Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

coefficient of utilization (CU)
the ratio of the lumens reaching the working plane to the total lumens generated by the lamp. This factor takes into account the efficiency and distribution of the luminaire, its mounting height, the room proportions, and the reflectances of the walls, ceiling, and floor.
constant-current transformer
two-coil transformer with a moveable secondary coil used to provide constant output current to
a variable load. Constant current is maintained by mounting both the primary and secondary coils on the center element of a shell-type core and allowing the secondary coil to move up and down with changes in demand for load current. Increasing current demand due to a reduction in load impedance causes the secondary coil to move away from the primary coil. Increasing the coil separation increases flux leakage and reduces the secondary output voltage. The reduced output
voltage counteracts the demand for more current. Increases in load impedance reverse the process. Movement of the secondary coil is controlled automatically by attaching the secondary coil to a counterweight and pulley assembly and orienting the coil windings such that their flux directions oppose. Increases in secondary current increase the magnetic repulsion between the coils, which, aided by the counterweight, moves the secondary coil away from the primary. Reductions in secondary current produce the opposite effect.
CU

See coefficient of utilization
cumulatively compounded
a compound-wound DC machine in which the flux produced by the MMF of the shunt field winding and the flux produced by the MMF of the series field winding are in the same direction.
current
the flow of charge, measured in amperes (1 ampere = 1 coulomb/s).
current density vector field
the field (commonly denoted J ) that is related to the electric field intensity vector field by the conductivity of the medium that the fields are located in. One of the quantities found on the right side of Ampere's Law. The units are (amperes/square meter).
current distribution factor
in economic dispatch studies, the proportion of a power line's total current which is contributed by a particular generating plant.
current limiting
the output current is limited to a preset level even under a shorted output condition. This can be accomplished by reducing the output voltage to prevent the current limit from being exceeded.
current limiting fuse
a fuse that limits the level of fault current from that which is available. It operates by developing a substantial voltage across the fuse following the melting of the fuse element.
current regulator
a device used to control the magnitude and phase of the current in DC, AC or other electrical variable speed drives. May use different control strategies like hysteresis current control or ramp comparison current control.
current transducer
a device used to measure current in a variety of applications including variable speed drives. May give out a current proportional to the measured current or a voltage proportional to the measured current. Electrical isolation may be obtained by using a current transformer (cannot be used at DC) or a hall effect transducer that can be used down to DC.
current transformer (CT)
(1) a transformer that is employed to provide a secondary current proportional to primary current flowing. The primary "winding" is often created by passing the system conductor or bus bar through an opening in the device
and the secondary is typically rated at a standard value to match common meters and display units. Current transformers are used in current measurement, protective relays, and power metering applications. The load (meter) on a CT should never be removed without first shorting the secondary of the CT, otherwise dangerous voltage levels may result when the load is removed.
(2) a device which measures the instantaneous current through a conductor of an electric power line and transmits a signal proportional to this current to the power system's instrumentation. current unit a protective relay that monitors the magnitude of a power transmission line's current flow.
current withstand rating
the current withstand rating of a device is the maximum short term current that can flow in the device without causing damage.
See ampacity
current-limiting device
when operating within its current-limiting range, a current-limiting device is open in 0.25 cycle or less and limits the maximum short-circuit current to a magnitude substantially less than the short-circuit current available at the fault point.
DC circuit
electrical networks in which the voltage polarity and directions of current flow remain fixed. Thus such networks contain direct currents as opposed to alternating currents, thereby giving rise to the term.
DC current constant
current 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 currents 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.
DC offset current
the exponentially decaying current component that flows immediately following a fault inception. DC offset is the result of circuit inductance, and is a function of the point in the voltage wave where the fault begins. The offset for a given fault can range from no offset to fully offset (where the instantaneous current peak equals the full peak-peak value of the AC current).

direct current machine
a DC machine is an electromechanical dynamo that either converts direct current electrical power into mechanical power (DC motor), or converts mechanical power into direct current electrical power (DC generator). Some DC machines are designed to perform either of these functions, depending on the energy source to the dynamo.
direct current motor
a rotation machine energized by DC electrical energy and used to convert electrical energy to mechanical energy.
direct-axis subtransient open-circuit time constant
a constant that characterizes the initial decay of transients in the d-axis variables of the synchronous machine with the stator windings open-circuited. The interval characterized is that immediately following a disturbance, during which the effects of all amortisseur windings are considered. A detailed (derived) closed-form expression for the subtransient open-circuit time constant of a machine with a single d-axis amortisseur windings is obtained by taking the reciprocal of the smallest root of the denominator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the field winding resistance is very small and the detailed expression simplified.
Powered by The Filipino Engineer