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across-the-line (ATL) 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) A current that alternates regularly in direction. Refers to a periodic current with successive half-waves of the same shape and area.
alternator A machine used to generate alternating current by rotating conductors through a magnetic field; an AC generator.
alternator periodic-time relationship The phase voltages of two generators running at different speeds.
alternators paralleled Alternators are connected in parallel whenever the power demand of the load circuit is greater than the power output of a single alternator.
ambient temperature The temperature surrounding a device.
amortisseur winding Consists of copper bars embedded in the cores of the poles of a synchronous motor. The copper bars of this special type of squirrel-cage winding are welded to end rings on each side of the rotor; used for starting only.
armature A cylindrical, laminated iron structure mounted on a drive shaft; contains the armature winding.
armature winding Wiring embedded in slots on the surface of the armature; voltage is induced in this winding on a generator.
automatic compensators Motor starters that have provisions for connecting three-phase motors automatically across 50%, 65%, 80%, and 100% of the rated line voltage for starting, in that order, after preset timing.
autotransformer A transformer in which a part of the winding is common to both the primary and secondary circuits.
auxiliary contacts--Contacts of a switching device in addition to the main circuit contacts; auxiliary contacts operate with the movement of the main contacts; electrical interlocks.
blowout coil Electromagnetic coil used in contactors and motor starters to deflect an arc when a circuit is interrupted.
branch circuit That portion of a wiring system that extends beyond the final overcurrent device protecting the circuit.
brushless excitation The commutator of a conventional direct-connected exciter of a synchronous motor is replaced with a three-phase, bridge-type, solid-state rectifier.
brushless exciter Solid-state voltage control on an alternator, providing DC necessary for the generation of AC.
bus A conducting bar of different current capacities, usually made of copper or aluminum.
busway A system of enclosed power transmission that is current and voltage rated.
capacitor A device made with two conductive plates separated by an insulator or dielectric.
centrifugal switch On single-phase motors, when the rotor is at normal speed, centrifugal force set up in the switch mechanism causes the collar to move and allows switch contacts to open, removing starting winding.
circuit breaker A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without injury to itself when properly applied within its rating.
cogenerating plants Diesel-powered electric generating sets designed to recapture and use the waste heat from both their exhaust and cooling systems.
commutator Consists of a series of copper segments insulated from one another and the mounting shaft; used on DC motors and generators.
compensator transformer A tapped autotransformer used for starting induction motors.
conductor A device or material that permits current to flow through it easily.
conduit plan A diagram of all external wiring between isolated panels and electrical equipment.
contactor An electromagnetic device that repeatedly establishes or interrupts an electric power circuit.
controller A device or group of devices that governs in a predetermined manner the delivery of electric power to apparatus connected to it.
counter-emf (CEMF) An induced voltage developed in a DC motor while rotating. The direction of the induced voltage is opposite to that of the applied voltage.
cumulative compound-wound generator or motor A series winding is connected to aid the shunt winding.
current The rate of flow of electrons measured in amperes.
current flow The flow of electrons.
DC exciter bus A bus from which other alternators receive their excitation power.
definite time A predetermined time lapse.
delta connection A circuit formed by connecting three electrical devices in series to form a closed loop; used in three-phase connections.
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 switch A switch intended to open a circuit only after the load has been thrown off by some other means; not intended to be opened under load.
drum switch A manually operated switch with 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.
dual-voltage motors--Motors designed to operate on two different voltage ratings.
duty cycle The period of time in which a motor can safely operate under a load. Continuous means that the motor can operate fully loaded 24 hours a day.
dynamic braking Using a DC motor as a generator, taking it off the supply 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 current--Current induced into the core of a magnetic device. Causes part of the iron core losses, in the form of heat.
efficiency The efficiency of all machinery is the ratio of the output to the input-that is, output/input = efficiency.
electric controller A device or group of devices that governs, in some predetermined manner, the electric power delivered to the apparatus to which it is connected.
elementary diagram (ladder diagram, schematic diagram, line diagram)
Represents the electrical control circuit in the simplest manner. All control devices and connections are shown as symbols located between vertical lines that represent the source of control power.
emergency generator system A generating set that functions as a power source in a health care facility, such as a hospital; a standby power system. In addition to lighting, the loads supplied are essential to life and safety.
engine-driven generating sets Generators with prime movers of diesel, gasoline, or natural gas engines, and the like.
exciter A DC generator that supplies the magnetic field for an alternator.
feeder The circuit conductor between the service equipment or the switchboard of an isolated plant and the branch-circuit overcurrent device.
field discharge switch Used in the excitation circuit of an alternator. Controls (through a resistor) the high inductive voltage created in the field coils by the collapsing magnetic field.
flux Magnetic field; lines of force around a magnet.
frequency Cycles per second or hertz.
fuse An overcurrent protective device with a circuit-opening fusible part that is heated and severed by the passage of overcurrent through it.
gear motor A self-contained drive made up of a ball-bearing motor and a speed-reducing gear box.
grounded Connected to earth or another conducting body that serves in place of earth.
growler An instrument consisting of an electromagnetic yoke and winding excited from an AC source; used to locate short-circuited motor coils.
hertz The measurement of the number of cycles of an alternating current or voltage completed in one second.
hysteresis Part of iron core losses.
identified conductor (neutral) A grounded conductor in an electrical system, identified with the code color white.
induced current--Current produced in a conductor by the cutting action of a magnetic field.
induced voltage--Voltage created in a conductor when the conductor interacts with a magnetic field.
induction Induced voltage is always in such a direction as to oppose the force producing it.
insulator Material with a very high resistance used to electrically isolate two conductive surfaces.
