I can only get a secondary voltage(230) if I put 12 volts on the primary slip rings
Do I need to excite the low voltage side? and if so how?Or is something missing in the wiring?
The wiring has been altered by the previous owner

You will need to provide more details as to what this is exactly... and what make and model and capacity if you know them.

I assume it's a generator. Some are self exciting others aren't.