Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ELECTRICAL POWER CONTROL SYSTEM
The present invention relates to an electrical power
control circuit and more particularly to an electrical power
control circuit far electrical lighting systems, for example
fluorescent lighting systems in large commercial buildings.
A known power control system for providing a reduced
voltage to fluorescent lamps in an electrical lighting
arrangement is disclosed in WO 88/03353. In this document,
a
transformer provides a reduced voltage which can be
supplemented by a further transformer up to a normal mains
voltage for the purpose of enabling the fluorescent lamps
to
strike. The further transformer is then disabled so that
the
reduced voltage is again applied for running the lighting
system thereby reducing power consumption. Of course any
voltage reduction should not result is a perceptibly dimmer
light output.
Another known power control system for providing a
reduced voltage to fluorescent lamps in an electrical lighting
system involves the use of a plurality of switchable
transformers which at start up are switched out so that a
normal mains voltage is applied directly to the lighting.
Then, they are switched in to provide the reduced lighting.
However, there will be a power surge generated when
disconnecting the transformer if it is operating. For example,
a l0 KVA transformer for a bank of up to 200 lamps, could
generate a surge of 400 amps when switched in this way.
Amongst other things, the switching contacts would rapidly
degrade leading to un-reliability. Thus, these type of systems
have not been used due to their failure rate.
It is an object of the present invention to provide an
electrical power supply circuit which overcomes the above
problems with switching transformers.
According to one aspect of the present invention there
is provided a method of controlling an electrical power system
for providing one of a plurality of selected voltages to
a
load, the method comprising the steps of:-
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2
(a) electric:ally connecting one ensl of a winding to the
posit=ive termina_I of ~: source of electrical power, the winding
being tapped at a predE>t~ermined position for sup~>lying an output
term~:na.1 with a select:E~cl voltage;
$ (b) enablimg a ~=erminal connection means to electrically
connEect the other end c~f said winding to a neutral terminal of
said source of a Lectrical power in response t;o supply of power
being required;
(c) excluding a predetermined number of turns of said
l~ windvyng in response to a request for another selected voltage;
( d ) monito ai ng fc>r at least one ty~~e o:1= f ault condition;
and
(e) electrically disconriect:ing the winding from the neutral
terrrunal and elec:tr_icalLy short-circuiting said ether end of the
15 winding to said predetermined position when a fault condition is
detected .
In this way, the present invention can provide a number of
different output ~~oLt ~~ges at the output terminal according to
demand. Furthermore, when a fault condition ~s monitored, a
20 fail:7,afe condition is provided wherein the effect. of the winding
is taken out of c=i rc:uit in a sa:>=e ~~ray by disconnecting the winding
from the neutral terminal an<:1 preventing turns of the winding
being open circc.n t. Accordingly, damage to t he winding and
circmitry of t=he system in general is avoided.
25 Preferably, step (c) comprises disablin~~ the terminal
connection means ':_; ele~tricall.y disconnect said ether end of said
winding from the rue-~utr<~:L terminal and enabling a switching means
to electrically connect to the neutral terminal to exclude the
predetermined number of turns c>1= said winding located from said
i~ othew end of t_he w:indincl.
Thus, it is L->ossi'h1e to short-circuit just the turns of the
wind~.ng toward th~~ ot:her end of the winding which sari connected to
the neutral terninal. 'this is effected towards the neutral
terminal end thereby enabling better performance from the
~i5 conneect ion means and swit thing me>ans since sma Ll er currents are
encountered.
Conveniently, step (e) comprises disabling said terminal
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connection means and said switching means and enabling a
further switching means to electrically short-circuit said
other end of said winding to said predetermined position.
A
In this way, the winding can be disconnected from the
neutral terminal ~in a safe and effective manner whilst
preventing turns of the winding from being open circuit.
In a preferred embodiment, the method further comprises
the step of monitoring for an increased load demand and
stopping step (c) in response to a predetermined load demand.
10. As a result, whilst a preferred (reduced) voltage can be
supplied during stable conditions, a relatively higher voltage
can be supplied when an extra load demand appears.
