Why does the open-circuit test essentially show only excitation losses and not in copper losses?

Oh, dude, the open-circuit test is like when you're checking out a car but not actually driving it - you're just seeing how it revs up without any load. So, it's all about measuring the excitation losses because there's no current flowing through the windings to cause copper losses. It's like checking out a car's engine without actually taking it for a spin - you're just seeing how it purrs without any real work.