Symbols and Diagrams for Motors 311

AC

Alternating current. Electricity that reverses direction at regularly recurring intervals of time. AC switches direction 60 times per second, or 60 hertz (Hz), in the US.

actuator

The part of a limit switch that contacts an object and triggers the switch. Limit switch actuators come in several different styles.

air flow switch

An automatic switch that senses the movement of air in a duct. Air flow switch symbols have a flag shape that represents a sail.

alternating current

AC. Electricity that reverses direction at regularly recurring intervals of time. Alternating current switches direction 60 times per second, or 60 hertz (Hz), in the US.

armature

The part of a motor in which a current is induced to create a magnetic field. The armature usually consists of a series of coils or groups of insulated conductors surrounding a core of iron.

ball float

A round floating component used in float switches to detect the level of a liquid. A ball float rests on the surface of the liquid.

battery

A device that converts chemical energy into electrical energy. Batteries are a source of direct current.

bimetallic strip

A component made of two metals with different expansion temperatures that curls when exposed to heat. Bimetallic strips are often used in temperature switches and circuit breakers to detect temperature.

circuit breakers

An overcurrent device with a bimetallic strip that bends and trips a switch to open a circuit. Circuit breakers detect excess current to prevent overheating in a circuit.

circuit logic

The reasoning behind the function of an electrical circuit or system. Circuit logic comprises all the principles required to understand electrical circuitry.

coil

Multiple loops of conducting wire used to create a magnetic field when current passes through it. Coils are often wrapped continuously around magnetic cores made of iron or steel.

compound motor

A DC motor that has both series and shunt field windings. Compound motors combine the advantages of the shunt and series motors.

contacts

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

control devices

A component in a circuit that controls the flow of electricity. Control devices determine when a circuit is energized.

control relay

An electrical relay that controls a circuit or other relays. Control relays can open and close many sets of contacts.

control transformers

An electromagnetic device that decreases motor voltage to usable levels for the motor control circuit. Control transformers also provide electrical isolation.

current

The flow of electricity. Current strength is called amperage and is measured in amperes.

delay-on-de-energize timer

A timing relay whose contacts change position immediately when the coil is energized, but delay changing position when the coil is de-energized. Delay-on-de-energize timers, or off-delay timers, include normally open time-delay opening (NOTO) and normally closed time-delay closing (NCTC) relays.

delay-on-energize timer

A timing relay whose contacts delay changing position when the coil is energized, but change position immediately when the coil is de-energized. Delay-on-energize timers, or on-delay timers, include normally open time-delay closing (NOTC) and normally closed time-delay opening (NCTO) relays.

diagram

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

diaphragm-operated switch

A type of pressure switch that senses small pressure changes at low pressures. Diaphragm-operated switch symbols include a semicircle that represents the diaphragm.

direct current

DC. Electricity that flows in one direction. Direct current does not reverse the direction of flow.

double-pole double-throw

DPDT. A switch with two movable contacts that each have two stationary contacts. Double-pole double-throw switches control four circuits, but only two at a time.

double-pole single-throw

DPST. A switch with two movable contacts that each have one stationary contact. Double-pole single-throw switches simultaneously control two different circuits.

electric motor

A machine that converts electricity into mechanical energy or motion. Electric motors operate on the principle of magnetic induction.

field

The part of a motor that creates a magnetic field around the armature. The field consists of electromagnets and windings.

float switch

An automatic switch that senses the level of liquid in a tank. Float switch symbols include a circle that represents a ball float.

flow switch

An automatic switch that senses the movement of air or liquid in a duct or pipe. Flow switch symbols include a triangular shape to represent a sail or paddle.

fuses

An overcurrent device with a metallic component that melts to open a circuit. Fuses detect excess current to prevent overheating in a circuit.

hydraulic systems

A power transmission system that uses the force of flowing liquids to transmit power. Hydraulic systems often use pressure switches.

ladder diagram

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

limit switch

A mechanical sensor that detects the presence or absence of an object. The limit switch symbol has a wedge that represents an actuator.

