Electrical Glossary


Accelerating Relay Any type of relay used to aid in starting a motor or to accelerate a motor from one speed to another. Accelerating relays may function by: motor armature current (current limit acceleration); armature voltage (counter emf acceleration); or definite time (definite time acceleration).

Accessory (control use) A device that controls the operation of magnetic motor control. (Also see Master Switch, Pilot Device, and Push Button.)

Across-the-line Method of motor starting which connects
the motor directly to the supply line on starting or running. (Also called Full Voltage Control.)

Actuator A device that creates mechanical motion by converting various forms of energy to rotating or linear mechanical energy.

Adjustable Speed Drive A mechanical, fluid or electrical device that variably changes an input speed to an output speed matching operating requirements.

AGMA (American Gear Manufacturers  Association) Standards setting organization composed of gear products manufacturers and users. AGMA standards help bring uniformity to the design and application of gear products.
Air-Over (AO) Motors for fan or blower service that are cooled by the air stream from the fan or blower.
Alternating Current (Ac) Current changing both in magnitude and direction; most commonly used current.

Alternator A machine used to generate alternating current by rotating conductors through a magnetic field.

Ambient Temperature The temperature surrounding a device. The temperature of the air which, when coming
into contact with the heated parts of a device, carries off its heat. Ambient temperature is commonly known as room temperature.

Ampacity The maximum current rating of a wire or cable..

Ampere Unit of electrical current.

Amplifier A device used to increase a signal.

Amplitude The highest value reached by a signal, voltage, or current.

AND Gate A digital logic gate that must have all of its inputs high to produce an output.

Anode The positive terminal of an electronic device.

Applied Voltage The amount of voltage connected to a circuit or device.

ASA American Standards Association.

Astable Mode The state in which an oscillator can continually turn itself on and off, or continually change from positive to negative output.

Atom The smallest part of an element that contains all the properties of that element.

Attenuator A device that decreases the amount of
signal, voltage, or current.

Armature The rotating part of a brush-type direct current motor.In an induction motor, the squirrel cage rotor.

Automatic Self-acting, operating by its own mechanism
when actuated by some triggering signal such as a change in current strength, pressure, temperature, or mechanical configuration.

Automatic Starter A self-acting starter which is completely controlled by master or pilot switches or other sensing devices; designed to control automatically the acceleration of a motor during the acceleration period.

Auxiliary Contacts Contacts of a switching device in addition to the main circuit contacts; auxiliary contacts operate with themovement of the main contacts.

Backlash Rotational movement of a gear reducers output shaft clockwise and counter clockwise, while holding the input shaft stationary. Usually expressed in thousandths of an inch and measure at a specific radius at the output shaft.

Barrier Charge The potential developed across a semiconductor junction.

Base The semiconductor region between the collector and emitter of a transistor. The base controls the current flow through the collector-emitter circuit.

Base Current The amount of current that flows through the base-emitter section of a transistor.

Bias A dc voltage applied to the base of a transistor to preset its operating point.

Bimetal Strip A strip made by bonding two unlike metals together that, when heated, expand at different rates. This causes a bending or warping action.

Blowout Coil Electromagnetic coil used in contactors and starters to deflect an arc when a circuit is interrupted.

Bounceless Switch A circuit used to eliminate contact bounce in mechanical contacts.

Branch Circuit That portion of a wiring system that extends beyond the final overcurrent device protecting the circuit.

Brake An electromechanical friction device to stop and hold a load. Generally electric release spring applied coupled to motor shaft.
 Breakdown Torque (of a motor) The maximum torque that will develop with the rated voltage applied at the rated frequency, without an abrupt drop in speed. (ASA)

Bridge Circuit A circuit that consists of four sections connected in series to form a closed loop.

Bridge Rectifier A device constructed with four diodes, which converts both positive and negative cycles of ac voltage into dc voltage.

Brush A Current-conducting material in a DC motor, usually graphite, or a combination of graphite and other materials. The brush rides on the commutator of a motor and forms an electrical connection between the armature and the power source.

Buchholz relay Gas-detector relay (known as a Buchholz relay) for conservator- type transformers, generally when rated 7.5 MVA and larger. The device is mounted in the pipe between the highest part of the transformer tank and the conservator. The relay is equipped with two sets of contacts, one for alarm upon gas accumulation and one for trip upon oil surge.

Busway A system of enclosed power transmission that is current and voltage rated.

Cad Cell A device that changes its resistance with a change of light intensity.

Canadian Standards Association (CSA) The agency that sets safety standards for motors and other electrical equipment used in Canada.

Capacitance The electrical size of a capacitor.

