Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

Thomson
Thomson, William (Lord Kelvin) (1824-1907) Born: Belfast, Ireland

is best known as a physicist who championed the absolute temperature system that now bears his name (Kelvin). Thomson did significant work to expand Faraday's ideas. It was Thomson's work that Maxwell would extend into his seminal publications on electromagnetics. Thomson received a knighthood for his theoretical suggestions for the use of low-voltage signals in
the trans-Atlantic telegraph cable. Thomson was proved correct when on a third attempt, a cable was laid and worked. An earlier high-voltage cable had failed.
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.
three-winding transformer
a transformer with three windings, typically primary, secondary, and tertiary. Common three phase, three winding transformers employ wye connected primary and secondary windings and a delta connected tertiary winding.

In some cases, an autotransformer is used to form the primary-secondary combination.
thyristor rectifier
a rectifier where the switches are thyristors. Thyristors are turned on by a gate trigger signal, and turned off by natural commutation. The output voltage is controllable by adjusting the firing angle of the trigger signal. The direction of the power flow is reversible when an inductive load is used. When the average power flow is from DC to AC, the rectifier is said to be operating in the line-commutated inverter mode.
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.
tie switch
a disconnect switch used on feeders and laterals to reconfigure distribution circuits to allow for line maintenance.
time delay
a time-current response characteristic, established by national standards, which means that a time-delay fuse is designed to carry five times rated current for 10 seconds before opening.
See envelope delay
time overcurrent (TOC) relay
an over-current relay that has intentional, selectable, time delay. The time delay is chosen so that the relay will operate more slowly than downstream relays or fuses, and more quickly than upstream relays or transformer fuses. Relay and fuse curves are generally displayed on time-current curves.
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-current characteristic curve
(1) a relay time-current curve is a curve showing the time versus current characteristic of a time overcurrent (TOC) relay.

(2) a fuse time-current curve shows the melting and clearing times of a given fuse or family of fuses.

(3) a coordination time-current curve shows the relationship of the operating and clearing characteristics of coordinating devices (TOC relays and fuses) on a power system.
time-current curve

See time-current characteristic curve
time-delay fuse

See dual-element fuse
time-delay relay
relay that responds with an intentional time delay.

1. in control circuits, time-delay relays are used to cause a time delay in the state of the relay when power is applied or removed to the relay actuator;

2. in power system protective relays, the response time usually depends on the magnitude of the measured value. If the measured value is a large multiple of the pickup value, then the relay operates or trips after a short time delay. For smaller multiples of pickup, the relay trips after a longer time delay.
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-close contact
a contact in which the desired time to close the contactor could be set by the user.
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