If 240 volts AC is the correct frequency for the transformer, and it's fed in to the 240 VAC transformer winding, the 100 volt transformer winding will output 100 volts. But if 240 VAC, 50/60 Hz is fed into the 100 volt winding - and the transformer is designed to be a normal 50/60 Hz transformer - the transformer core will saturate about halfway through each AC cycle. The result will be very high primary current when the transformer saturates.
