Page 70 - Electrical Engineering Dictionary

but the transformer is reduced in weight and available power gain ratio of power
size. Autotransformers are most suited for available from a network to the power avail-
relatively small changes in voltage. Three able from the source.
phase autotransformers are by necessity con-
nected in a wye configuration. avalanche breakdown process that oc-
curs in a semiconductor space charge region
autotransformer starter a single three- under a sufficiently high voltage such that the
phase autotransformer or three single phase net electron/hole generation rate due to im-
transformer used to start induction motors at pact ionization exceeds certain critical value,
a reduced voltage. causing the current to rise indefinitely due
to a positive feedback mechanism. The I-R
auxiliary memory See secondary heating caused during this process can per-
memory. manently degrade or destroy the material.
avalanche injection the physics whereby
auxiliary relay a relay employed in
electrons highly energized in avalanche cur-
power system protection schemes that does
rent at a semiconductor junction can pene-
not directly sense fault presence and loca-
trate into a dielectric.
tion. Typical auxiliary relays include lockout
relays, reclosing relays, and circuit breaker
avalanche photodiode (APD) a photodi-
anti-pump relays.
ode (detector) that provides internal current
gain. Used in optical communication sys-
auxiliary winding a winding designed to
tems when there is limited optical power at
be energized occasionally for a specific pur-
the receiver.
pose, such as starting a single-phase motor.
The power to the winding may be controlled
average optical power time average of
by various means including a timer, centrifu-
the optical power carried by a non-CW opti-
gal switch, current sensing relay, or voltage
cal beam.
(counter EMF) sensing relay.
average picture level (APL) describes
availability the probability that a system
the average (mean) changes in a video signal
is operating correctly and is available to per-
due to a changing brightness of the visual im-
form list functions at the instant of time t.
age. The APL is typically expressed in terms
Also defined as the value
ofapercentage(10-15%fordarkpicturesand
75-90% for bright pictures). Changes in the
1 − outage APL can effect linearity unless DC restora-
tion or clamping circuits are included in the
See also outage. video circuitry.
available bit rate (ABR) ATM conges- average power the average value, taken
tion control algorithm that enables a source to over an interval in time, of the instantaneous
discover the bit rate available between it and power. The time interval is usually one pe-
a destination in a network. The source trans- riod of the signal.
mits a resource manager cell containing the
desired bit rate; each switch this cell passes average-value model a mathematical
through adjusts the bit rate down to what it representation in which the average value of
can support. Upon reaching the destination, variables are used to model a system. In elec-
the cell contains the available bit rate and is tricmachinesanddrives, systemvariablesare
returned to the source. typically averaged over various switching in-

c
2000 by CRC Press LLC