Capacitive Any circuit or device having characteristics similar to those of a capacitor.

Capacitor A device made with two conductive plates separated by an insulator or dielectric.

Capacitor Start Motor A single-phase induction
motor with a main winding arranged for direct connection
to the power source and an auxiliary winding connected in series with a capacitor. The auxiliary winding is in the circuit only during starting. (NEMA).

Cathode The negative terminal of a device.

Cathode-Ray Tube (CRT) An electron beam tube in which the beam of electrons can be focused to any point on the face of the tube. The electron beam causes the face of the tube to produce light when it is struck by the beam.

Center-Tapped A transformer that has a wire connected
to the electrical midpoint of its winding. Generally the secondary is tapped.

Centrifugal Starting Switch A mechanism that disconnects the starting circuit of a motor when the rotor reaches approximately 75% of operating speed.

Charge Time The amount of time necessary to charge a capacitor.

Choke An inductor designed to present an impedance
to ac current, or to be used as the current filter of a dc power supply.

Circuit Breaker Automatic device that opens under abnormal current in carrying circuit; circuit breaker is not damaged on current interruption; device is ampere, volt, and horsepower rated.

Clock Timer A time-delay device that uses an electric clock to measure the delay period.

Cogging Non-uniform or erratic rotation of a direct current motor. It usually occurs at low speeds and may be a function of the adjustable speed control or of the motor design.

Collapse (of a magnetic field) When a magnetic field suddenly changes from its maximum value to a zero value.

Collector The semiconductor region of a transistor which must be connected to the same polarity as the base.

Commutator The part of a DC motor armature that causes the electrical current to be switched to various armature windings. Properly sequenced switching creates the motor torque. The commutator also provides the means to transmit electrical current to the moving armature through brushes that ride on the commutator.

Comparator A device or circuit that compares two like quantities such as voltage levels.
Conduction Level The point at which an amount of voltage or current will cause a device to conduct.

Conductor A device or material that permits current to flow through it easily.

Contact A conducting part of a relay which acts with another conducting part to complete or to interrupt a circuit.

Contactor A device that repeatedly establishes or interrupts
an electric power circuit.

Continuity A complete path for cur rent flow.

Controller A device or group of devices that governs, in a predetermined manner, the delivery of electric power to apparatus connected to it.

Controller Function Regulate, accelerate, decelerate, start, stop, reverse, or protect devices connected to an electric controller.

Controller Service Specific application of controller.
General Purpose: standard or usual service. Definite
Purpose: service condition for specific application
other than usual.

Counter Electromotive Force A Voltage that opposes line voltage caused by induced magnetic field in a motor armature or rotor.

Current The rate of flow of electrons. Measured in amperes.

Current Flow The flow of electrons.

Current Rating The amount of current flow a device
is designed to withstand.

Current Relay A relay that functions at a predetermined
value of current. A current relaymay be either an overcurrent relay or an undercurrent relay.

Dashpot Consists of a pistonmoving inside a cylinder
filled with air, oil, mercury, silicon, or other fluid.
Time delay is caused by allowing the air or fluid to
escape through a small orifice in the piston. Moving
contacts actuated by the piston close the electrical

Definite Time (or Time Limit) Definite time is a qualifying term indicating that a delay in action is purposely introduced. This delay remains substantially constant regardless of the magnitude of the quantity that causes the action.

Definite-Purpose Motor Any motor designed, listed, and offered in standard ratings with standard operating characteristics or mechanical construction for use under service conditions other than usual or for use on a particular type of application.

Delta Connection A circuit formed by connecting three electrical devices in series to form a closed loop. Most often used in three-phase connections.
Device A unit of an electrical system that is intended
to carry but not utilize electrical energy.

Diac A bidirectional diode.

Dielectric An electrical insulator.

Digital Device A device that has only two states of operation.

Digital Logic Circuit elements connected in such a manner as to solve problems using components that have only two states of operation.

Digital Voltmeter A voltmeter that uses a directreading,
numerical display as opposed to a meter movement.

Diode A two-element device that permits current to flow through it in only one direction.

Direct Current (Dc) Current that does not reverse its direction of flow. A continuous nonvarying current in one direction.

Disconnecting Means (Disconnect) A device, or
group of devices, or other means whereby the conductors
of a circuit can be disconnected from their
source of supply.

Drum Controller Electrical contacts made on the surface of a rotating cylinder or section; contacts made also by operation of a rotating cam.

Drum Switch A switch having electrical connecting parts in the form of fingers held by spring pressure against contact segments or surfaces on the periphery of a rotating cylinder or sector.

