Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

synchro
also called a selsyn (for self-synchronous). An AC servo machine used in pairs primarily for remote sensing and shaft positioning applications. Its construction is essentially that of a wound-rotor induction machine with either a single-phase or 3-phase rotor winding. Various stator and rotor interconnections are possible, depending on desired function and required torque.
synchro-check relay
a device used to monitor the frequency and phase angle of the voltages across an open circuit breaker.

Synchro-check relays are commonly used to prevent breaker closing or reclosing on excessive voltage or frequency difference.
synchronization
(1) a situation when two or more processes coordinate their activities based upon a condition.

(2) the process of determining (usually channel) parameters from a received signal, for example carrier frequency offset, carrier phase, or symbol timing.
synchronizing coefficient
electrical torque component in phase with the rotor angle.
synchronizing relay
a relay that monitors the voltage across an open circuit breaker to determine the frequency and phase relationship of the voltage sources on either side of the breaker. Synchronizing relays are used on generator breakers to bring the generator to the system frequency and to match the phase angle between the generator and system prior to closing the breaker.
synchronous condenser
an unloaded, over-excited synchronous motor that is used to generate reactive power.
synchronous machine
an AC electrical machine that is capable of delivering torque only at one specific speed (ns), which is determined by the frequency of the AC system (f) and the number of poles (P) in the machine. The relationship between synchronous speed and the other variables is

ns = 120f/P
synchronous motor
an AC motor in which the average speed of normal operation is exactly proportional to the frequency to which it is connected. A synchronous motor generally has rotating field poles that are excited by DC.
synchronous operation
an operation that is synchronized to a clocking signal.
synchronous reactance
the inductive reactance of the armature windings in synchronous machines under steady-state conditions. Designated by the symbol Xs,expressed in ohms per phase, the synchronous reactance is a function of the stator inductance and the frequency of the stator currents.
synchronous reference frame
a two dimensional space that rotates at an angular velocity corresponding to the fundamental frequency of the physical stator variables (voltage, current, flux) of a system.
In electric machines/power system analysis, an orthogonal coordinate axis is established in this space upon which fictitious windings are placed. A linear transformation is derived in which the physical variables of the system (voltage, current, flux) are referred to variables of the fictitious windings. See also arbitrary reference frame, rotor reference frame
See stationary reference frame
synchronous reluctance machine
a type of synchronous machine that has no rotor winding. The rotor consists of salient poles, which causes the reluctance to vary as a function of position around the airgap. When operated as a motor, a rotating magnetic field is established by the stator windings that causes a reluctance torque on the rotor as the path of lowest permeability stays aligned with the peak of the stator flux wave.
synchronous reluctance motor
a synchronous motor that depends on a reluctance variation on the rotor for the mechanism of torque production. The rotor shape is designed to provide a high difference in the reluctances between the d and q axes.
synchronous speed
speed of the rotating magnetic flux produced by three-phase currents in stationary coils in three-phase AC machines. The synchronous speed is calculated by a knowledge of the number of poles of the machine and the frequency of the stator currents as

Ns = 120fs/P
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