**Nyquist criterion**

The Nyquist criterion is a frequency domain tool which is used in the study of stability. To use this criterion, the frequency response data of a system must be presented as a polar plot in which the magnitude and the phase angle are expressed as a function of frequency.

**Nonlinear Resistance**

The property exhibited by devices whose resistance does not change uniformly with changes in voltage or current.

**Neutron**

A subatomic particle that is neither positively nor negatively charged and that has a mass approximately equal to that of a proton.

**Norton’s Theorem**

Norton’s Theorem states that any linear network can be reduced to a simple parallel circuit whose resistance RN is in shunt with a current source IN.

**Nucleus**

The center of an atom, containing proton (s) and neutron (s).

**Negative-sequence impedance**

The quotient of that component of negative-sequence sinusoidal voltage that is due to the negative-sequence component of the current divided by the negative-sequence component of the current at the same frequency.

**Negative-sequence network**

The equivalent representation of a power system constructed by using only the negative sequence impedances of the various components.

**Newton—Raphson method**

An iterative technique for solving a set of non-linear equations (eg… the power ﬂow equations) by solving a succession of linearized equations to derive improvements to the latest estimate of the solution.

**No-load characteristic**

The saturation curve of a machine at no load.

**No-load test**

A test applied to a machine on no load at rated voltage and frequency.

**Non-ideal or actual transformer**

A transformer having winding resistance and leakage flux, and a magnetic core having ﬁnite permeability and core losses.

**Normally excited**

The operating condition of a synchronous machine at Unity Power Factor.

**NEMA**

National Electrical Manufacturers Association is a group who publishes electrical and medical imaging standards.