Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

spectrum
(1) a range of electromagnetic energy ordered in accordance with their relative periodicity.

(2) the magnitude of the Fourier transform of a (deterministic) signal. The word spectrum is also used to refer to the power spectrum of a random process. spectrum analyzer a test system that measures RF or microwave devices in terms
of signal frequency and signal power.
speed droop
a linear characteristic that is provided to governors of two or more units operating in parallel for stable load division in case of load increase.
speed of light
(1) a scalar constant in vacuum roughly equal to 3x108 meters per second.

(2) the phase velocity representing the rate of advance of the phase front of a monochromatic light wave.
speed range
the minimum and maximum speeds at which a motor must operate under constant or variable torque load conditions.

A 4:1 speed range for a motor with a top speed of 1800 rpm means that the motor must be able to operate as low as 450 rpm and still remain within regulation specifications. The controllable speed range of a motor is limited by the ability to deliver required torque below base speed without additional cooling.
speed regulation
the variation of the output speed of an electromechanical device as the load on the shaft is increased from zero to some specified fraction of the full load or rated load. Usually expressed as a percentage of the no-load speed. A large speed regulation is most often considered as a bad regulation from a control point of view.
speed sensor
a device used to detect the speed of the rotor of an electric machine. Optical (strobe) and electromagnetic tachometers are commonly used.
speed servo
a servo where the speed is the controlled parameter.
See servo
sphere gap
a spark gap whose electrodes are metal spheres. A sphere gap with carefully-calibrated electrode spacing is used as a measuring instrument for voltages in the kilovolt to megavolt range.
spike

See surge
spike suppressor
any of several devices e.g., metal-oxide varistors that clamp short-duration power line overvoltages to an acceptable level.
spiral inductor
an integrated circuit implementation of a common electrical element that stores magnetic energy. Two extreme behaviors of an inductor are that it will act as a short circuit to low frequency or DC energy, and as an open circuit to energy at a sufficiently high frequency (how high is determined by the inductor value).

In an MMIC, a spiral inductor is realized by a rectangular or circular spiral layout of a narrow strip of metal. The value of the inductance increases as the number of turns and total length of the spiral is increased. Large spiral inductors are very commonly used as "bias chokes" to isolate the DC input connection from the RF circuit. Since a large valued inductor essentially looks like an open circuit to high frequency RF/microwave energy, negligible RF/microwave energy will leak through and interact with the DC bias circuitry.
splice
a permanent connection between two fibers made by melting or fusing the two fibers together in an electric arc or gas flame. Or they may be held together in a variety of mechanical devices that align the two fiber cores. In fusion splicing, connections can be achieved with losses < 0.1 dB.
SPST

See single-pole single-throw
spur
a conductor which branches off of a main line.
static var compensator
a device for fast reactive compensation, either inductive or capacitive, brought about by thyristor-based control of an effective shunt susceptance. It is typically used to regulate voltage at a bus on the high voltage transmission system.
step voltage
in power system safety studies, the voltage measured across two points on the ground which are separated by a distance equal to an average person's step while walking over the area in question.
step-down converter

See buck converter
step-up converter

See boost converter
subtractive polarity
polarity designation of a transformer in which terminals of the same polarity on the low-and high-voltage coils are physically opposite each other on the transformer casing. With subtractive polarity, a short between two adjacent terminals results in the difference of the two coil voltages appearing between the remaining terminals.
Subtractive polarity is generally used on transformers larger than 500 kVA and higher than 34.5 kV. Smaller units use additive polarity.
See additive polarity
subtransient impedance
the series impedance that a generator or motor exhibits during the subtransient period, typically the first few cycles of a fault. Subtransient impedances are generally used in calculating fault currents for determining instantaneous relay settings.
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