U.S. patent number 3,868,549 [Application Number 05/354,694] was granted by the patent office on 1975-02-25 for circuit for protecting contacts against damage from arcing.
This patent grant is currently assigned to Franklin Electric Co., Inc.. Invention is credited to Edward J. Schaefer, August L. Streater.
United States Patent |
3,868,549 |
Schaefer , et al. |
February 25, 1975 |
CIRCUIT FOR PROTECTING CONTACTS AGAINST DAMAGE FROM ARCING
Abstract
This disclosure deals with a circuit for substantially reducing
or eliminating switch contact damage due to arcing when opening or
closing the switch contacts under heavy AC current load conditions.
This is accomplished by providing a normally open shunt path across
the contacts and momentarily closing the path at the instant of
arcing. The shunt path includes contactless switch means which is
normally open. Opening of the switch contacts effects turning on of
the switch means and transfer of the load current thereto, thus
quenching the arc. At the next subsequent zero crossing of the AC
wave, the switch means is again opened. Similar operation occurs
when the switch contacts close and an arc starts to develop due to
contact bounce.
Inventors: |
Schaefer; Edward J. (Bluffton,
IN), Streater; August L. (Bluffton, IN) |
Assignee: |
Franklin Electric Co., Inc.
(Bluffton, IN)
|
Family
ID: |
23394524 |
Appl.
No.: |
05/354,694 |
Filed: |
April 26, 1973 |
Current U.S.
Class: |
361/13;
361/6 |
Current CPC
Class: |
H01H
9/542 (20130101); H01H 2009/546 (20130101) |
Current International
Class: |
H01H
9/54 (20060101); H02h 007/22 () |
Field of
Search: |
;317/11E,33SC
;307/136 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trammell; James D.
Attorney, Agent or Firm: Hibben, Noyes & Bicknell
Claims
We claim:
1. In a circuit including an alternating current supply, a load,
and switch contacts connected to carry load current when closed and
required to open under heavy current load conditions, the
improvement comprising contactless switch means adapted to be
connected to form a current path which bypasses said contacts, an
electrical device adapted to be connected in series with said load
and said contacts, said switch means including bias means connected
to respond to current flow through said device and to a potential
across said contacts, said switch means being biased to conduction
only when a potential exists across said contacts and current flows
through said device, whereby said switch means is biased to
conduction and load current is transferred to said path from the
time of initiation of an arc until the next subsequent zero
crossing of the alternating current.
2. Apparatus as in claim 1, wherein said switch contacts and said
device form a switch unit, and said device is a heater
resistor.
3. Apparatus as in claim 1, wherein said switch means includes main
terminals for carrying said load current and a control terminal for
turning on said switch means, said main terminals being adapted to
be connected to respond to a potential due to arcing, and said
control terminal being connected to said device, said device
providing energizing current to said control terminal.
4. Apparatus as in claim 3, wherein said device comprises a
resistor.
5. Apparatus as in claim 3, wherein said device comprises a current
transformer.
6. Apparatus as in claim 3, wherein said device comprises a
potential transformer and a resistor.
7. Apparatus as in claim 3, wherein said device comprises an
impedance and said main terminals are connected across both said
contacts and said impedance.
8. Apparatus as in claim 3, wherein said power terminals are
adapted to be connected directly across said switch contacts.
9. Apparatus as in claim 3, wherein said power terminals are
adapted to be connected across said switch contacts and said
device.
10. Apparatus as in claim 1, wherein said switch means is a solid
state device.
11. Apparatus as in claim 1, wherein said switch means comprises a
TRIAC having two main terminals and a gate, said main terminals
being adapted to be connected across said contacts and said gate
being adapted to be connected to respond to current flow through
said device.
12. Apparatus as in claim 1, and further including diode means
connected in series with said switch means to prevent current flow
through said switch means until said potential exceeds a minimum
value.
13. Apparatus as in claim 12, wherein said diode means comprises a
zener diode.
14. Apparatus as in claim 12, wherein said diode means comprises
two back-to-back parallel connected diodes.
