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Chapter 7 PLC and System Interfacing

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Presentation on theme: "Chapter 7 PLC and System Interfacing"— Presentation transcript:

1 Chapter 7 PLC and System Interfacing
Systems • Primary Systems • System Interfacing • Electrical Circuits • Basic Electrical Circuits • Improving Basic Electrical Circuits • Complex Electrical Circuits • Interfacing Circuits • Interface Devices • Electromechanical Relays • Solid-State Relays • Contactor Interfaces • Motor Starter Interfaces • Electric Motor Drive Interfacing

2 Primary systems transmit and control the movement of energy.
There are several types of primary systems used to produce work. A primary system is a system that transmits and controls the movement of some form of energy. Primary systems include electrical, electronic, fluid power, and mechanical systems. See Figure 7-1. Work is the movement of a load (in pounds) over a distance (in feet). Primary systems are used individually or in combination, or are interfaced to perform the required work.

3 Electrical systems use alternating current (AC) or direct current (DC) to produce power.
An electrical system is a primary system that produces work by transmitting and controlling the flow of electricity through conductors (wires). Either alternating current (AC) or direct current (DC) is used to produce the required power. See Figure 7-2.

4 Electronic systems monitor and control electricity to send and/or receive information, produce sound or vision, store data, control circuits, or perform other work. An electronic system is a primary system in which electricity is monitored and controlled to send and/or receive information, produce sound or vision, store data, control circuits, or perform other work. Electronic circuits use either analog or digital electrical signals. An analog signal is an electronic signal that has continuously changing quantities (values) between defined limits. A digital signal is an electronic signal that has two specific quantities that change in discrete steps (ON or OFF). See Figure 7-3.

5 Fluid power systems produce work by transmitting fluid under pressure through pipework.
A fluid power system is a primary system that produces work by transmitting fluid (gas or liquid) under pressure through pipework. A pneumatic system is a fluid power system that transmits power using a gas (air). A hydraulic system is a fluid power system that transmits power using a liquid (typically oil). See Figure 7-4.

6 Mechanical systems transmit power using gears, belts, chains, shafts, couplings, and linkages.
A mechanical system is a primary system in which power is transmitted through gears, belts, chains, shafts, couplings, and linkages. See Figure 7-5. Mechanical energy can be transmitted through multiple devices, such as from belt to gear to shaft.

7 Informational systems display electrical, electronic, fluid power, and mechanical quantities and conditions to indicate the status of a circuit, process, or application. An informational system is a primary system that monitors operations, displays quantities (values), and indicates the status of machines and processes. Electrical, electronic, fluid power, and mechanical quantities and conditions are displayed, indicating the status of the circuit, process, or application. The informational variables that are displayed and recorded include time, temperature, speed, weight, voltage, current, power, flow rate, level, volume, counts, color, brightness, and pressure. See Figure 7-6.

8 Interfacing systems interconnect primary systems of various types.
An interfacing system is a system or device that allows primary systems to work and/or communicate as one. Primary systems operating at one voltage level (12 V, 115 V) or of one type (electrical or hydraulic) are interconnected so that various parts of the systems can operate together. Interfacing systems allow parts of primary systems that cannot directly work together to be compatible. See Figure 7-7.

9 Combination systems such as industrial robots interconnect two or more primary systems (hydraulic, pneumatic, mechanical, electrical, electronic, digital, and welding), combining the individual advantages of each system to meet the requirements of a given application. A combination system is a system that interconnects two or more primary systems to combine the individual advantages of each system to meet the requirements of a given application. If the parts are compatible, the systems can be directly connected. However, combination systems are typically connected using interface devices. See Figure 7-8.

10 System interfacing permits devices and components of various levels of voltage, current, and power to work together as a system. System interfacing permits devices and components of various levels of voltage, current, and power to work together as a system. An interface device is an item that allows variously rated components to be used together in the same circuit. Interface devices convert one form or level of energy (electrical, electronic, fluid power, etc.) to another form. With interface devices, almost any machine or process change can be accommodated. See Figure 7-9.

11 Circuit conditioning is required any time the existing electrical power is not at the proper phase, voltage, or current level for the application. Circuit conditioning is required any time the existing electrical power is not at the proper phase, voltage, or current level for the application. The electrical power may need to be filtered, raised, lowered, or changed to correct the condition. Electronic systems often require voltage or current level adjustments, signal coding, signal de-bouncing, or filtering of the electrical power. Fluid power systems may require that pressure be raised or lowered, that flow rate be raised or lowered, or that temperature be lowered. Typical circuit conditioning in an electrical system requires changing (rectifying) AC to DC. Bridge rectifiers are the most efficient and common rectifier used in 1 rectification circuits. See Figure 7-10.

