ABSTRACT
An electric current (motion of charge) always creates an associated magnetic field. If a coil or other circuit lies within this field, and if the field changes with time, an electromotive force (voltage) is induced in the circuit. The magnitude of the induced voltage is proportional to the rate of change of flux d¢1dt (webers/sec) linking the circuit, and its polarity is such as to oppose the cause producing it. If no ferromagnetic materials (such as iron) are present, the rate of change of flux is proportional to the rate of change of current dildt which is producing the magnetic field. The proportionality factor relating the induced emf (voltage) to the rate of change of current is called the inductance. The presence of ferromagnetic materials greatly increases the strength of the effects, but also makes them significantly nonlinear, since now the flux produced by the current is not proportional to the current. Thus, iron can be used to get a large value of inductance, but the value will be different for different current levels. For small changes in current about some operating point, one can define an incremental inductance for a linearized analysis using our usual linearizing methods. Large current swings require a nonlinear treatment, and this can be easily studied using simulation methods (lookup table) if the variation of inductance with current has been measured. We will do such a nonlinear simulation shortly.
