Relays, Contactors, and Motor Starters 201

"Relays, Contactors, and Motor Starters" provides an overview of the primary components involved in electric motor control. Relays are electrical switches that control a circuit. When activated by current, a relay opens and closes a circuit to turn a larger current on or off. Contactors control current by conducting it through metal contacts that make or break electrical circuits. When combined with an overload relay, a contactor becomes a motor starter.

Working with relays, contactors, and motor starters requires technicians to understand how to properly care for such devices and how to operate them effectively. After taking this class, users will be able to describe the design and function of common relays, contactors, and motor starters, as well as the applications for each device.

Class Details

Class Name:
Relays, Contactors, and Motor Starters 201
Number of Lessons:
Related 1.0 Class:
Contactors and Motor Starters 250

Class Outline

  • Elements of Motor Control
  • Relays
  • Relays in Action
  • Contactors
  • Manual Contactors and Magnetic Contactors
  • Contacts
  • Elements of Motor Control Review
  • Arcing
  • Contact Bounce
  • Arc Suppression Methods
  • Overload Protection at Startup
  • Overload Relays
  • Thermal Overload Relays
  • Magnetic Overload Relays
  • Magnetic Overload Relay in Action
  • Arcing and Motor Overload Protection Review
  • Motor Starters
  • Across-the-Line-Starters
  • Reduced Voltage Starters
  • Motor Starters and Ratings Review
  • NEMA and IEC Standards and Ratings


  • Describe the primary components of motor control.
  • Distinguish between power relays and control relays.
  • Describe the operation of a relay.
  • Describe the advantages and disadvantages of contactors.
  • Distinguish between manual contactors and magnetic contactors.
  • Describe contacts.
  • Describe arcing.
  • Describe contact bounce.
  • Describe arc suppression methods.
  • Describe overload protection at startup.
  • Describe overload relays.
  • Describe thermal overload relays.
  • Describe magnetic overload relays.
  • Describe motor starters.
  • Describe across-the-line starters.
  • Describe the different types of reduced voltage starters.
  • Distinguish between NEMA and IEC standards and ratings.

