Industrial Motor Control: Glossary

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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 that connects the motor directly to the supply line on starting or running. (Also called Full Voltage Control.)

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.

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.

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

Automatic Starter--A self-acting starter that 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 the movement of the main contacts.

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 defect an arc when a circuit is interrupted.

Bounceless Switch--A circuit used to eliminate con tact 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.

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.

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.

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.

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.

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 that 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 current flow.

Controller A device or group of devices that governs, in a pre-determined 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.

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 relay may be either an overcurrent relay or an undercurrent relay.

Dashpot Consists of a piston moving 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 circuit.

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.

(NEMA) 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 direct reading, 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 cur rent 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 mo tor 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.

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.

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.

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) 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 Control 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.

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

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

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 (Gate) A digital logic gate that has an out put 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.

Kickback Diode A diode used to eliminate the volt age spike induced in a coil by the collapse of a magnetic field.

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

LED (Light-Emitting Diode) A diode that will pro duce light when current flows through it.

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

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 pre vent the operation of a push button.

Logic A means of solving complex problems through the repeated use of simple functions that 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 motor.

Magnet Brake Friction brake controlled by electro magnetic means.

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; de vice 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 kilovolt-amperes at 2,300 volts; 250,000 kilovolt-amperes 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 over load motor current.

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

Motor-Driven Timer A device in which a small pi lot 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 in crease of conductance. The increase of conductance causes a decrease in the voltage drop across the device.

NEMA National Electrical Manufacturers Association.

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

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 non-latching types of devices.

Off-Delay Timer 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.

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 ex cess 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 current flow.

Peak Inverse/Peak Reverse Voltage--The rating of a semiconductor device, which indicates the maxi mum 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 wave form 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 be tween 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 con tact, 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-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 flow in an AC circuit offered by pure inductance or pure capacitance.

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.

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 device.

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 cur rent 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 in-rush 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 pro duce 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 that 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.

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 electro magnets.

Sensing Device A pilot device that measures, com pares, or recognizes a change or variation in the system that 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 current 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 over load 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 A large 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.

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, current carrying coil that provides magnetic action to per form various work functions.

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

Solenoid Valve A valve operated by an electric solenoid.

Solid-State Devices--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: A device 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.

Stealer Transistor A transistor used in such a manner 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 in stalled 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 pre determined 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 a motor, 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.

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 per form function; motor driven; solenoid-actuated; electronic.

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

Transducer A device that transforms power from one system to power of a second system: for ex ample, 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 collector-emitter.

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 device.

Trip Free Refers to a circuit breaker that cannot be held in the on position by the handle on a sustained overload.

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.

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 mini mum 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--A series connection of resistors used to produce different values of voltage drop across them.

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. A voltage relay may be either an overvoltage or an undervoltage relay.

Voltmeter An instrument used to measure a level of voltage.

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

Watt--A measure of true power.

Waveform--The shape of a wave as obtained by plot ting 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 device until the AC waveform returns to zero.

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