Re: why is the core of a transformer laminated??

WONDERFUL question!!!  In fact, while the principle is simple, it took me 
a few days to come up with a coherent explanation of the factors involved.

Just as a reminder, a transformer is two (or more) coils of wire wound 
around a magnetic core.  An AC current passed through one coil creates a 
magnetic flux in the transformer coil which, in turn, induces an AC 
current in the additional windings.  So far, so good.

A problem arises, however, if the core is solid magnetic material.  What 
happens in this case is that stray magnetic fluxes arise with a solid 
coil which oppose the main magnetic flux that you're trying to generate, 
therby degrading the performance of the transformer.  These currents are 
called "eddy" currents and get larger as the size of the core gets 
larger;  the larger core size allows these eddy currents to flow within 
the core and get larger due to the fact that these eddy currents generate 
magnetic fields within the transformer, which cause additional electric 
currents to flow within the magnetic core, which causes additional eddy 
currents;  what you get is a vicious cycle arising.  The net result is 
that the performance of the transformer gets screwed up.

Laminations minimize the generation of eddy currents.  The key thing 
about laminations is that they are electrically *insulating*, so that 
eddy currents are restricted.  The thinner the laminations, the better 
since the physical thinness of the laminations restricts the physical 
size of the eddy current loop, thereby restricting the stray currents 
formed by the eddy current flux.  The higher the frequency that you want 
the transformer to operate at, the thinner the laminations must be.  
[It's also easier mechanically to form a magentic core from laminations 
than from a solid piece of magnetic material].  To give you an idea of 
the physical sizes involved, 50--60 Hz transformers generally use 
laminations of 0.007 to 0.014 inches in thickness.

The key thing to remember is that the laminations are electrically 
insulated from each other.

That's a brief explanation, and I hope that it helps!