Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

no load tap changer
device that provides for changing the tap position on a tapped transformer when the transformer is de-energized. Different taps provide a different turns ratio for the transformer.
no voltage holding coil
a holding coil that keeps the main-line contactor closed on zero voltage conditions. DC motor controllers that contain this feature are used in places where the motor is vital to the operation of a process. These controllers can maintain control to the motor under momentary line power loses, by using the CEMF of the coasting armature to keep power to the main-line coil/contactor. If power to the motor controller is not restored within a short period of time, the motor coasts to a speed where it can no longer keep the main-line contactor closed. At this point, the m-coil drops out to insure starting resistors are placed back in the circuit.
no-load test
measurement of input parameters of an induction motor while running at nearly synchronous speed, with zero output on the shaft. This test is used to determine the magnetizing reactance of the motor equivalent circuit. See also open-circuit test.
a symbol representing a physical connection between two electrical components in a circuit.
node analysis
a circuit analysis technique in which KCL is used to determine the node voltages in a network.
nominal voltage
a number given to a system to name its classification of voltage, such as rated values.
non-time delay fuse

See single-element fuse
refers to a three-phase electric power transmission line whose conductors are not transposed See transposition.
normal demagnetization curve
the second quadrant portion of the hysteresis loop generated when magnetic induction (B)is plotted against applied field (H ), which is mathematically related to the intrinsic curve; used to determine the performance of a magnet in a magnetic circuit.
normally closed contact
contact of a contactor that is closed when the coil of the contactor is deenergized and opened when the coil is energized.
normally closed, time to close
a relay that is closed when the power to its actuator is off, but has a time delay to close when power is removed from the actuator. When power is applied, the relay immediately opens.
normally open contact
a contact that is open under normal operating conditions and closes when an action is initiated in its controller. For a contact that is part of a relay, the contact remains open when the relay is deenergized and closes when the relay is energized.
normally open, time to close a time delay
relay that is open when the power to its actuator is off. When power is applied to the actuator, the relay remains open for an adjustable time delay, after which it closes. When power is removed from the actuator, the relay opens immediately.
Norton theorem
states that the voltage across an element that is connected to two terminals of a linear, bilateral network is equal to the short-circuit current between these terminals in the absence of the element, divided by the admittance of the network looking back from the terminals into the network, with all generators replaced by their internal admittances.
a disturbance of the normal voltage waveform of duration less than 0.5 cycles, is of a polarity that is opposite to the waveform and is hence subtracted from the normal waveform with respect to the peak value of the disturbance voltage.

See National Television System Committee
nuclear magnetic resonance
the phenomenon in which the resonant frequency of nuclear spin is proportional to the frequency of an applied magnetic field.
See magnetic resonance imaging
nuclear power plant
a thermal electric power plant in which the heat for steam turbines is produced by nuclear fission.
nuclear reaction
a reaction which causes changes in the nucleus of an atom, thus changing elements to another element or isotope, usually with the release of energy.
nuclear reactor
(1) an apparatus designed to facilitate, contain, and control a nuclear chain reaction.

(2) any heat-producing array of fissile radioactive materials constructed so as to produce a controlled chain-reaction.
null a point on the radiation pattern that corresponds to zero or minimum values.