MAR-APR 2019

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INTECH MARCH/APRIL 2019 23 Components and subsystems A machine's automation system primarily consists of power and control components. For a smaller machine, these may be housed in one panel (fig ure 1); whereas larger machines may require multiple panels, often one for control and another for power. The main subsystems and components of a machine automation system are: l power distribution l motor control and drives l safety system l programmable controllers l discrete and analog I/O l communication systems l human-machine interface (HMI) The power distribution subsystem feeds pow- er to components, such as motors, drives, and controllers. The control subsystem primarily consists of safety systems, programmable con- trollers, discrete and analog I/O, communica- tion systems, and HMIs. Let's look at each of these areas in more detail. Power distribution The National Electric Code (NEC, also NFPA 70) has much to say about using electricity properly to safe- guard persons and property. The code comes into play well before the power source connects to the machine control enclosure through a plug, discon- nect, or terminal block. At the machine, the NFPA 79: Electrical Standard for Industrial Machinery is the benchmark for industrial machine safety related to fire and electrical hazards. Some of the major re quirements in machine control power distribution discussed in these standards include using proper disconnect means, protecting personnel from con- tact with electrical hazards, and protecting equip- ment from overcurrent and overloads. The disconnect—whether a switch, circuit breaker, or cord with a plug—must be provided for any control enclosure fed with voltages of 50 VAC or more. It should be properly sized, positioned, wired, labeled, and, in some cases, interlocked to the enclosure door. Protecting personnel from contact with electrical hazards is always needed, both inside and outside a machine power or control panel. All conductors must be protected from contact by personnel. Most power distribution devices are designed to facilitate this level of protection, but live components, such as power buses, distribution blocks, and other power terminals, should be covered with a non - con ductive, see-through cover. Protecting equipment from overcurrent is criti- cal to reduce the chance of fire. Conductors and electrical components must be protected from FACTORY AUTOMATION overcurrent related to short circuits. Overcur- rent protection devices, such as fuses and cir- cuit breakers, must be sized based on conduc- tor current-carrying ca- pacity, device interrupt rating, maximum fault current, system voltage, load characteristics, and other factors. For power circuits, branch-circuit-rated devices must be used to meet current-limiting and ground fault protection requirements. Supplemental overcurrent protective devices are not suitable for use in these circuits but work well in downstream control circuits tapped from the load side of the branch circuit. Motor control and drives Motors have special needs in machine control. For every motor, a proper form of electrical control is required, from simple on/off to more complex variable speed applications. Motor control devices include manual motor starters, motor contactors and starters with overloads (figure 2), drives, and soft starters. A motor circuit must include both overcurrent (short circuit) and overload protection. This typi- cally consists of branch-circuit protection, such as properly rated fuses, and a motor starter with over- load protection devices, such as thermal overloads, but additional protection may be needed. Additional protection to consider for machine control components includes loss of cooling and abnormal temperatures. Ground fault protection is also needed, so a proper ground connection is important. Over, under, and loss of voltage must also be considered. Protection from lightning, overspeed, and loss of a voltage phase in three-phase supplies are additional con- siderations for proper machine control. Some motor control- lers, such as drives and combination control- lers, are self-protected. If this is the case, the device's rating or man- ufacturer's instruc- tions will clearly note it is suitable for output conductor protection. FAST FORWARD l Power needs to be distributed to a machine's motors, drives, controllers, and other components. l The machine's safety system must remove motion-causing energy when called upon, including both electrical and fluid power. l It is a good practice to have multiple Ethernet and serial ports available to integrate to a variety of equipment, computers, HMIs, and business and enterprise systems. Figure 1. For smaller machines, a single panel is often used to house both the power distribution system and the control components.

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