The Economics of System Breakpoint for AC Motor Voltages

Proper voltage selection for certain size ac motors is very important in providing the most overall economic installation within good engineering practice. In addition to determining motor equipment costs, there are certain areas of motor horsepower ratings where the engineer must analyze the application and situation present in his plant in order to select the most economical voltage and machine to do a job. A number of factors such as motor equipment costs, construction, plant systems, type of motor, application, efficiency, power factor, and safety and codes are discussed in relation to motor voltage selection. A study was made on relative costs for 440-and 2300-volt induction and synchronous motors, complete with controls and substations, covering a range from 50 to 800 horsepower. Cost curves at speed ratings from 1800 r/min to 720 r/min show the approximate horsepower breakpoint and cost relationships as a function of motor horsepower. A brief discussion is presented on equipment costs when operating ac motors from 4160 and 13 200 primary voltages