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Tahun 2010

Symbols

Symbols

Symbols

Symbols

Symbols

Reference Designators T transformer CR control relay R resistor C capacitor LS limit switch PB pushbutton S switch SS selector switch TDR or TR time delay relay M motor, or motor relay L indicator lamp or line phase F fuse CB circuit breaker OL overload switch or overload contact

Example : Power circuit for a motor

Direct-on-line (DOL) starter power circuit used for a three-phase induction motor. As shown, the three-phase power input is connected to the motor through a contactor. Power is passed to the motor when the contacts (of the contactor) are in closed condition. Protection devices such as fuses, and overload relays are provided in series with power conductors to detect unhealthy conditions during operation.

Control circuit

Control circuits hardware consists of relay contacts, wires, hardware timers, and counters, relay coils, etc. These consist of input contacts representing various conditions;

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.

AND’ operation circuita 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’.

‘OR’ operation circuit

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’.

‘AND with OR’ operation circuit 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’.

Three-phase motor with DOL starter

Three-phase motor with star–delta starter

Three-phase motor with star–delta starter

Example : Tank water level control

Design control circuit for ‘Tank water level control’. Operation sequence should be such that

• When the water level goes below the low-level limit, open the inlet valve of water tank.

• When water level goes above high limit is detected, close the inlet valve.

Example : Tank water level control

ladder logic

PLC terminology

PLC terminology may differ from relay terminology, but the control concepts are the same.

Ladder logic instructions

Ladder logic instructions

Ladder logic diagram

Ladder logic diagram

Ladder logic diagram

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