Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

reciprocity
(1) a consequence of Maxwell's equations, stipulating the phenomenon that the reaction of the sources of each of two different source distributions with the fields generated by the other are equal, provided the media involved have certain permeability and permittivity properties (reciprocal media). Referring to reciprocal circuits, reciprocity
states that the positions of an ideal voltage source (zero internal impedance) and an ideal ammeter (infinite internal impedance) can be interchanged without affecting their readings.

(2) in antenna theory, the principal that the receive and transmit patterns of an antenna are the same.

reciprocity in scattering law according to which the source and detector points can be exchanged, providing the source amplitude and phase are preserved.
reciprocity theorem
in a network consisting of linear, passive impedances, the ratio of the voltage introduced into any branch to the current in any other branch is equal in magnitude and phase to the ratio that results if the positions of the voltage and current are interchanged.
recloser
a self-contained device placed on distribution lines that senses line currents and opens on overcurrent. Reclosing is employed to reenergize the protected line segment in the case of temporary faults. Reclosers have the capability for fast tripping for fuse saving, and slow tripping to allow sectionalizing fuse operation for faults on laterals. The recloser will retrip on permanent faults and go on to lockout. Reclosers are suitable for pole mounting on overhead lines.
reclosing relay
an auxiliary relay that initiates circuit breaker closing in a set sequence following fault clearing. Reclosing relays are typically employed on overhead lines where a high proportion of the faults are temporary.
recoil permeability
the average slope of the minor hysteresis loop, which is roughly the slope of the major hysteresis loop at zero applied field (H ), and is most often used to determine the effect of applying and removing a demagnetizing field to and from a magnetic material.
reduced-voltage motor starter
a device designed to safely connect an electric motor to the power source while limiting the magnitude of its starting current. Various electromechanical configurations may be used: primary resistor, delta-wye, part-winding (requires special motor or dual voltage windings). Power electronic devices may also be utilized to gradually increase the applied voltage to system levels. The complete starter must also include fault and overload protection.
regulation
the change in voltage from no-load to full-load expressed as a percentage of full-load voltage.
regulator
a controller designed to maintain the state of the controlled variable at a constant value, despite fluctuations of the load.
relay
a device that opens or closes a contact when energized. Relays are most commonly used in power systems, where their function is to detect defective lines or apparatus or other abnormal or dangerous occurrences and to initiate appropriate control action. When the voltage or current in a relay exceeds the specified "pickup" value, the relay contact changes its position and causes an action in the circuit breaker. A decision is made based on the information from the measuring instruments and relayed to the trip coil of the breaker, hence the name "relay." Other relays are used as switches to turn on or off equipment.
reliability
the probability that a component or system will function without failure over a specified time period, under stated conditions.
reluctance
the resistance to magnetic flux in a magnetic circuit; analogous to resistance in an electrical circuit.
reluctance motor
a motor constructed on the principle of varying reluctance of the air gap as a function of the rotor position with respect to the stator coil axis. The torque in these motors arises from the tendency of the rotor to align itself in the minimum reluctance position along the length of the air gap.
reluctance torque
the type of torque a reluctance machine's operation is based upon.

A reluctance torque is produced in a magnetic material in the presence of an external magnetic field, which makes it to line up with the external magnetic field. An induced field due to fringing flux develops a torque that eventually twists the magnetic material around to align itself with the external field.
remote terminal unit (RTU)
hardware that gathers system-wide real-time data from various locations within substations and generating plants for telemetry to the energy management system.
repulsion-induction motor
a single-phase motor designed to start as a repulsion motor, then run as an induction motor. The rotor has a DC-type winding with brushes shorted together, in addition to the normal squirrel cage winding. Although it is an expensive design, it provides excellent starting torque with low starting current (similar to a universal motor) and relatively constant speed under load.
residual current circuit breaker
European term for ground fault interrupter.
residual magnetism
a form of permanent magnetism, referring to the flux remaining in a ferromagnetic material after the MMF that created the flux is removed. For example, if a bar of steel is surrounded by a coil and current is applied to the coil, the steel bar will create a magnetic field due to the rotation of the domains in the steel. After the current is removed, some of the domains will remain aligned, causing magnetic flux.
residual overcurrent relay
an overcurrent relay that is connected to sense residual current. Residual current is the sum of the three phase currents flowing in a current transformer secondary circuit, and is proportional to the zero sequence current flowing in the primary circuit at that point.
resistivity
(1) the product of the resistance of a given material sample times the ratio of its cross-sectional area to its length.

(2) an electrical material property described by a tensor constant indicating the impedance of free electron flow in the material. Resistivity relates the electric field strength to the conduction current, and can be expressed as the inverse of the conductivity.
resistance ratio
of the potential of an electrical current applied to a given conductor to the current intensity value.
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