[time-nuts] Transformer inrush current and transformer simulation

[Attila Kinali]

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.

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

 
> 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. 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.
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.


				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