I/O section (input/output section) This section of a programmable controller interfaces the PC to the electrical signals in the field. The input takes the appropriate field indication and converts it to a signal recognizable by the processor. The output takes a signal sent by the microprocessor and converts it to the proper signal for the field devices.
isolating transformer A transformer in which the secondary winding is electrically isolated from the primary winding.
jogging The quickly repeated closure of a controller circuit to start a motor from rest for the purpose of accomplishing small movements of the driven machine.
legally required standby generating systems--Systems required by municipal, state, federal, or other codes or government agencies having jurisdiction.
Lenz's law A voltage is induced in a coil whenever the coil circuit is opened or closed.
maintaining contact A small contact in the control circuit used to keep a coil energized, usually actuated by the same coil; also known as a holding contact or an auxiliary contact.
mechanical interlock A mechanical interlocking device assembled at the factory between forward and reverse motor starters and multispeed starters; it locks out one starter at the
beginning of the stroke of either starter to prevent short circuits and burnouts by the accidental closure of both starters simultaneously.
megohmmeter (megger) An electrical instrument used to measure insulation resistance.
megohms A unit of resistance equal to 1,000,000 ohms.
motor circuit switch (externally operated disconnect switch, exo) Motor branch-circuit switch rated in horsepower. Usually contains motor-starting protection; safety switch.
motor controller A device used to control the operation of a motor.
motorizing A generator armature rotates as a motor.
motor starter A device used to start and/or regulate the current to a motor during the starting period. It may be used to make or break the circuit and/or limit the starting current. It is equipped with overload protection devices, such as a contactor with overload relays.
multimeter Electrical instrument designed to measure two or more electrical quantities.
NEC National Electrical Code.
nonsalient rotor A rotor that has a smooth cylindrical surface. The field poles (usually two or four) do not protrude above this smooth surface.
normal field excitation The value of DC field excitation required to achieve unity power factor in a synchronous motor.
normally open, 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, and with no external forces applied. The terms apply only to nonlatching types of devices.
ohmmeter An instrument used to measure resistance.
oil (immersed) switch Contacts of a switch that operate in an oil bath tank. Switch is used on high voltages to connect or disconnect a load. Also known as an oil circuit breaker.
overload Operation of equipment in excess of normal, full-load rating, or of a conductor in excess of rated ampacity which, when it persists for a sufficient length of time, would cause damage or dangerous overheating.
overload protection (running protection) Overload protection is the result of a device that operates on excessive current, but not necessarily on a short circuit, to cause the interruption of current flow to the device governed.
parallel circuit A circuit that has more than one path for current flow.
permeability The ease with which a material conducts magnetic lines of force.
plugging Braking a motor by reversing the line voltage or phase sequence; motor develops a retarding force; a quick stop.
plugging relay A device attached to a motor shaft to accomplish plugging switches reversing starter to establish countertorque that brings the motor to a quick standstill before it begins to rotate in the reverse direction.
polarity Characteristic (negative or positive) of a charge. 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.
polyphase An electrical system with the proper combination of two or more single-phase systems.
power factor The ratio of true power to apparent power. A power factor of 100% is the best electrical system.
preventive maintenance Periodic inspections to prevent serious damage to machinery by locating potential trouble areas; preventing breakdowns rather than repairing them.
processor The microprocessor section of a programmable controller. It is the section of a PC that holds the programs, receives information, makes a decision, and delivers an output signal to some external electrical device.
programmable controller (PC) A microprocessor-based system used to control electrical operations. It is a control system controlled by software that can easily be altered to provide flexible control schemes.
programmer A device-either hand held or personal computer, or special monitor and keyboard-that allows a person to enter desired control programs to the microprocessor section of a PC.
pulse-width modulation (PWM) A process that controls the width of a pulse delivered to an AC motor. By modulating the width of several pulses and controlling the amplitude, a waveform that approximates a sinewave is produced at adjustable frequencies.
pushbutton A master switch; manually operated plunger or button for an electrical actuating device; assembled into pushbutton stations.
raceway A channel or conduit designed expressly for holding wires, cables, or busbars.
rating The rating of a switch or circuit breaker includes (1) the maximum current and voltage of the circuit on which it is intended to operate, (2) the normal frequency of the current, and (3) the interrupting tolerance of the device.
rectifier A device that converts AC into DC.
regulation Voltage at the terminals of a generator or transformer for different values of the load current; usually expressed as a percentage.
relay Used in control circuits; operated by a change in one electrical circuit to control a device in the same circuit or another circuit.
remote control Controls the function initiation or change of an electrical device from some remote place or location.
residual flux A small amount of magnetic field.
resistance starter (primary resistance starter) A controller to start a motor at a reduced voltage with resistors in the line on start.
rheostat--A resistor that can be adjusted to vary its resistance without opening the circuit in which it may be connected.
r/min (RPM)--Speed in revolutions per minute.
rotor--The revolving part of an AC motor or alternator.
salient field rotor Found on three-phase alternators and synchronous motors; field poles protrude from the rotor support structure. The structure is of steel construction and commonly consists of a hub, spokes, and a rim. This support structure is called a spider.