Duty Specific controller functions. Continuous (time) Duty: constant load, indefinite long time period. Short Time Duty: constant load, short or specified time period. Intermittent Duty: varying load, alternate intervals, specified time periods. Periodic Duty: intermittent duty with recurring load conditions. Varying duty: varying loads, varying time intervals, wide variations.

Dynamic Braking Using a dc motor as a generator, taking it off the line and applying an energy dissipating resistor to the armature. Dynamic braking for an ac motor is accomplished by disconnecting the motor from the line and connecting dc power to the stator windings.

Eddy Currents Circular induced currents contrary to the main currents; a loss of energy that shows up in the form of heat.

Efficiency  A ratio of the input power compared to the output, usually expressed as a percentage.

Electrical Interlocking Accomplished by control circuits in which the contacts in one circuit control another circuit.

Electric Controller A device, or group of devices, which governs, in some predetermined manner, the electric power delivered to the apparatus to which it is connected.

Electron One of the three major subatomic parts of an atom. The electron carries a negative charge.

Electronic Control Control system using gas and/or vacuum tubes, or solid-state devices.

Emitter The semiconductor region of a transistor which must be connected to a polarity different than the base.

Enclosure Mechanical, electrical, and environmental protection for control devices.

Endshield The part of a motor that houses the bearing supporting the rotor and acts as a protective guard to the internal parts of the motor; sometimes called endbell, endplate or end bracket.

Eutectic Alloy Metal with low and sharp melting point; used in thermal overload relays; converts from a solid to a liquid state at a specific temperature; commonly called solder pot.

Exclusive OR Gate A digital logic gate that will produce
an output when its inputs have opposite states of logic level.

Explosion-Proof Motors These motors meet Underwriters Laboratories and Canadian Standards Association standards for use in hazardous (explosive) locations, as indicated by the UL label affixed to the motor. Locations are considered hazardous because the atmosphere does or may contain
gas, vapor, or dust in explosive quantities.

Feeder The circuit conductor between the service equipment, or the generator switchboard of an isolated plant and the branch circuit overcurrent device.

Feeler Gauge A precision instrument with blades in thicknesses of thousandths of an inch for measuring clearances.

Filter A device used to remove the ripple produced by a rectifier.

Frequency Number of complete variations made by an alternating current per second; expressed in Hertz.  

(See Hertz)

Form Factor Indicates how much AC component is present in the DC output from a rectified AC supply. Unfiltered SCR (thyristor) drives have a form factor (FF) of 1.40. Pure DC, as from a battery, has a form factor of 1.0. Filtered thyristor and pulse width modulated drives often have a form factor of 1.05.

Full Load Amperes (FLA) Line current (amperage) drawn by a motor when operating at rated load and voltage on motor nameplate. Important for proper wire size selection, and motor starter or drive selection. Also called full load current.

Full Load Torque (of a motor) The torque necessary to produce the rated horsepower of a motor at full load speed.

Full Voltage Control (Across-the-line) Connects equipment directly to the line supply on starting.

Fuse An overcurrent protective device with a fusible member, which is heated directly and destroyed by the current passing through it to open a circuit.

Gain The increase in signal power produced by an amplifier.

Gate A device that has multiple inputs and a single output; or one terminal of some solid-state devices such as SCRs or triacs.

General-Purpose Motor Any open motor that has a continuous 40C rating and is designed, listed, and offered in standard ratings with standard operating characteristics and mechanical construction for use under usual service conditions without restrictions to a particular application or type of application. (NEMA)

Heat Sink A metallic device designed to increase the surface area of an electronic component to remove heat at a faster rate.

Hertz International unit of frequency, equal to one cycle per second of alternating current.

High Voltage Control Formerly, all control above 600 volts. Now, all control above 5,000 volts. See Medium Voltage for 600- to 5,000-volt equipment.

Holding Contacts Contacts used for the purpose of maintaining current flow to the coil of a relay.

Holding Current The amount of current needed to keep an SCR or a triac turned on.

Horsepower Measure of the time rate of doing work
(working rate).

Hysteresis Loop A graphic curve that shows the value of magnetizing force for a particular type of material.

IEC (International Electrotechnical Commission)  
The worldwide organization that promotes international unification of standards or norms. Its formal decisions on technical matters express, as nearly as possible, an international consensus.

IGBT Stands for isolated gate bipolar transistor. The most common and fastest-acting semiconductor switch used in pulse width modulated (PWM) AC drives.

Impedance The total opposition to current flow in an
electrical circuit.

Induced Current produced in a conductor by the cutting action of a magnetic field.

