Chapter 2 Part 1-Symbols-Abbreviations-Ladder Diagram

1. Motor Symbols

• Symbols represent components of a motor control system.

• They do not always look like real-world components but are standardized for clarity.

• Common motor symbols:

  • Disconnect Switch – Used to safely isolate electrical circuits.

  • Relay Coil (CR) – Controls other contacts in the circuit.

  • Normally Open (NO) & Normally Closed (NC) Contacts – Represent switch states in de-energized conditions.

2. Disconnect Switches

• Used to cut off power to a motor or electrical device.

• Essential for maintenance and safety.

• Often represented as a switch symbol with an open or closed connection.

3. Abbreviations for Motor Terms

Common motor control abbreviations:

• CR - Control Relay

• M1, M2, M3 - Motor Starters #1, #2, #3

• L1, L2 - Power Lines

• OL - Overload Relay

• NO / NC - Normally Open / Normally Closed Contacts

• PB - Push Button

• PL - Pilot Light

• TR - Transformer

• F - Fuse

• OLR - Overload Relay

• A1, A2 - Coil Terminals

• T1, T2, T3 - Motor Terminals

• K - Contactor

• R - Resistor

• S - Switch

• T - Timer

• X - Terminal Connection

• GND - Ground

• V - Voltage

• A - Amperage

4. Motor Ladder Diagrams

• Used to represent electrical control circuits.

• Components:

  • Rails (Vertical Lines): Represent power sources (L1 and L2).

  • Rungs (Horizontal Lines): Contain control circuit elements.

  • Loads: Components that consume power, such as motors, solenoids, and lights.

  • Control Devices: Push buttons, switches, and relays that operate the loads.

  • Nodes: Points where circuit devices attach to each other.

  • Contact Bars: Sections of a switch that make or break a connection.

• Ladder diagrams are read top to bottom, left to right.

• One exception to this rule is the placement of the normally closed contacts controlled by the motor overload protection device. These contacts are drawn on the right side of the motor starter coil.

5. Circuit Components & Functionality

• Loads: Devices consuming electrical power (e.g., motor coils, solenoids, pilot lights).

• Control Devices: Operate loads by completing or breaking circuits.

• Series vs. Parallel Loads:

  • Parallel loads receive full voltage and operate independently.

  • Series loads share voltage, which may be insufficient for proper operation.

• Short Circuits: Incorrectly placed contacts can cause unintended power surges.

6. Numerical Cross-Referencing

• Used to locate auxiliary contacts controlled by coils in a control circuit.

• The rung number of the controlling coil is noted in parentheses next to contacts on other rungs.

• Helps quickly identify which relay coil is operating a specific contact.

7. Wire Identification & Numbering

• Used to ensure proper circuit connections.

• Common wire numbering method:

  • Wires connected to L1 are numbered 1.

  • Wires connected to L2 are numbered 2.

  • Additional wires are numbered sequentially.

• Cross-referencing is used to locate relay contacts controlled by coils.

8. Control Transformers & Grounding

• Transformers: Used to step down voltage for control circuits.

• Proper grounding: Prevents accidental motor startup due to faults.

• Fault Path: A short circuit condition occurs when the control transformer fuse opens due to a fault.

9. Broken Lines in Ladder Diagrams

• Broken lines indicate mechanical functions, not electrical conductors.

• Used to show the mechanical link between components (e.g., pushbuttons with NO/NC contacts).

• A vertical broken line between forward and reverse pushbuttons means their contacts are mechanically connected.

10. Reading Ladder Diagrams

• Simplify the diagram by breaking it into smaller sections.

• Devices are always shown in their de-energized state.

• When a relay coil is energized, its contacts change state accordingly.

• Relay coils on one rung may have contacts on other rungs, requiring cross-referencing.