15. An AC power circuit comprising electrical contacts adapted to
be connected to a load and an AC power supply, an electrical device
connected in series with said load and said supply, contactless
switch means connected to form a current path which bypasses said
contacts and including bias means responsive to a potential across
said contacts and to current flow through said device, whereby said
switch means conducts only from the time of opening of said
contacts and initiation of an arc and until the next subsequent
zero crossing of the alternating current.
16. Apparatus as in claim 15, and further including diode means
connected in series with said switch means.
17. Apparatus as in claim 16, wherein said device comprises
resistance means connected in series with said contacts, said
switch means comprising a TRIAC having its gate connected to
respond to current flow through said resistance means.
18. Apparatus as in claim 17, wherein said contacts and said
resistance means form an overload switch.
19. Apparatus as in claim 16, wherein said device comprises a
resistor.
20. Apparatus as in claim 16, wherein said device comprises a
current transformer.
21. Apparatus as in claim 16, wherein said device comprises a
potential transformer and a resistor.
22. Apparatus as in claim 16, wherein said switch means is adapted
to be connected directly across said switch contacts.
23. Apparatus as in claim 16, wherein said switch means is adapted
to be connected across said switch contacts and said device.
24. An AC power circuit for an electric motor, comprising a motor
winding, overload switch contacts and a device mounted within said
motor, said winding and said contacts and said device being
connected in series and being adapted to be connected to an AC
power supply, switch means mounted within said motor and connected
to form a bypass current path around said contacts, said switch
means including bias means responsive to a potential across said
contacts and to current flow through said winding and said device,
whereby said switch means is biased to conduction by a potential
due to the initiation of an arc and continues to conduct until the
next zero crossing of the AC wave.
25. Apparatus as in claim 24, wherein said switch means comprises a
TRIAC, and further including diode means connected in series with
said TRIAC.
26. Apparatus as in claim 24, wherein said device comprises a
heater for heating said overload switch.
27. A circuit for preventing damage to switch contacts when opening
under heavy current load conditions, comprising an electrical
component adapted to be connected in series with said contacts and
a load, voltage and current responsive switch means adapted to be
connected to form a bypass current path around said contacts, said
switch means including main terminals adapted to be connected
across said contacts and a control terminal connected across said
component, said switch means being biased to conduction only when a
potential exists across said contacts due to arcing and when
current flows through said component, whereby said current load is
transferred from said contacts to said switch means from the
initiation of an arc and until the next subsequent zero crossing of
the alternating current.
28. Apparatus as in claim 6, wherein said main terminals are
connected across both said contacts and said current
transformer.
29. Apparatus as in claim 6, wherein said main terminals are
connected across said contacts only.
30. In a power circuit including an AC power supply, a load
connected to be energized by said supply, and switch contacts
connected in series with said supply and said load for controlling
the flow of current to said load, the improvement of means for
preventing arcing across said contacts during opening under heavy
load current conditions, comprising an electrical component
connected in series with said contacts and said load, a triac
including main terminals and a gate, said main terminals being
connected in said power circuit to form a current path around said
contacts, and said gate being connected to said component, whereby
said triac is turned on when opening of said contacts produces a
potential across said main terminals and current flow through said
component provides gate triggering current.
31. In a power circuit as in claim 30, wherein said main terminals
are connected directly across said contacts, said component is a
resistor, and said gate triggering current is in phase with the
voltage across said resistor.
32. In a power circuit as in claim 30, wherein said main terminals
are connected across both said contacts and said component, whereby
current flow through said triac bypasses both said contacts and
said component.
Description
In many electrical installations it is necessary to open or close
an electric switch under heavy current load conditions. For
example, in an AC electric motor including an overload switch, the
switch is designed to open and disconnect the motor winding from
the AC power supply to protect the motor during such conditions as
arise from a locked rotor, and with a current load that may be over
100 amperes when the switch opens. Arcing occurs when the switch
opens, and unless a large overload switch is used, the switch
contacts may be severly damaged or destroyed after opening a few
times under such conditions. In addition, arcing may also occur if
the switch contacts are closed under heavy current load conditions
because the contacts tend to bounce afer initial closure. When the
contacts initially close, current flows through them, and if the
contacts bounce open again immediately after initial closure, an
arc develops which can destroy the contacts.
