Saturable reactors are used to transfer a system voltage to a load through a typical series impedance division circuit. The saturable reactor is in series with the load impedance and the combination is connected across the system voltage. The magnitude of the voltage reaching the load is determined by the ratio of the load impedance to the total series impedance (load + reactor).

By reducing the impedance of the saturable reactor, the load impedance becomes a higher percentage of the total impedance and thus more of the input voltage is dropped (transferred) across the load. Since both the saturable reactor and the load may consist of resistive and reactive components, each device has an impedance that is the vector sum of its resistance and reactance.

The method used to change the impedance of the saturable reactor is to change its inductance, thus its inductive reactance and therefore its impedance. The inductance is changed by changing the permeability of the magnetic core.

With no DC control current present, the permeability of the core is at its highest value and the inductance is therefore at its highest value. An series analysis will show how much of the system voltage reaches the load under this condition. With the application of a DC control current significant enough to saturate the inductance, the impedance of the reactor is reduced to nearly the value of only its resistance. Under this excitation, the greatest value of the system voltage is dropped (transferred) to the load. For the full system voltage to reach the load, the load impedance must be significantly higher than the resistive component of the reactor.

A typical amplification factor for a 10 KVA saturable reactor, requiring 150 Watts of DC control for full output, is 10,000 / 150 = 67. This is why such a device is also known as a magnetic amplifier.