Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF T~E INVENTION
Field of the Invention
This invention relates generally to voltage
regulating systems employing tap-changing trans~ormers, and
more specifically, this invention relates to a fail-sa~e
circuit or verifying that the non-arcing switch in the
~ap-changing trans~ormer system is operating properly before
X'' ' ~
~ . . .
' '
7~
1 a movable tap contact is moved to change the tap setting of
the transformer.
Description of the Prior Art
In order to prevent the undesired, and sometimes
destructive, effects of line voltage varia~ions, various
types of voltage regulators have been developed. One type
of voltage regulator is the tap-chanqing transformer. ~his
type of regulator is particularly useful in connection with
the regulation of relatively high line voltages for the
provision of relatively high power requirements.
A tap-changing transformer voltage regulating system
employs a shunt winding, connected from the load line to the
common side of the source, or directly across the source, and a
series winding connected from the load line to the high or
energy side of the source. The series winding is tapped or
divided into a plurality of sections, wlth a stationary contact
associated with each division between sections. A movable tap
contact (or tap contacts)is automatically driven in response to a
change in the line voltage~ A reversing switch is automatically
actuated in order to cause the flux produced by the ~eries winding
to be either additive or subtractive with respect to the flux
of the shunt winding, hence increasing or decreasing the line
voltage, as required.
Inasmuch as the load current is passing through the
series winding, a tap change can result in the formation of
relatively large electrical arcs. Such arcs are destructive to
the movable tap contacts and the stationary contacts, thus
reducing the life of the tap switch. In addition, since the
tap-changing transformer is located in an oil bath, the action of
- 2 -
l the arc on the oil causes reduction of the oil to produce carbon
in a solid or particulate form and hydrogen in gaseous ~orm.
In order to minimize undesirable arcs at the tap
switch, a pair of tap contacts are individually connected to the
source through a line having a bridying reactor (or preventive
autotransformer). A non-arcing switch is connected between the
parallel tap contact lines, and each of the lines includes a
current redirecting switch on the source side of the non-arcing
switch. When it is desired to move a tap contact, a control
motor is energized to open the current redirecting switch in that
line to cause the current flow for that tap contact to pass
through the normally closed non-arcing switch. The non-arcing
switch is then opened to break the current flow to that tap
contact, after which the control motor transfers the tap contact.
The non-arcing switch is then closed, after which the current
redirecting switch for the tap contact that was moved is closed.
While this arrangement works quite well, failure or
improper operation of the non-arcing switch, or failure of a
current redirecting switch to open, will result in production of
undesired arcs at the tap-changing transformer contacts. Such
failures, and the resultant undesired arcs, could go undetected
for quite some time, thus resulting in considerable contact dama~e
and the reduction of oil. Accordingly, it is desirable to remove
the tap-changing transformer regulating system from the load
circuit and/or indicate an improper operation of the current
redirecting or non-arcing switches.
In general, the prior art reveals little in the way of
concrete efforts to overcome the problems of a failure in the
current redirecting and non-arcing switches. An example of one
approach that has been tried is illustrated in U.S. Patent No.
- 3 --
X
~75~
1 3,436,646-Prescott. This arrangement utilizes a nu~ber of
switches to tegt for circuit conditions after each desired open-
ing or closing o~ a current redirecting switch or the non-arcing
switch. Hence, as shown in FIGURE 2 of the patent, a sequential
checking of the current redirecting and transfer switches appears
to be achieved. While this reference at least shows an attempt t~
deal with the problem of a failure in a current redirecting and
non-arcing switches, it involves a rather complex arrangement that
provides more information than is actually required, so long as
the protecting system is operating. Accordingly, the prior art
does not disclose any simple, reliable arrangement or determining
that a tap-change can be effected without fear of an undesirable
arcing condition.
SUMMARY OE' THE INVENTION
The present invention obviates the deficiencies of the
prior art by providing a fail-safe circuit that tests circuit
conditions immediately prior to transfer of a movable tap contact
during a tap change and determines whether such a tap change can
be effected without arcing at the tap contacts. This desirable
protection is achieved by utilizing a test immediately prior to
the transfer of the movable tap contact in order to verify that
proper current redirecting switch and non-arcing switch operation
was achieved. If an improper switch condition is detected, the
tap-changing transformer is removed from the load circuit and/or
an indication of the defective switch operation is produced.