It would of course be possible to use a large heavy-duty switch
having an ample life at such current breaking loads, but quite
often the necessary space is not available for such a switch.
Further, even if space were not a problem, it would be advantageous
to be able to extend the life, or increase the rating, of a switch
of any given size.
In numerous other instances, contact-type switches are used to
interrupt current flow in a circuit, and arcing can reduce the life
of the switches.
It is therefore a general object of the present invention to
provide circuit means for use with switch contacts, to eliminate or
reduce damage due to arcing, thereby enabling the use of a smaller
switch than would otherwise be required and/or prolonging the life
of the switch.
The foregoing general object is attained by providing, in a circuit
including switch contacts connected to carry alternating current
and required to open under current load conditions, the improvement
comprising contactless switch means adapted to be connected in
parallel with said switch contacts, said switch means including
bias means connected to respond to current flow through said
contacts and to a potential across said contacts, said switch means
being biased to conduction only when a potential exists across said
contacts and current flows through said contacts, whereby said
switch means is biased to conduction and load current is
transferred thereto from the time of initiation of an arc until the
next subsequent zero crossing of the alternating current.
The foregoing and other aspects of the invention will be apparent
from the following detailed description taken in conjunction with
the accompanying figures of the drawings, wherein:
FIG. 1 is a schematic electrical diagram of a circuit embodying the
invention; and
FIGS. 2 through 11 are diagrams similar to FIG. 1 but showing
alternate forms of the invention.
While some of the forms of the present invention are illustrated
and described in connection with an overload switch for an electric
motor, it should be understood that the invention has utility in a
variety of other electrical installations where contacts are used
to interrupt current flow in a circuit.
The circuit shown in FIG. 1 includes a load 10 which is connectable
to be energized by an AC power supply 9, and by a switch 11
including contacts 12. A low value resistor 16 is connected in
series with the switch 11 for a purpose to be described
hereinafter. Connected in parallel with the switch 11 is
contactless switch means which preferably is of the solid state
type. The switch means may be a TRIAC, a pair of back-to-back
parallel connected SCRs or thyratrons, and in the present instance,
a TRIAC 17 is provided. One main or power terminal of the TRIAC 17
is connected to the juncture of the load 10 and the contacts 12,
and the other main or power terminal is connected to the juncture
of the contacts 12 and the resistor 16. The gate 18 of the TRIAC 17
is connected through a resistor 19 to the other side of the
resistor 16. The resistor 19 is provided to limit the TRIAC gate
current to within the gate dissipation rating of the TRIAC.
Considering the operation of the circuit illustrated in FIG. 1,
assume that the switch 11 has been closed to connect the load 10 to
the AC power supply 9, and that current is flowing through the load
10, the switch 11 and the resistor 16. The potential across the
resistor 16 energizes the gate 18 of the TRIAC 17, but no current
flows through the TRIAC 17 because the closed switch contacts 12
short the TRIAC 17. The TRIAC 17 requires a potential of at least
approximately 1.5 volts before it will conduct, and the potential
across the switch 12 is less than this value. Consequently, even
though the gate 18 is energized, the TRIAC 17 normally does not
conduct.
If the contacts 12 are opened, an arc starts to develop upon
initial opening movement of the contacts. The arc potential may be,
for example, about 30 volts which of course is sufficient to turn
on the TRIAC 17, and the current flow during initial arcing flows
through the resistor 16 and continues energization of the gate 18.
