Physics going on in a transformer

Yes Jon, but that suggests that the core was permanently magnetised. This is not the case with silicon steel used for transformers, the magnetic field is much too small to cause permanent magnetisation. The cause of the high peak current, as I said above, is that establishing a large field very quickly takes a very high current. Mike said that the highest rate of change of the magnetic field is at voltage zero crossings, but it is much higher still when directly connected to a high voltage, like the peak of a mains cycle, essentially instantly. Materials that remain magnetised are called hard magnetic materials, such as that used for permanent magnets, because they do not easily lose the magnetism, whereas soft ones (like silicon steel) do. It usually takes significant heat or physical deformation to demagnetise a hard material. Alloys such as AlNiCo, an aluminium-Nickel-Cobalt mixture make strong magnets, and some Rare Earth metals make the strongest ones, as used in modern permanent magnet motors. These would suffer demagnetisation very quickly in a motor where multiple magnetic fields interact, unless very hard magnetically.


Another point which I didn't make is that transformers of some size make increased noise as the load gets larger, a loud hum. This is because the magnetic field is making mechanical changes to the core, and it does this even if the laminations are tightly clamped. One rarely sees this in magnetism sections of textbooks, there are physical changes in the atomic crystal structure caused by the magnetic field. I have been looking at a number of advanced physics books to quantify the size of the change, but it seems it is not mentioned. Logically though something physical must be happening to make a noise. Perhaps these writers had never been close to a large transformer, certainly, small ones cannot be heard easily. You may like to look here Wikipedia Ref.

Yes Jon, but that suggests that the core was permanently magnetised. This is not the case with silicon steel used for transformers, the magnetic field is much too small to cause permanent magnetisation. The cause of the high peak current, as I said above, is that establishing a large field very quickly takes a very high current. Mike said that the highest rate of change of the magnetic field is at voltage zero crossings, but it is much higher still when directly connected to a high voltage, like the peak of a mains cycle, essentially instantly. Materials that remain magnetised are called hard magnetic materials, such as that used for permanent magnets, because they do not easily lose the magnetism, whereas soft ones (like silicon steel) do. It usually takes significant heat or physical deformation to demagnetise a hard material. Alloys such as AlNiCo, an aluminium-Nickel-Cobalt mixture make strong magnets, and some Rare Earth metals make the strongest ones, as used in modern permanent magnet motors. These would suffer demagnetisation very quickly in a motor where multiple magnetic fields interact, unless very hard magnetically.


Another point which I didn't make is that transformers of some size make increased noise as the load gets larger, a loud hum. This is because the magnetic field is making mechanical changes to the core, and it does this even if the laminations are tightly clamped. One rarely sees this in magnetism sections of textbooks, there are physical changes in the atomic crystal structure caused by the magnetic field. I have been looking at a number of advanced physics books to quantify the size of the change, but it seems it is not mentioned. Logically though something physical must be happening to make a noise. Perhaps these writers had never been close to a large transformer, certainly, small ones cannot be heard easily. You may like to look here Wikipedia Ref.