Relays, Contactors, and Motor Starters 201

AC

Alternating current. Electricity that reverses direction at regularly recurring intervals of time. AC switches direction at a frequency of 60 hertz in the United States.

Across-the-line starters

A motor starter that connects the motor directly to the power circuit on startup. Across-the-line starters do not limit inrush current.

Alloy

A metal consisting of a mix of two or more elements, one of which must be a metal. Common alloys include cast iron, bronze, brass, and different types of steel.

Alternating current

AC. Electricity that reverses direction at regularly recurring intervals of time. Alternating current switches direction at a frequency of 60 hertz in the United States.

Amperage

The strength or intensity of an electrical current. Amperage is measured in amperes (A).

Arc chutes

A set of parallel magnetic plates that break up electric arcs and absorb their energy until they are extinguished. Arc chutes are commonly found in circuit breakers and contactors.

Arc flash

An electrical explosion caused when an electric current travels through the air unintentionally. Arc flash releases intense light, heat, and energy that can cause potentially serious injuries to anyone nearby.

Arc suppression

Any method used to minimize the duration and strength of electrical arcs that form between contacts. Arc suppression mechanisms include arc chutes and blowout coils.

Armature

The movable part of a relay. The armature in an electromechanical relay moves the pole of a switch.

Automation

The use of self-regulated equipment, processes, or systems that meet manufacturing requirements with limited human intervention. Automation is an efficient means of performing manufacturing processes.

Autotransformer starters

A reduced voltage starter that uses autotransformers to step down startup voltage. Autotransformer starters are efficient and have adjustable settings for startup voltage.

Autotransformers

An electrical transformer with only one winding. Autotransformers reduce voltage with less energy loss than resistors.

Bimetallic strip

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

Blowout coils

An electromagnet that elongates electric arcs until they break. Blowout coils increase the resistance that arcs experience by increasing their length.

Break

To open an electrical circuit and stop the flow of current. Sets of contacts break electrical circuits when they open.

Circuit

A controlled path for electricity. Circuits usually include a source, path, load, and control.

Circuit breakers

A safety device that opens a circuit when it detects overcurrent conditions and can be reset afterward. Circuit breakers typically contain a bimetallic strip.

Coil

A length of conducting wire that is wrapped continuously around a magnetic core. Coils are used to create magnetic fields when current passes through them.

Conductive

Able to act as a path for the movement of electricity. Conductive materials can form magnetic fields when energized.

Contact arcing

The formation of small electric arcs between energized contacts. Contact arcing usually occurs as contacts open or close.

Contact bounce

Rebounding that occurs when contacts close. Contact bounce can cause additional electric arcs to form.

Contactors

A relay used to safely control a high-energy circuit. Contactors are also known as power relays.

Contacts

A conductive metal part used to make or break an electrical circuit. Contacts in switches open or close circuits by connecting with or separating from one another.

Control circuit

A circuit that uses control devices to energize and de-energize loads by controlling current flow to a power circuit. Control circuits usually carry lower voltages than power circuits.

Control relays

A relay that opens or closes a circuit in response to electrical signals from other devices. Control relays are used to control manufacturing processes automatically or remotely.

Current

The flow of electricity. Current strength is measured in amperes (A), or amps.

Delta

A connection of three conductors that forms a triangular series circuit. Delta connections in a motor starter allow full voltage.

Direct current

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

Distribution systems

A network of electrical components that provides electricity from the utility to specific end destinations. A distribution system uses generators, power lines, and transformers to deliver electric power to buildings and other structures.

Double-pole double-throw

DPDT. A switch with two movable contacts that each have two potential circuit paths. Double-pole double-throw switches have two poles that can each connect to two possible circuit paths.

Double-pole single-throw

DPST. A switch with two movable contacts that each connect to one circuit path. Double-pole single-throw switches simultaneously open and close two different circuits.

DPDT

Double-pole double-throw. A switch with two movable contacts that each have two potential circuit paths. DPDT switches have two poles that can each connect to two possible circuit paths.

DPST

Double-pole single-throw. A switch with two movable contacts that each connect to one circuit path. DPST switches simultaneously open and close two different circuits.

Dual-element time delay fuses

A single-use motor protection device that allows short surges of high current. Dual-element time delay fuses include components that protect against short circuits and components that protect against overload.

Electric arcs

The area in which electricity jumps from one conductor to another. Electric arcs produce heat and light.