Consequently, the TRIAC 17 is turned on instantly as soon as the
arc starts to develop, and the power current is transferred away
from the contacts 12 to the TRIAC 17. Therefore, the arc is
immediately quenched or extinguished, and the contacts 12 are not
damaged. The power current continues to flow through the TRIAC 17
until the next subsequent zero crossing of the AC wave, at which
time the TRIAC 17 is turned off because of the lack of a potential
across it. In the next subsequent AC half cycle, the AC line
potential appears across the TRIAC but it does not conduct because
no current flows through the resistor 16 as is required to energize
the gate 18. The gate current is in phase with the load current and
both reduce to zero at the same time, this being true because the
element 16 is a resistor. Consequently, the TRIAC 17 conducts for
less than one half of an AC wave, and while the current flow
through the TRIAC may be very high during this time, it is
relatively short and an ordinary TRIAC can handle such current flow
without the necessity of providing a sink to draw heat away from
the TRIAC. The TRIAC 17 is a bistable contactless switch which
includes bias means connected to respond both to the arc potential
across the switch 11 and to current flow through he switch.
If the switch 11 is later closed, current again flows therethrough,
and if the contacts 12 bounce immediately after closing, an arc
tends to develop during the bounce. However, the flow of current
through the resistor 16 occurring as soon as the contacts 12
initially close, causes the TRIAC gate 18 to be energized, and the
initiation of an arc at the time of the contact bounce results in a
potential across the TRIAC 17 which turns it on as described
previously. Consequently, the TRIAC 17 again conducts and draws the
current away from the switch 11, permitting the switch 11 to
reclose under relatively light current flow conditions. Of course,
as soon as the contacts 12 reclose, the TRIAC 17 is again shorted
and is turned off.
The foregoing sequence of contact closure and bounce, arcing,
turning on of the TRIAC, reclosure and turning off of the TRIAC,
occurs as described when all of the events occur within the same
half wave. If the bounce and arc occur in a first half wave and the
reclosure occurs in the next half wave, the TRIAC will be turned
off at the zero crossing and the arc may reestablish at the
beginning of the next half wave. In this event, the TRIAC would
again be turned on during the next half wave and would remain on
until the next zero crossing or until the contacts reclose,
whichever occurs first.
The size of the resistor 16 should of course be made no larger than
necessary, to prevent waste of power and generation of heat. Its
size is determined by the amount of gate current required to
trigger the TRIAC 17, and depending upon the TRIAC requirement, it
may be made very small. Further, with a proper choice of component
values, the current limiting resistor 19 may be eliminated in the
FIG. 1 circuit and in the other circuits disclosed herein.
In some instances, as where the switch 11 is a current overload
switch, a small voltage may develop across the TRIAC main terminals
in the absence of an arc when current values become large, and this
condition may exist for a number of AC cycles before the switch
contacts open. This small voltage may exceed the voltage required
to turn on the TRIAC and cause it to conduct for a number of
cycles, resulting in overheating of the TRIAC if adequate heat
sinking is not provided. The foregoing may be prevented by using
one of the forms of the invention shown in FIGS. 2 and 3.
The form of the invention shown in FIG. 2 includes a current
overload switch 21, having contacts 22 and a heater resistor 23,
connected in series with an AC power supply 24, an on-off switch
26, a motor winding 27, and a shunt resistor 28. The motor also
includes a rotor 29. A TRIAC 31 and a zener diode 32 are connected
in series and are connected across the overload switch 21. The gate
33 of the TRIAC is connected through a current limiting resistor 34
to the resistor 28. In one conventional type of overload switch,
the contacts 22 are part of a bimetal element which bends in
response to the heat generated by the heater 24. The overload
switch 21 is designed such that the bimetal element is normally
closed but deforms and opens the contacts 22 when sufficient
current flows through the heater 23. The switch 21 is designed to
open upon the occurrence of overload currents, as when the rotor 29
of the motor is locked. When the switch cools, the contacts 22
automatically close again.
In overload conditions but before the switch contacts 22 open, if a
voltage develops across the switch 21, the zener diode 32 prevents
any current flow through the TRIAC 31 unless the zener voltage plus
the TRIAC 31 turn-on voltage is reached. The voltage across the
switch 21 would normally be less than 2 volts, and a zener is
chosen having a higher voltage, thus preventing current flow
through the TRIAC until the switch contacts 22 open and an arc
starts to appear. The remainder of the operation is the same as
that described with regard to FIG. 1.
