A single phase two winding transformer is designed to operate at 400/200 volts, 50Hz.if the high voltage side is now energized from a 400Volts, 40 Hz source, the no load l.v side voltage would be what numerical value?

 

A single phase two winding transformer is designed to operate at 400/200 volts, 50Hz.if the high voltage side is now energized from a 400Volts, 40 Hz source, the no load l.v side voltage would be what numerical value?

We don't know. The transformer might be frequency dependent or it might not be frequency dependent.

 

A typical 50Hz power transformer core may saturate at 40Hz.
If so it would draw high current and the output voltage less than expected.

 

If it's a reasonably high capacity transformer, you will get 200 V on the secondary. Of course, there will be more core losses with more heat generation.