Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

three-lamp synchronizing
a method used to connect a three-phase power system in parallel to another one. In order to connect two systems, they must have the same voltage magnitude, frequency, and phase-shift. To determine that is the case, an open switch is connected between the phases of the two systems and a lamp is connected across the open switch pole in each phase.

If the criteria previously listed are met, the lamps will all be dark. If there is a difference in voltage, the lamps will glow. If there is a difference in frequency, the lamps will alternately glow and go dark in unison. Finally, if the two sides have different phase rotations, the lamps will blink sequentially as only one phase can be aligned at a time. In order to synchronize the two systems, it is necessary to close the contactor when the phase-shift is minimum, which means that the three lights are dark.
three phase fault
a fault on a three phase power line in which all three conductors have become connected to each other and possibly the ground as well.
three-phase inverter
an inverter with a three-phase AC voltage output.
three-phase rectifier
a rectifier with a three-phase AC voltage input.
three-point starter
a manual DC motor starter in which a handle is pulled to start the motor. The motion of the handle causes a contact to move across a variable resistance in the armature circuit to limit the starting current. When the handle is moved to its fullest extent, the resistance is out of the armature circuit and an electromagnet holds the handle in place. With a three-point starter, the electromagnet is in series with the shunt field and loss of the field will shut down the motor. The disadvantage is that if it is desired to weaken the field for speed control, the motor starter may drop out. Also, the three-point starter cannot be used on a series DC motor.
thyristor-controlled phase angle regulator
a phase shifting device used in transmission systems. The phase angle change is brought about by thyristor-based control.
thyristor-controlled series compensator
a capacitor bank installed in series with an electric power transmission line in which each capacitor is placed in parallel with a thyristor device. Each capacitor may thus be switched in or out of the line for some variable portion of the AC cycle so as to maintain the line's maximum power-carrying ability under varying load conditions.
time response
the system response in the time domain when a reference input signal is applied to a system. The time response of a control system is usually divided into two parts: the transient response and the steady-state response.
time-dependent dielectric breakdown
breakdown of a dielectric is marked by a sudden increase in current when an electric field is applied. The breakdown does not occur immediately upon application of the electric field, but at a period of time later that depends exponentially upon the magnitude of the field.
time-to-open contact
a contact in which the desired time to open the contactor could be set by the user.
torque pulsation
oscillating torque produced by the interaction between the air gap flux, consisting mainly of the fundamental component, and the fluxes produced by harmonics in the rotor. Torque pulsations can stimulate complex mechanical vibrations that can flex and damage rotor and turbine elements.
torque ripple
in variable speed motor drives, refers to the torque not being smooth as the rotor moves from one position to another. Torque ripple may be produced from space harmonics within the machine or time harmonics generated by the supply.
transient impedance
the series impedance that a generator or motor exhibits following the subtransient period but prior to the steady-state situation.
transient open-circuit time constant
See quadrature-axis transient open-circuit time constant and direct-axis transient open-circuit time constant.
transient operation
a power system operating under abnormal conditions because of a disturbance.
transient suppressor
a device connected to a piece of sensitive electrical equipment to reduce the amplitude of transient voltage excursions, thus protecting the equipment.
transmission line coupler
passive coupler composed of two or more transmissions spaced closely together where the proximity of the transmission lines allows signals to be coupled or transferred in part from one line to the other. The electrical length of the transmission lines is usually one quarter of a wavelength.
transposition
(1) the practice of twisting a three-phase power line so that, for example, phase A takes the place on the tower formerly occupied by phase B, phase B takes the place of phase C, and phase C occupies the former position of phase A.

(2) a point on a three-phase electric power line where the conductors are physically transposed for purposes of improving circuit balance transputer a class of CPU designed and manufactured by Inmos Corporation. The transputer was specifically designed to be used in arrays for parallel processing.
triplex cable
a cable used for residential or commercial service drops consisting of two or three insulated conductors spiralled around a bare neutral wire which provides support for the cable.
twelve-pulse converter
the combination of two 6-pulse converters connected through a Y-Y and a delta-Y transformer in order to cancel the characteristic 5th and 7th harmonics of the 6-pulse converters. The lowest characteristic harmonics with twelve-pulse converters under balanced conditions are the 11th and 13th harmonics. The converters are connected in parallel on the AC side and in either series or parallel on the DC side, depending on the required DC output voltage.
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