You can get 1500 VA torroids for 50/60Hz from Actown.
http://www.actown.com/download/Catalog/Oem_Design_Guide.pdf Run cooler, big but lighter.
". Increase ƒ
B would decrease, possibly resulting in lower core losses. However, as you
move to higher frequencies, core losses could become more significant.
A switch to ferrite will minimize these losses but at a cost of decreased B.
However, the efficiency gains from a higher frequency will more than offset
the lower B. The higher frequency would also allow for a smaller
transformer, N and/or Ac would decrease."
"Confusing…yes. Confusing to us…no " ... Actown Electricoil Inc
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for a sine wave
E = 4.44 B N Ac ƒ x 10-8
Where E is the induced voltage, volts
B is the maximum induction, gauss
N is the number of turns in the windings
Ac is the cross-section of the magnetic material, cm^2
ƒ is the frequency, Hz