Page 196 - Electrical Engineering Dictionary

the stator and carries DC current, and the ar- the video by establishing the DC level of the
mature winding is on the rotor. The current sync tips.
and voltage in the armature winding is actu-
ally AC, but it is rectified by the commutator DC servo drive a feedback, speed control
and brushes. drive system used for position control. Ser-
vos are used for applications such as robotic
DC motor a motor that operates from a actuators, disk drives, and machine tools.
DC power supply. Most DC motors have a
field winding on the stator of the machine DC test tests that measure a static param-
that creates a DC magnetic field in the airgap. eter, for example, leakage current.
The armature winding is located on the rotor
of the machine and the DC supply is inverted DC to RF conversion efficiency dimen-
by the commutator and brushes to provide an sionless ratio of RF power delivered to the
alternating current in the armature windings. load (p out ) versus total DC input power dis-
sipated in the amplifier (p DC ). With the DC
DC motor drive a converter designed to to RF conversion efficiency given my η DC
control the speed of DC motors. Controlled we have
p out
rectifiers are generally used and provide a η DC =
variable DC voltage from a fixed AC volt- p DC
age. Alternatively, a chopper, or DC–DC
converter, can be employed to provide a vari- DC transconductance for an FET de-
able DC voltage from a fixed DC voltage. vice under DC bias, the slope of the out-
put drain to source current (I DS ) versus in-
DC offset current the exponentially de- put gate to source voltage (V GS )forafixed
caying current component that flows imme- drain to source voltage (V DS ), expressed in
diately following a fault inception. DC off- siemens. Given by g m we have
set is the result of circuit inductance, and is

a function of the point in the voltage wave ∂I DS
g m =
where the fault begins. The offset for a given ∂V GS V DS = constant

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 voltage constant voltage with no vari-
ation over time. This can be considered in
DC restoration reinsertion of lost DC general terms as an alternating current (AC)
level information into a signal after using AC with a frequency of variation of zero, or a
signal coupling; in television applications, zero frequency signal. For microwave sys-
the DC component of a composite video sig- tems, DC voltages are provided by batteries
nal represents the average brightness of the or AC/DC converters required to “bias” tran-
picture. After AC coupling of the compos- sistors to a region of operation where they
ite video, the DC level includes the average will either amplify, mix or frequency trans-
luminance signal plus the fixed average of late, or generate (oscillators) microwave en-
the sync and lanking signals, causing picture ergy.
level racking errors. For a positive video sig-
nal, the average value of mostly white scenes DC–AC inverter See inverter.
will be slightly lower than it should be; for
mostly dark scenes, the DC average could be- DC–DC converter a switching circuit
come negative (due to the sync and blanking that converts direct current (DC) of one volt-
signals) when it should be slightly positive. age level to direct current (DC) of another
Clamping circuits restore the DC average of voltage level. A typical DC–DC converter

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2000 by CRC Press LLC