A transformer will be designed to operate at an AC voltage and Frequency. This has a direct influence on the magnetic parameters in the transformer core. If you start to drop your frequency, or raise your voltage you can 'saturate' the core whereby it is operating past a safe level and then all bets are off.
For a sine wave V = 4.4*B*A*N*F
Where:
V is operating voltage RMS
B is core flux density
A is area of core in m^2
N is number of turns
F is frequency
That's from memory and I think it's right. Anyone else, please jump in.
Rob.