Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

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.
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-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 variation
long duration a change of voltage RMS value from nominal for a time period greater than 1 minute, and can be used with the words showing a magnitude change such as overvoltage, or under-voltage.
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.
voltage
the potential to do work, voltage is the ratio of the energy available to the charge, expressed in volts.
voltage coefficient of resistance
the change in resistance per unit change in voltage, expressed as a percentage of the resistance at 10% of rated voltage.
voltage change
a deviation of the peak or RMS voltage between two levels that are of some fixed duration.
voltage dip

See sag
voltage distortion
a change from a nominal clean sinusoidal waveform
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 fed inverter

See voltage source inverter
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.
voltage interruption
the removal of the supply voltage from any phase, which is of momentary, sustained, of temporary duration.