Induction Motor The simplest and most rugged electric motor, it consists of a wound stator and a rotor assembly. The AC induction motor is named because the electric current flowing in its secondary member (the rotor) is induced by the alternating current flowing in its primary member (the stator). The power supply is connected only to the stator. The combined electromagnetic effects of the two currents produce the force to
create rotation.
Inductor A coil used to introduce inductance into an electrical circuit.

Input Power delivered to an electrical device.

Input Voltage The amount of voltage connected to a device or circuit.

Instantaneous A qualifying term indicating that no delay is purposely introduced in the action of a device.

Insulator A material used to electrically isolate two conductive surfaces.

Integral Whole or complete; not fractional.

Interlock To interrelate with other controllers; an auxiliary contact. A device is connected in such a way that the motion of one part is held back by another part.

Internal Relay Digital logic circuits in a programmable
controller that can be programmed to operate in the same manner as control relays.

Inverse Time A qualifying term indicating that a delayed
action is introduced purposely. This delay decreases as the operating force increases.

Inverter An electronic device that changes direct current to alternating current; in common usage, an AC drive.

Inverter (Gate) A digital logic gate that has an output opposite its input.

Isolation Transformer A transformer whose secondary winding is electrically isolated from its primary winding.

Jogging (Inching) Momentary operations; the quickly repeated closure of the circuit to start a motor from rest for the purpose of accomplishing small movements of the driven machine.

Jumper A short length of conductor used to make a connection between terminals or around a break in a circuit.

Junction Diode A diode that is made by joining two pieces of semiconductor material.

Kick-Back Diode A diode used to eliminate the voltage spike induced in a coil by the collapse of a magnetic field.

Kilowatt A unit of power equal to 1000 watts and approximately equal to 1.34 horsepower.

Lattice Structure An orderly arrangement of atoms in a crystalline material.

Led (Light-Emitting Diode) A diode that will produce light when current flows through it.

Limit Switch A mechanically operated device which stops a motor from revolving or reverses it when certain limits have been reached.

Load The work required of a motor to drive attached equipment. Expressed in horsepower or torque at a certain motor speed.

Load Center Service entrance; controls distribution; provides protection of power; generally of the circuit breaker type.

Local Control Control function, initiation, or change
accomplished at the same location as the electric controller.

Locked Rotor Current (of a motor) The steady-state current taken from the line with the rotor locked (stopped) and with the rated voltage and frequency applied to the motor.

Locked Rotor Torque (of a motor) The minimum torque that a motor will develop at rest for all angular positions of the rotor with the rated voltage applied at a rated frequency. (ASA)

Lockout A mechanical device that may be set to prevent
the operation of a push button.

Logic Ameans of solving complex problems through
the repeated use of simple functions which define
basic concepts. Three basic logic functions are: and,
or, and not.

Low Voltage Protection (LVP) Magnetic control
only; nonautomatic restarting; three-wire control;
power failure disconnects service; power restored by
manual restart.

Low Voltage Release (LVR) Manual and magnetic
control; automatic restarting; two-wire control;
power failure disconnects service; when power is
restored, the controller automatically restarts the

Magnet Brake Friction brake controlled by electromagnetic

Magnetic Contactor A contactor that is operated electromechanically.

Magnetic Controller An electric controller; device functions operated by electromagnets.

Magnetic Field The space in which a magnetic force exists.

Maintaining Contact A small control contact used to keep a coil energized; usually actuated by the same coil. Holding contact; Pallet switch.

Manual Controller An electric controller; device functions operated by mechanical means or manually.

Master Switch A main switch to operate contactors, relays, or other remotely-controlled electrical devices.

Medium Voltage Control Formerly known as High Voltage; includes 600- to 5000-volt apparatus; air break or oil-immersed main contactors; high interrupting capacity fuses; 150,000 kVa at 2,300 volts; 250,000 kVA at 4,000–5,000 volts.

Microprocessor A small computer. The central processing
unit is generally made from a single integrated circuit.

Mode A state or condition.

Monostable (Mode) The state in which an oscillator or timer will operate through only one sequence of events.

Motor Device for converting electrical energy to mechanical
work through rotary motion; rated in horsepower.

Motor Circuit Switch Motor branch circuit switch rated in horsepower; capable of interrupting overload motor current.

Motor Controller A device used to control the operation of a motor.

Motor-Driven Timer A device in which a small pilot motor causes contacts to close after a predetermined time.

Multispeed Motor A motor that can be operated at more than one speed.

Multispeed Starter An electric controller with two or more speeds; reversing or nonreversing; full or reduced voltage starting.

NAND Gate A digital logic gate that will produce a high output only when all of its inputs are in a low state.

Negative One polarity of voltage, current, or a charge.

