[time-nuts] Transformer inrush current and transformer simulation
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
> 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
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
> 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
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.
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> 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.
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