Page 152 - Electrical Engineering Dictionary

cal contacts that turn with the machine ro- the machine. It also, enables reversal of
tor. The contacts are the connection points current in the armature winding. See also
in a series-connected loop of the coils that commutation.
make up the rotor winding. The brushes,
sliding over these contacts, continually di- commutator film an oxide layer on
vide the loop into two parallel electrical paths the commutator surface, indicated by a dark
between the brushes. color or a “film,” that is required for proper
The brushes are positioned such that they commutator action and full loading of the
make contact with those commutator seg- machine. On a new DC machine commu-
ments that are connected to coils that are tator, or on a commutator that has just been
moving through a magnetic neutral point be- stoned, there is no “film” on the commutator.
tween poles of the machine’s field flux. As a It is advisable to refer to the manufacturer’s
result, all coils making up one parallel path technical manual for the proper procedure to
are always moving under a north magnetic “break in” the commutator and develop the
pole, and the others are always moving under film so the machine can be operated at rated
a south magnetic pole. The movement of the conditions.
commutator contacts underneath the brushes
automatically switches a coil from one path compact disk (CD) a plastic substrate
to the other as it moves from a north pole re- embossed with a pattern of pits that encode
gion to a south pole region. Since the coils audio signals in digital format. The disk is
in both paths move in the same direction, but coated with a metallic layer (to enhance its
through opposite flux regions, the voltages reflectivity) and read in a drive (CD player)
induced in the two paths are opposite. Con- that employs a focused laser beam and mon-
sequently, the positive and negative ends of itors fluctuations of the reflected intensity in
each path occur at the same points in the se- order to detect the pits.
ries loop, which are at the points where the
brushes contact the commutator. The brush compact disk-interactive (CD-I) a spec-
positions, thus, represent a unidirectional (or ification that describes methods for providing
DC) connection to the rotating coil. See also audio, video, graphics, text, and machine-
commutator. executable code on a CD-ROM.

commutation angle time in electrical de- compact range an electromagnetic mea-
grees from the start to the completion of the surement facility in which far-field condi-
commutation process. Also called overlap tions are achieved by the use of an offset
angle. parabolic reflector. The reflector is fed using
a source antenna or other subreflector sys-
commutativity a property of an opera- tem located at its focus. The term “compact”
tion; an operation is commutative if the re- range is used to describe the relative differ-
sult of the operation is not affected by any ence in its size compared to a true far-field
reordering of the operands of the operation. range requiring a large separation distance
Additions and multiplication are commuta- between the source antenna and the device
tive, whereassubtractionanddivisionarenot. under test to achieve the same far-field con-
ditions.
commutator a cylindrical assembly of
copper segments, insulated from each an- compactness measure an alternative
other, that make electrical contact with sta- name for circularity measure.
tionary brushes, to allow current to flow
from the rotating armature windings of a compander a point operation that loga-
DC machine to the external terminals of rithmically compresses a sample into fewer

c
2000 by CRC Press LLC