Did you use a mandolin to slice the potatoes? They need to be really,really thin.
220 to 230 C is going to be too hot for sweet potatoes and they will be prone to burning. You need a much lower heat, like the 300 F I used. I may bump that up to 325 F next time. The thing I've always disliked about Sweet potato fries is they always tasted somewhat burnt to me due to the higher frying temps.
