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Chapter 11 Troubleshooting PLC Hardware

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1 Chapter 11 Troubleshooting PLC Hardware
PLC Hardware Problems • Power Supply Problems • Troubleshooting PLC Input Sections or Modules • Troubleshooting PLC Output Sections or Modules

2 PLCs are designed for use with all common supply voltages, such as 230 V, 208 V, 115 V, and 24 V.
All electrical devices and components must be powered by a power supply that can deliver the correct voltage level with enough current capacity. When a PLC is used in a system, several individual power supplies must be considered. First, a PLC must be powered for internal circuitry operation to deliver output power for operating PLC input devices and/or output components. Often, the power supplied to a PLC is different than the power supplied to PLC input devices. For example, PLCs that have 115 VAC supply power can deliver 24 VDC output power for powering PLC input devices, such as proximity and photoelectric switches, and/or output components, such as lamps and starter coils. See Figure 11-1.

3 When problems occur with PLC power supplies, input and/or output modules, or any system components, a check of the supplied power should be performed over time using a test instrument with a recording function. For any reoccurring problems, such as damaged PLC power supplies, input and/or output sections or modules, or other normally reliable system components, a check of the supplied power is required. A check of the supplied power should be performed over time (8 hr,12 hr, or 24 hr) using a test instrument with a recording function, such as the MIN MAX recording function on digital multimeters or the record function on power quality meters. Digital multimeters can record voltage or current over time and power quality analyzers can record voltage, current, power (VA, watts, power factor), transients, and harmonics over time. See Figure 11-2.

4 Short circuits typically occur when conductor insulation is damaged, which allows a current-carrying conductor to come in contact with any grounded noncurrent-carrying metal parts. An overload is an overcurrent condition that occurs when circuit current exceeds normal PLC operating current and/or designed circuit current. Fuses and circuit breakers are used in electrical circuits to protect the circuits from short circuits and overloads. See Figure A short circuit is an overcurrent condition in which the current of a circuit leaves the normal current-carrying path by going around the load back to the power source or ground uncontrolled. Short circuits typically occur when conductor insulation is damaged, which allows a current-carrying conductor to come in contact with a neutral conductor, ground conductor, or any grounded noncurrent-carrying metal parts.

5 Voltage measurements can be used to test fuses that are connected to a circuit.
A fuse is an overcurrent protection device that includes a fusible link that melts and opens a circuit when an overcurrent condition occurs. Fuses are connected in series with the circuit being protected. Fuses protect a circuit from overcurrents and short circuits. Electrical circuits include fuses to protect the incoming power supply circuit, control circuits, and individual components such as PLC power supplies, PLC sections or modules, input devices, and output components. Fuses used to protect AC or DC circuits are tested using test instruments that can measure voltage. See Figure 11-4.

6 Resistance measurements are used to test fuses that have been removed from a circuit.
Fuses removed from a circuit can be tested using a test instrument set to measure resistance (ohmmeter). See Figure 11-5.

7 Circuit breakers perform the same function as fuses and are basically tested the same way.
A circuit breaker (CB) is a reusable overcurrent protection device that opens a circuit automatically at a predetermined overcurrent. CBs are connected in series with the circuit being protected. CBs (like fuses) protect a circuit from overcurrents and short circuits. CBs are thermally or magnetically operated and must be reset after an overload. Circuit breakers perform the same function as fuses and are basically tested the same way. See Figure 11-6.

8 Control transformers are typically used to step down 115 VAC, 208 V, 230 V, or 460 V to 12 VAC or 24 VAC for control circuit use in a PLC enclosure. In most circuits in which the load is rated higher than 115 VAC (208 V, 230 V, or 460 V) the control circuit is operated at a lower voltage level than the loads. A step-down control transformer is used to step down the voltage to the level required in the control circuit. In the past, the secondary side of the control transformer was rated at 115 VAC for most control circuits, but today 12 VAC and 24 VAC are becoming more common, with 24 VAC being the most common. See Figure 11-7.

9 When the power rating of a transformer is exceeded by placing too great a load on the transformer, the voltage on the secondary side will start to decrease and circuit problems will develop. All control transformers have a fixed power output rating (in VA or kVA). When the power rating of a transformer is exceeded by placing too great a load on the transformer, the voltage will start to decrease and circuit problems will develop. See Figure 11-8.

10 PLCs can have a single fixed voltage rating or a dual voltage rating
PLCs can have a single fixed voltage rating or a dual voltage rating. Dual voltage rated PLCs (115/230 VAC) have a selector switch (or movable links) to set the PLC to one of the possible voltage ratings. When PLC specifications are available, the manufacturer will list the exact voltage range in which a PLC properly operates. For example, a 115 VAC rated PLC may have an 85 VAC to 132 VAC listed range. Although the PLC should operate satisfactorily within the specified range, any voltage at either end of the rating requires the power supply feeding the PLC to be checked and corrected if possible. See Figure 11-9.