In the fail-safe circuit of this invention, an input
is obtained that corresponds to the energizing voltage applied
to an energizable device, such as a motor, that drives the movable
- 4 -
1 tap contact. (This motor can also be utilized, as in the pre-
ferred embodiment disclosed herein, to control the current
redirecting switches and the non-arciny switch.~ An input signal
for the circuit is derived ~rom this voltage by utilization of a
first rectifier circuit which provides a DC input signal. This
~C input signal is connected across a firgt gating device, such
as a silicon controlled rectifier (SCR). So long as the firs~
gating device remains unactuated, the input signal appear~ across
the first gating device, which appears as an open circuit.
A current transformer is located in series with the non-
arcing switch to monitor the current flow through t'ne non-arcing
switch. The output of ~he current transformer is applied across a
second rectifying circuit to produce a DC reference signal indic-
ative of the current flow through the non-arcing switch. This
reference or triggexing voltage is applied to the gate terminal of
the first gating device or SCR to trigger the first gating de~ice
into a conducting state when current flow is detected through the
non-arcing switch. The magnitude of the reference signal applied
to the gate of the first SCR is limited by a Zener diode.
Both the reference or triggering voltage and the input
voltage appearing across the first gating device are applied to
one side of a fail-safe control switch that is the opposite of the
non-arcing switch (i.e., the ~ail-safe control switch is open when
the non-arcing switch is closed and closed when the non-arcing
switch is open). As the non-arcing switch is opened immediately
prior to transfer of the movable tap contact, this means that the
fail-safe control switch can be utilized to test circuit conditions
immediately prior to such a tap contact transfer. The other side
of the fail-safe control switch is connected to apply a triggering
voltage to a second or output gating device, such as a silicon
X
l controlled rectifier (SCR), if an improper circuit condition is
detected. This output SCR produces an output signal i~ the tap
contact position should not be changed because of a circuit abnor-
mality. It should be noted that it is necessary to prevent the
input signal from triggering the first SCR through the path lead-
ing to the first side of the fail-safe control switch, and thus a
unidirectional conducting device, such as a diode, is located in
that lead.
If current flows through the non-arcing switch, as it
should lmmediately prior to a tap contact transfer ir the selected
current redirecting switch has been properly opened a~d the non-
arcing switch is functioning properly, the first SCR is triggered
into conduction to bypass the input signal and prevent it from
being applied to the fail-safe control switch. ~owever, if no
such current flow exists, the input signal is applied to the gate
terminal of the second SCR to produce an output signal. Also,
upon opening o~ the non-arcing switch to prevent current flow to
the tap contact being moved, current flow through the non-arcing
switch should rapidly decrease and the reference signal disappear
in a relatively short time (such as the time until the next
current zero, if a non-arcing switch held open by current flow
therethrough is utilized). Accordingly, a timing circuit includ-
ing an RC network and a breakdown device are connected to the gate
of the second SCR, so that if the reference signal does not dis-
appear within the predetermined time an output signal will beproduced by the second SCR.
Upon receipt of an output signal from the second SCR,
indicating an improper circuit performance, the tap-changing
transformer system is circumvented or removed from the load line
by connecting the source directly to the load. This is achieved
1 by an appropriate transfer switching arrangement. Thi~ trans~er
switch may be in the form o~ a contact block, mounted for actua-
tion upon production of an output signal, to connect the source
directly to the load. It may be desired to have an impedance
network, such as a plurality of resistors, sequentiaLly inserted
into the circui~between the source and ~he tap-changing trans-
former to reduce the effects of circulating currents produced
when the source connections are being transferred.
If the tap-changing transformer is energized under no-
load conditions, the lack of current flow through the non-arcing
'~ switch will result in the fail-safe circuit circumventing the
tap-changing transformer. Accordingly, a second current trans
former is loca~ed in the load line to detect load current. In
the absence of load current, a switch is actuated to prevent the
production of an input signal. Thus, the fail-safe circuit will
not produce an output signal to actuate the transfer switch to
connect the source directly to~ the load.