SCADA Acronym for supervisory control and data acquisition.
selsyn Abbreviation of the term self-synchronous. Selsyn units are special AC motors used primarily in applications requiring remote control. These units are also referred to as synchros.
semiconductors Materials that are neither good conductors nor good insulators. Certain combinations of these materials allow current to flow in one direction but not in the opposite direction.
separate control The coil voltages of a relay, contactor, or motor starter are separate or different from those at the switch contacts.
separately excited field The electrical power required by the field circuit of a DC generator may be supplied from a separate or outside DC supply.
series field In a DC motor, has comparatively few turns of wire of a size that permits it to carry the full-load current of the motor.
series winding Generator winding connected in series with the armature and load; carries full load.
service factor An allowable motor overload; the amount of allowable overload is indicated by a multiplier which, when applied to a normal horsepower rating, indicates the permissible loading.
short and ground A flexible cable with clamps on both ends used to ground and short high lines to prevent electrical shock to workers.
shunt--To connect in parallel; to divert or be diverted by a shunt.
silicon-controlled rectifier (SCR) A four-layer semiconductor device that is a rectifier. It must be triggered by a pulse applied to the gate before it will conduct electricity.
single phase--A term characterizing a circuit energized by a single alternating EMF. Such a circuit is usually supplied through two wires.
slip In an induction motor, slip is the difference between the synchronous speed and the rotor speed, usually expressed as a percentage.
slip rings Copper or brass rings mounted on, and insulated from, the shaft of an alternator or wound-rotor induction motor; used to complete connections between a stationary circuit and a revolving circuit.
solenoid An electromagnet used to cause mechanical movement of an armature, such as a solenoid valve.
solid state As used in electrical-electronic circuits, refers to the use of solid materials as opposed to gases, as in an electron tube. It usually refers to equipment using semiconductors.
speed control Refers to changes in motor speed produced intentionally by the use of auxiliary control, such as a field rheostat or automatic equipment.
speed regulation Refers to the changes in speed produced by changes within the motor due to a load applied to the shaft.
split phase--A single-phase induction motor with auxiliary winding, displaced in magnetic position from, and connected parallel to, the main winding.
standby power-generating system--Alternate power system for applications such as heating, refrigeration, data processing, or communications systems where interruption of normal power would cause human discomfort or damage to a product in manufacture.
starting current--The surge of amperes of a motor upon starting.
starting protection--Overcurrent protection is provided to protect the motor installation from potential damage due to short circuits, defective wiring, or faults in the motor controller or the motor windings. The starting protection may consist of a motor disconnect switch containing fuses.
stator The stationary part of a motor or alternator; the part of the machine that is secured to the frame.
synchronous alternators Frequencies, voltages, and instantaneous AC polarities must be equal when paralleling machines.
synchronous capacitor A synchronous motor operating only to correct the power factor and not driving any mechanical load.
synchronous motor A three-phase motor (AC) that operates at a constant speed from a no-load condition to full load; has a revolving field that is separately excited from a DC source; similar in construction to a three-phase AC alternator.
synchronous speed The speed at which the electromagnetic field revolves around the stator of an induction motor. The synchronous speed is determined by the frequency (hertz) of the supply voltage and the number of poles on the motor stator.
synchroscope An electrical instrument for synchronizing two alternators.
tachometer An instrument used to check the speed of a motor or machine.
three phase A term applied to three alternating currents or voltages of the same frequency, type of wave, and amplitude. The currents and/or voltages are one-third of a cycle (120 electrical time degrees) apart.
three-phase system Electrical energy originates from an alternator that has three main windings placed 120° apart. Three wires are used to transmit the energy.
thyristor An electronic component that has only two states of operation-on or off.
torque The rotating force of a motor shaft produced by the interaction of the magnetic fields of the armature and the field poles.
transfer switches--Switches to transfer, or reconnect, the load from a preferred or normal electric power supply to the emergency power supply.
transformer--An electromagnetic device that converts voltages for use in power transmission and operation of control devices.
transformer bank When two or three transformers are used to step down or step up voltage on a three-phase system.
transformer secondary winding The coil that discharges the energy at a transformed or changed voltage, up or down.
wheatstone bridge Circuit configuration used to measure electrical qualities such as resistance.
wiring diagram Locates the wiring on a control panel in relationship to the actual location of the equipment and terminals; made up of a method of lines and symbols on paper.
wound-rotor induction motor An AC motor consisting of a stator core with a three-phase winding, a wound rotor with slip rings, brushes and brush holders, and two end shields to house the bearings that support the rotor shaft.
wye connection (star) 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.