is provided with six first stator poles 3 and six second stator poles 4 each of the first stator poles 3 is wound with a respective excitation coil 5 6 7 for the three phases the coils are arranged in diametrically opposite pairs 5 6 7 such that the coils of each diametrically opposite pair are electrically connected in series with each other in an alternative embodiment diametrically opposite coils are connected in parallel a rotor 8 includes a non magnetic core 9 ten magnetically separated rotor segments 10 of magnetic material are secured in longitudinal keyways in the core 9 so that they are circumferentially arranged and separated from the stator poles 3 4 by an air gap 11 the rotor segments 10 are arranged to direct magnetic flux from a first stator pole 3 to an adjacent second stator pole 4 or vice versa as shown most clearly in fig3 a for clarity the rotor segments in fig1 have been shown hatched also for clarity the outer diameter of the non magnetic core 9 has been shown as less than the outer diameter of the rotor segments although in practice these would all lie on a common diameter to give a smooth rotor surface to generally avoid the problem of noise due to windage created by an uneven rotor surface the stator 2 and the rotor segments 10 are preferably built up from a stack of laminations of electrical sheet steel as is conventional with electrical machines in general it will be noted that all the rotor segments can be produced from the material inside the stator bore providing for an efficient use of lamination material the rotor segments 10 can be held in.