Another desirable feature is to utilize some form of
indicating arrangement to let an operator know that an improper
circuit condition has been detected in the tap-changing trans-
former system. One way to achieve this indication is to have the
output signal of the fail-safe circuit also actuate a switch to
cause the fuse in the motor circuit to blow, thus providing an
indication of improper operation as well as precluding actuation
of the motor until the defective switch operation has been remedied.
Another possibility would be to have an appropriate indicator,
such as a lamp, triggered by the output signal. Of course, there
may also be situations in which it would be desired to have the
tap-changing regulator continue operation, even at the risk of
contact damage and breakdown of the oil. In such a case, the
- 7 -
5i94
01 output signa]s of -the fai].-safe circuit could be uti]ized just to
02 trigger an appropriate indica-tor to alert an operator to the
03 necessity oE -taking some corrective action.
04 With the fail-saee circuit di~closed in -this
05 application, a potentially damaging tap change i9 preclucled by
06 determining that circuit conditions immediately prior to the tap
07 contact transfer are improper. The fail-safe circuit is
08 relatively noncomplex and reliable, while utilizing solid state
09 switching to minimize the possibility of improper fail-safe
circuit outputs.
11 More particularly, the invention is a transfer switch
12 for disconnecting a source of energy from a tap-changing
13 transformer regulating system having a non-arcing tap switch
14 arrangement, and for connecting the source directly to a load.
The switch is comprised of the combination of a movable contact
16 block, a contact block carrying member mounted for reciprocal
17 motion, and a latching arrangement to hold the contact block
18 carrying member in a given position when the source of energy is
19 connected to the transformer regulating system. The latching
arrangement is actuated to release the contact block carrying
21 member when the source is to be connected directly to the load.
22 Biasing apparatus urges the contact block carrying rnember away
23 from the given position to which it is constrained by the
24 latching arrangement, while damping apparatus limits the speed of
travel of the contact blocls carrying member under the force of
26 the biasing apparatus. The first stationary contact is connected
27 to the source and is in constant engagement with the contact
28 block. A second stationary contact is connected to a lead
29 extending to the non-arcing tap switch arrangement. A third
30 ' stationary contact is connected to the non-arcing tap switch
31 arrangement through an impedance, the contact block being in
32 _ ~ _
01 engagement with the fir.st, second, and third stationary contacts
02 when the contac-t block carrying member ig congtrained by the
03 latching arrangemen-t. ~ fourth stationary con-tact is connected
04 to the load. Release oE the contact block carrying rnember by the
05 latching arrangement resu~.ts in the contact block sequentially
06 disengaging the second contact, disengaging the thir~1 contact and
07 engaging the fourth contact.
08 These and other ob jects, advantages and features of
09 this invention will hereinaEter appear, and for purposes of
illustration, but not limitation, an exemplary embodiment of the
11 subject invention is shown in the appended drawing.
12 ~RIEF DESCRIPTION OF THE DRAWING
13 FIGURE 1 is a schematic circuit diagram of a
14 tap-changing transformer voltage regulating system with which the
present invention may be employed.
16 FIGURE 2 is a schematic circuit diagram of a
17 tap-changing control system generally illustrating connections to
18 the fail-safe circuit of the present invention.
19 FIGURE 3 is a schematic circuit diagram of a preferred
embodiment of the fail-safe circuit of the present invention.
21 FIGURE 4 is a schematic diagram of a preferred
22 embodiment of a transfer switch for utilization in connection
23 with the present invention.
24 FIGURE 5 is a schematic diagram corresponding to FIGURE
4 illustrating a different set o conditions.
26 FIGURE 6 is a schematic diagram corresponding to FIGURE
27
28
29
31 - 8a -
"~
5~L
l 4 showing yet another set of conditions.
FIGU~E 7 is a schematic diagram correspondinq to FIGURE
4 showing still another set of conditions.
- FIGURE 8 is a schematic circuit diagram illustrating a
modification of the preerred embodiment of FIGURE 4.
