Clamp meters: Peak vs. In-rush?

"Paul G." "Phil Allison"

( snip listing)

** Your simulation is of an inductor - and NOT a AC supply transformer primary as the question requires.

The differences are many and great and the switch on transient behaviour very different - mostly because a transformer's laminated iron core will saturate hard when a frequency just a little lower than it is designed for is applied.

I doubt that LTSPICE IV can even do such a simulation.

Its why I said to TRY it !!

** Yep - there is nowhere to stat demolishing a fallacy if no sensible case is posted that supports it.

Posting inane drivel like " .think about it " or " I am surprised you don't know this " is just pouring fuel on the fire.

..... Phil

"ehsjr"

** But we are discussing the behaviour of a AC supply TRANSFORMER - where core saturation IS the cause of inrush current surges.

The simplest way to think about it is to first note that the core of nearly any AC supply tranny operates on the edge of saturation ALL THE TIME when mains power is applied.

( Proof of this is that if one raises the applied voltage by about 20% or lowers the AC frequency by the same - the magnetising current drawn by the tranny will dramatically increase. )

The note that switching the supply on at a zero crossing means the *average value* of the wave will not become zero until a whole cycle has passed - while switching on at a peak means only half a cycle need pass for the same result.

The second case produces a very much smaller inrush surge as a result.

Interestingly, if the AC voltage applied to a supply tranny is half or less its rated operating voltage - inrush current surges barely exist.

..... Phil

LTSPICE has 2 methods for simulating saturation, if you run LTSPICE, and look for saturation in the help menu it gives you the details. One method is "based on a model first proposed in by John Chan et la. in IEEE Transactions On Computer-Aided Design, Vol. 10. No. 4, April 1991 but extended with the methods in United States Patent 7,502,723". It uses Hc (Coercive force), Br (Remnant flux density), Bs (Saturation flux density), Lm (Magnetic Length), Lg (Length of gap), A (Cross sectional area), N (Number of turns). That will be tricky for an inductor that's aready built. The other method uses a "flux" statement: L1 N001 0 Flux=1m*tanh(5*x) I1 0 N001 PWL(0 0 1 1) this didn't make a lot of sense to me..... Fortunately a search came up with:

which has a file that has many examples inside it to play with. It explains how to set up different saturation scenarios. A quicky simulation showed an enormous inrush current.

Another link which seems useful is:

(a WIKI for LTSpice) more: and another: whew! this is a lot of reading, and getting into the guts of LTSpice. Many years ago, I used PSPice (the full release, that worked under DOS, cost me well over $1000), it would do inductor saturation as well. Apparently the saturation models don't allow you to do mutual coupling, so you need to make the transformer equivalent circuit in which you can place the saturating inductance. There are details in the above links. Of course, it will be quite difficult to set up the parameters to do a reasonable simulation, you need to know a lot about the device you are simulating. Paul G.

"Paul G."

** Takes far less time to simply measure what happens with a real transformer - the results are always 100% trustworthy too.

.... Phil