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Motor Controls

Logic and Line Diagrams 312

Logic and Line Diagrams provides a comprehensive look at circuit logic and diagrams. The way a circuit functions depends on its circuit logic, which can be AND, OR, NAND, or NOR. The logic used in a circuit determines the layout of its corresponding line diagram. In general, line diagrams lay out the relationship between components on parallel lines. Line diagrams also include numbers to identify the location of components, the wires in the circuit, and the connections between components.

This class will familiarize users with the rules and conventions of line diagrams, as well as the different types of circuit logic. This knowledge will allow users to read line diagrams, which is essential when working with motors and especially motor controls.

Difficulty Advanced
Format Online
Number of Lessons 19
Language English

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Line Diagrams
Circuit Control
Rules for Line Diagrams
Line Diagram Rules Review
Line Numbers and Cross-Reference Numbers
Wire Numbers
Terminal Numbers
Identifying Circuit Numbers
Mechanically Connected Contacts
Component Identification Review
Logic Functions
AND and OR
NAND and NOR
Logic Functions Review
Control Circuit Steps
Memory
Troubleshooting
Circuit Steps, Memory, and Troubleshooting Review
Final Review

Describe line diagrams.
Describe control devices.
Describe rules for line diagrams.
Identify line numbers. Identify cross-reference numbers.
Identify wire numbers.
Identify terminal numbers.
Describe methods for identifying mechanically connected contacts.
Describe logic functions.
Describe AND and OR logic.
Describe NAND and NOR logic.
Describe control circuit steps.
Describe circuit memory.
Describe troubleshooting.

Vocabulary Term

Definition

action

The third step of a control circuit. Actions are the physical work a circuit performs as loads are energized.

AND

A type of circuit logic where two normally open contacts are connected in series. With AND logic, both contacts must be closed to energize the load.

automatic signal

A circuit input sent when a device detects a change in conditions. Automatic signals are sent by flow, pressure, and temperature switches.

auxiliary contact

A motor starter contact used to provide memory to control circuits. Auxiliary contacts close when a motor starter is energized, and they stay closed until another signal opens them.

circuit lines

The parallel horizontal paths in a line diagram on which circuit components appear. Circuit lines stretch between the live power line, L1, and the neutral return power line, L2.

circuits

A controlled path for electricity. A circuit includes a source, path, load, and control.

contacts

Connecting points between two conductors that allow electricity to flow when they are closed. Contacts prevent the flow of electricity when open.

control circuits

An electrical system that uses control devices to determine when loads are energized or de-energized by controlling current flow. Control circuits usually carry lower voltages than power circuits.

control device

An input device that controls the flow of current in a circuit. Control devices determine when loads are energized or de-energized.

control relays

CR. An electrically controlled mechanical device that controls one circuit by opening and closing contacts in another circuit. Control relays often use electromagnetic coils to open and close contacts.

cross-reference number

A number that identifies other circuit lines that electrically connect to the load in the referenced line. Cross-reference numbers appear on the right side of a line diagram.

cross-referencing

The practice of referring from one part of a diagram to another. Cross-referencing helps identify contacts in a line diagram.

dashed line method

A technique used to cross-reference mechanically connected contacts by drawing a dashed line between them. The dashed line method is used on simple diagrams when the contacts are located near each other on the diagram.

decision

The second step of a control circuit. Decisions are based on the received signals and the logic functions in a circuit.

diagrams

A drawing that illustrates the parts or operation of something. Motor diagrams use pictures and symbols to represent motor components.

digital multimeter

DMM. A device that can measure voltage, current, or resistance. A digital multimeter is used to troubleshoot a circuit with the tie-down method.

dual palm buttons

A device with two separate buttons that must be pressed simultaneously in order to power a stamping press. Dual palm buttons use AND logic.

flow switch

A switch that actuates automatically when it senses the movement of fluids. Flow switches sense the movement of air or liquid in a duct or pipe.

hard-wired

Having wires that are physically connected to other devices or wires so as to consistently function the same way. Hard-wired circuits perform the same tasks over and over again.

ladder diagrams

An electrical print that shows the logic of an electrical circuit or system using standard symbols. Ladder diagrams are also known as line diagrams.

lead

A conductive device within a circuit or device to which other components can be attached. A lead has either a negative or positive charge.

limit switch

A switch that detects the presence or absence of an object. Limit switches operate mechanically.

line diagram

An electrical print that shows the logic of an electrical circuit or system using standard symbols. Line diagrams are also known as ladder diagrams.

line number

A number that identifies a circuit line in a line diagram. Line numbers run sequentially down the left side of a line diagram.

load

A device that converts electrical energy to motion, heat, light, or sound. Loads include motors and pilot lights.

logic

The reasoning behind the functioning of something. The logic of a circuit comprises all the principles required to understand electrical circuitry.

logic function

A sequence of operations that a circuit performs. Logic functions follow a set, predictable pattern.

manual signal

A circuit input sent by physical action. Manual signals are sent by light switches and pushbuttons.

mechanical signal

A circuit input sent by the mechanical motion of a machine. A mechanical signal is sent by a gear contacting a limit switch.

memory

The ability of a circuit to store a signal and keep the load energized even after the signal is removed. Memory circuits use auxiliary contacts.

momentary contact

A contact that returns to its original position immediately after sending a signal. Momentary contacts close when a pushbutton is depressed but open as soon as it returns to its original position.

motor starter

An electrically operated switch that starts a motor when actuated. Motor starters use magnetic induction to provide the startup current for a motor.

