voltage polarity and directions of current flow
change continuously, and often periodically.
Thus, such networks contain alternating currents
as opposed to direct currents, thereby
giving rise to the term.
AC coupling a method of connecting two
circuits that allows displacement current to
flow while preventing conductive currents.
Reactive impedance devices (e.g., capacitors
and inductive transformers) are used to provide
continuity of alternating current flow
between two circuits while simultaneously
blocking the flow of direct current.
AC motor an electromechanical system
that converts alternating current electrical
power into mechanical power.
AC plasma display a display that employs
an internal capacitive dielectric layer
to limit the gas discharge current.
AC steady-state power the average
power delivered by a sinusoidal source to a
network, expressed as
P Dj V j j I j cos. /
where
p
2 j V j and
p
2 j I j are the peak
values, respectively, of the AC steady-state
voltage and current at the terminals. represents
the phase angle by which the voltage
leads the current.
AC/AC converter a power electronics
device in which an AC input voltage of some
magnitude, frequency, and number of phases
is changed to an AC output with changes to
any of the previously mentioned parameters.
AC/AC converters usually rectify the input
source to a DC voltage and then invert the
DC voltage to the desired AC voltage.
active power line conditioner a device
which senses disturbances on a power line
and injects compensating voltages or currents
to restore the line’s proper waveform.
active RC filter an electronic circuit
made up of resistors, capacitors, and operational
amplifiers that provide well-controlled
linear frequency-dependent functions, e.g.,
low-, high-, and bandpass filters.
air capacitor a fixed or variable capacitor
in which air is the dielectric material between
the capacitor’s plates.
air circuit breaker a power circuit
breaker where the power contacts operate in
air. Some versions employ an air blast to
extend and clear the arc on contact opening,
while others employ arc chutes with magnetic
or thermal assists.
air core transformer two or more coils
placed so that they are linked by the same flux
with an air core. With an air core the flux is
not confined.
air-blast circuit breaker a circuit breaker
in which the arc which forms between the
contacts on opening is extinguished with a
blast of high-pressure air.
air-gap voltage the internal voltage of a
synchronous machine that is generated by the
air gap flux. Also referred to as the voltage
behind leakage reactance.
algorithm (1) a systematic and precise,
step-by-step procedure (such as a recipe, a
program, or set of programs) for solving a
certain kind of problem or accomplishing a
task, for instance converting a particular kind
of input data to a particular kind of output
data, or controlling a machine tool. An algorithm
can be executed by a machine.
(2) in image processing, algorithms can be
either sequential, parallel, or ordered. In sequential
algorithms, pixels are scanned and
processed in a particular raster-scan order.
As a given pixel is processed, all previously
scanned pixels have updated (processed) values,
while all pixels not yet scanned have old
(unprocessed) values. The algorithm’s result
will in general depend on the order of scanning.
In a parallel algorithm, each pixel is processed
independently of any changes in the
others, and its new value is written in a new
image, such that the algorithm’s result does
not depend on the order of pixel processing.
In an ordered algorithm, pixels are put in
an ordered queue, where priority depends on
some value attached to each pixel. At each
time step, the first pixel in the queue is taken
out of it and processed, leading to a possible
modification of priority of pixels in the
queue. By default, an algorithm is usually
considered as parallel, unless stated otherwise
all-optical network an optical communications
network where the role of electronics
is reduced to basic supervisory and control
functions. All-optical devices are used exclusively
between the nodes to re-configure
the network which enables the greatest use of
fiber bandwidth.
alphanumeric mode relates to alphabetic
characters, digits, and other characters
such as punctuation marks. Alphanumeric
is a mode of operation of a graphic terminal
or other input/output device. The graphics
terminal should toggle between graphic and
alphanumeric data.
alternating current (AC) a periodic current
the average value of which over a period
is zero.
alternating current machine an electromechanical
system that either converts alternating
current electrical power into mechanical
power (AC motor), or converts mechanical
power into alternating current electrical
power (AC generator, or alternator). Some
AC machines are designed to perform either
of these functions, depending on the energy
source to the dynamo.
ammeter an instrument for measuring
electric current in amperes.
amplidyne a special generator that acts
like a DC power amplifier by using compensation
coils and a short circuit across its
brushes to precisely and fastly control high
powers with low level control signals
black start the task of re-starting an isolated
power system which is completely deenergized.
Most generating plants require
substantial external electric power to start.