In the form of the invention illustrated in FIG. 3, a pair of
diodes 36 and 37 are connected in back-to-back parallel relation,
and the parallel doides 36 and 37 are connected in series with a
TRIAC 38. The diodes have a forward bias voltage of about 1.5 to 2
volts, and the diodes 36 and 37 prevent current flow through the
TRIAC 38 until the potential across an overload switch 39 exceeds
the diode and the TRIAC minimum voltages. The remainder of the
circuit, and the operation, are similar to that of FIG. 2.
In the form of the invention illustrated in FIG. 4, a TRIAC 41 is
connected in parallel with the contacts 42 of an overload switch 43
which also includes a heater resistor 44. The gate 46 of the TRIAC
is connected through a current limiting resistor 47 to the side of
the heater resistor 44 which is opposite from the contacts 43. The
circuit shown in FIG. 4 may be used when the connection between the
contacts 42 and the resistor 44 is accessible. It will be apparent
that the circuit of FIG. 4 will operate similarly to the circuit of
FIG. 1, and that the heater resistor 44 both heats the contacts 42
and also serves the function of the resitor 16 of FIG. 1. In the
FIG. 4 circuit, the TRIAC turn-on voltage probably will not be
reached before the contacts 42 open, because the TRIAC is not
connected across the heater resistor 44. Therefore, it is less
likely that a diode arrangement as shown in FIGS. 2 and 3 would be
required, although a diode arrangement may be provided in the FIG.
4 circuit or any of the circuits disclosed herein.
The circuit illustrated in FIG. 5 includes an over-load switch 51
connected in series with a load 52 and an AC power supply 53, the
switch 51 including contacts 54 and a heater resistor 55. Also
connected in series with the overload switch 51 is a shunt resistor
57. A TRIAC 58 is connected across the switch 51, and the gate 59
of the TRIAC 58 is connected through a current limiting resistor 61
to the resistor 57.
It will be apparent that the construction and operation of the FIG.
5 circuit is similar to that of the FIG. 4 circuit, except that the
TRIAC gate current is provided by a shunt resistor separate from
the overload switch. In the arrangement shown in FIG. 5, the value
of the potential across the main or power terminals of the TRIAC 58
prior to the opening of the contacts 54 is not sufficient to turn
the TRIAC 58 on. If the TRIAC 58 were to be turned on with the
contacts 54 closed, a heat sink may be provided to carry heat from
the TRIAC 58 in the event it is on long enough for heating to
become a problem.
The circuit shown in FIG. 6 includes switch contacts 63 connected
in series with a load 64 and an AC power supply 66. A small current
transformer 67, including a primary winding 68 and a secondary
winding 69 is connected to supply gate energizing current to the
TRIAC 71. The primary winding 68 is connected in series with the
contacts 63 and the secondary winding 69 is connected through a
current limiting resistor 72 to the gate 73 of the TRIAC 71. The
amount of gate energizing power required is of course very low,
measured in milliwatts. The transformer 67 may therefore be in the
order of a 300 to 1 step down current transformer, for example.
Assuming that the switch contacts 63 are closed, current flows
through the load 64, the contacts 63 and the primary winding 68.
The alternating current flowing through the primary winding 68
induces current flow in the secondary winding 69 and supplies gate
energizing current to the TRIAC 71. The main terminals of the TRIAC
71 are connected across the switch contacts 63 and the primary of
winding 68, instead of across the switch contacts only as in the
circuit shown in FIG. 1. Since the transformer 67 primary voltage
drop may be made very small due to its very small rating (of the
order of milliwatts), this drop can easily be limited to a value
which will not cause the TRIAC 71 to conduct when the switch 63 is
closed. Moreover, the resultant gate current flowing in the
secondary winding 69 may be out of phase with and lead the main
current by an appreciable phase angle. However, since the main
terminals of the TRIAC are connected both across the primary of
transformer 67 and across the contacts 63, when the contacts 63
open and the load current is transferred to the TRIAC, the primary
winding of the transformer 67, and therefore also the gate 73, will
cease to carry current. With no gate energizing current, the flow
of current through the TRIAC cannot be re-established after the
power current reaches zero at the completion of the half cycle of
conduction. Thus, with the connection of the main terminals of the
TRIAC as shown in FIG. 6 rather than as shown in FIG. 5, the phase
angle of the gate current relative to the load current phase angle
is immaterial.
