Abstract :
Of the various systems for controlling electric motors, none surpass in interest and economic importance those which make use of electrically actuated switches controlled by automatic or semi-automatic means. A few years ago the drum controller was quite sufficient for any motor then in use. Since then the use of larger motors and the need for automatic features, unattainable in a simple manner with drum controllers, have resulted in the development of a great variety of magnetically controlled switches, and of various means for operating them in control systems.