In another embodiment, the method further comprises the
step of monitoring the voltage to said one end of the winding
and stopping step (c) in response to the voltage falling
below
a predetermined value.
As a result, whilst a preferred (reduced) voltage can be
supplied during stable conditions, a relatively higher voltage
can be supplied to compensate for when the input voltage
drops.
Conveniently, the method further comprises the step of
supplying said request for another selected voltage after
the
lapse of a predetermined time interval following supply of
power being required.
In this way, another voltage can be provided in a simple,
convenient and cost effective manner.
According to another aspect of the present invention
there is provided an electrical power control system for
providing one of a plurality of selected voltages to a load,
the electrical power control system comprising:-
a positive and neutral terminal for connection to a
source of electrical power;
an output terminal for supplying a plurality of selected
voltages;
,. 35 a winding having one end electrically connected to the
positive terminal and being tapped at a predetermined position
for supplying the output terminal with a selected voltage;
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a terminal c: ~nne..-.tion mean: capable of bEeing enabled to
ele~ct::rically connect t:rte ether end of the winding to the neutral
terminal;
a switching mean:: capable of being enabled to exclude a
S predcete:rmined number of turns of said winding in response to a
request for another selected vo:Ltage;
monitoring r~ieans f~~r moW .toeing at Least or:e type of fault
condition; and
further sw:~tching means capable of being enabled to
1~ elect:rically short-r_.ixw:mit said other end of the winding to said
predc-~~te:rmined position when a fault condition is detected.
In this way, different output. voltages can be provided at the
output germinal a~:~:ording to demand, yet when a fault condition is
moni.t ored, a f<~il ;~:Ee c:c:~nciition :i:> effected wherein t:he effect of
1S the winding is rer~u:wed in a safe way so that. damage to the winding
and circuit of the system is avoided.
Preferably, said ::witching means is connected to the neutral
terms nal to excl:ide the predetermined number o~- turns of said
winding from said other end of the winding.
2~ In one case, in response t:~:-~ monitoring of a f ault condition,
said monitoring m~~ans disables s<~id terminal connection means and
sa id switching me ins to el ectrica=ply discorwuect: said other end of
the winding from r_he rE-ut=ra.1 terminal and enables said further
switching means .
ZS In a preferred embodiment,. said monitorinc means further
comprises a curremt dem-~nd sensing means for sensing for transient
current changes in the ~~urrent demand by the load; wherein said
morl.it or:ing means d:iaables said >wi.tching means in response to
tra:~:~ient changes ~~n ca.m rent above a predetermim:d level .
In another pre:~c~rred e~rnbodiment, said monitoring means
further comprises a current overload monitor ing means for
monitoring current to t: rue winding; wherein said rconi.toring means
disa>;~:Les said terminal connection means and said switching means
to e1 ec:trically d iscorme°ct sai~:~ other end ~f-------
3S
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the winding from the neutral terminal and enables said further
switching means in response to a monitored current above
a
predetermined maximum level.
In still another preferred embodiment, said monitoring
means further comprises a voltage monitoring means for
monitoring voltage to said one end of the winding; and wherein
said monitoring means disables said switching means in
response to a voltage below a predetermined minimum.
Conveniently, said monitoring means further comprises
timer means for measuring the time starting from a supply
of
said a selected voltage; wherein said monitoring means enables
said switching means when said measured time exceeds a
predetermined time interval.
In one case, said timer means monitors a further time
starting from supply of said selected voltage; wherein said
monitoring means enables said switching means only when said
further time exceeds a further predetermined time interval
during which the voltage to said one end of said winding
has
not fallen below said predetermined minimum.
By having two time intervals arranged in this way,
unnecessary changes in the system are not made until stable
conditions have been attained.
It is preferred that the timer means is reset whenever
the switching means is disabled or said further switching
means is enabled.
Conveniently, the terminal connection means, the
switching means and the further switching means comprise
relay
contacts.
It is preferred that the system further comprises a zero
crossing detector so that movement of the relay contacts
can
take place at zero crossing points.
