A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integer number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.
Miniature synchronous motor used in analog clocks. The rotor is made of permanent magnet.
Small synchronous motor with integral stepdown gear from a microwave oven
Single-phase 60 Hz 1800 RPM synchronous motor for Teletype machine, non-excited rotor type, manufactured from 1930 to 1955
Externally excited motor, 1917. The exciter is on the left.
An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor therefore needs no electrical connections to the rotor. An induction motor's rotor can be either wound type or squirrel-cage type.
Three-phase totally-enclosed fan-cooled (TEFC) induction motor with end cover on the left, and without end cover to show cooling fan on the right. In TEFC motors, interior heat losses are dissipated indirectly through enclosure fins, mostly by forced air convection.
Cutaway view through stator of TEFC induction motor, showing rotor with internal air circulation vanes. Many such motors have a symmetric armature, and the frame may be reversed to place the electrical connection box (not shown) on the opposite side.
A model of Nikola Tesla's first induction motor at the Tesla Museum in Belgrade, Serbia
Squirrel-cage rotor construction, showing only the center three laminations