DETAILED DESCRIPTION OF THE PREFERRED_EMBODIMENT
A tap-changing transformer voltage regulating system,
of the type with which the fail-safe circuit of this invention is
utilized, is depicted in FIGURE 1. A shunt winding ll -is connec~d
between a load line 13 and a common line 15 of the source of power
(although shunt winding 11 could be connected directly across the
source). The shunt winding 11 is in parallel with whatever system
or systems is fed from the load line 13, such system or systems
being schematically indicated as a load 17.
A series winding l9 is connected to the high or energy
side of the source that is applied to terminal 21. The ~lux pro-
duced by series winding 19 either supports or opposes the flux of
the shunt winding 11, a reversing switch 23 determining whether
the flux of series winding 19 is going to be additive or subtrac-
tive with respect to the flux of shunt winding 11.
Series winding l9 is tapped or divided into a series of
sections permitting changes in the flux produced by the series
winding 19 to be made in incremental steps (to add to or subtract
from the flux of the shunt winding to keep the flux in the shunt
winding 11, and hence the load voltage, constant). These steps
are indicated by a series of stationary taps or contacts 25, 27,
29, 31, 33, 35 and 36. ~his last stationary contact 36 is a
"zero" position and is utilized during movement of switch 23. A
3~
1 pair of movable tap contacts 37 and 39 engage appropriate station-
ary contacts to provide for the flow of load current from terminal
21 to load 17, while providing the appropriate adjustment of the
load voltage. This current flow occurq through an appropriate
bridying reactor consisting, for example, of coils 41 and 43 wound
on a common core.
In order to preclude undesired arcing as a movable tap
contact 37 or 39 is transferred ~rom one stationary contact to
another, the non-arcing switch 45 is utilized. Non-arcing switch
45 may be any appropriate switch of this type, although-in the
preferred embodiment disclosed herein a non-arcing switch utilizi~g
solid state devices, such as a pair o~ SCR's, is employed. The
non-arcing qwitch 45 is utilized in conjunction with a pair of
current redirecting switches 47 and 49. Operation of the non-
arcing switch 45 is appropriately controlled, such as by theschematically illustrated control switch 51, which may be utilizPd
to gate the solid state devices of the non-arcing switch 45
utilized in this preferred embodiment.
To illustrate operation of the tap-changing transformer
voltage regulator, assume that the reversing switch 23 is in a
position to lower the output voltage (i.e., the flux of series
winding 19 opposes or subtracts from the flux of shunt winding 11).
If it is desired to increase the line voltage, this would mean
that the total opposing flux o series winding 19 should be
reduced, such as by moving the tap contact 37 from stationary
contact 29 to stationary contact 31.
With this assumed illustrative example, current redir
ecting switch 47 would be opened, so that the portion of the load
current passing through the switch 47 would be redirected through
the normally closed non-arcing switch 45. Then, the control
7 5 ~ ~
1 switch 51 would be opened to remove the gating signal from the
SCR's of non-arcing switch 45 to cause non-arcing switch 45 to
open at the next current zero, thug causing all of the load cur-
rent to pass through the tap contact 39. At this point, the
movable tap contact 37 may be moved to stationary contact 31
without any arcing. Control switch 51 would then be closed to
gate the SC~'s of non-arcing switch 45 and cause non-arcing switch
45 to return to the closed condition, at which time current would
again flow through the non-arcing switch 45 and the movable tap
contact 37. Current redirecting switch 47 would then be closed to
complete the tap change.
A system for mechanically e~fecting the movement of the
tap contacts 37 and 39 is illustrated in FIGURE 2. An energi~ble
device, such as a motor 53, is utilized to drive the movable tap
contacts 37 and 39, as well as the redirecting switches 47 and 49
and the control switch 51. When a tap-changing action is called
for, one o~ the control switches 55 or 57 will be closed, depend-
ing upon the direction in which the movable tap contact is to be
moved. These contacts 55 and 57 are actuated by appropriate time
delay relays. Upon closure of one of the control switches 55 or
57, the motor will be energized through a fuse 59, normally closed
contact 61, resistor 63, the appropriate control switch 55 or 57,
and a corresponding normally closed limit switch 65 or 67. In the
case of the control switch 55, the circuit would also include a
25 capacitor 69. Normally closed limit switches 65 and 67 would be
opened upon the movable tap contacts 37 and 39 reaching the last
stationary contact 25-35 in the direction of motion indicated by
the corresponding control switch 55 or 57.