Examples of the present invention will now be described
with reference to the accompanying drawing, in which:-
Figure 1 illustrates a first electrical power control
system embodying the present invention at start up;
Figure 2 illustrates the system of figure 1 after start
up;
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Figure 3 illustrates the system of figure 1 after
switching to output a reduced voltage;
Figure 4 illustrates a sub-circuit involved in
controlling operation of the system shown in figure 1;
Figure 5 illustrates a second electrical power control
system embodying the present invention at start up.
Referring to figure 1, a positive rail 1 has a positive
terminal L for connection to a source of electrical power (not
shown) and a neutral rail 2 has a neutral terminal N for
10. connection to the source of electrical power. A transformer
winding 3 has a positive end 13 connected to the positive rail
1 and a neutral end 14 connected both to a terminal connection
4 and a terminal 15. The terminal connection 4 can be
electrically connected to a terminal 5, which is connected to
the rail 2, by means of a relay contact 200A and the terminal
15 can be electrically connected to a terminal 7 by means of
a relay contact 300A. At a point 16 within the transformer
winding, a terminal 17 is connected. The terminal 17 can be
electrically connected to the terminal 5 by means of a relay
contact lOOA. The relay contacts lOOA, 200A, and 300A are all
normally open contacts. This is shown in figure 1. Only when
their respective coils 100, 200 and 300 (described
hereinafter) are energised, are the electrical connections
made.
The transformer winding 3 is tapped at a predetermined
point 18 which is connected to an output terminal T. In the
present embodiment, the transformer winding 3 has 126 turns
between point 16 and the neutral end 14, 126 turns between the
point 16 and tapping point 18, and 14 turns between the
tapping point 18 and the positive end 13. It will be apparent
therefore that by suitable operation of the relay contacts
100A and 200A, either the connection of the neutral end 14 to
the neutral rail 2 via terminal 5 or the connection of the .
point 16 to the neutral rail 2 via terminal 17 and 5 can take
place so that one of two selected reduced voltages can appear ,
at terminal T.
The relay contact 300A is operated to short circuit the
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turns of the winding between point 18 and the neutral end
14
so that these are not able to be open circuit which would
be
detrimental to the condition of the transformer winding 3.
A sub-circuit of a monitoring means control circuit is
connected between the rails 1 and 2. This sub-circuit
comprises a fuse 10 having one end connected to the rail
1 and
the other end connected to a terminal point of a normally
open
relay contact 600A. The relay contact 600A can make an
electrical connection to a terminal point which is connected
to one side of a heat sensor 12. The other side of the heat
sensor 12 is connected both to a coil 800 and to a terminal
point of a normally closed relay contact 300B. The relay
contact 300B can make an electrical connection to a terminal
point which is connected to a terminal point of a relay 500A
contained with a box generally identified by reference numeral
51. The relay contact 500A can make an electrical connection
either to a terminal point connected to the coil 100, which
is connected to the rail 2, or to both a lamp Am (Amber)
,
which is connected to the rail 2, and a terminal point
connected to the coil 200, which is connected to the rail
2.
A red lamp Rd is also connected from a point between fuse
and relay contact 600A, and the rail 2.
Another sub-circuit of the monitoring means control
circuit is also connected between the rails 1 and 2. This
sub-
25 circuit comprises a fuse 20 having one end connected to the
rail 1 and the other end connected to a terminal point of
a
normally closed relay contact 100B. The relay contact 100B
can
make an electrical connection to a terminal point which is
connected to a terminal point of another relay contact 200B.
30 The relay contact 200B can make electrical contact with a
terminal point which is connected to a fault condition unit.
The fault condition unit comprises a DC power supply
which provides a 12 volt supply to one terminal of a normally
open relay contact 800B. The relay contact 800B can make
an
35 electrical connection to a coil 900 which is connected to
the
rail 2. Another 12 volt supply is connected to one terminal
of a normally open relay contact 700A. The relay contact
700A
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can make electrical connection to the coil 300 which is
connected to the rail 2. A further 12 volt supply is connected
to a terminal of a normally closed relay contact 800A and a
terminal of a normally open relay contact 900A. The relay
contacts 800A and 900A can make electrical connection to one
terminal of a manual reset switch 20. The other terminal of
the manual reset switch 20 is connected to a coil 700 which
is connected to the rail 2.