12 Interface devices are required any time an input device or output component is not directly compatible with a PLC. In any system using a PLC, interface devices are required any time an input device is not directly compatible with the input section of the PLC. Likewise, an interface device is required any time an output component is not directly compatible with the output section of the PLC. For example, transformers, magnetic motor starters, and solenoid-operated fluid power valves are required interface devices for a PLC controlling a baking process. See Figure 7-11.

13 Interface devices change or condition a system in a variety of ways.
Interface devices change or condition a system. See Figure Interface devices can perform the following functions: • multiply the number of available output contacts • allow one voltage level to control another voltage level • allow a small current to control a large current • allow small DC electronic signals to control AC circuit loads of any size • change one voltage level to another voltage level • change the control logic of a circuit

14 Basic electrical circuits must include a source of electricity, a method of controlling the flow of electricity, and a component that converts electrical energy into some other usable form of energy. A circuit must also include protection device(s) to ensure that the circuit operates safely and within designed electrical limits. In addition, a basic circuit must also include a protection device (fuse or circuit breaker) to ensure that the circuit operates safely and within designed electrical limits. For example, a basic electrical circuit used to produce heat includes a power supply (voltage rating will vary depending on the design of the unit); a selector switch and temperature switch for controlling the flow of electricity; heating elements as the load; and circuit breakers and/or fuses for protecting the system from overloads. See Figure 7-13.

15 Basic PLC-controlled circuits with multiple loads wired in parallel have the loads wired separately to the PLC output terminals and the output terminals programmed to be parallel. A heating element control circuit is connected to a PLC so that the PLC controls the heating elements through its programming. The PLC requires two input terminals, one for the selector switch and one for the temperature switch, and two output terminals, one for the heating contactor coil and one for the heater “ON” lamp. See Figure Even though the hardwired heating element circuit and the PLC-controlled heating element circuit both operate in the same manner, there are differences between the two circuits.

16 Basic circuits are improved by adding additional protection, instrumentation for monitoring circuit parameters, and finer controls, and by interconnecting all basic circuits into a system. All electrical circuits begin as basic circuits. In a basic electrical circuit, electricity is delivered from the power supply, protected from overcurrents and overloads, controlled by switches, and used to produce work through output components. Basic circuits can be improved by adding additional protection (ground-fault detection and/or phase-loss protection); adding instrumentation for monitoring circuit parameters; adding controls for loads (dimming lamps, timing and sequencing load operations); and interconnecting all basic circuits in a system. See Figure 7-15.

17 PLCs and PCs are interconnected through communication lines to form various types and levels of networks. In a network, input devices such as pushbuttons, limit switches, temperature switches, smoke detectors, and pressure switches, and output components such as motors, lamps, solenoids, and alarms are interconnected to PLCs, PCs, and/or other electronic control and monitoring devices. PLCs are used to monitor the input device circuits and control the output component circuits. PCs, along with PLCs, are added to allow the gathering, monitoring, controlling, and displaying of system data. PLCs and PCs are interconnected through communication lines to form various types and levels of networks. See Figure 7-16.

18 Sensors and switches are typical input devices to a PLC, with lighting, motors and information displays being typical output components for a PLC. On a plant floor, input devices and output components are connected to PLC input and output modules (or sections). Various input module types allow analog and digital input devices to send information to the PLC. An analog input device sends a continuously changing variable signal to a PLC or PC. Temperature, pressure, flow, and level sensors are common analog input devices for a PLC-controlled system. ON/OFF switches such as pushbuttons, limit switches, and toggle switches are common digital input devices. A digital input device is a device that is either ON or OFF (open or closed). See Figure 7-17.

19 Relays, contactors, and motor starters are the most common interface devices used to control high-power loads. Once all loads (lamps, motors, or heating elements) are identified, the exact type of control devices and output components required to make the circuit operate as designed are selected. Low-power output components are connected directly to the output terminals of the PLC as long as the output component voltage, current, and power ratings are less than the maximum rating of the PLC output module. An interface device is required when the power requirements of the load are higher than the ratings of the output module. A high-power load is a load whose voltage and current rating is greater than the voltage and current rating of the PLC output. The most common interface devices used to allow PLCs to control high-power loads are relays, contactors, and motor starters. See Figure 7-18.

