Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

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.
differential protection unit
a protective unit based on the difference of currents flowing in and out of a protected zone.
differential relay
a differential relay is a protective relay that measures current going into a device from all sources by means of a network of paralleled current transformers. Ideally, the operational current is zero for normal conditions, and rises to a high value (proportional to fault current) when a fault comes on inside the differential zone.

Differential relays are commonly applied in bus protection, transformer protection, generator protection, and large motor protection.
differentially compounded
a compound 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 oppose each other. Most often obtained by incorrectly connecting the machine, the differentially compounded machine may demonstrate very erratic behavior.
digital tachometer
a device with a sensor that senses pulses from a rotating axis and converts them to digital output calibrated in rotations per minute (rpm).
digital voltmeter (DVM)
a modern solid-state device capable of measuring voltage and displaying the value in digitized form. The term is also used loosely for the digital multimeter, which can also measure current and resistance.
diode
a two-terminal device that permits the flow of electric current in only one direction.

Diodes are most often constructed by abutting n-type and p-type regions of a semiconductor, that has significantly higher electrical conductivity in one direction (forwardbiased) than the other (reverse-biased).

Diode devices may be specially designed for low-power, high switching speed applications (signal diodes) or higher-power applications (rectifier diodes).
diode detector
a device that by use of rectification and the use of inherent nonlinearity separates a modulating signal from its carrier.
diode rectifier
a circuit in which the output voltage is fixed by the circuit parameters and the load. The direction of power flow is not reversible. An example of a single-phase diode-bridge rectifier with a capacitor filter is shown. Note that the diodes are on only for a short duration, while the rectified line voltage is greater than the capacitor voltage.
direct axis (d axis)
the magnetic axis of the rotor field winding of a synchronous machine. The axis between the poles of a DC machine.
direct axis magnetizing (armature) reactance
a reactance that represents all the inductive effects of the d-axis stator current of a synchronous machine, except for that due to the stator winding leakage reactance. In Park's d-axis equivalent circuit of the synchronous machine, this reactance is the only element through which both the stator and rotor currents flow. Its value may be determined by subtracting the stator winding leakage reactance from the steady-state value of the d-axis operational impedance or from the geometric and material data of the machine.
direct axis synchronous reactance
the sum of the stator winding leakage reactance and the direct-axis magnetizing (armature) reactance of a synchronous machine. This represents the balanced steady-state value of the direct-axis operational impedance of the synchronous machine, and thus characterizes the equivalent reactance of the machine during steady-state operation.
direct axis transient
reactance a value that characterizes the equivalent reactance of the d-axis windings of the synchronous machine between the initial time following a system disturbance (subtransient interval) and the steady state. This reactance cannot be directly mathematically related to the d-axis operational impedance. However, in models in which the rotor windings are represented as lumped parameter circuits, the d-axis transient reactance is expressed in closed
form as the sum of the stator winding leakage reactance, and the parallel combination of the d-axis magnetizing reactance and the field winding leakage reactance.
direct burial
(1) the practice of burying a specially-armored power or communications cable in a ditch without the use of a surrounding conduit.
(2) a term applied to any cable which is meant for direct burial. direct control bottom, first, control layer
of a multilayer controller, directly responsible for adjusting the manipulated inputs to the controlled process; typical example of direct control is the regulation layer of an industrial control system, where the manipulated inputs are used to make the controlled variables follow the desired set point values.
direct converter
a frequency converter that converts an RF signal to a baseband signal directly in receivers. It converts a baseband signal to an RF signal directly in transmitters.
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 drive
a drive in which no gear reducer is used.