Job Roles



Vocabulary Term Definition
AC magnetic motor starters A type of starter for AC motors that combines a magnetic contactor and an overload relay. AC magnetic motor starters can be operated remotely.
across-the-line starters A motor starter in which the motor is directly connected to the power supply. Across-the-line starters allow full voltage into the motor when starting.
actuate To put a machine or device into motion or operation. Contacts can actuate output devices.
alternating current AC. A current formed when electrons flow in a one direction and then the opposite direction. Alternating current is not steady and must often be converted to DC.
amperes Amps. A unit of electrical measurement that describes both an amount of electricity and the time it takes for electricity to travel a specific distance. One ampere, or amp, equals one coulomb per second.
arc chutes An arc suppression device. Arc chutes extinguish arcs by channeling them into chambers above the contacts.
arc column A string-like spark of electricity that connects across the gap between two contacts. Arc columns develop when electricity flows via ionized air molecules or vaporized metal and results in damage to the contacts.
arc suppression Any method used for extinguishing electrical arcs between contacts. Arc suppression is necessary to ensure worker safety and prolong contact life.
arcing Overheating that occurs when electricity flows from one surface to another. Arcing is dangerous because it can cause injuries to operators and damage to equipment.
armature Any magnetic pole that causes mechanical motion. The armature is the mechanical piece that moves in relays and contactors.
bimetallic strip A strip made by bonding two unlike metals that expand at different rates when heated. A bimetallic strip curls when the rates of expansion are different.
bimetallic thermal overload relay A type of thermal overload mechanism that uses a strip composed of two different metals. When heated, the two metals in the bimetallic thermal overload relay expand at different rates, causing the strip to warp and create an opening in the circuit.
blow In fuses, to interrupt an electrical circuit due to a melted component. A fuse blows, or trips, when current exceeds a set limit.
blowout coils An arc suppression device that uses magnetic coils to create magnetic fields that push arcs upward until they break. Blowout coils are usually used for contactors and DC motor starters.
break The place at which a circuit can be opened or closed. Breaks can be opened or closed by many different types of switches.
circuit A completely enclosed path of various devices that contains an electrical current. Circuits normally include a source, path, load, and control.
circuit breakers A safety device that detects overcurrent in a circuit. Circuit breakers open circuits to avoid short circuits.
closed circuit A controlled path on which live electricity travels. A closed circuit can be formed by relays.
closed contacts The point at which two contacts connect with each other, allowing current to flow. Closed contacts create a circuit.
coils A bundle of wire that is wrapped continuously around a magnetic core. Coils are used to create a magnetic field when current passes through them.
conductive A material's ability to act as a path for the movement of electricity. Conductive materials are often types of metals.
contact bounce An unwanted effect that occurs when contacts close under high amounts of pressure and then rebound from each other due to the force. Contact bounce is undesirable because it can create secondary arcs, reduce contact life, and lead to pitting.
contactors A type of relay that is designed to handle heavy and fluctuating current loads. Contactors provide safe connection and disconnection to motor loads and use an electromagnetic coil to operate contacts.
contacts A conductive metal part in an electrical circuit. Contacts open or close circuits by connecting with or separating from one another.
control circuit A type of circuit 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 relays An electrical switch that opens and closes a circuit. Control relays can open or close one or many sets of contacts.
current The flow of electricity through a circuit. The amount of current in a circuit can fluctuate.
DC Direct current. A current formed when electrons flow in a single direction continuously. DC is controlled by coils on contactors.
deionization A process that removes ions in order to remove an electrical charge. Deionization can be used as an arc suppression method.
delta A connection of three components where a triangular series circuit is formed. Delta connections are used in wye delta starters.
direct current DC. A current formed when electrons flow in a single direction continuously. Direct current is controlled by coils on contactors.
dissipate To break up, disperse, and fade away. Arc suppression devices dissipate arcs that might form between contacts.
divert To change the path or movement of something. Arc suppression devices divert arcs that might form between contacts.
double-pole double-throw DPDT. A set of two moveable contacts that can break a circuit in two places each. A double-pole double-throw is a type of switch.
double-pole single-throw DPST. A set of two moveable contacts that can break a circuit in one place each. A double-pole single-throw is a type of switch.
dual voltage motors A type of three-phase motor that operates on two voltage levels. Dual voltage motors allow the same motor to be used with two different power line voltages.
dual-element time delay fuses A motor overload protection device that allows inrush current flow. The dual-element time delay fuse contains three elements that melt when met with inrush, and thus allow the motor time to start without the fuse blowing.
electric arc The area in which electricity jumps from one conductor to another to produce extreme heat and light. Electric arcs are used in welding and some industrial furnaces.
electromagnet A magnet formed from electric current. An electromagnet is typically formed by wrapping several windings of wire around an iron core.
electronic overload relay A type of relay that detects overload by monitoring motor current. Electronic overload relays are highly flexible and can be programmed to accommodate many applications.
elements Components situated at both ends of a dual-element time delay fuse to prevent motor overload. Elements melt when they experience overload but spare the fuses from blowing, allowing the motor time to start.
eutectic alloy A blend of metals that melt to activate a mechanical device. When eutectic alloys melt in overload relays, they signal the relay to open a circuit.
eutectic overload A type of thermal overload relay that uses a melting alloy to activate mechanical devices. A eutectic overload, also known as a melting alloy overload relay, opens a circuit in the case of overload.
fuses A safety device that detects excess current in a circuit. Fuses often have a component that melts and opens the circuit when overcurrent occurs.
heat sensitivity Susceptibility to bending when exposed to heat. Heat sensitivity leads to warpage, which is often caused by a physical twist or turn within a part due to internal stress.
horsepower A unit of measurement that indicates the amount of electrical power in larger devices. Horsepower is used instead of watts to describe the capacity of electric motors.
IEC The International Electrotechnical Commission. An international organization that prepares and publishes all standards for electrical, electronic, and related technologies. The IEC develops standards that are applied in Europe and other countries.
inrush current The initial surge of current into a motor. Inrush current can be up to ten times higher than the continuously needed current because of a lack of resistance.
insulation Material that is not conductive. Insulation prevents electrically charged components from contacting other components.
ionized A substance that exhibits a negative or positive charge. Ionization occurs after gaining or losing one or more electrons.
loads A circuit component that converts electricity into light, heat, or mechanical motion. Examples of loads include light bulbs, appliances, or other machines.
magnetic contactor A contactor that is operated remotely through solenoid action. Magnetic contactors offer convenience and safety to operators by allowing remote access to a circuit.