Negative Resistance The property of a device in which an increase of current flow causes an increase of conductance. The increase of conductance causes a decrease in the voltage drop across the device.

National Electric Code (NEC) A safety code regarding the use of electricity. The NEC is sponsored by the National Fire Protection Institute. It is also used by insurance inspectors and by many government bodies regulating building codes.

NEMA (National Electrical Manufacturers Association)
 A nonprofit trade organization, supported by manufacturers of electrical apparatus and supplies in the United States. Its standards alleviate misunderstanding and help buyers select the proper products. NEMA standards for motors cover frame sizes and dimensions, horsepower ratings, service factors, temperature rises and various performance characteristics.

NEMA Size Electric controller device rating; specific
standards for horsepower, voltage, current, and interrupting

Neutron One of the principal parts of an atom. The neutron has no charge and is part of the nucleus.

Nonautomatic Controller Requires direct operation to perform function; not necessarily a manual controller.

Noninductive Load An electrical load that does not have induced voltages caused by a coil. Noninductive loads are generally resistive, but can be capacitive.

Nonreversing Operation in one direction only.

NOR Gate A digital logic gate that will produce a high output when any of its inputs are low.

Normally Open and Normally Closed When applied to a magnetically-operated switching device, such as a contactor or relay, or to the contacts of these devices, these terms signify the position taken when the operating magnet is de-energized. The terms apply only to nonlatching types of devices.

Off-Delay Timers A timer in which the contacts
change position immediately when the coil or circuit is energized, but delay returning to their normal positions when the coil or circuit is de-energized.

Ohmmeter A meter used to measure resistance.

On-Delay Timer A timer in which the contacts delay
changing position when the coil or circuit is energized, but change back immediately to their normal positions when the coil or circuit is de-energized.

Open Circuit A break in an electrical circuit that prevents normal current flow.

Operational Amplifier (OP-AMP) An integrated circuit used as an amplifier.

Optoisolator A device used to connect sections of a circuit by means of a light beam.

Oscillator A device or circuit used to change dc voltage into ac voltage.

Oscilloscope An instrument that measures the amplitude
of voltage with respect to time.

Out-of-phase Voltage A voltage that is not in phase
when compared to some other voltage or current.

Output Devices Elements such as solenoids, motor
starters, and contactors that receive input.

Output Pulse A short duration voltage or current
which can be negative or positive, produced at the
output of a device or circuit.

Overload Protection Overload protection is the result
of a device that operates on excessive current, but not necessarily on short circuit, to cause and maintain the interruption of current flow to the device governed.
 NOTE: Operating overload means a current that is not in excess of six times the rated current for alternating-current motors, and not in excess of four times the rated current for direct-current motors.

Overload Relay Running overcurrent protection; operates on excessive current; not necessarily protection for short circuit; causes and maintains interruption of device from power supply. Overload Relay Heater Coil: Coil used in thermal overload relays; provides heat to melt eutectic alloy.

Overload Relay Reset Push button used to reset
thermal overload relay after relay has operated.

Panelboard Panel, group of panels, or units; an assembly
that mounts in a single panel; includes buses, with or without switches and/or automatic overcurrent protective devices; provides control of light, heat, power circuits; placed in or against wall or partition; accessible from front only.

Parallel Circuit A circuit that has more than one path for cur rent flow.

Peak Inverse/Peak Reverse Voltage The rating of a semiconductor device which indicates the maximum amount of voltage that can be applied to the device in the reverse direction.

Peak-To-Peak Voltage The amplitude of voltage measured from the negative peak of an ac waveform to the positive peak.

Peak Voltage The amount of voltage of a waveform measured from the zero voltage point to the positive or negative peak.

Permanent-split Capacitor Motor A single-phase induction
motor similar to the capacitor start motor except that it uses the same capacitance which remains in the circuit for both starting and running. (NEMA)

Permeability The ease with which a material will conduct magnetic lines of force.

Phase Relation of current to voltage at a particular time in an ac circuit. Single Phase: A single voltage and current in the supply. Three Phase: Three electrically- related (120-degree electrical separation)
single-phase supplies.

Phase-Failure Protection Phase-failure protection is provided by a device that operates when the power fails in one wire of a polyphase circuit to cause and maintain the interruption of power in all the wires of the circuit.

Phase-Reversal Protection Phase-reversal protection is provided by a device that operates when the phase rotation in a polyphase circuit reverses to cause and maintain the interruption of power in all the wires of the circuit.

Phase Rotation Relay A relay that functions in accordance
with the direction of phase rotation.

Phase Shift A change in the phase relationship between two quantities of voltage or current.

Photodetector A device that responds to change in light intensity.

