Low impact motor control with star-delta starting

Starting motors using 'across-the-line' techniques has a heavy impact on the power delivery system. For rural, power capacity limited and emergency delivery systems or storefront manufacturing operations, this is unacceptable. With the interest in world trading, exporters must be aware that most foreign (to the USA) countries are less tolerant of high motor starting currents than US domestic users. The technical expertise of the foreign plant maintenance staff may not support electronic starters which reduce starting currents. Three phase star-delta starting arrangements, a well known but often bypassed technique, can meet the requirements if the associated reduced starting torque is acceptable. This paper reviews the calculation methods for determining starting currents and torques. Procedures for selecting the correct motor overload devices, motor branch circuit wire sizes and starting device configurations are presented. Motor starting and protection diagrams are presented and discussed. The benefits and limitations of this starting technique are discussed.