Chapter 6 Electromechanical Systems 133 Copyright Goodheart-Willcox Co., Inc. Most motors used in industry operate using a three-phase ac power source. Three-phase ac motors are often called the workhorses of industry. The two basic types are induction motors and synchronous motors. Three-phase induction motors have a squirrel cage rotor. Since three-phase voltage is applied to the stator, no external starting mechanisms are needed. Three- phase induction motors are made in a number of sizes and have good starting and running torque characteristics. Three-phase induction motors are used in indus- trial applications, such as machine tools, pumps, elevators, hoists, and conveyors. In Figure 6-15, the three-phase power source is connected directly to the windings of the stator. Phase A is connected to windings A1 and A2, which are located 180° apart. Phase B connects to windings B1 and B2, also 180° apart. Phase C follows the same pattern. The windings are distributed 360° around the entire stator. The beginnings of each phase winding (A1, B1, C1) are located 120° apart. This is equivalent to 360° divided by three (or the total number of phases). Like- wise, the ends of the windings (A2, B2, C2) are located 120° apart. The stator wind- ings are energized with the three-phase voltages, causing the rotor to rotate in a constant direction. Reversing any two of the winding connections (A and B, B and C, or A and C) of a three-phase ac motor causes the rotor to rotate in the opposite direction. Three-phase synchronous motors are unique and very specialized. These motors deliver constant speed and can be used to correct power factors of three- phase systems. Direct current is applied to the wound rotor to produce an elec- tromagnetic field. Three-phase ac power is applied to the stator, which also has fi windings. Squirrel-cage rotor A1 A2 C2 B1 C1 B2 Distributive-wound stator pole Phase A Phase C Phase B Goodheart-Willcox Publisher Figure 6-15. Three-phase ac induction motors are widely used for industrial applications and do not require an external starting mechanism.