[kalee20]

It does sound like an induction motor - as others have said, this runs slightly below synchronous speed when loaded.

However, the Garrad SP25 mk 4 turntable (and possibly others) does run as a true synchronous motor, ie at synchronous speed - make sure your motor isn't like this! The Garrard motor has a squirrel-cage rotor - like an induction motor - and the stator has a couple of thick copper single-turn links part way through the pole pieces (so-called shading links). This gives an extra field, phase-shifted from the main field, and drags the rotor round in a definite direction when the stator is energised from a single-phase supply. The rotor runs up as an induction motor. However, the rotor also has a permanent magnet bonded to it, and as the speed nears the synchronous speed, this 'takes over'. The squirrel-cage internal currents drop to zero, and the steady field of the permanent magnet ensures the rotor is dragged round in step with the rotating field created by the stator.

It's instructive to brake thie motor spindle between fingers - you can feel the torque keeping the speed constant, and after a certain point, as synchronism is lost, the torque has a step change as the motor reverts to induction mode.

In principle, there would be nothing against having a 3-phase version of this, or using a capacitor to give the phase-shifted field instead of the shading links.

The deciding factor for your motor is, does it have means to provide a steady feild for the rotor, such as a permanet magnet? If so, it is synchronous.