Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

D'Arsenval meter
a permanent-magnet moving-coil instrument with a horseshoe form magnet. It measures direct current only.

damper winding
an uninsulated winding, embedded in the pole shoes of a synchronous machine, that includes several copper bars short-circuited by conducting rings at the ends, used to reduce speed fluctuation in the machine by developing an induction-type torque that opposes any change in speed.

damping
a characteristic built into electrical circuits and mechanical systems that prevents rapid or excessive corrections that may lead to instability or oscillatory conditions.

damping coefficient
electrical torque component in phase with the rotor speed.

dB
See decibel
dBc
ratio of the signal power (p) to a reference signal power (p_{ref}), usually the modulation carrier signal, expressed in decibels referenced to a carrier (dBc). Thus a harmonic signal that is 1/100th of the power in a desired fundamental signal is at -20 dBc.

P_{dBd} = 10 log_{10} ( p / p_{ref} )

dBm
Power ratio in decibels referenced to 1 milliwatt.

dBW
ratio of the signal power in watts (p watts)to a 1W reference power, expressed in decibels referenced to 1 W (dBW). Thus

1 watt signal power is equal to 0 dBW, and -30 dBW is equal to 0 dBm.

DC
direct current

See DC voltage
DC block
A circuit simulation component that behaves like a capacitor of infinite value.

DC chopper
a DC to DC converter that reduces the voltage level by delivering pulses of constant voltage to the load. The average output is equal to the input times the duty cycle of the switching element.

DC circuit
electrical networks in which the voltage polarity and directions of current flow remain fixed. Thus such networks contain direct currents as opposed to alternating currents, thereby giving rise to the term.

DC current constant
current 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 currents 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 generator commutator exciter
a source of energy for the field winding of a synchronous machine derived from a direct current generator. The direct current generator may be driven by an external motor, a prime mover, or by the shaft of the synchronous machine.

DC input power
the total DC or bias power dissipated in a circuit, which is usually dependent on signal amplitudes, expressed in watts. This may include input bias, bias filtering, regulators, control circuits, switching power supplies and any other circuitry required by the actual circuit. These considerations should be explicitly specified, as they will affect how efficiency calculations are performed.

DC link
the coupling between the rectifier and inverter in a variable speed AC drive.

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 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.

DC load flow
a fast method of estimating power flows in an electric power system in which the problem is reduced to a DC circuit, with line impedances modeled as resistances and all generator bus voltages presumed to remain at their nominal values.

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.