Back Emf and Saturation

Author: N.H. Crowhurst

The magnitude of the back emf developed in the primary by an applied a-c voltage varies with the rate at which the current developed by the voltage changes. The more rapidly this current changes, the greater the back emf produced. The property of the primary that relates the rate of change of the current in the primary windings to the resultant back emf is known as the primary Inductance. When the primary inductance is high, the back emf is high, and the magnetizing current is small.

The inductance of a coil determines the amount of voltage induced by a rate of current change

There is a limit to the magnetization that a core can take, called saturation. Below this point, the primary inductance remains nearly constant and is relatively high. When saturation occurs, the relation between voltage induced and the further increase in current needed to produce it ceases to be approximately constant. It is as if the magnetic material disappeared and we had only an air-core inductor.