Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

asynchronous
not synchronous.
asynchronous AC systems
AC systems either with different operating frequencies or that are not in synchronism.
asynchronous bus
a bus in which the timing of bus transactions is achieved with two basic "handshaking" signals, a request
signal from the source to the destination and an acknowledge signal from the destination to the source. The transaction begins with the request to the destination. The acknowledge signal is generated when the destination is ready to accept the transaction. Avoids the necessity to know system delays in advance and allows different timing for different transactions.

See synchronous bus
asynchronous circuit
(1) a sequential logic circuit without a system clock.
(2) a circuit implementing an asynchronous system. asynchronous demodulation a technique for extracting the information-carrying waveform from a modulated signal without requiring a phase-synchronized carrier for demodulation.
See synchronous demodulation
asynchronous machine

See induction machine
asynchronous operation
a term to indicate that a circuit can operate or a communication system can transmit information when ready without having to wait for a synchronizing clock pulse.
asynchronous system
a (computer, circuit, device) system in which events are not executed in a regular time relationship, that is, they are timing-independent. Each event or operation is performed upon receipt of a signal generated by the completion of a previous event or operation, or upon availability of the system resources required by the event or operation.
asynchronous transfer mode (ATM)
method of multiplexing messages onto a channel in which channel time is divided into small, fixed-length slots or cells. In ATM systems the binding of messages to slots is done dynamically, allowing dynamic bandwidth allocation. ATM is asynchronous in the sense that the recurrence of cells containing information from an individual user is not necessarily periodic.
direct axis synchronous reactance
the sum of the stator winding leakage reactance and the direct-axis magnetizing (armature) reactance of a synchronous machine. This represents the balanced steady-state value of the direct-axis operational impedance of the synchronous machine, and thus characterizes the equivalent reactance of the machine during steady-state operation.
permanent magnet synchronous machine
a polyphase AC motor with rotor mounted permanent magnets and sinusoidal distribution of stator phase windings. The field windings in the rotor are replaced by permanent magnets to provide the field excitation in these machines.
subsynchronous resonance
an electric power system condition where the electric network exchanges energy with a turbine generator at one or more of the natural frequencies of the combined system below the synchronous frequency of the system.
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.
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