As the motor 53 begins its rotation, it will close one
of the holding switches 71 or 73, corresponding to the direction
X ~ 11 -
l of motion of the motor. Closure of the appropriate switch 71 or
73 results in energization of the relay coil 75, ~Ihich opens the
normally closed switch 61, thus precluding operation changes while
a tap change i5 in progre~s. ~fter completion of the desired tap
change, the closed holding switch 71 or 73 will open to de-enerqize
coil 75 and cause switch 61 to return to its normally closed state
to permit initiation of another tap change.
A fail-safe circuit according to the present invention
is schematically illustrated at 77. This fail-safe circuit 77 is
ld shown in greater detail in FIGURE 3.
As may be seen in FIGUP~E 2, the voltage applied to motor
53 is also applied across terminals A and s of fail-safe circuit
77. Terminal B is connected to the common line of the motor
circuit, while terminal A is connected to one side of the motor
(autotransformer action will provide a voltage at this point
regardless of the diraction of motor energization). The motor
voltage across terminals A and B is applied to a rectifier circuit
83, as may be seen in FIGURE 3, which produces a DC input signal
on line 85. The DC input signal on line 85 is referred to common
line 87 of the fail-safe circuit and is applied across a first
gating device or SCR 89. In its non-conducting state, SCR 89
appears as an open circuit, so that the DC input signal appears
entirely across SCR 89 from the point 91 to the common line 87.
A current transformer 93 is located in the non-arcing
switch line, in order to monitor and detect any current flow
through the non-arcing switch 45. Current transformer 93 provides
a reference signal indicative of the current flow conditions in
non-arcing switch 45 at terminals C and D. The reference signal
produced by current transformer 93 as a result of monitoring
current conditions in the non-arcing switch 45 is applied to a
~ .
5'~
l second rectifier circuit ~5 to produce a DC reference or triyger-
ing voltage for SCR 89. The magnitude of this triggering signal
is limited by a Zener diode 97, which is connected in parallel
with a resistor 99 and a capacitor lOl. The triggering voltage
established by current transformer g3, rectifier current 95 and
Zener diode 97 is applied to the gate terminal or electrode 103
of SCR 89 through a resistor 105.
A switch 107 is utilized to track the opposite of the
non-arcing switch 45 (i.e., switch 107 is open when non-arcing
switch 45 is closed and vice versa). Since the control switch 51
directly controls and exhibits the same states as non-arcing
switch 45 (i.e., switch 51 is closed when non-arcing switch 45 is
closed and open when non arcing switch 45 is open), the switch 107
also reflects the opposite conditions from control switch 51, and
it is this relationship that is utilized in this preferred embodi~
ment, although any appropriate control arrangement may be utilized
to have switch 107 yield the opposite condition from non-arcing
switch 45. Switch 107 is connected between terminals E and F in
the fail-sae circuit 77.
The input signal appearing across SCR 89 at point 91 is
connected to terminal E through a resistor 109. Similarly, the
reference or triggering voltage across Zener diode 97, appearing
at point lll, is connected to terminal E through a line 113. In
order to prevent the input signal at point 91 from being fed back
through line 113 to trigger SCR 89, a unidirectional conducting
device or diode 115 is located in line 113.
When control switch 51 is opened to open non-arcing
switch 45, switch 107 is closed at this time. If current flowed
through non-arcing switch 45 prior to the opening of control
- 13 -
X
~ ~ ~ 7r~ 4
1 switch 51, which it should have, the triggering voltage at point
111 would have placed SCR 89 in a conducting state, essentially
shorting point 91 ~o the common line 87. In such a state, ~he
entire input slgnal between line 85 and common line a7 would
S appear across a resistor 117 located in line 85, so that no volt~
age would be connected to terminal E from point 91. However, if
there had been no current flow through the non-arcing switch 5;,
such as by a failure of the proper current redirecting switch to
open or a failure of the non-arcing switch 45, SCR 89 would not
have been gated on, so that the input voltage at point 91 would be
conveyed to terminal E. This input voltage would then be conveyed
thxough switch 107 and a resistor 119 to the gate terminal or
electrode 121 of an SCR 123. Accordingly, SC~ 123 would be trig-
gered into a conducting state to produce an output signal across
terminals G and H. This output signal would ~e provided by the
input voltage appearing from line 85 to common line a7. Therefore,
an output signal is produced to indicate that current flow did not
exist through the non-arcing switch when it should have, which
could mean that load current is still flowing through the tap con-
tact to be moved, and thus presents the possibility of an unde-
sired arc.