Thus a black start may be initiated by handstarting
gas turbine generators or by opening
the gates of a hydroelectric generator somewhere
in the system.
blackout total loss of power to the entire
power system.
blocked-rotor test an induction motor
test conducted with the shaft held so it cannot
rotate. Typically about 25% of rated voltage
is applied, often at reduced frequency and the
current is measured. The results are used to
determine the winding impedances referred
to the stator.
advancement of a country is measured by the index per capita consumption of electricity – more it is more advanced the country is.
analog-to-digital (A/D) conversion a
method by which a continuously varying signal
(voltage) is sampled at regularly occurring
intervals. Each sample is quantized to a
discrete value by comparisons to preestablished
reference levels. These quantized
samples are then formatted to the required
digital output (e.g., binary pulse codewords).
The A/D converter is “clocked” to provide
updated outputs at regular intervals. In order
not to lose any baseband information, sampling
must occur at a rate higher than twice
the highest incoming signal frequency component.
See also Nyquist rate.
analog-to-digital (A/D) converter a device
that changes an analog signal to a digital
signal of corresponding magnitude. This device
is also called an encoder, ADC, or A/C
converter.
armature reaction (1) inDCmachines, a
distortion of the field flux caused by the flux
created by the armature current. Armature
reaction in a DC machine causes lower flux
at one pole-tip and higher flux at the other,
which may lead to magnetic saturation. It
also shifts the neutral axis, causing sparking
on the commutator.
(2) in AC synchronous machines, a voltage
“drop” caused by the armature current.
In the steady state model of the synchronous
machine, the armature reaction is accounted
for by a component of the synchronous reactance.
armature voltage control a method of
controlling the speed of a DC motor by varying
the voltage applied to the armature while
keeping the voltage applied to the field circuit
constant.
armature winding an arrangement of
coils carrying the main current, typically
wound on the stator of a synchronous machine
or the rotor of a DC machine, in which
an alternating voltage is induced by the magnetic
field.
artificial intelligence the study of computer
techniques that emulate aspects of human
intelligence, such as speech recognition,
logical inference, and ability to reason from
partial information.
asynchronous transfer mode (ATM)
method of multiplexing messages onto a
channel in which channel time is divided into
small, fixed-length slots or cells. In ATM
systems the binding of messages to slots is
done dynamically, allowing dynamic bandwidth
allocation. ATM is asynchronous in
the sense that the recurrence of cells containing
information from an individual user is not
necessarily periodic.
automatic generation control (AGC)
phrase describing the computer-based process
by which electric utilities control individual
generating stations to maintain system
frequency and net interchange of power on a
highly interconnected transmission grid. Automatic
generation control (AGC) systems
monitor grid frequency, actual and scheduled
power flows, and individual plant output to
maintain balance between actual and scheduled
power production, both within transmission
control areas and at individual generating
stations. Control is generally accomplished
by adjusting the speed control (or
droop) characteristics of individual generating
units. Control actions are determined by
planned production schedules and power exchange
agreements among participating utilities.
autotransformer a power transformer
that has a single continuous winding per
phase, part of this winding being common
to both the primary and the secondary sides.
As a result, these voltages are not isolated but the transformer is reduced in weight and
size. Autotransformers are most suited for
relatively small changes in voltage. Three
phase autotransformers are by necessity connected
in a wye configuration.
average power the average value, taken
over an interval in time, of the instantaneous
power. The time interval is usually one period
of the signal.
bandwidth (1) the frequency range of
a message or information processing system
measured in hertz.
(2) width of the spectral region over which
an amplifier (or absorber) has substantial
gain (or loss); sometimes represented more
specifically as, for example, full width at half
maximum.
(3) the property of a control system or
component describing the limits of sinusoidal
input frequencies to which the system/
component will respond. It is usually
measured at the half-power points, which are
the upper and lower frequencies at which the
output power is reduced by one half. Bandwidth
is one measure of the frequency response
of a system, i.e., the manner in which
it performs when sine waves are applied to
the input.
(4) the lowest frequency at which the ratio
of the output power to the input power
of an optical fiber transmission system decreases
by one half (3 dB) compared to the
ratio measured at approximately zero modulation
frequency of the input optical power
source. Since signal distortion in an optical
fiber increases with distance in an optical
fiber, the bandwidth is also a function of
length and is usually given as the bandwidthdistance
product for the optical fiber in megahertz megahertz
per kilometer.
bandwidth efficiency the ratio of the
information rate in bits per second to the
required bandwidth in hertz for any digital
modulation technique.
battery one or more cells connected so
as to produce energy.
baud the signaling rate, or rate of state
transitions, on a communications medium.
One baud corresponds to one transition per
second. It is often confused with the data
transmission rate, measured in bits per second.
beta particle an electron or positron emitted
from a radioactive source.
binary-coded decimal (BCD) (1) a
weighted code using patterns of four bits to
represent each decimal position of a number.
