Methods of varying the speed of alternating-current motors

Electrical developments of recent years have been toward the extension of the use of alternating currents in every field where practicable, due to the advantages of distributing systems and the simplicity of the induction motor. For constant speed work, the field is well covered by the squirrel cage and polar wound motors. Various methods have been proposed for varying and adjustable speed work and this paper is to discuss the methods used and proposed.