In this example we have three interlocks: Motor Vibration High, Motor Temperature High & Overload trip. If there is no interlock or all working normal, then PLC sends a start command to substation where motor feeder is installed. Lets say all interlocks are healthy, so PLC sends the permissive signal to motor feeder.įirst PLC checks the Local/Remote status, if it is in Remote then proceeds further.Īgain it checks for any active trips/interlocks. For normal or low capacity motors these are very rarely used, again depends on our applications, industry, requirements etc. For High capacity motors, start permissive signals will be used. Generally we call this as “Start Permissive”, as name implies it is only required for starting the motor, after motor started the status of this permissive signal will not be considered by motor feeder (Only required for starting the motor). if all the interlocks are healthy then only permissive signal will be sent to motor feeder. In PLC, Start permissive will be used as a extra safety & for checking the interlock status. For starting the motor, the permissive must be healthy otherwise motor feeder will be de-energized or will not start the motor. Here we are sending an Permissive signal ( Start Permissive) to the motor feeder. Now Local/Remote selection switch is in Remote mode. This is a safety concern.Īlso Read : How a 4-20mA Transmitter Works ? Whatever mode it is, the Stop commands will be accepted by motor feeder and stops the motor immediately. Note : Local/Remote Selection will not be applied for Emergency Stop or Stop Commands either from PLC or LCP. If field operator pressed the start push button from the field LCP then motor will not be started as the selection is in remote mode.ĭepends on the Local/Remote selection switch status the motor feeder will decides which signals to be consider i.e.
PLC and ignore the signals from LCP.įor example : if Local/Remote selection Switch in Remote Mode.
Similarly if Local/Remote Selection switch in Remote Mode then motor feeder will consider signals from Remote i.e.
If Local/Remote selection switch is in Local mode then motor feeder will only consider signals from LCP and ignore the commands from PLC. The motor feeder also sends a Local/Remote status to PLC. In common practice, this LCP has emergency stop & Local/Remote selection switch also. Similarly if motor feeder receives start & stop command inputs from local control panel (LCP) which is installed in field (near to motor) then we call them as Local Start & Local Stop signals. If motor feeder receives start & stop command inputs from PLC then we call them as Remote Start & Remote Stop signals. These are mounted on the motor feeder panel (in addition to LCP). Note : the motor feeder may have start, stop, some other trips indications like overload etc… in the motor feeder panel which are not shown in the fig.
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