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Re: Power transformer saturation


Hi Bob, Malcolm and all,

On the transformer saturation topic, here are a few thoughts of mine:

A given transformer is most likely to saturate when its primary is run
overvoltage and there is NO LOAD current drawn from the secondary.
This results in the highest flux density in the iron core.  If there
is not enough iron,  saturation will start to occur.  A good example
of this is the Microwave Oven Transformer.  These are built to a low
budget and contain the bare minimum of iron to minimise weight.  If
you run one of these with its secondary open circuit, you will notice
it growl when you feed it with much more than its rated input voltage.
This is accompanied by a steep rise in magnietizing current, (The tell
tale signs of core saturation.)

As load current is drawn from the secondary of a transformer,  it
generates a magnetic flux which opposes the flux due to the primary.
This loading takes the core FURTHER AWAY FROM SATURATION.  A good
example of this is the (CT) current transformer CT. The current in the
primary induces a corresponding current in the secondary. The net flux
in the core is almost zero.

The VA rating of a given transformer is usually determined by choosing
an acceptable temperature rise in the copper windings.  A bigger core
lets you use thicker copper,  and hence lower resistance.  Therefore
there is less temperature rise for a given VA,  (or more VA for a
tolerable temperature rise if you like.)

A larger cross-section core also improves coupling between the primary
and secondary windings.  This results in less leakage reactance and
improves load regulation.  This last point is of little importance to
coilers because we usually trash the regulation by adding big ballast
inductors in series anyway !  Nevertheless it is an important factor in
the design of power transformers.

I agree with Malcolm.  Don't worry about over current saturating the
core. Worry about over voltage.   But, remember that the temperature
rise is proportional to I^2.  Twice the rated current will cause four
times the intended temperature rise.

*** HOWEVER ***

It is possible to saturate the core of a power transformer in Tesla
Coil operation quite easily due to the inductive kick provided by the
ballast !

During each charging cycle,  energy is stored in the ballast.  After
each firing of the spark gap,  this energy is released.  After every
firing,  the ballast acts as a source and adds to the voltage from the
AC supply.  So if you use a 240V transformer on a 240V AC supply it is
very possible to saturate the transformer due to the inductive kick
of the ballast.  This is something which I am tackling now with a TC.

If you dont believe this,  try measuring the RMS voltage across the
LOW VOLTAGE primary of your power transformer AFTER THE BALLAST.
It may be higher than you would guess,  once the gap starts firing.

It is possible to eliminate this saturation by careful choice of
ballast,  capacitor and break rates.  The voltage waveform can be made
decidedly spikey,  such that it has a high peak value but a low average
value.  The high peak gives maximum energy per bang,  and the low
average keeps saturation at bay.

I have rambled too much already,  but I hope this has shed some light.

Comments welcomed,
Flamesuit at the ready,

                                                        Cheers,

                                                        -Richie,