if the transformer converts 240v ac to 8V ac, ratio 30:1 then it should step up the voltage by the same ratio, i.e, 15V ac to 450v ac.but it's not the case !!
no, transformers work in reverse only as long as the voltages don't exceed those in the spec. And only as long as the power does not exceed that of the original. In this case you are applying 15 volts to a winding made for 8 volts, and expecting 450 volts on a winding made for 240 volts. Won't work. You should check the current levels also, as I suspect your load is larger than the transformer is designed for also. .
It will, but you have to realize that the series resistance of the primary (which is now your secondary) is getting in the way. I am going to guess that this is a very small transformer, capable of one or two VA at most. The rating is a little tricky too. A 240V-8V transformer puts out a significantly higher voltage, maybe 20-30V, with no load. It's rated at 15V because under a load, with magnetic and resistance losses, the effective turns ratio is lower. Used in reverse, the opposite problem exists, because your actual turns ratio is much higher than 30:1. Edit: If it wasn't for silly ideas a lot of inventions wouldn't have been made. The secondary probably doesn't have enough turns to provide adequate self-inductance at 240 volts. It would just act like a short and burn out. There is also a possibility that there is not enough dielectric strength between adjacent windings or the frame to prevent arcing, but most transformers are designed to withstand thousands of volts so this is less likely. If possible, I'd recommend trying lower voltages (perhaps 8V from another transformer), and measuring the current with the primary (now the secondary) open. Put a load resistor in there to start just in case. If you're drawing more than a few milliamps, don't try hooking it up to the mains.