A current sensor 21 in the form of a toroid is wound
10. around the rail 1. The output of the sensor 21 is connected
to a first sub-circuit generally identified by reference
number 52 and shown in detail in figure 4. As can be seen, the
output of sensor 21 is connected to a conversion network 24.
The network converts the current signal from sensor 21 and
provides an output comprising a voltage which is proportional
to the current flowing along the rail 1. The voltage output
from the network 24 is connected to a step sensor 22 and a
level sensor 23.
The step sensor 22 detects the rise in level of the input
value from the network 24 against the preceding input value.
In this way, it is possible to detect when the load connected
to terminal T varies so that an increased voltage may be
required, for example in the case of fluorescent lighting, the
variation in load implies switching on of lighting.
To avoid incorrect sensing due to transients on the line
due to switching of inductive components, a null circuit can
be included which effectively stops the sensing for a brief
period of time during switching of, say, relay contact 500A.
Each time the step sensor 22 detects an increase in
current, a signal is sent to short timer 25 which is reset and
started. The output of short timer 25 is sent to gate logic
26 for controlling a switch 27 to enable or disable the coil
500.
The level sensor 23 detects an initial current level and
outputs a signal to a gate 28 for controlling a switch 29 to
enable or disable the coil 600. In the event that the current
level exceeds a predetermined maximum, the ,level sensor 23
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outputs a signal to the gate logic 26.
A voltage sensor 30 detects the voltage on the positive
rail 1 via a wipe located on the relay contact 600A. When the
voltage drops below a certain level, a signal is sent to gate
logic 26 and also to a long timer 31 which is reset and
started.The output of the long timer is sent to the gate logic
26.
The electrical power control system described with
reference to figure 1 operates as follows. Figure 1
illustrates the initial position when power is first supplied
to terminals L and N. In the initial 4 to 8 ms, an initial
current flow occurs along rail 1 and through some turns of the
winding 3 of the transformer to the output terminal T since
the relay contacts 100A, 200A and 300A are in their normally
open position, but those turns do not offer any significant
impedance for such a short amount of time. In addition lamp
Rd is lit via fuse 10 showing not only the presence of a
supply voltage, but that fuse l0 has not blown. The current
sensor 21 senses this flow of current. As a result, the level
sensor 23 outputs a signal to gate 28 along line 40. The logic
of gate 28 provides a signal to switch 29 so that coil 600 is
supplied with current so as to energise the coil and hence
close relay contact 600A.
As a result, a circuit is formed through fuse 10 and the
now closed relay contact 600A. Current can therefore flow
through the heat sensor 12, which detects a cool condition of
the winding 3 at start up, through the normally closed relay
contact 300B, and through relay contact 500A which is
electrically connected to coil 200. Current also flows through
the heat sensor to the coil 800. In addition, the lamp Am is
lit.
Since coil 200 is now carrying current, the relay contact
200A closes to electrically connect the terminals 4 and 5
together so that the neutral end 14 of the winding 3 of the
transformer is connected to the rail 2. Accordingly, current
flows through all the turns of the winding 3. Thus, a voltage
appears at terminal T which comprises 252/266 of the voltage
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at terminal L. The supply of this voltage is indicated by the
lighting of lamp Am.
Since coil 800 is now carrying current, the relay contact
800B closes and the relay contact 800A opens. However, current
will not flow for long through fuse 20 because with the
energisation of the coil 200, the relay contact 200B opens.
It will be appreciated that coils 700 and 900 are designed to
be slow to operate in response to energisation (say 100 ms)
so that the reaction of their respective relays does not take
place before the relay contact 200B opens. Thus, there is no
risk that coil 300 may become energised to close relay contact
300A. The above situation is shown ~in figure 2.
As noted above, the current sensor 21 senses the initial
flow of current through rail 1. As a result, the step sensor
22 detects a step in the current and outputs a signal to short
timer 25 and a signal to gate logic 26 along line 41. By means
of the gate logic 26, the presence of a signal on line 41
inhibits switch 27 from energising coil 500, which remains in
its initial position. However, once the step sensor has
detected the initial flow of current for a predetermined time,
no further step is detected and hence the signal on line 41
disappears.