The circuit illustrated in FIG. 7 is generally similar to that
illustrated in FIG. 6 but includes a voltage transformer rather
than a current transformer. The FIG. 7 circuit includes switch
contacts 76, a load 77 and an AC power supply 78, The components
76, 77 and 78 being connected in series with a small shunt resistor
79. A volage transformer 81 is provided, having its primary winding
82 connected across the resistor 79 and its secondary winding 83
connected to the gate 84 of a TRIAC 86. The value of the resistor
79 may be much smaller than the shunt resistor of the circuits of
the character shown in FIG. 1, for example, and the transformer 81
may be, for example, a one-to-10 step up transformer. The power
terminals of the TRIAC 86 are connected across the contacts 76 and
the resistor 79.
With the contacts 76 closed, current flows through the resistor 79
and the potential across the resistor 79 induces gate energizing
current in the secondary winding 83. The remainder of the operation
is similar to that of the circuit illustrated in FIG. 6.
It might be mentioned that the circuits including transformers are
advantageous in that the amount of power losses and heating,
inherent in purely resistive shunts, are greatly reduced. Further,
because of the reduction in power rating to only slightly greater
than the milliwatts requirement of the TRIAC gate circuit, the
transformer design can be made very simple, will require much less
space, and is inexpensive as compared to resistive shunts of the
required rating.
The circuit illustrated in FIG. 8 includes switch contacts 88
connected in series with a load 89, an AC power supply 91 and a
small shunt resistor 92. A TRIAC 93 has its power terminals
connected across both the contacts 88 and the shunt resistor 92,
and the gate 94 of the TRIAC 93 is connected through a current
limiting resistor 95 to the juncture of the contacts 88 with the
resistor 92.
When the contacts 88 are closed, current flows through the contacts
88 and the resistor 92, and the potential across the resistor 92
supplies gate energizing current to the TRIAC 93. The TRIAC 93 is
of course normally turned off when the contacts 88 are closed, but
a heat sink may be provided to protect the TRIAC 93 in the event it
turns on prior to the opening of the switch contacts 88.
The TRIAC is turned on by an arc potential and by the gate
energizing current. After the arc is extinguished, the TRIAC is
turned off at the next zero crossing.
The circuit shown in FIG. 9 includes a current transformer similar
to that shown in FIG. 6, but the TRIAC main terminals are connected
across the switch contacts only, rather than across both the
contacts and the transformer as in FIG. 6. The FIG. 9 circuit
includes switch contacts 98 connected in series with a load 99, an
AC power supply 101, and the primary winding 102 of a current
transformer 103. The secondary winding 104 of the transformer 103
is connected through a current limiting resistor 106 to the gate
107 of a TRIAC 108. The power terminals of the TRIAC 108 are
connected directly across the contacts 98.
The operation of the circuit shown in FIG. 9 will be the same as
that of the circuit illustrated in FIG. 6, with the exception that
the potential across the TRIAC main terminals will be lower than
the potential across the main terminals of the TRIAC 71 because the
TRIAC 98 is connected across the switch contacts 98 only. Further,
in the circuit shown in FIG. 6, after the contacts 63 open and
while the TRIAC 71 is still conducting, gate energizing current is
no longer provided by the transformer 67. This is in contrast to
the circuit shown in FIG. 9 where the TRIAC current flows through
the transformer primary winding 102 and thereby maintains gate
energizing current while the TRIAC 108 is conducting. After the
TRIAC has been turned on, at the next zero crossing of the AC wave,
the gate energizing current should also be substantially zero to
turn the TRIAC off and maintain it off. In this circuit the
transformer 103 must be designed to maintain its secondary current,
which supplies the gate circuit, substantially in phase with the
load current. This is in contrast to the circuit of FIG. 6 where
the gate current phase angle is independent of the load current
phase angle, as previously described in connection with FIG. 6.
