An excitation system (15) is disclosed for providing excitation to a main rotating electrical machine (2). The excitation system comprises an exciter (50) and an auxiliary generator (52). The exciter and the auxiliary generator have separate stator cores (14, 18) and share a common rotor core (16). The common rotor (16) core may be located between the two stator cores (14, 18). This may help to optimize space, improve material usage and reduce the total rotating mass. A mounting arrangement for the common rotor core is also disclosed.