Photodiode A diode that conducts in the presence of light, but not in darkness.

Pilot Device Directs operation of another device.
Float Switch: A pilot device that responds to liquid levels. Foot Switch: A pilot device operated by the foot of an operator. Limit Switch: A pilot device operated by the motion of a power-driven machine; alters the electrical circuit with the machine or equipment.

Plugging Braking by reversing the line voltage or phase sequence; motor develops retarding force.

Pneumatic Timer A device that uses the displacement of air in a bellows or diaphragm to produce a
time delay.

Polarity The characteristic of a device that exhibits opposite quantities, such as positive and negative, within itself.

Pole The north or south magnetic end of a magnet; a terminal of a switch; one set of contacts for one circuit of main power.

Potentiometer A variable resistor with a sliding contact, which is used as a voltage divider.

Power Factor A comparison of the true power (WATTS) to the apparent power (VOLT AMPS) in an ac circuit.

Power Rating The rating of a device that indicates the amount of current flow and voltage drop that can be permitted.

Pressure Switch A device that senses the presence
or absence of pressure and causes a set of contacts to
open or close.

Printed Circuit A board on which a predetermined
pattern of printed connections has been formed.

Proton One of the three major parts of an atom. The
proton carries a positive charge.

Pull Out Torque Also called breakdown torque or maximum torque, this is the maximum torque a motor can deliver without stalling.
Pull-up Torque (of alternating-current motor) The
minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown occurs. (ASA)

Push Button A master switch; manually-operable plunger or button for an actuating device; assembled into push-button stations.

RC Time Constant The time constant of a resistor and capacitor connected in series. The time in seconds is equal to the resistance in ohms multiplied by the capacitance in farads.

Reactance The opposition to current flowin an ac circuit
offered by pure inductance or pure capacitance. The opposition to a flow of current other than pure resistance. Inductive reactance is the opposition to change of current in an inductance (coil of wire). Capacitive reactance is the opposition to change of voltage in a capacitor.

Reactors are utilized to provide inductive reactance in power circuits for a wide variety of purposes, including fault-current limiting, inrush-current limiting (for capacitors and motors), harmonic filtering, VAR compensation, reduction of ripple currents, blocking of power-line carrier signals, neutral grounding, damping of switching transients, flicker reduction for arc-furnace applications, circuit detuning, load balancing, and power conditioning.Reactors can be installed at any industrial, distribution, or transmission voltage level and can be rated for any current duty from a few amperes to tens of thousands of amperes and fault-current levels of up to hundreds of thousands of amperes.

Rectifier A device that converts alternating current into direct current.

Regulator A device that maintains a quantity at a predetermined level.

Relay Operated by a change in one electrical circuit to control a device in the same circuit or another circuit; rated in amperes; used in control circuits.

Reluctance The characteristics of a magnetic field which resist the flow of magnetic lines of force through it.

Remote Control Controls the function initiation or change of an electrical device from some remote point, or location.

Remote Control Circuit Any electrical circuit that
controls any other circuit through a relay or an equivalent

Residual Magnetism The retained or small amount of remaining magnetism in the magnetic material of an electromagnet after the current flow has stopped.

Resistance The opposition offered by a substance or body to the passage through it of an electric current; resistance converts electrical energy into heat; resistance is the reciprocal of conductance.

Resistance Start Induction Run Motor One type of split-phase motor that uses the resistance of the start winding to produce a phase shift between the current in the start winding and the current in the run winding.

Resistor A device used primarily because it possesses the property of electrical resistance. A resistor is used in electrical circuits for purposes of operation, protection, or control; commonly consists of an aggregation of units.
Starting Resistors Used to accelerate a motor from rest to its normal running speed without damage to the motor and connected load from excessive currents and torques, or without drawing undesirable inrush current from the power system.
Armature Regulating Resistors Used to regulate the speed of torque of a loaded motor by resistance in the armature or power circuit.
Dynamic Braking Resistors Used to control the current and dissipate the energy when a motor is decelerated by making it act as a generator to convert its mechanical energy to electrical energy and then to heat in the resistor.
Field Discharge Resistors Used to limit the value of voltage that appears at the terminals of a motor field (or any highly inductive circuit) when the circuit is opened.
Plugging Resistors Used to control the current and torque of a motor when deceleration is forced by electrically reversing the motor while it is still running in the forward direction.

Rheostat A resistor that can be adjusted to vary its
resistance without opening the circuit in which it
may be connected.

Ripple An ac component in the output of a dc power
supply caused by improper filtering.

RMS Value The value of ac voltage that will produce
as much power when connected across a resistor as
a like amount of dc voltage.

