Your description sounds similar to the AC output tachgen I've seen on some small motors. They are using a (ceramic? ferrite?) magnetic rotor attached to the motor shaft, with a coil wound on a bobbin. There are 2 steel disks with fingers bent to surround the rotor (forming interlaced stator poles) which are probably what's inducing the AC into the coil (my guess). These are on Oriental Motor AC motors, where the output is 0 to about 100V for the input into their variable speed modules.
I've also seen a little (synchronous, I suppose) AC gearmotor (approx. a 2" cube) produced like this. It has two rotors and two coils on bobbins (2 leads per coil). It's labeled Hurst Mfg Corp, Princeton Indiana (USA). It's a 115VAC powered, and utilizes a 0.68uF capacitor when operated as a motor. I haven't actually spun it with a meter attached to see what the output would be (it's mounted on the back end of a motorized variac/autotransformer).
WB .................