Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

DC motor
a motor that operates from a DC power supply. Most DC motors have a field winding on the stator of the machine that creates a DC magnetic field in the airgap. The armature winding is located on the rotor of the machine and the DC supply is inverted by the commutator and brushes to provide an alternating current in the armature windings.

DC motor drive
a converter designed to control the speed of DC motors. Controlled rectifiers are generally used and provide a variable DC voltage from a fixed AC voltage. Alternatively, a chopper, or DC-DC converter, can be employed to provide a variable DC voltage from a fixed DC voltage.
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).

DC servo drive
a feedback, speed control drive system used for position control. Servos are used for applications such as robotic actuators, disk drives, and machine tools.
DC test
tests that measure a static parameter, for example, leakage current.
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.
DC-AC inverter

See inverter
DC-DC converter
a switching circuit that converts direct current (DC) of one voltage level to direct current (DC) of another voltage level. A typical DC-DC converter includes switches, a low pass filter (to attenuate the switching frequency ripple), and a load. The size of magnetic components and capacitors can be reduced and bandwidth can be increased when operating at high frequency. Most DC-DC converters are pulse-width modulated (PWM), while resonant or quasi-resonant types are found in some applications. Commonly used topologies include the buck converter, boost converter, buck-boost converter, and Cuk converter. Isolation can be achieved by insertion of a high frequency transformer.
the removal of some government controls on public utilities, generally including the unbundling of certain services, the dismantling of vertically-integrated utilities, and the introduction of competition among various utility companies for customer services.
dead band
(1) the portion of the operating range of a control device or transducer over which there is no change in output.
(2) referring to an automatic controller behavior, a range of values of the controlled variable in which no corrective action occurs. This type of controller behavior is responsible for the time lag, called dead zone lag, which can cause instability of the controlled system if other conditions are present.

See dead zone
dead end
an installation in which an electric power line terminates at a pole or tower, typically for purposes of structural stability.
dead man
(1) a stand on which to rest a utility pole when setting the pole by hand.
(2) a buried log used as a guy anchor. dead-end shoe a fixture for securing a wire or strain insulator to a utility pole.
dead tank breaker
a power circuit breaker where the tank holding the interrupting chamber is at ground potential. Oil circuit breakers, for example, are typically dead tank breakers.
dead zone

See dead band
decibel (dB)
a unit of measure that describes the ratio between two quantities in terms of a base 10 logarithm. For example, the ratio between the power level at the input and output of an amplifier is called the power gain and may be expressed in decibels as follows:

G(dB) = 10 log10 ( Pout / Pin )

Terms such as dBm, dBuV, dBW indicate that the decibel measurement was made relative to an established standard. A common power measure reference is 0 dBm, which is defined to be 1 mW (milliwatt, 0.001W). A common voltage reference is 1 μV (1 microvolt).
from the number system that has base 10 and employs 10 digits.
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.
definite-purpose motor
any motor design, listed and offered in standard ratings with standard operating characteristics, with special mechanical features for use under service conditions other than usual or for use on a particular type of application.
demand meter
an electric meter which shows both the energy used and the peak power demand in a given period.
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.