Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

National Fire Protection Association (NFPA)
sponsor and publisher of the National Electrical Code and other safety standards.
National Television System Committee (NTSC)
a body that recommended the standard for colored television broadcasts in the U.S. in 1953. NTSC video contains 525 lines, a field rate of 59.94 fields/second, a horizontal frequency of 15,734.264 Hz, and an interlaced color subcarrier frequency of 3.579545 MHz. The NTSC format is also in use by many countries other than the U.S.
natural commutation
commutation of current from one switching device to another in a power electronics converter at the instant the incoming voltage has a higher potential than that of the outgoing wave, without the aid of any commutation circuitry.
See commutation
nautical mile (nm)
1,852 meters. Approximately 1 minute of arc on the earth's surface.
NEC

See National Electrical Code
negative sequence
the set of balanced but reverse sequence (acb) components used in symmetrical component analysis. Normal load currents contain no negative sequence current.
negative sequence overcurrent relay
a protective relay that senses and operates on negative sequence overcurrent. Typical applications include the sensing of unbalanced faults and the protection of synchronous and induction machines from rotor overheating.
negative transition angle
the angular portion of the time based output signal that has a negative slope, expressed in degrees. This quantity could be loosely interpreted as the "trailing edge" angle.
negative-positive-zero (NPO)
an ultra-stable temperature coefficient (@ 30 ppm/OC from -55OC to 125OC) temperature-compensating capacitor.
negative-sequence impedance
the impedance offered by a circuit when negative-sequence currents alone flow through it, expressed in ohms. The impedance is complex, with its real part being the circuit resistance and imaginary part, which is a function of frequency and inductance referenced as negative-sequence reactance, also expressed in ohms.
negative-sequence reactance
inductive reactance offered by a circuit for the flow of negative-sequence currents alone. Expressed in ohms, the inductive reactance is a function of frequency and the inductance of the circuit
to negative-sequence current flow.
See negative-sequence impedance
NEMA

See National Electrical Manufacturers Association
NEMA code letter
the nameplate letter designation used to indicate the input kilowatt-amp rating of a motor under locked-rotor, or starting conditions.
NEMA size
a standard-size device, such as a motor controller. The NEMA size establishes the rating of the device.
See National Electrical Manufacturers Association
NEMA type
for induction motors NEMA establishes five types of induction motors (A, B, C, D, and E) that have different torque-speed characteristics to account for various types of loads.
NFPA

See National Fire Protection Association
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.
node
a symbol representing a physical connection between two electrical components in a circuit.
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