Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

air-gap voltage
the internal voltage of a synchronous machine that is generated by the air gap flux. Also referred to as the voltage behind leakage reactance.
arrester discharge voltage
the voltage in an arrester during a surge.
automatic voltage regulator (AVR)
an automatic feedback control system that is responsible for maintaining a scheduled voltage either at the terminals of a synchronous generator or at the high-side bus of the generator step-up transformer. The control is brought about by changing the level of excitation.
branch voltage
the voltage across a branch of a circuit.
DC voltage constant
voltage with no variation over time. This can be considered in general terms as an alternating current (AC) with a frequency of variation of zero, or a zero frequency signal. For microwave systems, DC voltages are provided by batteries or AC/DC converters required to "bias" transistors to a region of operation where they will either amplify, mix or frequency translate, or generate (oscillators) microwave energy.
dropout voltage
the voltage level where proper equipment operation is hindered.
equivalent noise voltage (ENV)
a noise voltage source that is effectively in series with either the inverting or noninverting input terminal of the op amp and represents the total noise contributed by the op amp if the inputs were shorted.
flat voltage start
the usual initial assumption made when beginning a power-flow study. All voltages are assumed to be 1.0 p.u., and all angles are assumed to be zero.
half-wave voltage
the voltage required to produce an amount of refractive index change in a medium that will retard the phase of a traversing optical wave.
high-voltage DC (HVDC)
transmission transmission of electric power (at typically 500-1500 kV) using DC rather than AC. This can be desirable for several reasons:

1. For economic reasons when a large amount of power is to be transmitted over a long distance, i.e., 300-400 miles, or via
underwater cables;

2. For the connection of asynchronous AC systems; and

3. Improved transient stability and dynamic damping of the electrical system oscillations.
induced voltage
voltage produced by a time-varying magnetic flux linkage.
See Faraday's law.
induction voltage transformer
specially constructed transformer with a rotating primary coil that is used to provide voltage regulation on individual power circuits. The secondary of an induction regulator is mounted on the stationary shell of cylindrical core, and the primary is mounted on a movable, center rotor. In the neutral position, the magnetic axes of the primary and secondary coils are oriented 90 degrees to each other, reducing the magnetic coupling to zero. In this position, energizing the primary does not induce voltage in the secondary; however, rotating the primary coil in either direction from the neutral position creates mutual flux linkage and causes a secondary voltage to appear.

Rotation in one direction causes secondary voltage to be in phase with the primary; rotation in the opposite direction causes secondary voltage to be out of phase with the primary. Voltage regulation is provided by connecting the primary coil across the line to be regulated and connecting the secondary coil in series with the load. By positioning the primary coil based on load demand in the line, secondary voltage can be used to adjust line voltage either up or down. Induction regulators are also equipped with a short-circuited coil mounted on the primary in spatial quadrature with the primary coil. In the neutral position, this coil has maximum coupling with the secondary coil, which minimizes the inductive reactance in the load line due to the secondary coil.
Kirchoff's voltage law (KVL)
a fundamental law of electricity that states that the sum of the voltage drops and rises in a closed loop must equal 0.
line-to-line voltage
a voltage measurement of a three phase line made between any two conductors.
low voltage holding coil
a holding coil that keeps the main-line contactor closed on low voltage conditions. Controllers that contain this feature are used in places where the motor is vital to the operation of a process, and it is necessary to maintain control of the motor under low voltage conditions.
mains voltage
European term for the voltage at the secondary of the distribution transformer.
momentary overvoltage
an increase in voltage above the system's specified upper limit for more than a few seconds. Generally a rather loosely-defined term.
no voltage holding coil
a holding coil that keeps the main-line contactor closed on zero voltage conditions. DC motor controllers that contain this feature are used in places where the motor is vital to the operation of a process. These controllers can maintain control to the motor under momentary line power loses, by using the CEMF of the coasting armature to keep power to the main-line coil/contactor. If power to the motor controller is not restored within a short period of time, the motor coasts to a speed where it can no longer keep the main-line contactor closed. At this point, the m-coil drops out to insure starting resistors are placed back in the circuit.
nominal voltage
a number given to a system to name its classification of voltage, such as rated values.
a voltage having a value larger than the nominal voltage for a duration greater than 1 minute.