Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

critical clearing angle
(1) following a balanced three-phase fault at the stator terminals of a synchronous machine, the maximum value of the angular position of the rotor prior to the removal (clearing) of the fault such that the rotor will obtain synchronous speed without slipping poles following the removal (clearing) of the fault. The corresponding time for the rotor to achieve this angle is specified as the critical clearing time.
(2) the largest allowable angular deviation from synchronism that may be borne by a power system such that the system remains stable: the edge of instability.
crossarm
a transverse, generally insulated member mounted horizontally on a utility pole. It carries insulators and allows wide spacing of overhead conductors.
crossarm brace
a brace, often insulated, which keeps a crossarm from rotating on its attachment bolts.
CT

See current transformer
CU

See coefficient of utilization
cumulatively compounded
a compound-wound DC machine in which the flux produced by the MMF of the shunt field winding and the flux produced by the MMF of the series field winding are in the same direction.
current
the flow of charge, measured in amperes (1 ampere = 1 coulomb/s).
current density vector field
the field (commonly denoted J ) that is related to the electric field intensity vector field by the conductivity of the medium that the fields are located in. One of the quantities found on the right side of Ampere's Law. The units are (amperes/square meter).
current distribution factor
in economic dispatch studies, the proportion of a power line's total current which is contributed by a particular generating plant.
current limiting
the output current is limited to a preset level even under a shorted output condition. This can be accomplished by reducing the output voltage to prevent the current limit from being exceeded.
current limiting fuse
a fuse that limits the level of fault current from that which is available. It operates by developing a substantial voltage across the fuse following the melting of the fuse element.
current regulator
a device used to control the magnitude and phase of the current in DC, AC or other electrical variable speed drives. May use different control strategies like hysteresis current control or ramp comparison current control.
current transducer
a device used to measure current in a variety of applications including variable speed drives. May give out a current proportional to the measured current or a voltage proportional to the measured current. Electrical isolation may be obtained by using a current transformer (cannot be used at DC) or a hall effect transducer that can be used down to DC.
current transformer (CT)
(1) a transformer that is employed to provide a secondary current proportional to primary current flowing. The primary "winding" is often created by passing the system conductor or bus bar through an opening in the device
and the secondary is typically rated at a standard value to match common meters and display units. Current transformers are used in current measurement, protective relays, and power metering applications. The load (meter) on a CT should never be removed without first shorting the secondary of the CT, otherwise dangerous voltage levels may result when the load is removed.
(2) a device which measures the instantaneous current through a conductor of an electric power line and transmits a signal proportional to this current to the power system's instrumentation. current unit a protective relay that monitors the magnitude of a power transmission line's current flow.
current withstand rating
the current withstand rating of a device is the maximum short term current that can flow in the device without causing damage.
See ampacity
current-limiting device
when operating within its current-limiting range, a current-limiting device is open in 0.25 cycle or less and limits the maximum short-circuit current to a magnitude substantially less than the short-circuit current available at the fault point.
cylndrical-rotor machine
a synchronous machine with a cylindrical rotor containing a distributed field winding and an essentially uniform air-gap. This design is limited to two and four pole machines (3600 and 1800 rpm at 60 Hz) and is usually used in large generators.
See salient-pole rotor machine
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.

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