At the same time as the current sensor 21 senses the
initial current, the voltage sensor 30 senses a voltage above
a predetermined minimum level and outputs a signal to the long
timer 31 and to the gate logic 26 along line 42.
Once the short timer 25 has timed out, a signal is output
to the gate logic 26 along line 43. However, switch 27 does
not energise coil 500 until the long timer 31 also times out
and outputs a signal along line 44. In this way, there is no
undue energisation of coil 500 during periods of voltage
instability. Nevertheless, once the voltage has become stable
and remains so, the short timer 25 controls energisation of
coil 500.
In summary, the gate logic 26 will not operate to turn
on switch 27 if there is a signal on line 41 indicating a step
in current demand or if there is no signal on line 42 which
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indicates insufficient voltage or if both the short timer
and long timer 31 have timed out and output signals on their
y respective lines 43 and 44.
When the gate logic criteria have been met, then switch
5 on of switch 27 occurs so that current flows through coil
500.
As a result, relay contact 500A is moved to electrically
connect to the relay terminal which is connected to the coil
100. Thus, current no longer passes through coil 200 which
becomes de-energised whilst coil 100 now becomes energised.
10 As a result, relay contact lOOA closes and relay contact
200A
opens. Thus, the turns of the winding 3 between the point
16
and 14 are eliminated. Consequently. a voltage appears at
terminal T which comprises 126/140 of the voltage at terminal
L. It will be appreciated that it is preferred that the relay
15 contact 100A closes before the relay contact 200A opens.
This
situation is shown in figure 3.
In addition to the above relay contact movements, it will
be understood that whilst relay contact 200B now closes and
relay contact 100A opens, there remains no current flow
20 through the circuit incorporating these relay contacts.
The circuit of this embodiment incorporates fault
monitoring so as to provide a number of safety features.
In particular, the present embodiment can provide a
failsafe condition in the event of failure of the relay
25 contact operating coils, general overloading of the system,
a fault external to the system creating an overload condition,
a fault in the winding causing a thermal build up and
operating the heat sensor 12, a fault causing the fuse 10
to
blow, a disconnection in the sub circuit wiring causing the
relay contacts lOOA or 200A to release, and any failure which
causes the winding to go open circuit.
The appearance of the failsafe condition is described
below with reference to a number of examples. As long as
current is flowing through rail 1 below a predetermined level,
coil 600 remains energised and the relay contact 600A is
closed. However, when the level sensor 23 detects a current
above a maximum permissible current, a signal is output to
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gate 28 along line 45 and the logic of gate 28 makes switch
29 turn off so that coil 600 is no longer energised. As a
result, relay contact 600A opens which de-energises coils 100,
200 and 800. As a consequence, the relay contacts 100A and
200A open and the'relay contacts 100B, 200B and 800A close.
The latter three relay contacts closing provides for a -
flow of current which energises coil 700 via manual reset
switch 20. Thus, after about 100 ms, the coil 700 causes the
relay contact 700A to close which provides a flow of current
through coil 300. As a result, the relay contact 300A closes
to connect terminals 15 and 7 thereby putting a short circuit
across the primary turns of the winding 3 between points 18
and 14. Consequently, the magnetic field is collapsed so that
the winding 3 ceases to operate as a transformer and offers
substantially no impedance between points 13 and 18.
Since the full input voltage now appears at terminal T,
closing relay contact 300A has the effect that the electrical
power supply system of the present invention is bypassed. In
addition, damage to the winding 3 that could otherwise occur
from being open circuit is avoided so that a failsafe
condition can be provided. In this respect, the situation of
leaving such an open circuit should be considered. If an open
circuit occurs for any length of time, there will be a voltage
drop between points 13 and 16, in the present case 24 volts,
so that the electrical power supply system of the present
invention is not bypassed and hence a true failsafe condition
is not provided. Furthermore, there will be a reversing
energisation of the winding which will lead to an unpleasant
and disturbing vibration in the form of a hum or buzz. In
addition, the winding will eventually reach a saturation
voltage across the open circuit part of the winding. This
saturation voltage can reach quite high values, in the present
case of the order of 760 volts, which is not only potentially q
very dangerous to anyone who should accidentally touch the
system but can also produce sparking due to breakdown of the
insulation thereby producing a winding insulation failure.
