Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

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-toground 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 converter

See buck-boost transformer
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.
up-down transformer

See buck-boost transformer
UPS

See uninterrupted power supply
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.
V-V transformer

See open-delta transformer
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.
VAR

See volt-ampere-reactive
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.
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