Saturation is most clearly seen in the magnetization curve (also called BH curve or hysteresis curve) of a substance, as a bending to the right of the curve (see graph at right). As the H field increases, the B field approaches a maximum value asymptotically, the saturation level for the substance. Technically, above saturation, the B field continues increasing, but at the paramagnetic rate, which is 3 orders of magnitude smaller than the ferromagnetic rate seen below saturation.
The relation between the magnetizing fieldH and the magnetic flux densityB can also be expressed as the magnetic permeability: μ = B / H. The permeability of ferromagnetic materials is not constant, but depends on H. In saturable materials the permeability increases with H to a maximum, then as it approaches saturation inverts and decreases toward zero. Further increase in H will not cause an increase in B as the permeability is too small.
Different materials have different saturation levels. For example, high permeability iron alloys used in transformers reach magnetic saturation at 1.6 - 2.2 teslas (T), whereas ferrites saturate at 0.2 - 0.5 T. The amorphous metal alloy Metglas saturates at only 125 milliteslas.
Effects and uses
Saturation limits the maximum magnetic fields achievable in ferromagnetic-core electromagnets and transformers to around 2 T, which puts a limit on the minimum size of their cores. This is why high power utility transformers are so large.
In electronic circuits, transformers and inductors with ferromagnetic cores operate nonlinearly when the current through them is large enough to drive their core materials into saturation. This means that their inductance and other properties varies with changes in drive current. In linear circuits this is usually considered an unwanted departure from ideal behavior. When ACsignals are applied, this nonlinearity can cause the generation of harmonics and intermodulation distortion. To prevent this the level of signals applied to iron core inductors must be limited so they don't saturate. To lower its effects, an air gap is created in some kinds of transformer cores.
On the other hand, saturation is exploited in some electronic devices. Saturation is employed to limit current in saturable-core transformers, used in arc welding. When the primary current exceeds a certain value the core is pushed into its saturation region, limiting further increases in secondary current. In a more sophisticated application, saturable core inductors and magnetic amplifiers use a DC current through a separate winding to control an inductor's impedance. Varying the current in the control winding moves the operating point up and down the saturation curve, controlling the AC current through the inductor. These are used in variable fluorescent lightballasts, and power control systems.
