On the other hand, saturation is exploited in some electronic devices. To lower its effects, an air gap is created in some kinds of transformer cores. To prevent this, the level of signals applied to iron core inductors must be limited so they don't saturate. When AC signals are applied, this nonlinearity can cause the generation of harmonics and intermodulation distortion. In linear circuits this is usually considered an unwanted departure from ideal behavior. This means that their inductance and other properties vary with changes in drive current. 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 is one reason why high power utility transformers are so large. 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. Saturation occurs when practically all the domains are lined up, so further increases in applied field can't cause further alignment of the domains. The stronger the external magnetic field, the more the domains align. When an external magnetizing field H is applied to the material, it penetrates the material and aligns the domains, causing their tiny magnetic fields to turn and align parallel to the external field, adding together to create a large magnetic field B which extends out from the material. Their tiny magnetic fields point in random directions and cancel each other out, so the material has no overall net magnetic field. Before an external magnetic field is applied to the material, the domains are oriented in random directions. It is a characteristic particularly of ferromagnetic materials, such as iron, nickel, cobalt and their alloys.įerromagnetic materials like iron that show saturation are composed of microscopic regions called magnetic domains that act like tiny permanent magnets that can change their direction of magnetization. Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetizing field H cannot increase the magnetization of the material further, so the total magnetic field B levels off.
Since winding resistance may be very low, current can be very high unless limited by impedances external to the inductor.Ĭore saturation can occur by imposing too low a frequency at designed voltage OR too high a voltage at designed frequency on a transformer or by imposing too high a DC current on a choke coil.
That is, when inductive reactance disappears due to core saturation, the voltage applied to the circuit is opposed only by the I*R drop of the winding and whatever external impedances are in the circuit. The current is simply responding to instantaneous circuit conditions. I think it is erroneous to refer to "current saturation." It is the core that saturates. Magnetic amplifiers had the advantage of extreme ruggedness and immunity to voltage spikes that can wipe out semiconductor circuits. Very tightly controlled, gradual saturation characteristics are required in magnetic amplifiers that were once heavily used in naval gun control applications but which are now pretty much obsolete, I believe. Abrupt core saturation is a desirable characteristic in, for example, timing circuits, blocking oscillators, or self-switching power inverters but very undesirable in signal transformers. The designer will select a core material with properties appropriate for his application. Saturation can occur gradually or suddenly depending upon the properties of the core material. Since there are no more domains available to be forced into alignment with the flux lines, inductive reactance disappears, current is no longer opposed by inductive reactance, and current abruptly spikes upward until limited by the DC resistance of the conductor in the winding and by external circuit conditions. At some point, however, all or most domains have been brought into alignment with the flux lines and a further increase in volt*time product cannot cause a further increase in the number of aligned domains. For a given core material and winding configuration, the larger the core, the greater the number of domains available to be aligned with the lines of flux and the greater the voltage*time product the core can support without saturation. On a little more intuitive level, inductive reactance occurs as a change in current in the winding on a ferromagnetic core causes a change in the number of magnetic domains in the core that are aligned with the lines of magnetic flux caused by the winding.
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