A control
circuit is for the automatic control of equipment, for safety interlocking, and
sequencing the operations of the plant equipment and machines.
Control
circuits hardware consists of relay contacts, wires, hardware timers, and counters,
relay coils, etc. These consist of input contacts representing various
conditions; the output coils are energized or de-energized depending on the
input conditions represented by the control circuit.
Input
contacts represent the binary state of the condition:
• True or
false
• On or off.
There are
two types of contacts NO (normally open) and NC (normally closed).
• Input
contact: These are contacts of relays, contactors, timers, counter, field instrument
switches, pressure switches, limit switches, etc.
• Output
coil: These have two states – On or Off. Output coil can be auxiliary contactor
or Main contactor coil.
A few simple
control circuits are shown in figure below to represent logical AND, OR, and such
conditions.
1. ‘AND’
operation circuit
Figure
2.3(a) shows a simple control circuit (AND operation) with two input contacts (NO)
representing two conditions that must be true to complete the circuit to switch
on the output relay coil and change the state of output from ‘Off’ to ‘On’.
2. ‘OR’
operation circuit
Figure
2.3(b) shows a circuit with three input contacts (NO) representing that at
least one of the three conditions should be true to complete the circuit to
switch On the relay coil and change output state from ‘Off’ to ‘On’.
3. ‘AND with
OR’ operation circuit
Figure
2.3(c) shows a control circuit, consisting of a combination of AND and OR operations.
There are
two parallel (OR condition) paths with two input contacts (NO) connected in series
in each path representing AND conditions.
The path for
coil K3 will be completed when one of the path conditions comes true. The
circuit then will switch ‘On’ the relay coil and change the output state from
‘Off’ to ‘On’.
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