Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

coefficient of thermal expansion (CTE) mismatch
the difference between the coefficients of thermal expansion of two components, i.e., the difference in linear thermal expansion per unit change in temperature. (This term is not to be confused with thermal expansion mismatch).
commutating inductance
in switched circuits (converters, inverters, etc.), the inductance that is in series with the switching elements during the process of commutation from one topological state to another. This inductance results in noninstantaneous commutation due to the fact that current in an inductor cannot change instantaneously.
compound-connected DC machine
a direct current machine with two field windings in which one field winding is connected
in series and one field winding is connected in parallel (shunt) with the armature winding. The shunt winding may be connected ahead of the series winding (long-shunt connection), or behind the series winding (short-shunt connection).
compound-rectifier exciter
a source of field current of a synchronous machine derived from the phase voltages and currents of the machine. The phase voltages and currents of the machine are fed through transformers, then rectified in order to provide DC quantities to the field winding. The components of the exciter are the transformers (voltage and current), rectifiers (including possible gate-circuitry), and power reactors; exclusive of all input control elements.
conductance
(1) the reciprocal of resistance.
(2) a characteristic that describes the availability and the mobility of conduction electrons within a material. The values range from zero for a perfect insulator to infinity for a perfect conductor. The units are siemens.
(3) the ability of a substance to carry a thermodynamic flow, such as current, heat, energy, etc. conducted emission an RF current propagated through an electrical conductor.
conducted noise
unwanted electrical signals that can be generated by power electronic switching circuits. Conducted noise can travel through the circuit cables as common-mode or differential mode currents and can interfere with control circuits or other electronic equipment.
conductivity
(1) the reciprocal of resistivity.
(2) a measure of a material's ability to conduct electrical current. Conductivity . is the ratio of the conduction current to the electric field in Ohm's Law: Jc = σE
cone of protection
a method used to determine the extent of protection to surrounding structures afforded by a tall, grounded structure like a steel tower. Proposed prior to the "rolling ball" model, this method suggests that any structure which can fit within a right circular cone whose vertex is at the top of the tower will be protected from lightning strikes by that tower. The angle of the cone's vertex is a matter of some controversy.

See rolling ball
convective heat transfer
the process by which a moving fluid transfers heat to or from a wetted surface.
corona effect
flow of electrical energy from a high-voltage conductor to the surrounding ionized air. This effect only becomes significant for potentials higher than 1000 V. This effect is characterized by a faint glow, a crackling noise and conversion of atmospheric oxygen to ozone.
crest factor
the ratio of the peak value of a signal to its RMS value.
CT

See current transformer
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 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.
DC link inductor
an inductor used on the output of a controlled rectifier in AC current source drives to provide filtering of the input current to the current source inverter. If used in conjunction with a capacitor, then it is used as a filter in voltage source drives.
derating factor
the fraction (or percent) of nominal rating to which a specified quantity must be reduced due to unusual operating conditions. Examples of conditions that may require application of a derating factor are high altitude, high ambient temperature, frequent motor starting, and "plugging" operation of a motor.
dielectric
(1) a medium that exhibits negligible or no electrical conductivity and thus acts as a good electrical insulator.
(2) a medium characterized by zero conductivity, unity relative permeability, and a relative permittivity greater than one. Also known as an insulator.
dielectric constant
(1) a quantity that describes how a material stores and dissipates electrical energy.
(2) ratio of the electrical capacity of a condenser, which has a given material as the dielectric, to the capacity of an identical condenser, but with air as the dielectric.
(3) permittivity of a medium normalized to the permittivity of free space; a measure of the response of a dielectric to an applied electric field.
(4) an electric property of an insulator or semi-conducting material, which describes how differently electric fields will behave inside of the material as compared to air. As an example, er = 12.9 for GaAs as compared to er = 1 for air. In integrated circuits, an effective dielectric constant (eeff) is used, since the electric fields supported by the signals traveling through the conductors on the circuit flow through both air and the insulator or semiconductor simultaneously.
differential protection
a protective relaying scheme in which the currents entering and leaving the protected line or device are compared.
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