Safety Switch Enclosed manually-operated disconnecting
switch; horsepower and current rated; disconnects
all power lines.

Saturation The maximum amount of magnetic flux a
material can hold.

Schematic An electrical diagram which shows components
in their electrical sequence without regard
for physical location.

Selector Switch A master switch that is manually operated; rotating motion for actuating device; assembled into push-button master stations.

Service Factor for Motors A measure of the overload capacity built into a motor. A 1.15 SF means the motor can deliver 15% more than the rated horsepower without injurious overheating. A 1.0 SF motor should not be loaded beyond its rated horsepower. Service factors will vary for different horsepower motors and for different speeds.

Semiautomatic Starter Part of the operation of this type of starter is nonautomatic while selected portions are automatically controlled.

Semiconductor A material that contains four valence electrons and is used in the production of solid-state devices. The most common types are silicon and germanium.

Semi-magnetic Control An electric controller in which functions are partly controlled by electromagnets.

Sensing Device A pilot device that measures, compares,
or recognizes a change or variation in the system
which it is monitoring; provides a controlled
signal to operate or control other devices.

Series-Aiding Two or more voltage producing devices
connected in series in such a manner that their
voltages add to produce a higher total voltage.

Series Circuit An electric circuit formed by the connection
of one or more components in such a manner
that there is only one path for cur rent flow.

Service The conductors and equipment necessary to
deliver energy from the electrical supply system to
the premises served.

Service Equipment Necessary equipment, circuit
breakers, or switches and fuses with accessories
mounted near the entry of the electrical supply; constitutes
the main control or cutoff for supply.

Service Factor (of a general-purpose motor) An
allowable overload; the amount of allowable overload
is indicated by a multiplier which, when applied
to a normal horsepower rating, indicates the
permissible loading.

Shaded-Pole Motor A single-phase induction motor
provided with an auxiliary short-circuited winding
or windings displaced in magnetic position from the
main winding. (NEMA)

Shading Loop Alarge copper wire or band connected
around part of a magnetic pole piece to oppose a
change of magnetic flux.

Short Circuit An electrical circuit that contains no resistance to limit the flow of current.A fault or defect in a winding causing part of the normal electrical circuit to be bypassed, frequently resulting in overheating of the
winding and burnout.

Signal The event, phenomenon, or electrical quantity
that conveys information from one point to another.

Signal Generator A text instrument used to produce a low-value, ac voltage for the purpose of testing or calibrating electronic equipment.

Silicon-Controlled Rectifier (SCR) A four-layer semiconductor device that is a rectifier and must be triggered by a pulse applied to the gate before it will conduct.

Sine-Wave Voltage A voltage waveform; its value at any point is proportional to the trigonometric sine of the angle of the generator producing it.

Slip Difference between the rotor rpm and the rotating
magnetic field of an ac motor.

Snap Action The quick opening and closing action of a spring-loaded contact.

Solder Pot See Eutectic Alloy.

Solenoid A magnetic device used to convert electrical energy into linear motion. A tubular, currentcarrying coil that provides magnetic action to perform various work functions.

Solenoid-and-Plunger A solenoid-and-plunger is a olenoid provided with a bar of soft iron or steel called a plunger.

Solenoid Valve A valve operated by an electric solenoid.

Solid-StateDevices Electronic components that control
electron flow through solid materials such as crystals; e.g., transistors, diodes, integrated circuits.

Special-Purpose Motor A motor with special operating
characteristics or special mechanical construction, or both, designed for a particular application and not falling within the definition of a general-purpose or definite-purpose motor. (NEMA)

Split-Phase A single-phase induction motor with auxiliary
winding, displaced in magnetic position from, and connected parallel to, the main winding. (NEMA)

Starter A starter is a controller designed for accelerating a motor to normal speed in one direction of rotation. NOTE:Adevice designed for starting a motor in either direction of rotation includes the additional function of reversing and should be designated as a controller.

Startup The time between equipment installation and the full operation of the system.

Static Control Control system in which solid-state devices perform the functions. Refers to no moving parts or without motion.

StealerTransistor Atransistor used in such amanner as to force some other component to remain in the off state by shunting its current to electrical ground.

Step-Down Transformer A transformer that produces a lower voltage at its secondary winding than is applied to its primary winding.

Step-Up Transformer A transformer that produces a higher voltage at its secondary winding than is applied to its primary winding.

Surge A transient variation in the current and/or potential
at a point in the circuit; unwanted, temporary.

Switch A device for making, breaking, or changing the connections in an electric circuit.

