Magnetic amplifiers utilize the concept of magnetic saturation. The heart of the magnetic amplifier is the saturated core reactor. When a saturated reactor is used in a circuit to amplify, the entire circuit becomes a magnetic amplifier.
We know that,
This tells us that, under condition of saturation, when flux is essentially constant, inductance becomes very low. So, if a magnetic field is externally induced into the core, reactance of the coil is reduced. The idea of varying inductance is shown below:
As Idc is increased, more flux is created in the core.This process can continue until the core is saturated, at that time not much change will occur in flux by the current from a.c. source, Iac. This means that the inductor is now essentially an air-core coil with much less inductance and consequently, less reactance than before—so a higher Iac results. The d.c. supply is used to control the alternating current source. By having higher number of turns on the d.c. winding, saturation can be accomplished with relatively small currents.
However, if the core is just barely saturated, the opposition to the a.c. will be different during each half cycle. This is true because one half of the cycle will tend to bring the core out of saturation when the flux is in opposite direction of existing flux and drive it further into saturation when the flux directions are same.
On a basic saturated core reactor, there are many more turns on the d.c. (control) side than on a.c. (controlled) side. This is, in effect, a step-up relationship so that a very high a.c. voltage could result in control winding. The solution to this problem is to arrange the windings so that the a.c. component cancel in the control winding but control winding can still saturate the core. A possible arrangement is shown below:
A more proper way is turn-by-turn cancellation.
In these, first magnetic amplifier is used to feed error signal to field winding of machine and it is further amplified by the machine at armature winding. An schematic is shown below:
These amplifiers can be of single-stage or multi-stage type.