magnetic contactor A contactor that is operated remotely. Magnetic contactors offer convenience and safety to operators by allowing remote access to a circuit.
magnetic contactors A contactor that is operated remotely through solenoid action. Magnetic contactors offer convenience and safety to operators by allowing remote access to a circuit.
magnetic field A force of attraction that surrounds magnets or an electrical field. Magnetic fields are created by electricity.
magnetic overload relay A type of overload relay that senses the strength of a magnetic field produced by current flow. Magnetic overload relays shut motors down when the magnetic field is too strong.
manual contactor A type of contactor that requires a person to physically operate it. Manual contactors include mechanical switches or buttons that close or open a circuit.
manual controller A mechanical button or switch that a person must engage physically to open and close a circuit in a manual contactor. Manual controllers are components of manual contactors.
manual starters A type of across-the-line AC motor starter that must be physically started or stopped using a switch directly on the starter. Manual starters provide both motor overload and electric shock protection.
melting alloy overload relay A type of thermal overload relay that uses a melting alloy to activate mechanical devices. A melting alloy overload relay, also known as a eutectic overload, opens a circuit in the case of overload.
motor control components A device that performs specific functions within electric motors. Motor control components include relays, contactors, and motor starters.
motor overload protection The use of devices to open a circuit in the event of overload. Motor overload protection prevents motor windings from excess current over time while current is contained in the motor.
motor starter A device that starts a motor when triggered. Motor starters are rated by current or horsepower and serve as a form of motor protection.
motor starters An electrically operated switch that uses magnetic induction to provide startup current to a motor. Motor starters do not have the current capacity to start motors on their own and require other control components to do so.
motors A machine that converts one form of energy, such as electricity, into mechanical energy or motion. Motors operate on the principle of magnetic induction.
moveable contacts A contact on an armature that is mobile. Moveable contacts connect to matching stationary contacts.
NEMA The National Electrical Manufacturers Association. An association that sets standards for electrical equipment used in the United States. NEMA devices are usually more rugged and expensive than those rated by the IEC.
normally closed contacts NC contact. A device that keeps a circuit connected during normal operation. Normally closed contacts are disconnected to open the circuit when a relay is activated.
normally open contacts NO contacts. A device that keeps a circuit disconnected by preventing current from flowing. Normally open contacts must be closed for a closed circuit to form.
open contacts Contacts separated by space from one another, barring current flow. Open contacts prevent circuits from forming.
output devices A device that performs a mechanical action. Output devices must receive an electrical signal to act.
overload The excess current in a closed circuit over time. Overload is caused by current building up in a motor.
overload relays A relay that attaches to a contactor in order to create a motor starter. Overload relays protect the motor from overload by disconnecting the power to the motor and stopping its operation.
part-winding starters A reduced voltage starter that applies power to one set of windings and then to the other as the motor comes up to speed. This process of part-winding starters allows the windings to produce reduced starting current and torque.
pawl A hinged or pivoted device that fits into the notch of a ratchet wheel, gear, or bar. A pawl and ratchet work together to produce forward motion or to prevent backward motion.
pitting The corrosion of a metal that takes place in specific locations on a workpiece or component. Pitting shows on surfaces as small cracks or indentations on a surface.
power circuit A type of circuit that carries power to electrical loads. Power circuits often carry high voltages and consist of incoming main power, a motor starter, and the motor.
power relays A relay with heavy-duty contacts that are rated 15 amperes or higher. Power relays are also known as contactors.
primary resistor reduced voltage starters A motor starter with resistors that fight inrush current. Primary resistor reduced voltage starters offer motors smooth acceleration at startup with torque and voltage that increase gradually.
pushbutton A power control that activates or deactivates a component or system. Push buttons are operated by hand and usually have two positions.
ratchet wheel A toothed wheel that uses a pawl to keep from turning in one direction. The ratchet wheel is often used when dealing with systems that lift heavy weights.
reduced voltage starters A motor starter that reduces the power coming into the motor when it is initially started. Reduced voltage starters assist in motor protection.
reduced voltage starters A motor starter that reduces the power coming into the motor when it is initially started. Reduced voltage starters protect against shocks to large horsepower motors.
relays An electric switch that opens and closes a circuit using an electromagnetic coil. Relays can open or close one or many sets of contacts.
resistors An electronic component that regulates, limits, and opposes the flow of electrical current. Resistors tend to convert electrical energy into heat.
selector switch A switch that can be turned to different positions. In each position, a selector switch makes a connection with a particular set of contacts.
solenoid principle The use of a coil that enables voltage to change electrical energy. The solenoid principle uses magnetic fields to convert electrical energy to mechanical energy.
stationary contacts A contact that remains in a fixed position during an operation. Stationary conects are often connected to matching moveable contacts.
supply voltage Current that powers a motor. Supply voltage is often disconnected by protective devices to stop damage to a motor and its components.
switch A control device that closes or opens a circuit to energize or de-energize a circuit. A switch can be manual, mechanical, or automatic.
thermal overload relay A device that disconnects a motor from its power circuit when the relay senses excess current in the form of heat. Thermal overload relays contain heaters.
transformers A device that transfers electrical energy from one circuit to another, without changing the frequency, using electromagnetic induction. Transformers are most often used to change the line voltage.
trip In fuses, to interrupt an electrical circuit due to a melted component. A fuse trips, or blows, when current exceeds a set limit.
trip bar A mechanical part in a magnetic overload relay that tilts during overload and releases a set of NC contacts. Trip bars break circuits.
trip time The time it takes for a device to open a circuit in the event of an overload. Trip time varies from device to device.
tripping The process by which a device opens a circuit. Tripping occurs in response to an overload.
vaporizes The process by which a liquid becomes a gas. Vaporization of metal can form an arc.
voltage A measure of electrical pressure or potential known as electromotive force. Voltage is measured in volts.
volts A unit of measurement for electromagnetic force or pressure. Volts indicate voltage.
warp To bend something that was formerly straight. Warpage is often caused by a physical twist or turn within a part due to internal stress.
windings A conducting coil in a motor that is wound around the armature. Windings may be used to transfer voltage in transformers.
windings The conducting coils in a motor that are wound around the armature. Windings may be used to transfer voltage in transformers.
wye delta starters A type of reduced voltage starter in which the windings form the shape of the letter Y and then a delta. Wye delta starters reduce inrush current and are best for applications with slow and frequent starts.