The inside surface of the insulating cylinder 1 has, in a
respective plane for each of the phases to be switched, an array of
fixed tap contacts 16 electrically connected to the respective taps
of the control winding of the tapped transformer to be switched.
The fixed tap contacts 16 are engageable by respective contacts 17
and 18. These contacts 17 and 18 are provided horizontally offset
from each other on the contact support such that rotation of the
switching shaft 2 and thus of the contact support 3 makes at least
one of the contacts 17 or 18 touch the adjacent new fixed tap
contact 16 before the other of these contacts has left the old
fixed tap contact. The contact 17 thus works as switching contact,
the other contact 18 as auxiliary contact.
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FIG. 2 shows in top view the two contacts 17 and 18 next
to each other on the contact support 3 and the fixed tap contact
16. In the embodiment of FIG. 2 the contact 17 acting as switch
contact is formed in order to carry the most possible current of
two connected parts 17.a and 17.b. Each of the contacts 17 and 18
is formed of a respective upper contact part 17.1 or 18.1 as well
as a lower contact part 17.2 and 18.2. In the particular embodi-
ment of FIG. 2 of the switching contacts, the contact 17 thus is
formed altogether of four parts, namely two upper contact parts
l7.la and l7.lb as well as two lower contact parts 17.2a and 17.2b.
This double construction of the contact 17 acting as switching
contact is advisable for various embodiments but not essential to
the invention. The upper contact parts 17.1 and 18.1 and the lower
contact parts 17.2 and 18.2 of the contacts 17 and 18 are pivotal
about a separate pivot axes 19, 20, 21, and 22 on the contact
housing 7. The are urged toward each other by springs 23, 24, 25,
and 26 toward the fixed tap contact 16 engaged between them.
In other words: the upper contacts 17.1 and 18.1 as well
as the other contact parts 17.2 and 18.2 press with a defined
contact force on both sides of the respective fixed tap contact 16.
As a result of the described pivoting about the separate axes 19,
20, 21, and 22, sliding on the fixed tap contact 16 is possible.
In FIGS. 1 and 2 only the front or upper contact, pivots, and
springs can be seen.
Furthermore each contact support 3 carries two vacuum
switches 27 and 28 held by respective upper and lower mounting
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collars 29, 30, 31, and 32 so that the cuffs 33 and 34 as well as
the actuating stems 35 and 36 of the vacuum switches 27 and 28
extend upward. Of these described parts only the front one is
shown in FIG. 1. In order to operate the actuating stems 35 and 36
of the vacuum switches 27 and 28, there are two levers 37 and 38
that each have a free end carrying a respective roller bolt 41 and
42 with a respective control roller 39 and 40. On their other ends
they are effective on the respective already described actuating
stems 35 and 36. Both levers 37 and 38 are pivotal on respective
pivots 43, 44; it is also possible to provide a common pivot. The
control rollers 39 and 40 of the levers 37 and 38 themselves ride
on a control ring 45 which has an upper control surface 46 as well
as a lower control surface 47. The control ring 45 extends radi-
ally inward from the inner surface of the insulating cylinder 1.
FIG. 1 shows how the first control roller 39 rolls on the lower
control surface 47 and the second control roller 40 on the upper
control surface 46. The two control surfaces 46 and 47 thus serve
to actuate the vacuum switches 27 and 28 in that as the respective
control rollers 39 and 40 ride onto a bump the respective levers 37
and 38 will pivot about their pivots 43 and 44 and thus operate the
actuating stem 35 or 36 of the respective vacuum switch 27 or 28.
In addition on the inside of the insulating cylinder 1 is
an output contact ring 48 that has an outwardly leading contact
element 49 and serves for connection to the load. In particular
the control ring 45 and the output contact ring 48 can be formed as
a single part of a conducting material as shown in FIG. 1. The
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output contact ring 48 on its side is associated with a mechanical
output contact 50 that is provided on the contact support 3 and
that like the already described contacts 17 and 18 is formed of an
upper output contact part 50.1 and a lower contact part 50.2. Both
of these output contact parts 50.1 and 50.2 are similarly mounted
on respective pivots 53 and 54 and are pressed together by respec-
tive springs 51 and 52 so that they grip the output contact ring 48
with a defined contact pressure.
Finally the contact support 3 has two further rollers 55
and 56 that roll on opposite faces of the output contact ring 48
ant thus guide the entire contact support 3. With this described
system tolerance problems of all kinds are compensated for, in
particular bending of the long switching shaft. The contact
supports 3 pivotal on the switching shaft 2 are in any case so
guided by the output contact ring 48 that in spite of described
tolerance problems there is an exact control of the rollers 39 and
40 and thus actuation of the vacuum switches 27 and 28, in spite of
their limited actuation strokes.
FIG. 3 shows a further embodiment of a load selector
according to the invention. Here, unlike the above-described
embodiment, the contact support 103 is not pivotal but instead is
axially slidable. Inside the insulating cylinder 101 there is
again a centered switching shaft 102 that carries a longitudinally
slidable contact support 103. The longitudinal slidability is
effected by means of a guide 104. In this system also there are
two upright vacuum switches, of which only the front vacuum switch
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105 is shown. In each plane there is also an annular array of
fixed tap contacts 106 on the wall of the insulating cylinder 101
which are also engaged by a switching contact and an auxiliary
contact of which in the figure only an upper contact part 107.1 and
the respective lower contact part 107.2 are shown. Similarly there
is again inside the insulating cylinder 101 an output contact ring
108 that is flanked by an upper output contact part 109.1 and a
lower output contact part 109.2. The entire contact support is
guided in this system by a roller 110 which rides on the surface of
the output contact ring 108. The vacuum switches are controlled by
two levers 115 and 116 which have on their free ends respective
control rollers 113 and 114 that bear on a lower shaped control
surface 111 or an upper shaped control surface 112 and thus actuate
the actuating stems~of the vacuum switches, of which only the
actuating stem 117 of the front illustrated vacuum switch 105 is
shown.
FIG. 4 schematically shows the circuit that is formed by
the load selector according to the invention. The electrical
connections between the contacts 17 and 18 and the vacuum switches
27 and 28 and from there to the output contact 50 and thus to the
output contact ring 48 are not shown in corresponding FIG. 1.
FIG. 5 shows a circuit where as shown in FIG. 2 the
switch contact 17 is formed of two adjacent contact parts 17a and
17b. In this embodiment corresponding to FIG. 2 the electrical
connections are also all that is shown. It is clear that the basic
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operation is not different, that it is achievable by the circuit of
FIG. 4 of that of FIG. 5.
An exemplary switching sequence is shown in FIG. 6.
Thus a load selector with different switching steps is
illustrated. Such.a load selector with varying switching steps is
shown in principle in EP 0,160,125. Thus the average offset
between the fixed tap contact connected to the primary winding and
the two adjacent fixed step contacts is larger than the average
distance between the remaining fixed tap contacts. This allows the
load selector to be used also in transformers with high nominal
voltage ratings; since they allow considerable continuous peak
voltage to be handled. This reference also discloses particular
fixed tap contacts that are variously formed for such a load
selector.
In any case it is particularly advantageous that the
fixed tap contacts 16 are not arcuate, as in the state of the art,
so as to conform to the curvature of the insulating cylinder 1, but
instead are straight as shown in FIG. 2. While according to the
prior art the respective upper and lower contact parts 17.1, 18.1
and 17.2, 18.2 run continuously in the same track on the fixed
arcuate tap contacts, the straight shape ensures that continuously
other points of the surface of the fixed tap contacts 16 are
engaged, reducing their wear.
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