Small self-starting synchronous time motors

The paper deals with small self-starting synchronous motors such as are employed in time switches, meters, recording instruments, and other timing devices.In order to keep the paper within reasonable bounds it was felt generally advisable to confine the notes to selfstarting motors, since they are of more general application. The small hand-starting motors as used in domestic clocks, and any device which requires external starting means, are excluded.The paper gives a resume of some designs available, and includes general test-results obtained.It then continues with detailed data on the hysteresis type of motor recently investigated by the authors, and endeavours from the results obtained to substantiate Steinmetz's theory of this type of motor expounded in 1900. Various new materials, such as cobalt steel, aluminium-nickel (Al-Ni) steel, and magnetic silver alloys, are considered.The hysteresis coefficients for cobalt steel and aluminium-nickel steel were determined experimentally and are given.The nine designs described are chosen to give typical examples of the designs available, and in the short description of each some of the difficulties of design and manufacture are explained.The question of running-in torque, which is a very important factor to be considered, has been thoroughly investigated.The use of permanent magnets as synchronous-motor rotors has been studied from theoretical and practical viewpoints, and the conclusions are given.The requirements of the motor bearings are stated and typical examples are shown.The test methods used in the laboratory for measuring the various torques are explained.The test figures and curves given are those obtained by the authors from the particular models available in the laboratory. They may not, however, represent the maximum figures obtainable, as it is quite conceivable that variable results may be obtained due to inconsistencies in manufacture.In conclusion, brief notes will be found of some specialized applications of these small motors.