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Description  |
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FIELD OF THE INVENTION
This invention is directed to the control of a relay connected to a load,
by controlling the operating or dropout point of the relay so that it
opens or closes at a zero current point in the AC load circuit to
eliminate arcing at the relay contacts.
BACKGROUND OF THE INVENTION
Current is delivered to an alternating current load through the contacts of
a relay. When the load is to be switched off or on, the relay is actuated
or deactuated. When the relay contacts open while current is flowing
therethrough, an arc is drawn until the next zero current point, and this
arc burns the relay contacts. Similar arcing occurs if contact closure is
made at a non-zero current point. It is this arc burning of the relay
contacts which reduces relay life. Accordingly, to achieve greater relay
life, it is desirable to reduce or eliminate arcing upon the opening or
closing of the relay contacts.
SUMMARY OF THE INVENTION
In order to aid in the understanding of this invention, it can be stated in
essentially summary form that it is directed to an apparatus and method
for relay control which achieves minimized relay arcing by operating the
relay so as to open or close its contacts at the zero current point in the
AC load circuit. The method and apparatus include a device for observing
and a means for measuring the time of arcing and a control circuit for
receiving the relay open command signal and sending an operating command
so that the relay opens or closes its contacts at or near the zero current
point of the AC load circuit.
It is, accordingly, an object and advantage of this invention to provide an
apparatus and method for controlling a relay which supplies an alternating
current load which maximizes relay life by minimizing arcing at the relay
contacts when switching the load off or on.
It is another object and advantage of this invention to provide an
apparatus which has a radiation sensor positioned to receive radiation
emitted from a relay contact arc and has a control circuit connected to
the radiation sensor so as to shift the relay operating command to achieve
relay contacts opening or closing at or near zero current crossing.
The features of the present invention which are believed to be novel are
set forth with particularity in the appended claims. The present
invention, both as to its organization and manner of operation, together
with further objects and advantages thereof, may be best understood by
reference to the following description, taken in conjunction with the
accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of an example of a relay installation,
employing the method and apparatus for relay contacts or control in
accordance with this invention.
FIG. 2 is an isometric view of a plug-in relay for use with the method and
apparatus of this invention.
FIG. 3 shows a printed wiring board in which the relay of FIG. 2 is plugged
in, including part of the apparatus of this invention.
FIG. 4 is an isometric view of the sensor secured to the printed wiring
board of FIG. 3.
FIG. 5 is an enlarged side-elevational view of the relay on the printed
wiring board with parts broken away.
FIG. 6 is an end-elevational view thereof with parts broken away.
FIG. 7 is an electrical schematic block diagram showing an apparatus
embodying this invention.
FIG. 8 is a wave form diagram showing the current versus time and showing
the relay command signal versus the same time base.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The system for which the apparatus and method are intended and for which
they improve operating life comprises an AC power source represented by
lines 10 and 12 in FIG. 7. Line 10 is connected to moving contact 14 which
is connected to contact 16 upon closing of moving contact 14. The fixed
contact 16 is connected by line 18 to a load 20. The load 20 may be any
type of utilization circuit, including circuits having inductive and/or
capacitive elements therein although, for convenience, the present
invention will be described in connection with an inductive load. The
utilization circuit 20 broadly represents any alternating current load.
The utilization circuit is connected back to AC line 12 either directly,
as indicated, or through additional contacts acting in parallel with the
contacts 14 and 16.
As is conventional in relays, a solenoid coil 22 has its plunger
mechanically connected to the moving contact 14 so that electrical
actuation of the coil causes actuation of the moving contact. For
convenience of description of the conditions for and operation of the
present invention, the relay illustrated in FIG. 7 will be considered as
"normally closed" in the sense that, when relay coil 22 is not energized,
the contact 14 is closed as by a conventional spring (not shown). The
relay can be constructed in the opposite manner so that, with deactuation
of the coil, the contact will be opened by the spring and, in such a case,
is a "normally open" contact. Command line 24 carries a signal to energize
the solenoid and actuate the relay. In conventional systems, the command
line 24 may be directly connected to relay coil 22 so that, when a signal
is present, the coil is energized and the contacts are opened.
