an electrical machine that converts single-phase alternating current into polyphase (usually three-phase) alternating current without altering its frequency. The splitting is carried out by means of an asynchronous or a synchronous machine having a single-phase stator winding that is connected to the power supply. This winding is sometimes called the motor winding because it provides for the rotation of the machine’s rotor.

The pulsating magnetic field produced by the stator winding can be regarded as the superposition of two counterrotating magnetic fields. There is a forward field, whose intensity vector rotates in the same direction as the rotor, and a backward field. The backward field is almost entirely compensated (damped) by the field from the currents induced in the short-circuited winding of the revolving rotor; the resultant field of the stator and rotor windings therefore has an intensity vector that is rotating at the frequency of the single-phase alternating current. At right angles to the motor winding on the stator, there is a generator winding in which the rotating magnetic field induces an alternating current whose vector is shifted in phase by 90° with respect to the vector of the supply current. With suitable interconnections, the motor and generator windings are thus a source of polyphase current. The generator winding is also used for asynchronous starting of the phase splitter from a single-phase supply.

Phase splitters are used in electrified railroad transport to convert the single-phase current of the contact network into the three-phase current that supplies the auxiliary asynchronous motors of electric locomotives and electric trains.