It should be noted that the energisation of coil 300
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opens relay contact 300B so that electrical operation of
coils
100 and 200 and their respective relay contacts is inhibited.
If the current flowing along rail 1 drops again, the signal
along line 45 disappears and gate 28 turns switch 29 back
on
so that coil 600 is again energised. This leads to a closing
of relay contact 600A with the effect that relay contact
300A
opens and either relay contact 100A or 200A closes depending
upon the output from logic gate 26. Preferably, the sub-
circuit shown in f figure 4 is arranged such that the relay
contact 200A closes when current flows again along rail 1.
This can be achieved by making sure that long timer 31 is
reset, say by interrupting the voltage sensing of voltage
sensor 30. In this respect, it will be noted that regardless
of the current flow, if the voltage on rail 1 drops below
the
predetermined level, long timer 31 is reset so that relay
contact 500A automatically returns to the position connected
to coil 200.
When the electrical power supply system of the present
invention is in use, if the heat sensor 12 breaks due to
overheating, current no longer flows to coils 100, 200 and
800
with the result that relay contacts lOOA, 200A and 800 open.
Thus, relay contact 300A is closed with the same effects
as
above.
When the heat sensor 12 again detects an appropriate
temperature and closes, current can again flows to coil 800.
As a result, relay contact 800A opens breaking the current
path to coil 700. This results in its relay contact 700A
opening so that current no longer flows to coil 300. The
effect of this is for its relay contact 300B to close to
again
provide current to energise coil 100 or 200. It will be
appreciated that although relay contact 800B is closed, coil
900 is slow to operate so that relay contact 900A does not
operate in time to provide an alternative current path to
coil
700. Thus, the system is restarted.
Another fault monitoring concerns the situation if either
relay contacts 100A or 200A should open due to mechanical
or
electrical failure. Although contact 800B is closed due to
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current flowing through coil 800, coil 900 is not provided
with current because either relay contact 100A or 200B is
open. However, with the mechanical or electrical failure, that
r
open relay contact will close so that current is now supplied
to coil 900. After about 100 ms, relay contact 900A will close
so that current is supplied to coil 700 via manual switch 20
which eventually causes relay contact 300A to operate as
above. It should be noted that this locks the system so that
physical inspection of the system is required. However, power
l0 will still be supplied to the load connected to terminal T.
In a similar manner, should relay contact 800A or coil
800 fail, a similar failsafe condition can still be attained.
It will be appreciated that operation of relay contact
300A whilst relay contacts lOOA or 200A are actuated is
prevented not only electrically, but also mechanically by
interlocking the contacts so that relay contact 300A is
positioned between the relay contact 100A and 200A so that
operation of either of them inhibits operation of relay
contact 300A and operation of relay contact 300A inhibits
relay contact lOOA and 200A.
It will also be appreciated that once the failsafe
condition has been attained, the system can be returned to
normal running by actuation of the reset switch 20 which
breaks the current supply to coil 700 which will then break
the supply of current to coil 300 so that relay contact 300A
opens and either relay contact 100A or 200A closes.
Figure 5 illustrates a second embodiment of the present
invention wherein common components with the first embodiment
bear common reference numerals.
Referring to figure 5 it can be seen that the sub-circuit
containing fuse 20 has been modified. In particular, the fault
condition unit has been changed. The relay contact 200B is now
connected to one terminal of a normally open relay contact ,
1000A and to a coil 1000 which is connected to the rail 2. The
relay contact 1000A can make electrical connection to one ,
terminal of the relay contact 800B, to one terminal of the
normally open relay contact 700A, to one terminal of the
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normally closed relay contact 800A, and to one terminal of
the
normally open relay contact 900A. The remaining connections
are common to figure 1.