Upon opening of the control switch 51, the non-arcing
switch 45 should open to terminate current flow to the tap contact
37 and 39 ~hat is to be moved, assuming that the current flow has
been properly redirected through the non-arcing switch 45. If for
some reason this current flow doe not terminate rather quickly
(i.e., within one-half cycle, the maximum time required to reach
the next current zero), the possibility of undesired arcing during
moving of the tap contact still exists. Therefore, it is also
necessary to produce an output ~rom the fail-safe circuit 77 upon
- 14 -
~ ' .
~ r
1 the occurrence of this eventuality.
To achieve this function, the reerence or triggering
voltage at point 111 is conveyed to the terminal E and thence
through switch 107 and to a timiny circuit o~ resis~or llg and a
capacitor 127. In addition, the timing circuit incorporates a
breakdown device 129. Thus, if the voltage appearing at point 111
is still great enough after a time determined by resistor 119 and
capacitor 127 to cause breakdown of device 129, SCR 123 will be
triggered to provide an output signal across terminals G and H.
(A resistor 125 is included to provide a bleed path for capacitor
127, but as this resistor 125 is very large in comparison to re
sistor 119, it has very little effect on the changing time of
capacitor 127.)
The output signal appearing at terminals G and H may be
applied to a coil 131 to actuate a transfer switch arrangement of
the type disclo~ed in FIGURE 4. Coil 131 could also be utilized to
actuate an indicating device,leither in conjunction with the trans-
fer switch of FIGURE 4 or independently thereof. For example, coil
131 could be utilized to close fuse actuating switch 133 of FIGUR~
3. Closure of switch 133 would short the motor voltage to the
common line, resulting in activation or blowing of the fuse 59.
Blowing of fuse 59 would serve the dual function of providing an
indication of trouble to an operator, while also disconnecting
motor 53 to prevent its actuation of movable tap contacts 37 and
39. Of course, other type~ of indicating devices could also be
utilized, such as a lamp or an audible signal.
One problem that might be encountered is that if the tap-
changing transformer was utilized without a load (i.e., operating
into a no-load condition), the lack of current flow through the
non-arcing switch 45 would cause the fail-safe circuit 77 to
produce an output signal. In order to preclude this type of
X - 15 -
1 operation, a current transformer 135 is located in load line 13
If current is ~lowing in load line 13, current transormer 135 will
produce a signal to be applied to the rec~ifier formed by diodes
137, 139, 141 and 1~3. ~rhis signal will energize the relay coil
145 to close the normally open switch 147 in line 149 conveying
voltage from the motor to terminal A. (Switch 147 could also be
located at any other appropriate posi~ion in the circuit, such as
in line 85.) However, if there is no load current, the normally
open switch 147 will preclude the production of an input signal on
line 85. There~ore, the absence of a triggering voltage at point
111 will not result in the production of an ou~pu~ signal across
terminals G and H.
With respect ~o the preferred embodiment of the transfer
switch illustrated in FIGURE 4, it may be noted that a frame hav-
15 ing two elements 151 and 153 is schematically illustrated. A
current conducting contact block 155 is mounted on a contact block
carrying member 157, which extends through the frame portions 151
and 153. Contact block 155 has a first portion 159 adapted to
engage stationary contacts 161 and 163 and a second por~ion 165
20 adapated to engage stationary contacts 167 and 169. One end of
contact block carrying member 157 has a latching portion 171 which
is engaged by armature 173 actuated by coil 131.
Adjacent the other end of contact block carrying member
157 there is located a dashpot 175, which includes a cylinder 177
25 and a piston 179. Dashpot 175 is located internally of frame
portion 151, while externally of this frame portion 151 there is
located a compression spring 181 positioned between frame portion
151 and a stop 183 on the contact block carrying mem~er 157.
