Transformer Frequency

The voltage rating will have to go down proportionately, as Vrms=4.44*B*A*N*f

B, A, N fixed value by design.

Yes you can .If winding/Insulation of transformer are able to withstand the increased current (due to decrease in winding reactance).

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Correct me, if i am WRONG .

Punctuation Mark plays an important role..... !

It is not just the reduction in reactance, the flux in the core will be higher and nearer to saturation, when core saturates, the output voltage waveform will be distorted, causing high harmonics , overheating may result. When transformer core saturates, even without loading, the transfomer may be damaged.

At about 2/3's the input voltage would work too.

(but there's no alphabet in that equation.)

At reduced output.

TCM has the only correct answer, and here's the alphabet soup he's referring to. The Voltz/Hz ratio should not be allowed to exceed the ratio at rated voltage and rated frequency. So for numbers' sake 120V/60Hz=2 therefore at 40Hz 2=V/40 or V=80, which is 2/3 of 120 (you could also take 40/60 and get the same answer).

Why is V/Hz important? Because of operating too far into the hysteresis curve of the laminations of the core and the subsequent distortion of the sine wave and resulting high harmonic levels and heating of the core and surrounding structures..