Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

diversity selection
a form of diversity reception in which the receiver selects the strongest signal among the copies received. The weaker signals are simply ignored.
equipment misoperation due to an interruption, noise, or sag.
dropout current
the current at which a magnetically-operated device will revert to its de-energized position.
dropout voltage
the voltage level where proper equipment operation is hindered.
dual-cage rotor
a three-phase induction motor rotor with two separate squirrel cage windings, that give the effect of varying rotor resistance. The outer cage has high resistance to obtain high starting torque, while the inner cage has low resistance to reduce losses at full load.
dual-element fuse
a fuse constructed with two different types of fusible elements in series. One element is designed to melt very quickly in the presence of fault current. The other is designed to melt after a time delay when exposed to overload conditions. The fusible elements are somewhat similar in operation to the thermal and magnetic elements of an inverse-time circuit breaker. Dual-element and time-delay are often used interchangeably.
a method of winding the armature of a commutated electric machine such that the number of parallel electrical paths between brushes is double that provided by a simplex winding. Duplex windings are constructed by placing consecutive coils in alternate coil slots and continuing the winding twice around the rotor, filling the empty slots on the second pass. The result is two complete, identical windings between brush positions rather than the one winding that is produced when coils are placed in adjacent slots.
See simplex
dust cover
a cover to protect the terminals of a pad-mount transformer.
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.
duty cycle
(1) the ratio of the turn-on time of a semiconductor switch to the sum of the turn-on and turn-off times;

Turn-on / (Turn-on + Turn-off)

(2) the mode of operation that an electric machine is classified, in consideration of thermal limits, e.g., continuous operation, intermittent operation.

See duty ratio
duty ratio

See duty cycle
dynamic brake
the braking operation of a machine by extracting electrical energy and then dissipating it in a resistor.
dynamic braking
since AC motors do not have separate field excitation, dynamic braking is accomplished by continuing to excite the motor from the drive. This causes a generative current to the drives DC intermediate bus circuit. The dynamic brake resistors are then placed across the DC bus to dissipate the power returned. The brake resistor is usually switched by a transistor or other power switch controlled by the drive.
a term used to describe any of a variety of rotating machines that convert mechanical to electrical energy, or less commonly, electrical to mechanical energy. Dynamos typically consist of a stationary structure, called the stator, supporting a rotating element called the rotor. Energy conversion occurs via Faraday induction. A field winding (or in some smaller machines, permanent magnets) is mounted on one of the mechanical structures and produces a magnetic flux.

An armature winding is mounted on the other structure, and rotation of the rotor produces relative motion between the field flux and the coils of an armature winding, inducing a Faraday voltage in the armature coil. This Faraday induced voltage is the source of electrical energy at the dynamo output.
a rotating device used to measure the steady-state torque and power output of rotating machines. Dynamometers
generally provide precise control of the load torque applied to a test machine, and power output is determined through precise speed measurements.