In addition to the above, a green lamp Gr is connected
across the coil 100 and a blue lamp is connected across the
coil 300. Thus, when lamp Rd is lit, a user knows that the
system is connected into circuit and that a voltage exists
on
rails 1 and 2 and that fuse 10 has not blown, when lamp Am
is
lit that a voltage resulting from relay contact 200A is being
1Q provided at the output terminal T, when lamp Gr is lit that
a voltage resulting from relay contact 100A is being provided
at the output terminal T, and when lamp B1 is lit that a
fault
condition has occurred.
It will be apparent that at initial start up of the
embodiment in figure 5, current flows through relay contacts
lOOB and 200B through to coil 1000. However, coil 1000 is
slow
to operate so that relay contacts 100B or 200B open before
relay contact 1000A can close. Thus, the various functions
of
the fault condition unit do not have current supplied to
them.
In the circumstances of a fault condition, the effect is
to close both the relay contacts 100B or 200B so that current
is supplied to coil 1000. After the built in time delay,
relay
contact 1000A closes to supply current to the fault condition
unit so that it can operate as described above.
It will be understood that the embodiment
illustrated shows an application of the invention in one
form
only for the purposes of illustration. In practice, the
invention may be applied to different configurations, the
detailed embodiments being straightforward for those skilled
in the art to apply.
For example, whilst the embodiments described are
connected to operate so that relay contact 200A disconnects
as relay contact 100A connects, relay contact 200A can be
left
connected whilst relay contact 100A connects.
In addition, whilst two relay contacts 100A and 200A are
provided to enable the supply of two selected voltages at
terminal T, further relay contacts can be provided to enable
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the supply of more than two selected voltages.
Whilst the embodiments are described for use with a mains
supply of 240 volts at 50 cycles, other mains voltages and
frequencies can be used, for example, 110 volts or 277 volts '
at 60 cycles.
The embodiments described are fully automated with
automatic reset and constant sensing for faults. However,
whilst the present embodiment describes the switching from the
relay contact 100A to the relay contact 200A in the
circumstances of when power demand occurs when switching a
load connected to terminal T, when a low incoming voltage
occurs, when any failure in the sub circuit of figure 4 occurs
or when any circuit fault creating current fluctuation in
excess of a predetermined level, costs can be saved by
incorporating fewer responses to these circumstances. For
example, in simpler forms of the invention, some of these
aspects can be omitted to save costs, say the short and long
timer can be replaced by a simple time delay relay to switch
relay contact 500A. Similarly, the voltage sensor and .step
sensors shown in figure 4 can be omitted.
In addition, the relay contact 500A in box 51 is shown
as a relay contact which can be operated by a coil. It will
be appreciated that control of the operation of the relay
contact within box 51 can take many forms. For example, it can
be dependent on a complex of timers, for example as shown in
figure 4, or it can be dependent on a time delay relay. The
latter is particularly appropriate for the control of loads
having just one or two units, such as street lighting.
Although mechanically operated relay contacts could be
employed, it will be apparent that electronically operated
switches could be used as an alternative. However, it should
be noted that by having the relay contacts 100A and 200A
located at the neutral end of the winding 3, much smaller
switching currents are encountered than with prior art
arrangements. Indeed, by use of the present invention, it has
been possible to dramatically reduce the power rating of the
relay contacts required. For example, a 20 KVA system can be
CA 02226498 1998-O1-07
W O 96!03018 PCTlGB95/OI729
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handled with the relay contact rating of a 2 KVA system
without the deterioration normally associated with switching
large inductive loads. Thus, extremely high reliability is
i
assured.
Whilst the current sensor 21 is located on the rail 1,
it will be appreciated that the current sensor could be
located on the rail connected to terminal T.
Thus, the present embodiment provides a system which can
output a voltage which can be switched between a level
approximating to mains voltage (or a chosen voltage) and a
fully reduced level at switch on of the load, and to a reduced
voltage value which does not produce a noticeable drop in
effect on the load, say illumination of lighting, but which
provides a substantial improvement in economy whilst all the
time providing a secure and reliable failsafe condition in the
event of a fault thereby enhancing the safety of the system
and ensuring that the system complies with various legal
requirements.
It will be apparent that although the present invention
has been described in connection with an fluorescent lighting,
it will be apparent that the present invention can be applied
to other lighting systems and other loads in general.