NAND

A type of circuit logic where two normally closed contacts are connected in parallel. In NAND logic, both contacts must be opened to de-energize the load.

Normally closed. An electrical contact that generally allows electricity to flow. An NC contact symbol has parallel lines with a diagonal line crossing them.

Normally open. An electrical contact that does not generally allow electricity to flow. An NO contact symbol has parallel lines with an opening between them.

NOR

A type of circuit logic where two normally closed contacts are connected in series. In NOR logic, only one contact must be opened to de-energize the load.

normally closed

NC. An electrical contact that generally allows electricity to flow. A normally closed contact symbol has parallel lines with a diagonal line crossing them.

normally open

NO. An electrical contact that does not generally allow electricity to flow. A normally open contact symbol has parallel lines with an opening between them.

NOT

A type of circuit logic that uses normally closed contacts. NOT logic functions are either NAND or NOR.

numerical cross-reference method

A technique used to cross-reference mechanically connected contacts by indicating associated contacts' line numbers and pointing toward the lines. The numerical cross-reference method is used on complex diagrams when the connected contacts are several lines apart.

A type of circuit logic where two normally open contacts are connected in parallel. With OR logic, only one contact must be closed to energize the load.

overload condition

A situation in which current exceeds recommended levels. Overload conditions can cause devices to overheat.

overload contacts

OL. A normally closed contact that opens only when an overload condition is present. Overload contacts are sometimes placed in series between a load and the neutral line of a circuit.

overload relay

A control relay that protects a motor from overheating due to overload conditions in the machinery. Overload relays connect to contacts that open when overload conditions are detected.

parallel

An electrical route that has multiple paths. Parallel circuit lines in line diagrams each have their own number.

pilot lights

A type of indicator light found on machines. Pilot lights typically indicate when a device is energized or operating.

pressure switch

A switch that actuates automatically when it senses changes in pressure. Pressure switches are used in pneumatic and hydraulic systems.

pushbuttons

A manual control device that opens or closes a circuit when pressed. Pushbuttons can be normally open or normally closed, like other switches.

rails

The vertical paths in a line diagram that represent a circuit's power supply. The rails consist of L1, the live power line, and L2, the neutral return power line.

series

An electrical route that has only one path. Series connections in line diagrams may have multiple control devices but not multiple loads.

signal

The first step of a control circuit. Signals are inputs that change the condition of a circuit.

specifications

A description of the essential physical and technical properties of a circuit or a device. Specifications are sometimes called specs.

stamping presses

A machine tool that moves up and down continuously to cut and shape sheet metal using a die set. Stamping presses often require dual palm buttons for starting.

switches

A control device that opens and closes contacts in a circuit. Switches can operate manually, mechanically, or automatically.

symbols

A sign or mark that represents something else. Symbols are often used to represent components in diagrams.

temperature switch

A switch that actuates automatically when it senses changes in temperature. Temperature switches are often used in heating and cooling systems.

terminal

A connecting point in a circuit where a wire can be attached to connect a component. Terminals are either negative or positive.

terminal block

A device that joins wires or cables. Terminal blocks typically snap into a metal rail or are mounted on the panel of a control enclosure.

terminal number

A number assigned to a contact point of a component to help identify the connection points in a circuit. Terminal numbers are assigned by the manufacturer of a device or by the engineer of a terminal block.

thermostat

A device that regulates temperature. A thermostat can be controlled manually or automatically.

tie-down method

A process of troubleshooting a circuit with a digital multimeter (DMM). The tie-down method works by connecting one lead of a DMM to the neutral line of a circuit and systematically moving the other lead along the circuit to find the malfunctioning part.

troubleshooting

The systematic elimination of the various components of a system, circuit, or process to locate a malfunctioning part. Troubleshooting a motor circuit often involves using electrical diagrams as a guide to find the problem area.

voltage

The electrical pressure or potential that pushes electrons through a conductor. Voltage is measured in volts (V) and is also called electromotive force.

wire number

A number that identifies a specific wire in a line diagram to indicate how components are connected. Wire numbers start with 1 on the top left of a diagram and may be circled on older diagrams.