Induction-type synchronous motors

The induction-type synchronous motor is a polyphase motor and has a cylindrical rotor. It is started as an induction motor of the slip-ring type and synchronizes itself when direct current is switched into the rotor. The characteristics and construction of this type of motor are compared with those of other polyphase motors. The self-synchronizing feature of this type of machine is discussed in detail so as to present a full physical conception of the phenomena which occur, and the subject is illustrated by means of oscillograms. A mathematical expression is deduced for the limiting conditions beyond which synchronizing is uncertain. The considerations which determine the selection of the excitation voltage are discussed, as well as the method of control and other practical points affecting the installation of these machines.