The current through the electromagnet (and its magnetic field) switch direction as the AC voltage swings between positive and negative.
Driven by the switching magnetic field, for 60 Hz power this motor will try to spin at 3600 RPM (60 cycles/second x 60 seconds/minute).
An increase in voltage (but not frequency) increases torque. This pushes the motor closer to the ideal 3600 RPM speed (despite heavier loads) but not beyond that speed.
AC motors of other speeds can be built by removing the bottom permanent magnet, and replacing it with one or more AC powered electromagnets. Depending on the orientation of these new electromagnets, for 60 Hz power those motors will have full speeds of 1800 RPM (3600/2), 1200 RPM (3600/3) and other values.
