Abstract :
The paper deals with the practical development of an electric motor starter from the first principles relating to current, torque, and resistance values, to the data required by the salesman. First it describes the method adopted of working in percentages for all calculations; defines what is meant by starting, accelerating and running torques; refers to the lack of data regarding starting conditions; and gives tables of stored energy in rotors, for the normal load torque during acceleration and for the stored energy of a number of typical machines. Accelerations are classified as ¿natural regular¿, ¿natural irregular¿ ¿forced regular¿ and ¿forced irregular¿ and typical diagrams with automatic contactor and handoperated starters are shown. A formula is given for estimating accelerating times and torques, and the number of notches from the point of view of breaking capacity is then considered. The rating of the resistance elements, and points arising out of continuous and intermittent rating, together with diagrams for air cooling, and radiation and diffusion curves for oil-immersed resistances, are next dealt with. Certain difficulties for the salesman in tabulating the horse-power and resistance sizes are referred to, and the necessity of higher standard temperature-rises is urged. Unbalanced rotor currents and the methods of calculating suitable resistance values are dealt with, and the accuracy which can be obtained on test is shown. Eddy-current, rheostatic stator, auto-transformer and star-delta starters are referred to, and the entirely different type of current and torque curves obtained are shown. Methods of rapidly determining the starter proportions for various duties are given. Certain points regarding the rating of auto-transformers are raised, and a test showing the effect of unequal tap voltages is given.
