Transformer inrush current
Hello,
I'm trying to fully understand physiscal phenomen that is behing large
inrush magnetizing current in transformers. I know that many people
make parallel explanation with motor inrush current but I don't think
it's fully correct.
In my opinion explanation should go something like this:
In Faraday's law, induced emf is e = -dflux/dt that is negative rate
of change of magnetic flux. If we have power transformer which is
connected to AC voltage then magnitude of inrush current will depend
on instantaneous AC voltage and on remanent induction in feromagnetic
(iron ) core. If supply AC voltage is at maximum or minimum it's
magnetic flux is at rate of zero. So if we connect AC voltage to power
transformer when that voltage iz zero, there will be no inrush current
with high magnitude at all because dflux/dt is zero, so induced emf
(el.mot. force) will be zero. In this case there will be not transient
at all. On the other hand, highest transient will be when applied
voltage is about zero, because dflux/dt is at its maximum. Remanent
induction will only make things worse, because it will drive core to
saturation much sooner.
Since it is very unlikey that transformer will be switched on to AC
voltage at maximum value, it is almost always the case that iron core
will experience saturation. Because el.magnetic charactersitcs of iron
core is highly nonlinear, saturation will produce higher harmonics.
So, all in all we can say that large inrush magnetizing current will
always exists, and that it will be full of higher harmonics due to
nonlinear magnetic charactersitcs of fermagnetic materials.
Do you share this opinion or maybe there is some other physical
phenomen that is not included in this explanation.
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