Ladder logic circuits
The designing, programing, testing, commissioning, troubleshooting, and maintenance of control logic are much easier using the ladder logic programs in a PLC than in hard-wired circuit.
We have discussed the ladder logic instructions before and looked at the simple ladder logic programs in Chapter 2. Let us consider a ladder logic program for a typical control
circuit of the DOL starter for a three-phase motor as shown in Figure 7.1(a).
For a three-phase motor with a DOL starter, the following input and output signals are configured in the PLC.
Digital inputs
1. Control voltage ON and control fuse ‘F3’ OK (normally open)
2. Motor overload ‘F2’ (normally closed)
3. Motor stop ‘S0’ (normally closed)
4. Motor start ‘S1’ (normally open)
5. Main contact on feedback (normally open).
Digital output
Main contactor on
The ladder logic program Instructions for the DOL starter control circuit with a maintained contact control is shown in the Figure 7.4.
As various inputs for the control circuit in the PLC has been gathered, it will be for generating alarms. These alarms can be indicated with the help of ‘Indication Lamps’ that can be mounted on the motor starter panel in addition to the start and stop pushbuttons. These alarms can help in faultfinding. The following alarms can be configured using the inputs already available in the PLC. The PLC outputs can be generated to drive these alarm indication lamps.
In case of the DOL starter for a three-phase motor, the following alarms can be configured to indicate faults:
• Alarm indication lamp-1: Control voltage OFF
• Alarm indication lamp-2: Motor tripped on overload
• Alarm indication lamp-3: Main contactor feedback fault.
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