two major type of power source are used in inducstrial appcations: alternating current (ac) and current (dc). alternating current source are common in inducstrial installations and residences. these sources can either be single-phase or multiphase system. single-phase power source are common in residences, where the demand for electric power is limited. muliphase are used in high power source consumption appcations. the most common type of power source in the United States is the three-phase, 60 Hz power source.In Europe, most of the Middle East, Africa, and Asia, the frequency is 50 Hz.