[time-nuts] Transformer inrush current and transformer simulation

Mike Monett timenuts at binsamp.e4ward.com
Tue Jun 7 20:31:44 EDT 2016


> On    Fri,    03   Jun   2016   12:37:26   -0400    "Mike  Monett"
> <timenuts at binsamp.e4ward.com> wrote:

>> I found  a  significant  error in  the  LTspice  analysis.  I was
>> wondering how the current could jump instantaneously at zero when
>> the voltage is applied at the peak. That violates magnetism.

>> It turns  out  it doesn't. When LTspice  starts  an  analysis, it
>> first calculates the operating point. For the Sine voltage source
>> at 90  degrees, it applies the full voltage across  the  load. In
>> this case, it was 169.7V across 1 ohm, resulting in 169.7 Amps.

>> That is what was plotted, and is a significant error.

> Actually, spice  (the  engine behind LTspice) does  a  DC analysis
> before almost  all  modes of operation. This DC  analysis  has the
> intention to start the circuit from a steady-state point  and thus
> to reduce simulation time. In order for this to work properly, you
> have to  specify  the  DC voltage  and  currents  for  all sources
> correctly. Spice messes this up at times making the first  part of
> a transient simulation worthless (it has even worse  problems when
> you do  an  AC analysis). Additionally LTspice hides  too  much of
> these small  complications for the problems to be  visible  to the
> untrained eye  and  also at times makes it harder  to  provide the
> correct values. Thus, caution is advised.

After starting  with Intusoft in 1985, moving to  Microcap  in 1991,
having a  brief  fling  with PSpice around  1998,  and  switching to
LTspice in 2006, I can say LTspice has the easiest and  fastest data
entry of any SPICE program I have tried.

There is no problem with specifying the sources in  LTspice. Nothing
is hidden.  The  setup   menus   are   extremely  easy  to  view and
understand. If you wish, you can have the input parameters displayed
on screen, as I have done with two of the functions.

LTspice checks all the information given, and if it detects an error
it generates an error message and won't run.

My original  problem  was not the setup menus.  It  was  picking the
wrong model.

> The general  rule of "Never trust a simulation you  haven't forged
> yourself" applies.

Most people would be hard put to do a hand simulation of  a wideband
op amp in a closed-loop feedback network. That is what SPICE is for.

>> Out of  13  examples I analyzed, I found only  one  that involves
>> unloaded transformers.

>> I found  many references that discuss transformer  inrush current
>> caused by  core saturation. This is a serious problem as  it puts
>> stress on the components and reduces operating life.

> I only  had a quick glance at your webpage, but it seems  that you
> used the  standard LTspice transformer model.  Unfortunately, this
> is not a good model to study this kind of behaviour. For  one, the
> only loss  considered  in the model is  the  winding  coupling, it
> doesn't even directly consider resistive losses in the windings.

You would  be advised to learn LTspice as it would save you  a great
deal of misconceptions about how it works.

The winding  resistances  are included in  the  inductor  model. You
specify them as needed. I usually used a series resistance of 1 Ohm,
but changed  it  in some examples to suit the  application.  You can
also specify the parallel winding capacitance and resistance.

> In this  case,  the two most important effects  that  you  need to
> include are  saturation and core losses, which are  both frequency
> dependent. The  cores  of electric machine  transformers  are very
> poor when it comes to their "high" frequency behaviour. Where high
> frequency starts somewhere closely above mains frequency.  Ie 1kHz
> is already  so  far off that somewhere around  90%  of  the energy
> would be dissipated in the core.

I was not interested in examining the frequency response, saturation
effect or core losses. These are only important after the  core goes
into saturation.

I was  only interested in the result of switching at the peak  or at
the zero  crossing. This is clearly defined at the beginning  of the
document.

> The sharp  rise  in voltage and the  leading  inrush  current have
> frequency components that are way higher than mains frequency.

> Hence the  linear model you used will give inaccurate  results, to
> put it mildly.

> Unfortunately, building an accurate transformer model in  spice is
> not easy and depends on higher order functions that might or might
> not be  available in the flavour you use. Not to mention  that you
> will need  to  have  good   (measured)  numbers  on  the non-ideal
> behaviour of a transformer, which are also not easy to get by.

The saturation  and  core  losses   are  outside  the  scope  of the
investigation. The  investigation was only to examine the  effect of
switching at  the  peak or at the zero  crossing.  This  was clearly
stated at the beginning of the paper.

My analysis  correctly defined an unloaded transformer  as  the only
case where  switching  at  the peak or the  zero  crossing  made any
difference. This was the goal, and it was met.

I also showed that very few solid state switches were available that
switched at  the peak, that most vendors simply supply  devices that
switch at  the  zero  crossing and state to get  a  model  that will
accept the  surge currents, that switching at the  peak  could cause
severe surge  currents with capacitive loads, and that  I  could not
find any reference that stated switching at the peak would not cause
core saturation.

Your comments   offer   no   additional   information  regarding the
advisability of  switching  at the peak or  the  zero  crossing. The
information you  do supply is irrelevant to the problem,  and mostly
irrelevant to LTspice.

Attila Kinali

> It is upon moral qualities that a society is ultimately founded.
> All the  prosperity and technological sophistication in  the world
> is of no use without that foundation.

>                - Miss Matheson, The Diamond Age, Neil Stephenson

You need to consider getting new sigs. The two you post  have little
or nothing  to  do  with timenuts, and I'm  sure  everyone  has them
memorized by now.

MRM



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