(2) decimal digits 0 to 9, encoded by their
four-bit binary representation. Thus: 0 =
0000, 1 = 0001, 2 = 0010, 3 = 0011, 4 = 0100,
5 = 0101, 6 = 0110, 7 = 0111, 8 = 1000, 9 =
1001.
binocular vision the use of two images
of a scene, taken (often simultaneously) from
two different positions, to estimate depth of
various point features, once correspondences
between pairs of image features have been
established.
Bode plot a graphical characterization
of the system frequency response:
the magnitude of the frequency response
jH.j!/j;−1 < ! < 1 in decibels, and
the phase angle 6 H.j!/;−1 < ! < 1,
are plotted.
breakdown voltage the reverse biased
voltage across a device at which the current
begins to dramatically deviate and increase
relative to the current previously observed at
lower voltages close to the breakdown voltage.
This effect is attributed to avalanche or
zener breakdown. It is usually specified at a
predetermined value of current.
In a diode, applying a voltage greater than
the breakdown voltage causes the diode to
operate in the reverse breakdown region.
brightness the perceived luminance or apparent
intensity of light. This is often different
from the actual (physical) luminance, as
demonstrated by brightness constancy, Mach
band, and simultaneous contrast.
C
buck-boost transformer a special purpose
2- or 4-coil transformer used to produce
modest increases or decreases in the utilization
voltage at a load site. The low-voltage
coil(s), which typically have rated voltages
of 5% to 15% of the high-voltage coils, and
in use, the high- and low-voltage, coils, are
connected in series to produce an autotransformer
arrangement. If primary voltage is applied
to the high voltage coil and load voltage
is taken from the series coil combination, the
low-voltage coil adds to, or boosts, the load
utilization voltage. Conversely, reductions
in load utilization voltage occur when these
primary and secondary connections are reversed
causing the low-voltage coil to buck
the supply voltage. A typical 4-coil buckboost
transformer would have two 120 V primary
coils and two 12 V secondary coils,
which could be used to produce voltage ratios
of (120/132), (120/144), (240/252), and
(240/264).
C-band microwave frequency range,
3.95-5.85 Ghz
cache an intermediate memory store having
storage capacity and access times somewhere
in between the general register set and
main memory. The cache is usually invisible
to the programmer, and its effectiveness
comes from being able to exploit program locality
to anticipate memory-access patterns
and to hold closer to the CPU: most accesses
to main memory can be satisfied by the cache,
thus making main memory appear to be faster
than it actually is.
cage-rotor induction motor an induction
motor whose rotor is occupied by copper or
aluminum bars, known as rotor bars, instead
of windings. Also commonly referred to as
a squirrel-cage induction motor.
capacitance the measure of the electrical
size of a capacitor, in units of farads.
Thus a capacitor with a large capacitance
stores more electrons (coulombs of charge)
at a given voltage than one with a smaller
capacitance.
In a multiconductor system separated by
nonconductive mediums, capacitance (C)
is the proportionality constant between the
charge (q) on each conductor and the voltage
(V ) between each conductor. The total
equilibrium system charge is zero. Capacitance
is dependent on conductor geometry,
conductor spatial relationships, and the material
properties surrounding the conductors.
Capacitors are constructed as two metal
surfaces separated by a nonconducting electrolytic
material. When a voltage is applied
to the capacitor the electrical charge accumulates
in the metals on either side of the nonconducting
material, negative charge on one
side and positive on the other. If this material
is a fluid then the capacitor is electrolytic;
otherwise, it is nonelectrolytic.
capacitor-start induction motor (CSIM)
a single-phase induction motor with a capacitor
in series with its auxiliary winding,
producing nearly a 90_ phase difference between
the main winding and the auxiliary
winding currents at starting. This results in
a high starting torque, so this motor is used
for hard-to-start loads. The auxiliary winding
and capacitor are removed from the circuit
by a centrifugal switch as the machine
approaches operating speed.
capacitive reactance the opposition offered
to the flowof an alternating or pulsating
current by capacitance measured in ohms.
cathode ray tube (CRT) a vacuum tube
using cathode rays to generate a picture on a
fluorescent screen. These cathode rays are in
fact the electron beam deflected and modulated,
which impinges on a phosphor screen
to generate a picture according to a repetitive
pattern refreshed at a frequency usually
between 25 and 72 Hz
chain reaction a process in which highenergy
neutrons emitted from fissile radioactive
material are directed into more fissile
material such that more neutrons are emitted.
The process creates heat which is used
to power thermal power plants.
characteristic impedance inherent property
of a transmission line that defines the
impedance that would be seen by a signal
if the transmission line were infinitely long.
If a signal source with a “source” or “reference”
impedance equal to the characteristic
impedance is connected to the line there will
be zero reflections.
