This document defines over 80 electrical terms, including: - A torus, which is a donut-shaped magnetic core used in electric transformers. - Total harmonic distortion, which is an electromagnetic disturbance level expressed as a ratio calculated as a percentage of the fundamental component. - A tower, which is a tall structure for elevating electric transmission lines, distinguished from a pole by its greater height and complexity. - A transducer, which is a device that converts a physical quantity into an electrical signal, such as microphones, pickups, and loudspeakers. - A transformer, which is a device with two or more coils wound on an iron core that provides an efficient means of changing voltage and

1. Electrical Terms & Some Definition
 Torus - a donut-shaped magnetic core used in electric transformers.
 Total harmonic distortion - disturbance level an electromagnetic disturbance level due
to all emissions from equipment in a system. This is expressed as a ratio of the RMS
value of the harmonic content to the RMS of the fundamental and is calculated as a
percentage of the fundamental component.
 Tower - a structure for elevating electric transmission lines, distinguished from a pole cf
by its greater height and structural complexity.
 Trace length - the physical distance between electronic components connected by a
circuit path.
 Transducer - a device that converts a physical quantity into an electrical signal.
Typically, transducers are electromechanical energy conversion devices used for
measurement or control. Transducers generally operate under linear input-output
conditions and with relatively small signals. Examples include microphones, pickups,
and loudspeakers.
 Transducer gain - ratio of the power delivered to the load to the power available from
the source.
 Transformation ratio - dimensionless ratio of the real parts of the load and source
impedance.
 Transformed circuit - an original circuit with the currents, voltages, sources, and
passive elements replaced by transformed equivalents.
 Transformer - a device that has two or more coils wound on an iron core. Transformers
provide an efficient means of changing voltage and current levels, and make the bulk
power transmission system practical. The transformer primary is the winding that
accepts power, and the transformer secondary is the winding that delivers power. The
primary to secondary voltages are related by the turns ratio of the coils. The
corresponding currents are related inversely by the same ratio.
 Transformer differential relay - a differential relay specifically designed to protect
transformers. In particular, transformer differential relays must deal with current
transformer turns ratio error and transformer inrush and excitation current.
 Transformer fuse - a fuse employed to isolate a transformer from the power system in
the event of a transformer fault or heavy overload.
 Transformer vault - a fireproof enclosure in which power transformers containing oil
must be mounted if used underground or indoors.

2.  Transient
(1)the behavior exhibited by a linear system that is operating in steady state in
moving from one steady state to another. For stable systems, the transient will
decay while for unstable system it will not, and thus the latter never reach another
steady-state operation. See also settling time and time constant.
(2)any signal or condition that exists only for a short time.
(3)an electrical disturbance, usually on a power line.
(4)refers to momentary overvoltages or voltage reductions in an electric power
system due to lightning, line switching, motor starting, and other temporary
phenomena.
 Transient current - the fault current that flows during the transient period when the
machine apparent impedance is the transient impedance.
 Transient fault - a fault that can appear (e.g., caused by electrical noise) and disappear
within some short period of time.
 Transient impedance - the series impedance that a generator or motor exhibits
following the subtransient period but prior to the steady-state situation.
 Transient open-circuit time constant - See quadrature-axis transient open-circuit time
constant and direct-axis transient open-circuit time constant.
 Transient operation - a power system operating under abnormal conditions because of a
disturbance.
 Transient reactance - the reactance offered for the transient currents in synchronous
machines. Referred to by the symbol Xs, the transient reactance is a function of the stator
frequency and the transient inductance. Xs is comparatively smaller in comparison to the
steady-state inductive reactance of the machine.
 Transient short-circuit - time constant pass and stop bands are critical. A longer filter
length generally implies that the filter can have a steeper transition band to a similarly
shaped shorter length filter.
 Transient stability - the ability of a power system to remain stable following a system
disturbance.
 Transient suppressor - a device connected to a piece of sensitive electrical equipment to
reduce the amplitude of transient voltage excursions, thus protecting the equipment.

