Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

DC link capacitor
a device used on the output of a rectifier to create an approximately constant DC voltage for the input to the inverter of a variable speed AC drive.

DC machine
an electromechanical (rotating) machine in which the field winding is on the stator and carries DC current, and the armature winding is on the rotor. The current and voltage in the armature winding is actually AC, but it is rectified by the commutator and brushes.
DC-AC inverter

See inverter
definite time DC motor acceleration
when DC motors accelerate during their starting sequence, starting resistors are removed from the armature circuit in steps. In definite time DC motor acceleration (also referred to as open loop DC motor acceleration), the starting resistors are removed in definite time increments, whether the motor is actually accelerating or not.
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.
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).
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 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-axis subtransient reactance
the high-frequency asymptote of the d-axis operational impedance of a synchronous machine. This value characterizes the equivalent reactance of the d axis of the machine during the initial time following a system disturbance. In models in which the rotor windings are represented as lumped parameter circuits, the d-axis subtransient 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 d-axis rotor leakage reactances.
dissipation factor (DF)
the ratio of the effective series resistance of a capacitor to its reactance at a specified frequency measured in percentage.
See loss tangent
dust-ignition-proof machine
a machine designed with a casing or specialized enclosure to safely contain any internal ignition or flammable substances or components, and prevent them from igniting external flammables such as explosive gases, vapors, and dust particles.
electric furnace
a method of smelting metals and applying high heat for industrial processes which makes use of the heat from an electric arc struck between (typically) carbon electrodes.
electrolytic capacitor
a capacitor solution between two electrodes or plates of a capacitor, at least one of which is covered by a dielectric.
electron impact excitation
excitation of an atom or molecule resulting from collision by an electron.
electronically commutated machine
a DC machine with rotor-mounted permanent magnets, and concentrated (square) stator windings. The machine fundamentally behaves like a DC machine with linear speed and torque characteristics, and hence is also called a brushless DC machine. The machine is distinguished from conventional DC machines in its substitution of a six-switch inverter for brushes and copper commutator bars and in its need for the rotor position information in real time.
energy compaction
in a transformation, the concentration of the input signal energy into a relatively small part of the transformed signal. A linear transform of a random vector compacts the signal energy when the energy or variance of a small number of transform coefficients is large relative to the variance of the other coefficients.
equivalent reactance
the reactance of the windings of an electromagnetic machine reflected to one side (component) of the machine.
See equivalent impedance
explosion-proof machine
National Electrical Manufacturers Association (NEMA) classification describing an electrical machine that is totally enclosed and whose enclosure is designed to withstand an internal explosion of a specified gas or vapor that may accumulate within the enclosure. The specification also requires that the design prevent ignition of the specified gas or vapor surrounding the enclosure due to sparks, flashes, or explosions of the specified gas within the enclosure.
externally vented machine
classification describing an electrical machine constructed with an open frame in which ventilation air is forced through the machine by blower(s) mounted outside the machine enclosure.