20 Electromechanical relays have sets of contacts that are closed by magnetic force.
A control relay is a device that controls an electrical circuit by opening and closing contacts in another circuit. Control relays can be electromechanical or solid-state. An electromechanical relay is a switching device that has sets of contacts that are closed by magnetic force. In an electromechanical relay circuit, a coil is used to close normally open contacts with a magnetic force that develops when the coil is energized. Control relays are described by the number of poles, throws, and breaks the relay has. See Figure See Appendix.

21 Solid-state relays have an input voltage range, such as 3 VDC to 32 VDC, that allows a single solid-state relay to be used with most electronic circuits and PLC output modules. A solid-state relay is a relay that uses electronic switching devices in place of mechanical contacts. In a solid-state relay, the coil is replaced by an input circuit and the contacts are replaced with a solid-state switching component (transistor, SCR, or triac). See Figure 7-20.

22 The type of relay used depends on the life expectancy, electrical requirements, and cost requirements of the application. The type of relay used for an application depends on the life expectancy, electrical requirements, and cost requirements of the application. See Figure Electromechanical relays are typically rated for 250,000 operations, and solid-state relays can be rated for billions of operations. Either type of relay can be used in applications that require a nominal amount of switching operations and where switching time is not important. However, in applications that require thousands of switching operations a day, such as with flashing lights and high-speed production lines, solid-state relays are used.

23 Any load that can cause a problem to the output section or module of a PLC must include an interface device between the PLC and the load. Although there are applications using low-power devices in which the output load can be directly connected to a PLC output section or module safely, any load that can cause a problem must include an interface device between the PLC and the load. See Figure For example, solenoids can be rated for low-power while energized, but the coil of a solenoid draws high current when energized and produces high voltage spikes (transients) when de-energized.

24 The output side of a solid-state relay can be used to control high-voltage medium-power loads such as 3f heating elements. A few solid-state relays are available with multiple output (multicontact) switching devices. Solid-state relays with three output contacts are typically used to control medium-power loads. The low-voltage input (coil) side of a solid-state relay is connected to the output module or section of the PLC. The output side of a solid-state relay can be used to control high-voltage medium-power loads such as 3 heating elements. See Figure 7-23.

25 Contactors are designed to control high-power, non-motor loads.
A contactor is a control device that is designed to control high-power, non-motor loads. Contactors are relays that have high-current contacts. Contactors control high-power loads and/or many loads on an individual circuit (several loads connected in parallel). The most common use for contactors is in high-power lighting circuits. See Figure 7-24.

26 Contactors allow PLC output circuitry to be rated much lower than the loads that the PLC is controlling. Contactors can also be used as heating contactors. Heating elements draw high currents in order to produce heat. A 1200 W heating element will draw 10.5 A on a 115 V circuit, or 5.2 A on a 230 V circuit. The high current cannot be passed through PLC output contacts. See Figure 7-25.

27 Magnetic motor starters use a small control current to energize a coil to send high power to a motor. To allow a PLC to control high-power (horsepower) motors, magnetic motor starters (mechanical starters) or electric motor drives (solid-state starters) are traditionally used. A magnetic motor starter is a motor starter that has an electrically operated mechanical switch (contactor) with overload protection. Magnetic motor starters consist of electrical contacts, a coil to magnetically open and close the contacts, and overload protection. Magnetic motor starters use a small control current to energize a coil to send high power to a motor. See Figure 7-26.

28 When the coil of a motor starter and the control circuit require lower voltage, a step-down transformer is used to reduce the high voltage to a low voltage. The voltage of a magnetic motor starter coil is typically lower than the voltage used by motors. When the coil and control circuit require lower voltage, a step-down transformer is used to reduce the high voltage to a lower voltage. See Figure 7-27.

29 Electric motor drives control motor speed by controlling the frequency of the electricity to a motor. AC drives control and monitor motor speed by converting incoming AC voltage to DC voltage and then converting the DC voltage back to a variable-frequency AC voltage. To change the speed of a motor, electric motor drives vary the frequency (Hz) of the electricity to the motor. A standard 60 Hz AC-rated motor operates at full speed when connected to 60 Hz, at half speed when connected to 30 Hz, and at one-quarter speed when connected to 15 Hz. See Figure 7-28.

30 Any type of switch contacts (pushbutton, pressure switch, PLC output contacts) can be used to control an electric motor drive. Electric motor drives are replacing magnetic motor starters and control circuits. Similar to magnetic motor starters and PLCs, electric motor drives typically have switches remotely located as input devices. However, unlike magnetic motor starters that require an external power supply to energize the magnetic coil, electric motor drives supply voltage to control switches for input signals. See Figure 7-29.


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