3.  Transmission line
(1) an arrangement of two or more conductors used to convey electromagnetic
energy from one point to another.
(2) conductive connections that guide signal power between circuit elements.
 Transmission line coupler - passive coupler composed of two or more transmissions
spaced closely together where the proximity of the transmission lines allows signals to
be coupled or transferred in part from one line to the other. The electrical length of the
transmission lines is usually one quarter of a wavelength.
 Transmission line filter - a microwave device that is made up of sections of
transmission lines so as to act as a filter in the microwave frequency range.
 Transposition
(1) the practice of twisting a three-phase power line so that, for example, phase A
takes the place on the tower formerly occupied by phase B, phase B takes the
place of phase C, and phase C occupies the former position of phase A.
(2) a point on a three-phase electric power line where the conductors are
physically transposed for purposes of improving circuit balance transputer a
class of CPU designed and manufactured by Inmos Corporation. The
transputer was specifically designed to be used in arrays for parallel
processing.
 Triac - a power switch that is functionally a pair of converter-grade thyristors connected
in anti-parallel. Triacs are mainly used in phase control applications such as dimmer
switches for lighting. Because of the integration, the triac has poor reapplied dv=dt, poor
gate current sensitivity at turn-on, and longer turn-on time. They are primarily used for
AC power control with resistive loads, such as in light dimmers.
 Triplex cable - a cable used for residential or commercial service drops consisting of two
or three insulated conductors spiralled around a bare neutral wire which provides
support for the cable.
 Turbogenerator - a generator driven by a steam-turbine engine.
 Turns ratio - the ratio of the number of turns between two coupled windings, e.g., for a
transformer, it is the ratio of number of turns of the primary winding to the number of
turns of the secondary windings. For an induction machine, it is the ratio of the number
of turns of the stator winding to the number of turns of the rotor winding.
 Tween - the rotor and rotating magnetic flux of the stator due to increases in shaft load
in a synchronous machine.

4.  Twelve-pulse converter - the combination of two 6-pulse converters connected through
a Y-Y and a delta-Y transformer in order to cancel the characteristic 5th and 7th
harmonics of the 6-pulse converters. The lowest characteristic harmonics with twelve-
pulse converters under balanced conditions are the 11th and 13th harmonics. The
converters are connected in parallel on the AC side and in either series or parallel on the
DC side, depending on the required DC output voltage.
 Two-lamp synchronizing - the process to connect two three-phase power systems in
parallel using the same procedure as for three-lamp synchronizing except that lamps are
placed across only two phases of the switch. See also three-lamp synchronizing.
 Ultra-High Frequency (UHF) - electromagnetic spectrum with frequencies between 300
MHz and 3000 MHz or wavelengths between 10 cm and 100 cm. Also called as
decimetric waves.
 Ultrasound - an imaging modality that uses reflected high-frequency sound energy to
image the interface between materials with different acoustic impedances.
 Ultraviolet - a term referring to wavelengths shorter than 400 nm, but longer than 30
nm. The region 400–300 nm is the near ultraviolet, 300–200 is the middle ultraviolet; and
200–30 nm is the far ultraviolet or vacuum.
 Ultraviolet laser - laser producing its output in the ultraviolet region of the spectrum.
 Unbalanced line - refers to a signal carrying line where one of the conductors is
connected to ground. Contrast with balanced line.
 Unbalanced magnetic pull - a phenomenon in electric machines arising from the rotor
not being symmetrical with respect to the stator or the axis of the rotor and stator not
being coincident. Results in a higher pulling force on the side with the smaller airgap,
resulting in additional bearing stresses.
 Under excited - a condition of operating a synchronous machine, in which the current to
the DC field winding is insufficient to establish the required magnetic flux in the air-gap.
As a result, the machine requires reactive power from the AC system. An underexcited
synchronous motor operates at a lagging power factor, as it appears as an inductive load
to the AC system. An under-excited synchronous generator operates with a leading
power factor, since it must deliver power to a leading (capacitive) system.
 Under frequency relay - a protection device that curtails loads in an area that is deficient
in generation. Lower generation compared to load demands give rise to lower frequency
and a frequency threshold can be used by the relay to initiate load shedding in order to
balance generation and demand.
 Underground distribution - a class of electric power distribution work, typically used
in densely-populated urban business districts, in which conductors are carried in