The circuit shown in FIG. 10 includes a voltage transformer similar
to that shown in FIG. 7. The FIG. 10 circuit includes switch
contacts 111 connected in series with a load 112, a power supply
113, and a very small or low value resistor 114. The primary
winding of a transformer 116 is connected across the resistor 114
and the secondary winding of the transformer 116 is connected
through a resistor 117 to the gate 118 of a TRIAC 119. The main
terminals of the TRIAC 119 are connected directly across the switch
contacts 111.
The operation of the FIG. 10 circuit is similar to that of the
circuit shown in FIG. 9 but differs in that the gate energizing
current is provided by the potential across the resistor 114. In
FIG. 10, and also in FIG. 7, the winding ratio of the transformer
is determined by the value of the shunt resistor connected in
series with the switch contacts, the amount of gate energizing
current required, and by the amount of current expected to flow
through the resistor during arcing conditions. However, the
transformer 116 must be designed to maintain the gate current
substantially in phase with the load current, as is also true of
the transformer 103 in FIG. 9.
FIG. 11 shows a circuit including switch contacts 121 which are
connected in series with a load 122, an AC power supply 123 and a
reactive impedance which in the present instance is a winding 124.
A TRIAC 126 has its main terminals connected across both the
contacts 121 and the reactive impedance 124 which, in order to
maintain losses and heating to a minimum should be highly reactive.
The gate 127 of the TRIAC 126 is connected through a resistor 18 to
the juncture of the contacts 121 and the winding 124. The reactance
of the winding 124 would cause the TRIAC 126 gate current to be
greatly out of phase with the load current, but, for the same
reasons explained in connection with the circuit shown in FIG. 6,
since there will be no gate current once the load current has been
transferred to the circuit path including the TRIAC 126, there will
be no re-establishment of the TRIAC current after it reaches the
first zero crossing following the transfer of the load current. If,
in order to obtain sufficient gate current, the voltage across the
winding 124 became high enough to cause the TRIAC 126 to conduct
while the contacts 121 are closed, a diode arrangement as shown in
FIGS. 2 or 3 could be connected in series with the TRIAC 126.
It will be apparent from the foregoing that novel and useful
circuits have been provided for preventing or substantially
reducing damage to switch contacts which are required to open and
close under heavy current load conditions. By using a circuit of
the character described, a relatively small switch may be used
which is able to accommodate overload currents but which may not be
able to handle the opening and closing conditions encountered with
conventional circuits. In installations where the amount of space
is a critical factor, this is an important consideration. For
example, an overload switch mounted within an electric motor must
be quite small because of the limited amount of space available. By
using the present invention, a small switch may be used and its
life may be greatly extended compared to conventional
constructions, because of the reduction or substantial elimination
of contact damage due to arcing. The heat sink requirements of the
TRIAC are negligible because of its short conducting time. In all
of the forms disclosed herein, the contact-type switch is protected
by a contactless switch which is normally open but which is closed
upon the occurrence of an arc voltage and an arc current. Further,
a variety of circuits or devices have been disclosed for supplying
gate energizing current to a TRIAC. The gate energizing device is
required to supply gate energizing current at the instant of
arcing, and as mentioned, a variety of devices, such as a surge
transformer connected to supply gate current only at the instant of
contact opening, may be used for this purpose. Broadly, the
circuits include contactless switch means connected to form a
bypass current path around the contacts to be protected, the switch
means including a gate or control terminal for closing it. In the
circuits shown in FIGS. 1 to 5, 9 and 10, it is important that the
TRIAC gate current be in phase, or substantially in phase, with the
load current because the TRIAC gate current continues after the
contacts open and the arc is extinguished, making it possible for
the TRIAC to re-establish conduction in subsequent cycles. In the
remaining circuits disclosed herein, the phase angle of the gate
current is unimportant but care should be taken to prevent the
TRIAC from conducting when the contacts are closed .
* * * * *