Prev: Design Electronic Vehicle Anti-theft Gadget
Next: What's on your test bench?
From: Paul G. on 16 Nov 2009 23:01
On Sun, 15 Nov 2009 11:05:48 +1100, "Phil Allison" <phil_a(a)tpg.com.au>
wrote:

>
>"Paul G."
> "Phil Allison"
>>"Shaun"
>>>>
>>>> I looked up inrush current for transformers and I stand corrected, it's
>>>> maximum will occur at the voltage zero crossing point. I thought it was
>>>> the same as an L R circuit in which case if switch closes as the peak of
>>>> the ac waveform it causes the maximum transient.
>>>
>> A bit off topic....
>> At first thought, it seems counter-intuitive that zero-crossing is
>> the worst case for inrush current, so I used LTSPICEIV to simulate the
>> situation. As usual Phil is correct! here's the .asc file for the
>> simulation, note that you must set the inductor current to zero for
>> the initial condition.
>>
> ( snip listing)
>>
>> In the sim above, is a 1H inductor in series with 5 ohms, with
>> 110vac (160v peak) applied. The voltage is set to start at zero
>> crossing.
>
>** 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 !!
>

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:
http://www.plcdrives.com/forum/f34/re-simulating-non-linear-magnetics-ltspice-29584/
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:
http://ltwiki.org/index.php5?title=Main_Page (a WIKI for LTSpice)
more:
http://www.electronicskb.com/Uwe/Forum.aspx/design/45226/Inductor-saturation-in-LTspice
and another:
http://www.electronicskb.com/Uwe/Forum.aspx/cad/538/Simulating-non-linear-magnetics
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.





From: Phil Allison on 16 Nov 2009 23:19

"Paul G."
>
> 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.
>

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



..... Phil


First  |  Prev  | 
Pages: 1 2 3 4 5
Prev: Design Electronic Vehicle Anti-theft Gadget
Next: What's on your test bench?