5. conduits under streets between manholes and submersible distribution transformers are
mounted in underground vaults.
 Underground residential distribution - practices involved in the underground
distribution of electric power to residential subdivisions through direct-buried cables
and pad mound transformers.
 Undervoltage - a voltage that is less than nominal for a time greater than 1 minute.
 Undervoltage relay - a protective relay that operates on low voltage or loss of voltage.
 Unforced system - a dynamic system where all of the external sources of excitation are
identically zero.
 Ungrounded system - an electrical distribution system in which there is no intentional
connection between a current-carrying conductor and ground.
 Uninterrupted Power Supply (UPS)
(1) a power supply designed to charge an energy storage medium, while providing
conditioned output power, during the presence of input power and to continue
providing output power for a limited time when the input to the supply is
removed. These power supplies are typically used in critical applications to
prevent shut-down of these systems during power failures, power surges, or
brownouts.
(2) a device that provides protection for critical loads against power outages,
overvoltages, undervoltages, transients, and harmonic disturbances. A typical
UPS is a rectifier supplied battery bank for energy storage, and a PWM inverter-
filter system to convert a DC voltage to a sinusoidal AC output. UPS systems can
be on-line, as shown in the figure, where the UPS inverter powers the load
continuously, or off-line where the load is connected directly to the utility under
normal operation and emergency power is provided by the UPS.
 Unsymmetrical fault - a fault on a three-phase power line in which the fault current is
not equal in all three phases, e.g., a single-line-to-ground, double-line-to -ground or line-
to-line fault.
 Unsymmetrical load - a load which forces the currents in the three-phase power line
which supplies it to be unequal.
 Up-down counter - a register that is capable of operating like a counter and can be
either incremented or decremented by applying the proper electronic signals.
 Utilization factor - the ratio of the maximum demand on the system vs the rated
capacity of the system.
 Utilization voltage - the voltage across the power input terminals of a piece of electrical
equipment.

6.  Vacuum capacitor - a capacitor with a vacuum between its plates.
 Vacuum circuit breaker - a power circuit breaker where a vacuum chamber is used as
an insulating and arc clearing medium.
 Vacuum insulation - any insulation scheme which depends upon the dielectric
capabilities of a high vacuum.
 Vapor cooling - a cooling technique for power vacuum tubes utilizing the conversion of
hot water to steam as a means of safely conducting heat from the device and to a heat
sink.
 Varactor - a reverse biased PN or Schottky diode that uses the voltage variable depletion
region as a tuning element or as a nonlinear frequency multiplier.
 Varactor diode - a diode designed to have a repeatable and high capacitance vs. reverse
voltage characteristic. A two terminal semiconductor device in which the electrical
characteristic of primary interest is the voltage dependent capacitance.
 Varactor tuner - a tuning circuit at the input of a television receiver that uses a varactor
diode. The tuning capability comes from the characteristic of a varactor, or varicap, to
function as a voltage-sensitive capacitance.
 Variable frequency drive - electric drive system in which the speed of the motor can be
varied by varying the frequency of the input power.
 Variable speed AC drive - an AC motor drive that is capable of delivering variable
frequency AC power to a motor to cause it to operate at variable speeds. Induction
motors and synchronous motors are limited to operation at or near synchronous speed
when a particular frequency is applied. Variable speed drives rectify the incoming AC
source voltage to create a DC voltage that is then inverted to the desired frequency and
number of phases.
 Variable speed DC drive - a DC motor controller that allows the DC motor to operate
over a wide speed range. A common type of variable speed DC drive uses a separately
excited DC motor. Armature voltage control is used to provide operation below base
speed, and field weakening is used to provide operation above base speed.
 Variable Speed Drive (VSD) - See variable speed AC drive or variable speed DC drive.
 Vector - a quantity having both magnitude and direction.

7.  Vector controlled induction motor - a variable speed controller and motor in which the
magnetizing and torque producing components of current are controlled separately.
Some vector drives requires rotor position sensors. Vector controlled induction motors
can operate over a wider speed range, and may produce rated torque even at zero
speed, much like a DC motor. Thus, vector controlled induction motors are often used
for applications that might otherwise require a DC motor drive.
 Velocity of light - in vacuum, a constant equal to 2.997928 x 108 meters/second. In other
media, equal to the vacuum value divided by the refractive index of the medium.
 Very Small Aperture Terminal (VSAT) - a small earth station suitable for installation at
a customer’s premises. A VSAT typically consists of an antenna less than 2.4 m, an
outdoor unit to receive and transmit signals, and an indoor unit containing the satellite
and terrestrial interface units.
 Vibration damper - any of a number of devices mounted on a power line to reduce
vibrations caused by wind.
 Volt-Ampere-Reactive (VAR) - a unit of power equal to the reactive power in a circuit
carrying a sinusoidal current when the product of the root-mean-square value of the
voltage (expressed in volts), the root-mean square value of the current expressed in
amperes), and the cosine of the phase angle between the voltage and the current, equals
one; the unit of reactive power in the International System. Also expressed as megavars
and kilovars.
 Volta - Volta, Alessando (Corte) (1745–1827) Born: Como, Italy
is best known for the invention of a number of practical devices including the first
battery (voltaic pile), a simple electrometer for measuring current and electrophorus.
Volta was not a theoretical physicist, but a good researcher. He was able to follow up
Benjamin Franklin’s early work and that of Luigi Galvani by devising devices and
experiments that allowed him to explore the physics. Volta is honored by having his
name used as the unit of electromotive force, the volt.