A stator-fed half-speed synchronous motor

It is well known that an induction motor with an asymmetrically connected secondary circuit will run at a fractional slip rather greater than 0.5 but that the asynchronous performance is not wholly satisfactory. The defects arise chiefly, though not entirely, from a rotor field component which rotates with a small angular velocity (1¿2s)¿, and is responsible for reflecting undesirable low-frequency currents into the primary supply system. These disadvantages are avoided by superimposing a stationary magnetic field which fixes the (1¿2s)¿ field in space and thus compels the rotor to run at s=0.5, i.e. at exactly half speed. Further, the power factor can be controlled by altering the d.c. excitation in the usual way, and good efficiencies can be achieved. Test results are quoted for three different types of rotor and compared with a first-order theory which provides suggestions for improved rotor design and further developments.