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.

liquid flow switch

An automatic switch that senses the movement of liquid in a pipe. Liquid flow switch symbols have a flag shape that represents a paddle.

loads

A component in a circuit that converts electricity into light, heat, or mechanical motion. Loads include light bulbs, appliances, and machines.

magnetic field

The area in and around a magnet in which a magnetic force exists. Magnetic fields exhibit the powers of attraction and repulsion.

magnetic overload relay

An overload relay that senses the strength of the magnetic field a motor's current produces. Magnetic overload relays are often used in areas that experience extreme changes in temperature.

maintained contact device

A control device that closes or opens the circuit when actuated and remains in the actuated position to maintain the circuit's status. Maintained contact devices include emergency stop buttons.

momentary contact device

A control device that closes or opens the circuit only when it is actuated. Momentary contact devices include start and stop pushbuttons.

mushroom head

A type of pushbutton with a large round head. Mushroom head pushbuttons are easy to operate and often used for emergency stops.

National Electrical Manufacturers Association

NEMA. An organization that sets standards for electrical equipment used in the United States. The National Electrical Manufacturers Association provides commonly used electrical symbol standards.

NC

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

NCHO

Normally closed held open. A switch with contacts that are generally closed but held open when the circuit is de-energized. NCHO switch symbols show the movable contact above and away from the stationary contact.

NO

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

NOHC

Normally open held closed. A switch with contacts that are generally open but held closed when the circuit is de-energized. NOHC switch symbols show the movable contact touching the bottom of the stationary contact.

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 closed held open

NCHO. A switch with contacts that are generally closed but held open when the circuit is de-energized. Normally closed held open switch symbols show the movable contact above and away from the stationary contact.

normally closed time-delay closing

NCTC. A timing relay whose contacts open immediately when the coil is energized and pause before closing when the coil is de-energized. Normally closed time-delay closing relays are a type of off-delay timer.

normally closed time-delay opening

NCTO. A timing relay whose contacts pause before opening when the coil is energized and close immediately when the coil is de-energized. Normally closed time-delay opening relays are a type of on-delay timer.

normally open

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

normally open held closed

NOHC. A switch with contacts that are generally open but held closed when the circuit is de-energized. Normally open held closed switch symbols show the movable contact touching the bottom of the stationary contact.

normally open time-delay closing

NOTC. A timing relay whose contacts pause before closing when the coil is energized and open immediately when the coil is de-energized. Normally open time-delay closing relays are a type of on-delay timer.

normally open time-delay opening

NOTO. A timing relay whose contacts close immediately when the coil is energized and pause before opening when the coil is de-energized. Normally open time-delay opening relays are a type of off-delay timer.

off-delay timer

A timing relay whose contacts change position immediately when the coil is energized, but delay changing position when the coil is de-energized. Off-delay timers, or delay-on-de-energize timers, include normally open time-delay opening (NOTO) and normally closed time-delay closing (NCTC) relays.

on-delay timer

A timing relay whose contacts delay changing position when the coil is energized, but change position immediately when the coil is de-energized. On-delay timers, or delay-on-energize timers, include normally open time-delay closing (NOTC) and normally closed time-delay opening (NCTO) relays.

output devices

A device that performs a mechanical action after receiving the electrical signal to do so. Output devices include machines such as motors.

overload condition

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

overload relay

OL. A control relay that protects a motor from overheating due to overload conditions in the machinery. Overload relays can be thermal or magnetic.

paddle

A device used to sense changes in liquid flow. Paddles are represented by flag shapes on liquid flow switch symbols.

parallel

An electrical system that has multiple routes for the flow of electricity. Parallel windings consist of many turns of thin wires.

pictorial diagram

An electrical print that shows the external appearance of each component in a circuit. A pictorial diagram is much like a photograph of the circuit.

pilot lights

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

pneumatic systems

A power transmission system that uses the force of flowing gases to transmit power. Pneumatic systems often use pressure switches.

pole

The number of movable contacts controlled by a switch. The pole includes all of the movable contacts that belong to a single circuit.

power sources

The device that provides electrical energy to a circuit. Power sources include batteries, generators, and other devices.

