Prepare and Interpret Technical Drawing

Prepare and Interpret Technical Drawing

Common Competency

Standard Language of IEC

IEC: International Electrotechnical Commission, since 1993 international standard IEC 61131 for programmable logic controllers

Function block diagram (FBD)

Structured Text (ST)

Ladder Diagram (LD)

Sequential Function Chart (SFC)

Instruction List (IL)

American Symbols

European Symbols

01

Ladder Basics

Ladder Diagram

The LD originated in the graphical representation of electrical control systems using relays (relay-based logic). It is mostly used for discrete automation and is ideal for sequential control with interlocks.

Ladder circuits have some common characteristics. They are: 1. Power bar on the far left 2. Current flow from the switches combined to allow flow 3. One or more relay coils at the right 4. Neutral or negative power bar on the right

+ info

What is a Rung?

A rung of ladder diagram code can contain both input and output instructionsInput instructions perform a comparison or test and set the rung state based on the outcome

• Normally left justified on the rung

Output instructions examine the rung state and execute some operation or function

• In some cases output instructions can set the rung state
• Normally right justified on the rung

Series Vs Parallel Operations

Ladder Diagram input instructions perform logical AND and OR operations in and easy to understand format

• If all Input Instructions in series must all be true for outputs to execute (AND)
• If any input instruction in parallel is true, the outputs will execute (OR)

Paralleling outputs allows multiple operations to occur based on the same input criteria

Ladder Logic Execution

• Rungs of Ladder diagram are solved from Left to right and top to bottom

• Branches within rungs are solved top left to bottom right

Latching Circuit

Ladder Logic

latch circuit is used to carry out such an operation. It is a self-maintaining circuit in that, after being energized, it maintains that state until another input is received.

When the pushbutton (PB) is pushed, the circuit is completed and current flows energizing the coil (CR1).

Latching Circuit

Start Stop

latch circuit is used for the circuit used to carry out such an operation. It is a self-maintaining circuit in that, after being energized, it maintains that state until another input is received.

The function can be used to start and stop anything like a motor start/stop.

Power Circuit

The power circuit of a three-phase motor consists of the following items:

• 3PST three-phase disconnect
• Overcurrent protection
• Horsepower-rated motor starter contacts
• Overload relay heaters

Motor Control Power Circuit

Control Circuit

This system can provide a lower voltage for the control circuit, but requires additional wires and breakers to be installed at the main-load centre to supply the control circuit.

https://opentextbc.ca/basicmotorcontrol/chapter/control-circuits/

Motor Control Control Circuit

Function Block Diagram

Function Block Diagram programming is a language in which elements appear as blocks that are connected together resembling a circuit diagram.

Ladder Logic vs. Function Block Diagram

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