BFoelsch wrote:

Perhaps we are slightly confused with terminology here.

Saturation is a property of magnetic materials, and is dependent on flux
density.

I think that John Byrns meant to imply that the "degree of saturation," as
caused by the magnetic flux, is independent of secondary current (or the
reflected primary current). He is correct.

An unloaded transformer draws a current based on its inductance, the applied
frequency and voltage. The resulting magnetic induction is the product of
the current in amperes multiplied by the number of turns. The transformer
designer puts enough turns onto the primary to establish a flux density
somewhat less than saturation. Failure to do this will result in saturation
and loss of inductance, which will cause high current and overheating of the
primary, even in a no-load condition.

The kicker is that, once the transformer is loaded, the flux (and hence the
flux density) is proportional to the primary ampere-turns MINUS the
secondary ampere-turns. However, any secondary ampere-turns CREATE an equal
number of primary ampere-turns ( by increasing the primary current!). As any
number subtracted from itself is zero, the current drawn from the secondary
of a transformer has almost no effect on the flux density within the core,
and hence has almost no effect on the "degree of saturation." The flux is
determined solely by the unloaded inductance, the applied voltage, and the
frequency.

The OP's issue of running a winding at 2X voltage will result in disaster in
a commercial transformer design. Standard designs will not accomodate a 2X
increase in flux density without saturation.

Well said.

Transformers get hot in two different ways.
First is due to iron losses, and if the primary has low inductance, then
the unloaded primary current is high, and the iron gets hot, even if the wire
size is large enough to prevent the copper getting hot.
Second is due to unloaded current, and loaded current heating the copper.

A normally designed transformer has perhaps a total of 20% of losses, ie, its
only 80% efficient,
so if 100 watts goes in, only 80 watts come out.
This tranny would need to be able to dissipate 20 watts of heat.
So the iron losses might be 10% of the total losses.
Therefore 10 watts are lost in the iron.
The 10 watts of heat lost in the core is constant, and present even with no
load.
:Let's assume there is no saturation, ie, harmonic distortion currents in the
winding
are less than 7%.


Therefore, if the input voltage is 120v, and there is no load then the
idle current is 10/120 = 0.083 amps.
Therfore the impedance of the primary is 120/0.083 = 1,440 ohms.
Therefore the inductance of the primary coil is 1,440/ ( 6.28 x 60Hz ) = 3.82
Henrys.
Now if we used twice the primary turns, the inductance would rise four times,
and ZLp would rise four times, and magnetising idle current would be a quater,
and iron
losses would be much smaller.
But the load current ability would be reduced dramatically.
So the VA of the tranny would reduce, but the efficiency might rise.

Or if we wanted to maintain the VA rating, and reduce iron losses, then simply
increasing core size will do it, so use a taller stack of laminations.
There are limits, since as we increase the iron stack, the length of turns and
winding resistance increases, so to increase VA, and to get more efficiency,
and to lower temperature, we must increase the window size, wire size, and
perhaps stack.

The use of GOSS reduces losses, but this material still saturates like all other
iron
products, and runs noisily if the B of the iron is too high.

I suggest all those who are confused go read the text books,
before deciding to use transformers for purposes for which they were not
deigned.

Patrick Turner.






"William Sommerwerck" wrote in message
...
I'm always open to learning new things, but since when is core saturation
independent of current?

Obviously (?), all other things being equal, a transformer with more iron
will
require a higher current to reach full magnetizing flux.

By the way, no one (myself included) mentioned that a transformer designed
for
60Hz operation might not work very well on 50Hz -- and almost all European
AC is
50Hz.


Core saturation is independent of current, and depends
on the voltage and frequency.