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Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

T
common symbol for temperature, usually expressed in degrees Kelvin.
T-bracket
a metal frame which holds a lightning arrester and a cut-out to the top of a utility pole.
T-connection
term often used to describe, with some ambiguity, two distinct transformer connections — one to simply convert voltage levels in a 3-phase power system, and the other to convert between 3-phase and a 2-phase voltages. Both connections use only two single-phase transformers, one called the main and the other the teaser, arranged in a T
configuration. Details of each configuration are described below. These connections are also often referred to as Scott connections, since they were first proposed by Charles F. Scott.

Conversion of 3-phase voltage levels: In this configuration, the main transformer in the T-connection is a center-tapped unit that is connected between two lines of the three-phase system. The teaser transformer is connected between the center-tap of the main transformer and the third line of the 3-phase power system. Additionally, the coils of the
teaser transformer have 86.6% of the turns in the corresponding coils in the main transformer. The result is a balanced three-phase voltage on the secondary. In most applications, the main and teaser transformers are actually identical, full voltage units with center taps and two 86.6% taps, one with respect to each terminal. This allows main and teaser units to be interchanged, plus it provides for true, 3-phase, 4-wire system with a neutral connection.
See teaser transformer
tachometer
a instrument used to measure the speed of a rotating device. Several types of tachometers are available. Friction devices are placed against the shaft of the device being measured. Others used magnetic variation or reflected light pulses to determine the speed. Tachometer generators are mounted on the shaft of the device being measured and provide a voltage proportional to the speed. Tach generators are often used in servo systems.
tachometer-generator
a small generator that is connected to the shaft of a rotating machine and produces an output voltage directly proportional to the rpm of the machine. Typically used for closed-loop speed control.
tag
(1) that part of a memory address held in a direct mapped or set associative cache next to the corresponding line, generally the most significant bits of the address.

(2) a field attached to an object to denote the type of information stored in the object. The tag can flag control objects to prevent misuse. Tags can be used to identify the type of each object and thereby to simplify the instruction set, since, for example, only one ADD instruction would be necessary if each numeric object were tagged with its type (integer vs. real, for example).

(3) a temporary sign which is affixed to a network device to identify particular instructions. An example of this might be placing a tag which indicates "Do Not Close" on a circuit breaker which has been opened to permit downstream work.
tail biting
a frame-by-frame transmission scheme for which the data is convolution-ally encoded so that the encoder begins and ends in the same state, which, however, is unknown to the decoder. The advantage of this scheme is that no tail (overhead) is added to the data to force the encoder into a (by the decoder) known state. Compare with fractional rate loss.
tank
the container for the coils and core of a transformer, which is usually oil-filled for insulation and cooling.
tap
a connection (actually one of several) to a coil, allowing the number of turns in the coil to be varied.
tap changer
a device to change the tap setting on a transformer coil, allowing voltage control. See also tap, tap changing under load.
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