Chapter 6 Electromechanical Systems 121 Copyright Goodheart-Willcox Co., Inc. power, electrons flow first fl in one direction and then in the opposite direction. fi Each repeated pattern of direction change is called a cycle, Figure 6-2. The alter- nating current required for an automated manufacturing system may be either single-phase or three-phase. Single-phase ac power consists of one alternating current, Figure 6-2. Three-phase ac power consists of three alternating currents that vary by 120°. Three-phase alternating current is ordinarily used for larger systems because it can handle heavier power needs. Three-phase motors are dis- cussed later in this chapter. Some machines used for automated manufacturing require direct current (dc). In direct current, electrons fl ow in only one direction. Many robotic systems have fl motors and other parts that use direct current. The process of converting alternat- ing current to direct current is called rectification. fi Rectifi is the most conve- fication nient and inexpensive method of providing dc energy to machines. 6.1.2 Transmission Path The transmission path is a subsystem that provides a channel for the transfer of energy. It begins at the energy source and continues through the system to the load device. The term load refers to a part (or parts) designed to produce work. This path may be a single feed line, electrical conductor, light beam, or pipe. Some systems may have a supply line and a return line between the source and the load. There may also be a number of alternate transmission paths in the system. These may be connected in series to a number of small load devices or in parallel to many independent devices. 6.1.3 Control The control alters the fl ow of power and causes some type of operational change fl in the system, such as changes in electric current, hydraulic pressure, light inten- sity, or airflow. Control devices operate within the transmission path. In its sim- fl plest form, control occurs when a system is turned on or off. This type of control can take place anywhere between the source and the load device. TIME V O LTA GE 0 +V –V Cycle 1 Cycle 2 Cycle 3 Goodheart-Willcox Publisher Figure 6-2. This illustration depicts the voltage cycles in ac power. Each cycle contains two changes in direction—a high peak and a low peak.