Electric motors

A machine that converts electricity into mechanical energy or motion. Electric motors are common power sources for mechanical systems.

Electric shock

The flow of electricity through the body. Severe electric shock can be fatal.

Electrical faults

Any condition that causes an abnormal flow of current in a circuit. Common types of electrical faults include open circuit faults and short circuit faults.

Electromagnet

A magnet that gains an attractive force only when current passes through it. An electromagnet is typically formed by wrapping a current-carrying coil around an iron core.

Electromechanical relays

EMR. A relay that uses electric current to create a magnetic field that moves contacts. Electromechanical relays are inexpensive and often used in motor controls.

EMRs

Electromechanical relay. A relay that uses electric current to create a magnetic field that moves contacts. EMRs are inexpensive and often used in motor controls.

Energy efficiency

A measure of a system's energy output compared to the total energy supplied to it. Motor control devices optimize the energy efficiency of electric motors.

Fuses

A single-use safety device that opens a circuit when it detects excess current. A typical fuse has a component that melts to open the circuit.

Heating element

A device that converts electricity into thermal energy. Heating elements emit heat when current passes through them.

Horsepower

hp. A unit used to measure power in large electrical devices such as motors. One horsepower equals 33,000 foot-pounds of work per minute, or 746 watts.

Industrial robots

A reprogrammable machine often used for repetitive, precise tasks in a manufacturing setting. Industrial robots can perform tasks that would be dangerous to humans.

Inrush current

The initial surge of current into a motor upon startup. Inrush current can be 10 or more times higher than the normal operating current.

Insulated

Covered in a material that prevents the passage of electricity. Insulated plates are used in arc chutes to break up arcs.

International Electrotechnical Commission

IEC. An international organization that prepares and publishes standards for electrical, electronic, and related technologies. The International Electrotechnical Commission develops standards that are applied in Europe and other countries.

Load

The opposition to an applied force, such as weight or resistance. The load of a motor may refer to the electrical power that it consumes or the mechanical power that it outputs.

Magnetic fields

The area in and around a magnet in which a magnetic force exists. Magnetic fields attract and repel magnetic materials.

Magnetic induction

The use of magnets to cause voltage in a conductor. Magnetic induction is used by electric motors to convert electrical energy into movement.

Magnetic overload relays

An overload relay that uses excess current to create a magnetic field that moves contacts. Magnetic overload relays move the armature a greater distance than electromechanical relays.

Motor controls

Any device used to start, stop, or change the speed, torque, or direction of a motor. Motor controls are essential for operating manufacturing equipment such as conveyor belts and industrial robots.

Motor starters

A device used to turn a motor on and off. A motor starter consists of a contactor combined with an overload relay.

National Electrical Manufacturers Association

NEMA. An organization that sets standards for electrical equipment used in the United States. The National Electrical Manufacturers Association provides ratings and standards for various types of motor control devices.

NC

Normally closed. An electrical contact that is positioned to allow current flow. NC contacts only open to stop current flow when they are activated.

NO

Normally open. An electrical contact that is positioned to prevent current flow. NO contacts only close to allow current flow when they are activated.

Normally closed

NC. An electrical contact that is positioned to allow current flow. Normally closed contacts only open to stop current flow when they are activated.

Normally open

NO. An electrical contact that is positioned to prevent current flow. Normally open contacts only close to allow current flow when they are activated.

On/off switches

A simple control device that closes or opens a circuit. On/off switches are often single-pole single-throw.

Overcurrent

Excess current flowing through a circuit. Overcurrent conditions can cause circuits to overheat and catch fire.

Overload

A situation in which too much current flows through a motor, causing it to overheat. Overload can damage motors and can create safety hazards for operators.

Overload relays

A protective relay that opens a motor circuit when overcurrent conditions persist for too long. Overload relays prevent a motor from overheating due to overloading.

Part winding starters

A reduced voltage starter that lowers startup voltage by only energizing one winding. Part winding starters can only be used with motors that have two stator windings.

Phases

A conductor in an AC electrical system with its own specific voltage. Most AC electrical systems are three-phase.

Poles

A movable contact controlled by a switch. The number of poles in a switch indicates the total number of movable contacts.

Power

The rate at which a device converts electrical energy into another form, such as motion, heat, or light. Power in motors is measured in watts (W) or horsepower (hp).

Power circuit

A circuit that carries high-energy electricity to a load. Power circuits often have very high voltage and require additional safety precautions.