FIG. 8 represents by its curve 26 the current supplied in the AC lines 10
and 12. The current line 26 is optimally a sine wave and is shown in the
single cycle in FIG. 8, with a positive current pulse from 0 to 180
degrees and a negative current pulse from 180 to 360 degrees. A zero
crossing of the current is achieved at 0, 180, and 360 degrees. The
current curve is shown as an idealized sine wave in FIG. 8 for convenience
of illustration. The middle part of FIG. 8 represents the voltage to relay
coil 22 which, in this example, is of the "normally closed" type so that
it requires actuation voltage to the relay to open the contacts. In its
low state represented by line 28, the relay is closed. At the time
represented by rising line 30, the voltage to relay 22 goes to the high
state represented by line 32 and, after a very short period of time of
less than a half cycle, the relay contacts are opened. As is wellknown,
when relay contacts are opened with current flowing therethrough, an arc
is drawn, and this arc burns the relay contacts and reduces the life of
the relay. In conventional circuits, there is no control over the
actuation timing of the relay with respect to the current wave
therethrough. Thus, the time 30 may occur at any point in the cycle
between 0 and 360 degrees, and the chance that it will occur at one of the
zero current crossings, such as indicated by line 27, is small. For this
reason, the life of a conventional relay is greatly reduced.
FIG. 1 shows relays 34, 36 and 38 plugged into printed wiring board 40. The
printed wiring board is a dielectric substrate carrying on one or both
surfaces signal and load wiring for the relays plugged therein. Relay 42
is the same as relays 34, 36 and 38 and is shown in FIG. 2 in a position
where it is unplugged from the printed wiring board and, in FIGS. 3, 5 and
6, in a position where it is plugged into the printed wiring board. The
relays 34, 36, 38 and 42 are examples of commercially available relays
which can be employed in connection with the method and apparatus of this
invention. The relay 42 contains the contacts 14 and 16 and coil 22. Relay
pins 44 and 46 are connected to the coil and insert into board pin sockets
48 and 50, respectively. As is seen in FIGS. 1 and 3, pin socket 48 is
connected to bus 52, which may connect to similar pin sockets with respect
to other relays. The bus 52 is connected to control circuit 54 of this
invention, generally indicated in FIG. 7. The socket 50 is also connected
to the control circuit 54 so that the control circuit controls the
energization of pins 44 and 46 and, consequently, the energization of
relay coil 22. Each relay on the board is connected to be operated by the
control circuit. The control circuit 54 is the control circuit for relay
42 and any other relays in its group. The pin 56 shown in FIG. 2 is the
lead 18 in FIG. 7, which is connected to the fixed contact 16 and is
connected to the utilization circuit. In the present instance, relay pin
56 is connected to the stationary contact of the relay and is received in
socket 18 in FIG. 3, which is part of the line to the utilization circuit.
Relay pin 58 is connected to the moving contact of the relay and plugs
into the socket 10 in FIG. 3 which forms part of the alternating current
line 10 seen in FIG. 7. Relay 42 is thus a plug-in relay unit which plugs
into corresponding sockets in the printed wiring board to receive
actuation signals from the control lines in the printed wiring board and
to control the flow of power in the power lines in the printed wiring
board.
Relay 42 and each of the other relays controlled in accordance with the
apparatus and method of this invention has its contacts visible. In the
relay 42, this is achieved by the provision of a window opening 60 in line
with the relay contacts, as seen in FIGS. 5 and 6. The window opening may
contain a convex lens 62 to collimate the optical emissions of arcing at
the contacts. Positioned to receive the visible arcing emissions is a
light-sensing semiconductor device 64. The light-sensing device is
conveniently positioned on the lower side of board 40. The board is
provided with an opening 66 in alignment with the opening 60. As seen in
FIG. 7, the light-sensing device 64 sends a signal through line 68 to the
control circuit with its signal corresponding to the arcing of the
contacts 14 and 16. While a visible signal is described, arc radiation in
other frequencies can be detected and employed as arc sensors.
It should be noted that, with a sensor (such as light-sensing device 64 for
example) at each relay, each relay can be controlling a load having
different characteristics from the other loads, such as inductive or
capacitive characteristics, and each relay is controlled for "no arc" at
its load contacts.
Referring to FIG. 7, it is seen that the AC lines 10 and 12 also feed
signal transformer 72 which has its secondary connected through a pair of
IN 4004 diodes 74 and 76. Both diodes are connected to the gate of MPS
6566 transistor 78. The collector line 80 of the transistor has a high
pulse at each voltage zero crossing of the supply lines 10 and 12.
Connected between the signal transformer at the lines is full-wave
rectifier bridge 82 which supplies power to regulator 84 which supplies
voltage to the circuit.