Chernobyl typically refers to a fire at a
nuclear power plant near Kiev in the Republic
of the Ukraine.
circuit breaker a circuit breaker is a device
that makes and breaks the electrical contact
between its input and output terminals.
The circuit breaker is capable of clearing fault
currents (tripping) as well as load currents.
The circuit breaker consists of power contacts
with arc clearing capability and associated
control and auxiliary circuits for closing
and tripping the breaker under the required
conditions.
clamping the process of fixing either the
minima or maxima of a voltage.
code division multiple access (CDMA)
a technique for providing multiple access to
common channel resources in a communication
system. CDMAis based on spread spectrum
techniques where all users share all the
channel resources. Multiple users are distinguished
by assigning unique spreading codes
to each user. Traditionally, individual detection
is accomplished at the receiver through
correlation or matched filtering.
cognitive map the cognitive map, introduced
by R. Axelrod to study decision making
processes, consists of points, or nodes,
and directed links between the nodes. The
nodes correspond to concepts. See also fuzzy
cognitive map.
cogeneration (1) any of a number of
energy generation systems in which two (or
more) forms of energy are produced in forms
practical for use or purchase by an end user.
Typical systems produce electrical energy for
sale to a utility and process steam for local
space heating or other process uses. Cogeneration
Cogeneration designs are generally adopted to increase
the overall efficiency of a power generation
process.
(2) typically, the production of heat energy,
e.g. to heat buildings, as an adjunct to
the production of electric power.
compact disk (CD) a plastic substrate
embossed with a pattern of pits that encode
audio signals in digital format. The disk is
coated with a metallic layer (to enhance its
reflectivity) and read in a drive (CD player)
that employs a focused laser beam and monitors
fluctuations of the reflected intensity in
order to detect the pits.
complementary metal oxide semiconductor
(CMOS) (1) refers to the process that
combines n-channel and p-channel transistors
on the same piece of silicon (complementary).
The transistors are traditionally made
of layers of metal, oxide, and semiconductor
materials, though the metal layer is often replaced
by polysilicon. There are a number
of variations such as HCMOS, high-speed
CMOSwhich scales down the elements compared
to the standard MOS process and thus
increases the speed and reduces the power
consumption for each transistor in the CPU.
(2) a CMOS memory device used in computers
to store information that must be available
at startup. The information is maintained
in the device by a small battery.
conductivity (1) the reciprocal of resistivity.
(2) a measure of a material’s ability to conduct
electrical current. Conductivity _ is the
ratio of the conduction current to the electric
field in Ohm’s Law:
Jc D _E
constant-current transformer two-coil
transformer with a moveable secondary coil
used to provide constant output current to
a variable load. Constant current is maintained
by mounting both the primary and secondary
coils on the center element of a shelltype
core and allowing the secondary coil to
move up and down with changes in demand
for load current. Increasing current demand
due to a reduction in load impedance causes
the secondary coil to move away from the
primary coil. Increasing the coil separation
increases flux leakage and reduces the secondary
output voltage. The reduced output
voltage counteracts the demand for more current.
Increases in load impedance reverse the
process. Movement of the secondary coil is
controlled automatically by attaching the secondary
coil to a counterweight and pulley assembly
and orienting the coil windings such
that their flux directions oppose. Increases
in secondary current increase the magnetic
repulsion between the coils, which, aided by
the counterweight, moves the secondary coil
away from the primary. Reductions in secondary
current produce the opposite effect.
control system (1) the entity comprising
the controlled process and the controller.
Control system is influenced by the environment
of the process both through the free inputs
to the process itself and through any current
information concerning the behavior of
these free inputs that is made available to the
controller.
(2) an arrangement of interconnecting elements
that interact and operate automatically
to maintain a specific system condition or
regulate a controlled variable in a prescribed
manner.
controlled rectifier a rectifier that uses
switching elements that have forward voltage
blocking capability to allow a variable
voltage DC output.
controller (1) the entity that enforces the
desired behavior — as specified by the control
objectives—of the controlled process by
adjusting the manipulated inputs. The values
of these inputs are either predetermined or
decided upon (computed) using on-line, i.e.,
real time, decision mechanism of the controller—
based on the currently available information.
See also controlled variable.
(2) a device that generates the input to the
plant or process. The role of the controller
is to force the controlled variable of the plant
or process to behave in a desired manner.
(3) a unit that directs the operation of a
subsystem within a computer. For instance,
a disk controller interprets data access commands
from host computer (via a bus), and
sends read/write, track seeking, and other
control signals to the drive. During this time,
the computer can perform other tasks, until
the controller signals DATA READY for
transfer via the CPU bus.
cooling tower a reinforced-concrete,
spool-shaped structure in a thermal power
plant in which condenser cooling water
is itself cooled by convectively-driven air
streams
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