The operation of the transfer switch of FIGURE 4 may now
be understood by reference to FIGURE 4 in connection with FIGURES
5-7. In FIGURE 4, the position of the transfer switch during nor-
mal operation is depicted. In such position, contact block
- 16
~ .
l 155 connects the source applied to terminal 185 to terminal
21 (FIGURE 1). In addition to the simple conducting
path through contacts 161 and 167 to terminal 21, there is
a parallel path through contact 163 and an impedance,
such as a resistor 187. The purpose of the impedance or
resistor 187 is to limit the circulating current and the
resulting arc that results upon actuation of the transfer
switch.
Upon production of an output signal by fail-safe
circuit 77, coil 131 is energized to withdraw armature 173
from engagement with latch portion 171. As a result,
compression spring 181 forces the contact block carrying
member 157 to the left (FIGURE 4 orientation). As may be
seen in FIGURE 5, the first effect of this travel is to remove
lS the portion 165 of contact block 155 from contact 167.
However, portion 159 of contact block 155 i5 still in engagement
with contact 163, so that current flow to the tap-changing
transformer continues through resistor 187. At this point,
portion 165 of contact block 155 has not yet been brought into
engagement with contact 1690
Further motion of contact block 155 (as illustrated
in FIGURE 6), brings portion 165 of contact block 155 into
engagement with contact 169. Portion 159 still engages contact
163, so that the circulating current is limited by resistor 187.
Also at this point, piston 179 of dashpot 175 is beginning to
compress the fluid in cylinder 177 to slow the motion of the con-
tact block carrying member 157 produced by spring 181.
Finally, as seen in FIGURE 7, portion 159 of contact
block 155 is removed from contact 163, so that the source on
terminal 185 is connected directly to a terminal 189, to which
- 17 -
,7~6,~
l is connected the load line 13. Hence, the tap-changing
transformer has been circumvented an~ the source connected
directly to the load line. Daghpo~ 175 also serves to cushion
the impact at this final position to prevent contact bounce
and damage to switch elements. After correction of the problem,
the transfer switch may be manually reset by urging contact
block carrying member 157 against the force of spring 181 to
permit the armature 173 to once again engage the latching portion
171.
The impedance or resistor 187 could also involve a
network in which the current to the tap-changing transformer
would be decreased in sequential steps. A resistive network
for achieving this procedure is schematically illustrated in
FIGURE 8. Accordingly, a resistor 191 is connected to a
15 stationary contact 193, a resistor 195 is connected to a
stationary contact 197, a resistor l99 is connected to a
stationary contact 201, and a resistor 203 is connected to a
stationary contact 205. Purely for illustrative purposes,
resistors l91 and 195 are 0.1 ohm, resistor 199 is 0.3 ohm and
20 resistor 203 is 0.8 ohm. With such an arrangement, the
initial movement of contact block 155 would cause it to leave
contact 167 so that current flow would be through the 0.1 ohm
of resistor 191. Furthex motion of contact block 155 would
bring it into engagement with contact 197, so that current flow
25 would be through the parallel path of resistors 191 and 195,
with a resistance of 0.05 ohm. Continued motion of contact
block 155 would remove it from contact 193, so that all current
flow would be through the 0.1 ohm of resistor 195. At the
point that contact block 155 was brought into engagement with
30 both contacts 197 and 201, all current flow would still continue
- 18 -
1 through resistor 195. As contact block 155 leaves contact
197, the resistance would increase to the 0.4 ohm of resis~ors
195 and 199. As in the case of contacts 197 and 201,
engagement of contact block l5S with contacts 201 and 205
continues the current flow through the 0.4 ohm of resistors
195 and 199. As contact block 155 is disengaged from contact
201, the total resistance in the circuit from terminal 185
to terminal 21 increases to the 1.2 ohm of resistors 195,
199 and 203. And finally, the contact block is brought into
engagement with contact 169 to connect the source on terminal
185 directly to the load line of terminal 189.
It should be understood that various modifications,
changes and variations may be made in the arrangements,
operations and details of construction of the elements disclosed
herein without departing from the spirit and scope of this
invention.
- 19 -