pressure switch

An automatic switch that senses the pressure in a pneumatic or hydraulic system. Pressure switch symbols include a semicircle that represents a diaphragm.

protective devices

A circuit component that prevents damage, injury, and/or fire. Protective devices include grounding circuits and overcurrent devices.

pushbutton

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

relay

An electrically controlled mechanical device that controls one circuit by opening and closing the contacts in another circuit. Relays use electromagnetic coils to open and close contacts.

rotor

The rotating part of a motor. The rotor connects to an output shaft that drives the load.

sail

A device used to sense changes in air flow. Sails are represented by flag shapes on air flow switch symbols.

schematic diagram

An electrical print that uses symbols to represent all electrical components in a circuit. Schematic diagrams show the electrical relationships of all components but not their physical locations.

selector switch

A rotating manual switch that makes connections with different contacts when turned to different positions. Selector switches generally have between one and eight positions.

sensor switches

A control device that actuates when it detects changes in environmental conditions. Sensor switches include limit, float, flow, pressure, and temperature switches.

series

An electrical system that has only one route for the flow of electricity. Series windings consist of a few turns of thick wire.

series field

A winding designed to be connected in series with the armature of a DC motor or generator. Series fields consist of a few turns of thick wire.

series motor

A DC motor that has field windings connected in series with the armature. Series motors provide very high startup torque but must never be run without a load.

shunt field

A winding designed to be connected in parallel with the armature of a DC motor or generator. Shunt fields consist of many turns of thin wire.

shunt motor

A DC motor that has field windings connected in parallel with the armature. Shunt motors are commonly used because of their excellent speed regulation.

sine wave

The most common type of AC waveform. A sine wave consists of 360 electrical degrees and is produced by rotating machines.

single-phase motor

A low-horsepower AC motor that operates on current with only one sine wave. Single-phase motors are often used for simple residential appliances like furnaces, washing machines, and air conditioners.

single-pole double-throw

SPDT. A switch with one movable contact and two stationary contacts. Single-pole double-throw switches alternately control two different circuits.

single-pole single-throw

SPST. A switch with one movable contact and one stationary contact. Single-pole single-throw switches are typical on/off switches.

solenoid

An output device that uses a coil to convert electrical energy into linear mechanical motion. Solenoids are often used to control valves in fluid power systems.

squirrel cage motors

An AC induction motor with a rotor that consists of metal bars connected at either end. The squirrel cage motor is the most common AC motor.

starting winding

An auxiliary stator winding used in single-phase motors that receives current during motor startup. The starting winding is disconnected when the motor reaches 60-80% of the full-load speed.

switch

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

symbol

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

temperature switch

An automatic switch that senses changes in temperature. Temperature switch symbols use a bent line to represent a bimetallic strip.

thermal overload relay

An overload relay that senses how much heat a motor's current produces. Thermal overload relays shut down the motor when they detect excessive heat.

three-phase motor

A popular AC motor with a continuous series of three overlapping AC cycles offset by 120 electrical degrees. Three-phase motors are used for all large AC motor applications.

throw

The number of stationary contacts that correspond to a switch's movable contacts. The throw includes all of the stationary contacts for one or many circuits.

timing relay

A control relay that delays changing the position of its contacts instead of opening or closing them immediately. Timing relays may delay changing position either when the coil is energized or when it is de-energized.

torque

A force that produces rotation. Torque determines how much work a motor or generator can accomplish.

volts

A unit used to measure electromotive force or pressure, which is called voltage. One volt of force is needed to cause one coulomb to do one unit of work.

winding

Wire wrapped around a core or into a coil that is used to conduct current. Windings form electromagnets and magnetic fields in motors and other devices.

windings

Wire wrapped around a core or into a coil that is used to conduct current. Windings form electromagnets and magnetic fields in motors and other devices.

wiring diagram

An electrical print that shows the physical locations and electrical relationships of all electrical components in a circuit. Wiring diagrams use symbols, like schematic diagrams, but they are much less common.

wound rotor motor

An AC induction motor with a rotor that contains windings. Wound rotor motors produce high startup torque.