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Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

UL
See Underwriters Laboratory
UL classes
a classification system established by Underwriters Laboratory (UL) for the purpose of defining certain operating characteristics of low voltage fuses. UL classes include G, J, L, CC, T, K, R, and H.
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.
unbalanced operation
in an n-phase system (n> 1), a condition in which the phase voltages (currents) are either

1. not equal-amplitude sinusoids or

2. have phase angles displaced by a value other than that specified for balanced operation.

The term "unbalanced" is also used to describe a machine that has unsymmetrical phase windings.
underexcited
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.
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