Safety relay circuit for large power contactors

Details Safety relay circuit for large power contactors Safety relay circuit for large power contactors

2002-05-10

2003-08-26

Sircus, Brian (Department: 2836)

Electricity: electrical systems and devices

Control circuits for electromagnetic devices

For relays or solenoids

C361S166000, C361S192000

Reexamination Certificate

active

06611416

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
1. Field of the Invention
The present invention relates to relay circuits and, in particular, to safety relay control circuits that are employed to monitor the status of power contactors.
2. Background of the Invention
In many industrial and other systems, high levels of power are required by loads, and must be repeatedly switched on and off with respect to the loads. Power contactors are commonly employed to provide this switching function. Although rare, it is occasionally possible that one or more of the contacts within such power contactors, which are normally open, become welded closed due to arcing that occurs during the switching process. Because such welding of one or more contacts precludes those contacts from being switched off, and thus precludes those contacts from being employed to switch off the power to the loads, it is important that welded contacts be identified soon after a welding event occurs so that the welded contacts can be replaced.
In order to identify welded contacts, safety relay circuits can be employed. Referring to
FIG. 1
, a Prior Art safety relay circuit
100
is shown that is capable of monitoring whether any of three power contacts
102
,
104
and
106
have become welded closed. As shown, each of the three power contacts
102
,
104
and
106
governs whether a corresponding single-phase of power provided by a respective line
112
,
114
and
116
is provided to a high-power, three-phase load
108
. The safety relay circuit
100
, upon identifying whether any of the three power contacts
102
,
104
or
106
is welded closed, provides a signal to an operator (or other controller or control system) indicating the welded state of the power contact(s), typically by preventing the operator from restarting a system
110
of which the three-phase load
108
and the safety relay circuit
100
form a part.
The safety relay circuit
100
, which is shown in a ladder diagram format in
FIG. 1
, operates as follows. As shown, an operator can press an ON button
118
so that a first port
124
of a standard safety relay
120
is coupled to a power (or voltage or current) source
122
. In the present embodiment, the standard safety relay
120
is an A-B 440R-ZBL220224 safety relay manufactured by the Allen-Bradley Company of Milwaukee, Wis. In alternate embodiments, the safety relay
120
can be another type of safety relay. In the embodiment shown, the safety relay
120
is configured to apply a voltage to a second port
126
once the voltage source
122
is coupled to the first port
124
.
The safety relay circuit
100
further includes a normally-closed auxiliary contact
130
coupled between the second port
126
and a third port
128
of the safety relay
120
. Assuming that the auxiliary contact
130
is in fact in its closed position, the third port
128
experiences a voltage change as the voltage is applied by the safety relay
120
to the second port
126
. The safety relay
120
is further coupled to ground
123
at a fourth port
132
. If each of the above-described events occurs, the safety relay
120
switches from an off state to an on state. Also as shown, the safety relay
120
in this embodiment includes several other ports, including ports
125
,
127
,
129
and
131
, between which are coupled a set of emergency stop switches
133
, which allow for the safety relay to be switched to its off state in an emergency. Further, the safety relay circuit
100
is coupled to certain of the lines
114
,
116
by way of a power transformer
135
, one side of which is coupled between the power source
122
and ground
123
, and the safety relay circuit further includes one or more protective fuses
137
that can have a variety of current tolerances.
As shown on line
5
of the ladder diagram, the safety relay
120
has fifth and sixth ports
140
and
134
, respectively, between which are coupled first and second normally-open contacts
136
and
138
, respectively. The fifth port
140
is additionally coupled to the power source
122
while the sixth port
134
is additionally coupled to a first coil
142
, which is coupled between the sixth port and the ground
123
. In accordance with the ladder diagram, the first and second normally-open contacts
136
,
138
within the safety relay
120
close when the safety relay switches to its on state, such that the power source
122
is electrically coupled to the first coil
142
. The first coil
142
, which is thus coupled between the voltage source
122
and ground, consequently conducts and is energized.
When the first coil
142
is energized, several contacts switch from their normal positions to their excited positions. First, each of the power contacts
102
-
106
switches to its closed position, such that the three single-phases of power associated with the lines
112
-
116
are provided to the three-phase load
108
. Additionally, in accordance with the ladder diagram, the normally-closed auxiliary contact
130
is opened and a normally-open sustaining contact
143
that is coupled in parallel with the ON button
118
is closed. The sustaining contact
143
thus maintains the electrical coupling between the first port
124
and the power source
122
even though the operator releases the ON button
118
.
The safety relay
120
is designed to operate so that, as long as the voltage source
122
continues to be coupled to the safety relay
120
, the safety relay remains in its on state such that the contacts
136
,
138
remain closed and the first coil
142
remains conductive, even though the auxiliary contact
130
is opened. That is, the auxiliary contact
130
need only be closed at the time that the first port
124
is coupled to the voltage source
122
in order for the safety relay
120
to enter its on state, and need not remain closed thereafter in order for the safety relay
120
to remain in its on state. In order to deenergize the first coil
142
, and consequently to open the power contacts
102
-
106
, open the sustaining contact
143
and close the auxiliary contact
130
, the operator presses an OFF button
144
. This causes a decoupling of the power source
122
from the first port
124
, which causes the safety relay
120
to return to an off state so that both of the contacts
136
,
138
open.
The safety relay circuit
100
is usually capable of providing an indication to an operator when one or more of the power contacts
102
-
106
has welded and will not return to an open position. As shown in
FIG. 2
(Prior Art), each of the power contacts
102
-
106
and the auxiliary contact
130
are physically positioned within a single power contactor
146
along with the coil
142
. All of the power contacts
102
-
106
and the auxiliary contact
130
are physically coupled to one another so that, during proper operation, the contacts are all in their normal positions, in their excited positions, or in between their normal and excited positions.
Usually, when the first coil
142
becomes conductive or energized, all of the contacts
102
-
106
and
130
move to their excited positions. Also, the power contactor
146
is spring-loaded such that, when the first coil
142
becomes non-conductive or deenergized, the contacts
102
-
106
and
130
usually all return to their normal positions. However, if any one or more of the power contacts
102
-
106
become welded in their respective closed positions, the spring-loading within the power contactor
146
is insufficient to cause the contacts to return to their normal positions. That is, one of the power contacts
102
-
106
becomes locked in its excited, closed position, while the auxiliary contact
130
becomes locked in its excited, open position.
Because welding of any of the power contacts
102
-
106
causes the auxiliary contact
130
to become locked in its open position, the safety relay circuit
100
is able to provide the operator with an indication that a welding event has occurred. Specifically, because the auxiliary contact
130
is locked in its open position, the safety relay
120

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