The input to rectifier 76 is also connected to the input of IN 4004 diode
85, which has its output connected through a network to the base of MPS
6566 transistor 86. The collector of transistor 86 and the signal line 80
are both connected to the base of MPS 6566 transistor 88. The function of
this circuit is to cancel alternate pulses in line 88 and invert. The
output signal in line 90 is high with a low pulse at alternate voltage
zero crossings.
The sensing device 64 shown in FIG. 7 is a light-sensing LPT 80
photo-transistor, and as previously indicated, a plurality of such
photo-transistors may be respectively observing a plurality of relay
contacts for arcing. Several are shown, but the control circuit may
control only a single relay and, in such a case, only one photo-transistor
is required. On the other hand, while it is most convenient to observe the
arc by means of its visible light output, other types of sensors may be
alternatively employed, so long as they indicate the timing and duration
of the arc. Line 68 is connected to the gate of MPS 6566 transistor 92,
and the collector of this transistor is also connected to signal line 90.
The result is that the signal in line 90 is held low for the duration of
any arc observed by the sensor 64.
Line 90 is connected to terminal 14 of 8039 microprocessor. Microprocessor
94 is programmed as a logic device. Whenever it is instructed to open a
relay contact by a command signal in command line 24, the logic device
actuates relay opening at a subsequent zero crossing indicated in line 90.
If an arc is observed by the sensor 64, or companion sensor, the logic
device measures the duration of the arc and places the duration
information in memory for that relay. Each subsequent relay opening
command causes the logic device to look to the memory for that relay and
add the remembered time to the zero voltage crossing point signaled by
line 90 before actuating the relay by an actuation signal in line 44.
Thus, the relay 42 is caused to open its contact 14 at a zero current
crossing 27 (FIG. 8) so that there is no opening arc. This results in long
relay life.
Referring again to FIG. 8, the bottom waveform illustrates the voltage
condition for drop-out operation of a "normally open" contacts relay, with
the relay contacts closed at line 29, the voltage dropping at line 31, to
the de-actuated relay voltage at line 33, and the contacts opening at zero
current time line 27.
As a particular utilization circuit, a 16 ampere, 230 volt "Calrod" heater
load is supplied through a suitable plug-in relay. Without the apparatus
and method of this invention, the normally expected relay life is about
200,000 cycles. Since the contact burning rate is extremely small when the
apparatus and method of this invention are employed, relay life under the
same circumstances but with the inventive adjustment of the actuation
point for no arcing, of about 5,000,000 cycles can be achieved without
relay failure. The greater the current in the load circuit, the more
arcing occurs with the consequence of more contact damage. The amount of
arcing is a function of the load reactance. A purely resistive load would
produce minimum relay contact damage, but even a "Calrod" heater load is
sufficiently inductive to cause relay contact damage of the amount noted
above. Thus, different relay life improvement can be seen with different
loads.
In most relays, the pull-in or drop-out time changes slowly due to the
amount of cycling required. Such is especially true in permanently
connected equipment. For example, in the present case, the utilization
circuitry 20 is considered to be the heater in a deep-fat fryer and the
command signal in line 24 is a signal related to the deep-fat temperature
so that the temperature of the deep fat can be carefully controlled. In
other types of cooking or other utilizations, the command signal 24 may be
a timing circuit which terminates power to the utilization circuit. In
permanent or semi-permanent installations, circuit and equipment
parameters change only slowly, and thus the observation of the arc need
not be automatic but may be periodically done by the cooker operator, the
manager, or maintenance personnel. In such manually adjusted equipment,
the observer would make a manual adjustment on the control circuit to
adjust the point at which energization of coil 22 occurs as a function of
the current curve 26 so that the contact 14 opens at zero arc. It must be
observed that the energization in lines 44 and 46 does not occur at 0,
180, or 360 degrees on the current wave, but occurs sufficiently before
so that the contacts actually open at the zero point. In such a case, the
control circuit 54 would be one in which a command signal is received in
line 24 signalling the desire for relay opening, followed by an analysis
of the voltage curve, and when the voltage curve reaches a predetermined
point on its cycle, relay coil 22 is energized so that the moving contact
opens when the current curve is at one of its zero points. The manual
adjustment would adjust the point on the voltage curve at which the
solenoid energization occurs. The automatic case is the same, but the arc
is seen by a detector and the adjustment is automatically achieved.
This invention has been described in its presently contemplated best mode,
and it is clear that it is susceptible to numerous modifications, modes
and embodiments within the ability of those skilled in the art and without
the exercise of the inventive faculty. Accordingly, the scope of this
invention is defined by the scope of the following claims.
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