Chapter 5: Control Logic
Chapter Overview
Chapter 5 will cover control logic. Control logic describes the established and universal set of symbols and rules on how line diagrams are laid out. Without a standardized set of rules to follow in all control circuits, it would be very difficult to build and, especially, troubleshoot control systems. Worksheet information link.
Key points to remember:
Electrical circuits can be grouped into three basic sections: signal, decision, and action.
Common logic functions include:
- AND
- OR
- NOT
- NOR
- NAND
Basic Rules of Line Diagrams:
- Only one load is allowed per line.
- If more than one load is required, loads must be connected in parallel.
- Control devices are connected between L1 and the operating coil (or load).
- Each line in a ladder diagram should be numbered, starting at the top and reading down.
Control Logic
Control logic is considered the “language of controls.” It refers to the specific set of rules that govern all motor control systems. Think of control logic as the fundamental guidelines you will always follow when working with ladder diagrams commonly used in motor control systems. An industry-standard set of symbols and rules ensures that motor control circuits are accurately interpreted and implemented across different disciplines and languages.
Simply put:
- Series control in circuits is considered “AND” logic.
- Parallel control in circuits is considered “OR” logic.
Wire Reference Numbers in Motor Control Diagrams
Wire reference numbers are commonly used in motor control ladder diagrams to distinguish and separate different parts of the circuit. Each number identifies an individual wire or group of wires within a specific section of the circuit. Diagrams are numbered starting from the top, working left to right, and are structured much like how books are read in western cultures.
Every time you cross a component in the diagram, a new wire—and consequently a new corresponding number—is required.
Editing note: include reference here for wire reference numbers.
Image origin and reference: Numbering System – Basic Motor Control (opentextbc.ca)
Electrical Circuit Basics
All electrical circuits can be broken down into three main sections: signal, decision, and action. Think of these sections as follows:
- Circuit initiation: switches and pushbuttons,
- Circuit control: timing and sequencing, and
- Circuit operation: running and reversing.
It is crucial to become familiar with how diagrams are organized, as this standardization ensures efficient communication, coordination, and safe implementation of electrical systems. Without this organization, installing, maintaining, and troubleshooting electrical systems would become increasingly difficult.
Troubleshooting is the systematic process of eliminating various parts of a system to isolate a problem. Your ability to troubleshoot effectively is directly related to your understanding of the specific system you’re working with. Identifying issues in your electrical installations before calling for an inspection can save time and prevent costly setbacks in the field.
Common problems in electrical circuits include short circuits, open circuits, and single phasing in three-phase systems:
- A short circuit occurs when the required load in a circuit is bypassed, causing a fault that can lead to arcing and damage to systems and components.
- Ground faults happen when a live conductor unintentionally touches anything connected to the earth or “ground.”
- Phase-to-phase (or line-to-line) short circuits are more dangerous than ground faults because they involve potentially twice as much energy, as two separate circuits come into contact, compared to a single circuit connecting to the ground.
Keep in mind:
When testing live circuits, it is essential to check from ground to each line and then line-to-line due to the potential danger of phase-to-phase faults. An electrician can determine if there is a single-phasing problem or if a fuse is blown by simply adhering to this rule of thumb when taking electrical measurements or troubleshooting.
Looking Deeper
It is important to become familiar with the basic rules concerning motor control diagrams that you learn, as all diagrams will follow the same basic rules regardless of geographic location or spoken language. This universal language is an important and effective way of safely communicating the purpose and intended operation of all control circuits. These same rules will also carry over to more complex motor control systems programmed and controlled with PLC’s (programmable logic controllers).
Common language of all control systems used to set foundational rules regarding the design and implementation of logic diagrams
Specialized schematic drawings depicting two vertical rails and horizontal rungs used to convey the logic of a control system.
The manufacture’s terminal numbers that are used to identify and separate the different components included in a ladder diagram.
A dangerous condition that occurs in three-phase systems where a single phase is lost, usually due to power outage, causing a harmful imbalance in the system.