Transformers

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Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi all,
         A few days ago I said something about the power capability 
of transformers in reply to someone else and I remember saying that 
the core flux goes down under load (?). I wish to correct this 
statement. The core flux doesn't go down. But it doesn't go up 
either. It remains at the same level that would be generated by the 
magnetizing force if the transformer was unloaded. What actually 
happens is that in loading the secondary, an opposing flux is 
generated which in turn causes primary current to rise in order to 
cancel it out so *nett* core flux remains constant. If a transformer 
core is not saturated by applying the magnetizing force alone, it 
never will. As I said once before, you can model a loaded transformer 
as an inductor (the primary inductance) in parallel with a resistance 
which reflects the load current.

      I then went on to gripe about the winding window (available 
space for windings) and wish to say something about that also. I 
design and wind a lot of transformers and it is a constant bugbear 
for me that I have to live with the window available for a given 
core. From the lamination manufacturer's point of view, there is a 
sound economic reason why the limitation exists. Without exception, 
all the E-I laminations I've seen for various core sizes are formed 
from single rectangles of metal sheet in that the I's are stamped out 
from a pair of E's. This means that the only waste material is from 
the bolt holes in the laminations which is rather efficient and leads 
to a constant pole area for a particular set of E's and I's. However, 
it leads to the limited winding window that I alluded to.

Malcolm