Power relays

A relay used to safely control a high-energy circuit. Power relays are also known as contactors.

Power tools

A tool powered by an external source of energy. Power tools often use electric motors to convert electricity into motion.

Primary resistor starters

A reduced voltage starter that uses resistors to lower startup voltage. Primary resistor starters are simple and have high starting torque.

Programmable logic controllers

PLCs. A processor-driven device that uses logic-based software to provide electrical control to a machine or process. Programmable logic controllers can be used to automate processes.

Protective relays

A relay designed to open a circuit when it detects dangerous conditions. Protective relays help prevent hazards caused by overcurrent and electrical faults.

Ratchet wheel

A toothed wheel that can only turn in one direction. Ratchet wheels are often held in position by another component and only turn when the component is removed.

Reduced voltage starters

A motor starter that limits inrush current by lowering voltage during startup. Reduced voltage starters are necessary to prevent damage in large and high-power motors.

Relay

A switch controlled by electrical current. Relays may be electromechanical, thermal, or solid-state.

Resistance

The opposition to current flow. Resistance is measured in ohms (Ω).

Resistors

A device used in circuits to limit current flow. Resistors can be used to prevent damage from high current.

Rotor

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

Selector switches

A control device that can be turned to different positions to close different circuits. Selector switches have two or more throws.

Semiconductor devices

An electrical component made from semiconductor materials. Semiconductor devices like diodes, transistors, and integrated circuits (ICs) have no moving parts.

Sensors

A device that detects a change in a physical stimulus and turns it into a signal that can be measured or recorded. Sensors can be connected to relays to automate manufacturing processes.

Single-pole double-throw

SPDT. A switch with one movable contact and two potential circuit paths. Single-pole double-throw switches are often selector switches.

Single-pole single-throw

SPST. A switch with one movable contact and one circuit path. Single-pole single-throw switches are typically on/off switches.

Solid-state overload relays

An overload relay that uses semiconductor devices to monitor current and open circuits. Solid-state overload relays often have adjustable trip times.

Solid-state relays

SSR. A relay that uses semiconductor devices to open or close circuits. Solid-state relays are fast and contain no moving parts but are more expensive than other relay types.

SPDT

Single-pole double-throw. A switch with one movable contact and two potential circuit paths. SPDT switches are often selector switches.

SPST

Single-pole single-throw. A switch with one movable contact and one circuit path. SPST switches are typically on/off switches.

Stator

The stationary part of a motor. The stator surrounds the motor's rotor.

Step down

To decrease the voltage of electrical current using a transformer. Stepping down is more energy efficient than using a resistor.

Surges

A sudden, large, but typically brief increase in voltage or current in an electric circuit. Surges can damage circuit components.

Switches

A device that opens and closes a circuit. Switches control when a circuit is energized.

Thermal overload relays

An overload relay that uses excess current to create heat that melts or bends components. Thermal overload relays often use a bimetallic strip or a melting alloy to break a circuit.

Thermal relays

A relay that uses electric current to create heat that melts or bends components to move contacts. Thermal relays are often used as protective relays.

Three-phase

Alternating current electricity that consists of three overlapping voltages. Three-phase electricity is used for all large AC motors and is the standard power supply that enters homes and businesses.

Throws

A stationary contact connected to a circuit path. The number of throws in a switch indicates the total number of potential circuit paths.

Timing relays

A control relay that delays changing the position of its contacts instead of opening or closing them immediately. Timing relays are often used in reduced voltage motor starters.

Torque

A force that produces rotation. Torque is measured in pound-feet (lb.-ft.) in the English system and newton-meters (Nm) in the metric system.

Trip

To automatically open a circuit. Protective relays trip to prevent damage and potential injury.

Trip time

The time it takes for an overload relay to open a circuit in the event of overcurrent. Trip time is measured in seconds.

Voltage

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

Voltage drop

A decrease in voltage that occurs as electricity passes through resistance. A large voltage drop across one load in a circuit can reduce the voltage available to other loads.

Wear

The gradual removal or decay of material caused by contact, corrosion, and friction. Wear occurs in contacts due to arcing and friction.

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.

Wye

A connection of three conductors in which one end of each conductor is connected to a common central point. Wye connections in a motor starter can lower the voltage.

Wye-delta starters

A reduced voltage starter that uses a wye connection to lower startup voltage and then switches to a delta connection. Wye-delta starters can only be used with three-phase AC electricity.