Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

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.
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.
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.
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
vacuum capacitor
a capacitor with a vacuum between its plates.
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.
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.
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-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 collapse
the rapid and uncontrollable drop of bus voltage due to a slight increase in load at the bus, generally characterized by inadequate reactive support in a high-load area.
voltage dip

See sag
voltage drop
the difference in potential between the two ends of the resistor measured in the direction of flow current. The voltage drop is V = IR, where V is voltage across the resistor, I is the current through the resistor, and R is the resistance.
voltage instabilityproximity index
an index that gives an indication of the amount of real or reactive power margin available in the system before a voltage collapse occurs.
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