New methods of improving the synchronizing torque and weight efficiency of self-excited synchronous induction motors

Polyphase self-excited synchronous induction motors, including the latest types, are analysed with special reference to weight efficiency, conversion efficiency, and configuration of synchronizing torque. It is shown that the weight efficiency of such machines is low compared with that of polyphase asynchronous motors, because the secondary must be designed to carry polyphase load currents as well as unidirectional load and exciting currents. The alternating currents must be carried in a polyphase winding, the unidirectional currents in a single-phase winding. Although the latter may be a part of the former, it nevertheless follows that it is practically impossible to make the maximum synchronous torque of a given frame equal to the maximum asynchronous torque of the same frame. It is further pointed out that, even with a unidirectional and pulsating synchronizing torque, such motors do not run asynchronously with a sufficiently uniform peripheral speed of the revolving member to permit of the general utilization of their asynchronous overload capacity. To overcome these drawbacks the author has conceived the idea of providing self-excited synchronous induction motors with a constant synchronizing torque, to run the machines synchronously from no load to a little beyond full load, and to run them asynchronously for all higher loads. The manner in which a constant synchronizing torque is produced is explained in detail and it is shown how the somewhat conflicting requirements of synchronizing torque and acceptable compounding characteristic are harmonized. Several embodiments of the resulting new motors are described, their synchronous performance being analysed by means of the circle diagram.