Switchboard A large, single panel with a frame or assembly of panels; devices may be mounted on the face of the panels, on the back, or both; contains switches, overcurrent, or protective devices; instruments accessible from the rear and front; not installed in wall-type cabinets. (See Panelboard)

Synchronous Speed The speed of the rotating magnetic
field of an ac induction motor.

Tachometer Generator Used for counting revolutions per minute. Electrical magnitude or impulses are calibrated with a dial-gauge reading in rpm.

Temperature Relay A relay that functions at a predetermined
temperature in the apparatus protected. This relay is intended to protect some other apparatus such as a motor or controller and does not necessarily protect itself.

Terminal A fitting attached to a circuit or device for
convenience in making electrical connections.

Thermal Compound A grease-like substance used
to thermally bond two surfaces together for the purpose
of increasing the rate of heat transfer from one
object to another.

Thermal Protector (as applied to motors) An inherent
overheating protective device that is responsive to motor current and temperature. When properly applied to amotor, this device protects the motor against dangerous overheating due to overload or failure to start.

Thermistor A resistor that changes its resistance
with a change of temperature. Are conductive ceramic materials, whose resistance remains relatively constant over a broad temperature range, then changes abruptly
at a design threshold point, creating essentially a solid-state thermal switch. Attached control modules register this abrupt resistance change and produce an amplified output signal, usually a contact closure or fault trip annunciation. Thermistors are more accurate and faster responding than thermostats.

Thermostat A protector, which is temperature-sensing only, that is mounted on the stator winding. Two leads from the device must be connected to a control circuit, which initiates corrective action. The  customer must specify if the thermostats are to be normally closed or normally open.

Thyristor An electronic component that has only
two states of operation, on and off.

Time Limit See Definite Time.

Timer A pilot device that is also considered a timing relay; provides adjustable time period to perform function; motor driven; solenoid-actuated; electronic.

Torque The torque of a motor is the twisting or turning
force which tends to produce rotation.

Transducer A device that transforms power from
one system to power of a second system: for example,
heat to electrical.

Transformer An electromagnetic device that converts
voltages for use in power transmission and operation
of control devices.

Transient See Surge.

Transistor A solid-state device made by combining
three layers of semiconductor material. A small
amount of current flow through the base-emitter can
control a larger amount of current flow through the

Triac A bidirectional, thyristor device used to control
ac voltage.

Trigger Pulses A voltage or current of short duration
used to activate the gate, base, or input of some electronic
Trip Free Refers to a circuit breaker that cannot be
held in the on position by the handle on a sustained

Troubleshoot To locate and eliminate the source of
trouble in any flow of work.

Truth Table A chart used to show the output condition
of a logic gate or circuit as compared to different
conditions of input.

Underwriters Laboratories (UL) Independent United States testing organization that sets safety standards for motors and other electrical equipment.

Undervoltage Protection The result when a device
operates on the reduction or failure of voltage to
cause and maintain the interruption of power to the
main circuit.

Undervoltage Release Occurs when a device operates
on the reduction or failure of voltage to cause
the interruption of power to the main circuit, but
does not prevent the reestablishment of the main circuit
on the return of voltage.

Unijunction Transistor (UJT) A special transistor
that is a member of the thyristor family of devices
and operates like a voltage-controlled switch.

Valence Electron The electron in the outermost shell
or orbit of an atom.

Variable Resistor A resistor in which the resistance
value can be adjusted between the limits of its minimum
and maximum value.

Varistor A resistor that changes its resistance value
with a change of voltage.

Volt/Voltage An electrical measure of potential difference,
electromotive force, or electrical pressure.

Voltage Divider Aseries connection of resistors used
to produce different values of voltage drop across

Voltage Drop The amount of voltage required to
cause an amount of current to flow through a certain
value of resistance or reactance.

Voltage Rating A rating that indicates the amount of
voltage that can safely be connected to a device.

Voltage Regulator A device or circuit that maintains
a constant value of voltage.

Voltage Relay A relay that functions at a predetermined
value of voltage.Avoltage relaymay be either
an overvoltage or an undervoltage relay.

Voltmeter An instrument used to measure a level of

Volt-Ohm-Milliammeter (VOM) A test instrument
so designed that it can be used to measure voltage,
resistance, or milliamps.

Watt A measure of true power.

Waveform The shape of a wave as obtained by plotting
a graph with respect to voltage and time.

Wye Connection A connection of three components
made in such a manner that one end of each component
is connected. This connection generally connects
devices to a three-phase power system.

Zener Diode A special diode that exhibits a constant
voltage drop when connected in such a manner that
current flows through it in the reverse direction.

Zener Region The region current enters into when it
flows through a diode in the reverse direction.

Zero Switching A feature of some solid-state relays
that causes current to continue flowing through the
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