Question

what's nominal excitation current?

Answer 1

Electric machines can be divided in permanent magnet ones and the ones excited with rotor current (of course there are other ways to classify the machines, but this is one). Nominal excitation current is a machine-specific value, which tells how large current does the excitation coils take in typical operating condition. For more abou excitation, read:
http://en.wikipedia.org/wiki/Excitation_%28magnetic%29
http://en.wikipedia.org/wiki/AC_motor