Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

tachometer
a instrument used to measure the speed of a rotating device. Several types of tachometers are available. Friction devices are placed against the shaft of the device being measured. Others used magnetic variation or reflected light pulses to determine the speed. Tachometer generators are mounted on the shaft of the device being measured and provide a voltage proportional to the speed. Tach generators are often used in servo systems.
tachometer-generator
a small generator that is connected to the shaft of a rotating machine and produces an output voltage directly proportional to the rpm of the machine. Typically used for closed-loop speed control.
thermal reactor
a reactor which maintains a critical reaction with thermal neutrons.
time-current characteristic curve
(1) a relay time-current curve is a curve showing the time versus current characteristic of a time overcurrent (TOC) relay.

(2) a fuse time-current curve shows the melting and clearing times of a given fuse or family of fuses.

(3) a coordination time-current curve shows the relationship of the operating and clearing characteristics of coordinating devices (TOC relays and fuses) on a power system.
time-to-close contact
a contact in which the desired time to close the contactor could be set by the user.
time-to-open contact
a contact in which the desired time to open the contactor could be set by the user.
trace length
the physical distance between electronic components connected by a circuit path.
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.
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.
utilization factor
the ratio of the maximum demand on the system vs the rated capacity of the system.
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.
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 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.
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-meansquare 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.
voltage-behind-reactance model
a representation of a machine in which the stator voltage equations are modeled as a voltage source in series with a reactance (and typically a resistance). The voltage source represents the back emf present on the stator windings due to the coupling between the stator and rotor circuits. In synchronous machine modeling, several different voltage-behindreactance models have historically been used, wherein approximations are used to represent the machine in various detail.
winding factor
a design parameter for electric machines that is the product of the pitch factor and the distribution factor.
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