DC motors that have their magnetic fields produced by coils as described earlier can, in principle, be powered from AC as well, since if the applied voltage reverses (as it does in ac), then the current in both the rotor and stator coils will reverse together, leading to no change in the direction of the torque produced. This is not the case if the magnetic fields are produced by permanent magnets instead of coils. In practice, energising a motor designed for DC with AC may not be very efficient because the manufacturer may not have made the motor from materials that reduce eddy current losses, expecting this not to be important (which it isn't for DC).
The main distinction between a motor and a generator is that, in a motor the rotr back emf is less than the applied voltage by an amount equal to the voltage drop across the armature (and field coil if series connected) resistance whilst in a generator the back emf is larger than the supplied voltage for the same reason. If the stator windings of the AC alternator described above were energised by ac, then
the machine would work as a motor instead, provided the rotor was turning at exactly the right speed so that each time a rotor pole approached a stator winding, the field direction in that winding was such that a torque was produced in the direction of motion. For this to occur, the rotor must turn at the same speed that would be needed to create the frequency of voltage being applied to the stator. For this reason, such a machine is called a 'synchronous motor'.
A brushless version is often preferred in which the rotor field is produced by permanent magnets rather than a coil. The power available from such motors is limited to <50kW.The main problem with this type of motor is running it up to the synchronous speed in the first place. If it is not very close to the synchronous speed, it will not turn at all. An auxiliary motor is required to achieve the synchronous speed, after which the main motor can take over. Because of these starting problems, it has limited use, mainly applications that do not require the motor to start and stop very often. More recently, this problem can be overcome by using modern power electronic drives that start the motor with very low frequency ac and gradually increase the frequency of the supply as the rotor speeds up, keeping the rotor close to synchronism. The motor is usually fitted with a rotor position sensing device so that the electronics can check that the rotor is indeed staying synchronous with the applied stator frequency.
To avoid such problems, a very widely used alternative form of synchronous motor is the 'induction motor'