Page 188 - Electrical Engineering Dictionary

current transducer a device used to mea- take turns providing the conducting path for
sure current in a variety of applications in- the current sources, and the current in the
cluding variable speed drives. May give out a timing capacitor changes direction each half
current proportional to the measured current of the period. If the current sources are
or a voltage proportional to the measured cur- controlled by an external voltage, the circuit
rent. Electrical isolation may be obtained by becomes a voltage-controlled multivibrator.
using a current transformer (cannot be used Current- and voltage controlled multivibra-
at DC) or a hall effect transducer that can be tors are a vital part of phase-locked loops and
used down to DC. frequency synthesizers.

current transformer (CT) (1) a trans- current-limiting device when operating
former that is employed to provide a sec- within its current-limiting range, a current-
ondary current proportional to primary cur- limiting device is open in 0.25 cycle or less
rent flowing. The primary “winding” is of- and limits the maximum short-circuit cur-
ten created by passing the system conductor rent to a magnitude substantially less than
or bus bar through an opening in the device the short-circuit current available at the fault
and the secondary is typically rated at a stan- point.
dard value to match common meters and dis-
play units. Current transformers are used in current-mode control the inductor cur-
current measurement, protective relays, and rent is sensed to control the pulse width.
power metering applications. The load (me- The switch is turned on by a constant fre-
ter) on a CT should never be removed without quency clock and turned off when the in-
first shorting the secondary of the CT, oth- ductor current reaches the control reference.
erwise dangerous voltage levels may result This method produces subharmonic oscilla-
when the load is removed. tions if the steady-state duty ratio is greater
(2) a device which measures the instanta- than or equal to 0.5. An artificial stabilizing
neous current through a conductor of an elec- ramp is necessary to achieve stable control
tric power line and transmits a signal propor- over the full range of duty ratios. When the
tional to this current to the power system’s slope of the ramp is equal to the falling slope
instrumentation. of the inductor current, one cycle response is
achieved. (i.e., the input perturbations are re-
current unit a protective relay that mon- jected, and the current reference is followed
itors the magnitude of a power transmission within one cycle).
line’s current flow.
current-source inverter (CSI) an in-
current withstand rating the current verter with a DC current input. Current-
withstand rating of a device is the maximum source inverters are most commonly used in
short term current that can flow in the device very high power AC motor drives.
without causing damage. See ampacity.
cursor (1) the symbol on a computer
current-controlled multivibrator a mul- screen that indicates the location on the
tivibrator with oscillation period controlled screen that subsequent input will affect.
by the current that is charging and discharg- (2) a movable, visible mark used to indi-
ing a timing capacitor. Most of the circuits cateapositionofinterestonadisplaysurface.
are based on two symmetric controlled cur- (ANSI).
rent sources connected by a timing capaci-
tor, and a steering circuit (connected between curvature a geometric property that de-
power supply voltage and this capacitor), scribes the degree that a surface or a curve is
which includes two switches. The switches bent. The curvature of a curve is the magni-

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