11 Output power supplies of PLCs are typically used to supply voltage to input devices connected to the PLCs. Some PLCs supply an output voltage that can be used to supply power to input devices and/or output components. See Figure When a PLC supplies an output voltage, the output power supply will have both a voltage rating and a current rating. For example, typical PLC output ratings are 24 VDC/200 mA. The output voltage rating is what the voltage output of a PLC will actually be, but the current rating is a range. For example, a 200 mA rating means that any amount of current from 1 mA to 200 mA can safely be drawn from the power supply.

12 The listed current rating of a power supply must be accepted as the maximum amount of current that can safely be provided from a PLC output power supply. An in-line ammeter is typically used to test for output power supply current. Some applications will require a clamp-on ammeter to be used. See Figure

13 When an input signal is sent to a PLC, the PLC conditions, filters, and optically isolates the signal. Input devices, such as limit switches and temperature switches are connected to the input section or module of a PLC. When an input signal is sent to a PLC, the PLC detects the current flow and voltage and processes the signal. See Figure

14 When troubleshooting PLC input sections, programming diagrams and manufacturer troubleshooting charts indicate how input devices are connected and which output components are being controlled. Troubleshooting the input section or module of a PLC requires knowledge of how the programmed circuit should be operating and how the PLC input section or module works. The PLC program can be viewed on the monitor or printed out. Using a programming diagram when troubleshooting a PLC input section or module aids in understanding how the input devices are connected and which loads are being controlled. Manufacturers also provide troubleshooting charts to help isolate input section or module problems. See Figure See Appendix.

15 Testing input modules requires that power supplies and input devices be tested and that status lights of input modules and symbols on computer monitors be checked. Signals and information are sent to a programmable controller using input devices such as pushbuttons, limit switches, level switches, and pressure switches. The input devices are connected to the input section or module of the programmable controller. When a PLC does not receive the proper information from input devices (opening and closing contacts as designed) or the input section or module is not operating correctly the controlled system cannot operate properly. See Figure

16 All input devices and the PLC program must operate correctly in order for an automated circuit to operate properly. Input devices such as pushbuttons, limit switches, pressure switches, and temperature switches are connected to the input section or module of a PLC. Input devices send information and data concerning circuit and process conditions to the PLC. The PLC processor receives the information from the input devices and executes the program. All input devices and the PLC program must operate correctly for the circuit to operate properly. A major advantage to using a PLC to control a circuit/system is that all the PLC inputs and outputs can be monitored. The ability to monitor the inputs of a circuit is an advantage when troubleshooting a problem. See Figure

17 Proper heat sinking and cooling is required with solid-state devices to eliminate any potential heat problems. In a solid-state switch, there are no moving contacts. Instead, a solid material is used that can be switched from a very high resistance to a very low resistance. There are numerous advantages to solid-state switching, such as an extremely long operation life, much faster operating speed (ON/OFF), and no contact bouncing. However, there are a few disadvantages to solid-state switching. Because solid-state “contacts” never go to 0 Ω of resistance when closed, there is always a voltage drop, which produces heat at the contacts. Proper heat sinking and cooling is required with solid-state devices to eliminate any potential heat problems. See Figure

18 The current that flows through an “open” solid-state switch is called leakage current. Leakage current can turn on the input circuitry of a PLC, affecting system operation. Some loads, such as PLC input circuitry, require a very small amount of current to turn ON and can be affected by solid-state device leakage current. PLC input terminals only require a small amount of current to signal the presence of an input signal from an input device. Thus, leakage current from a solid-state switch such as a proximity switch can send a false signal to a PLC input section or module. See Figure

19 Testing the leakage current of solid-state devices requires that the OFF state leakage current measurement be compared to the specified minimum operating current of the PLC input module. The load resistor acts as an additional lower resistance load, which allows the leakage current to flow through the lower resistance path. Typically, a 10 kΩ to 20 kΩ resistor is used to solve a leakage current problem. See Figure

20 Signals from the CPU of a PLC are sent through logic circuits to output section status lights and opto-isolation circuits before being sent to output terminals. Output components are connected to the output section or module of the PLC. The output signal flows from the PLC processor control circuits out to the output drivers in the opposite direction of the input signal. See Figure

21 When troubleshooting PLC output sections, programming diagrams and manufacturer troubleshooting charts help the troubleshooter/technician understand how the input devices are connected and which output components are being controlled. Troubleshooting the output section or module of a PLC requires knowledge of how the programmed circuit should be operating and how the PLC output section or module works. The programmed circuit can be viewed on the monitor and/or printed out. Using the programmed circuit when troubleshooting the PLC output section or module is necessary to understand when the output components are to be energized based on input device conditions. Manufacturers also provide troubleshooting charts to help isolate output section or module problems. See Figure See Appendix.

22 Testing output modules requires that power supplies and output components be tested and that status lights of output modules and symbols on computer monitors be checked. A PLC turns the output components (loads) in a circuit ON and OFF according to the program. The output components are connected to the output section or module of the programmable controller. When an automated system is not producing quality work, the problem may lie in the output section or module, output component, or controller. See Figure

23 All output components and the PLC program must operate correctly in order for an automated system to operate properly. Output components such as motor starters, solenoids, contactors, and lights are connected to the output section or modules of a programmable controller. An output component performs the work required for an application. The processor energizes and de-energizes the output components according to the program. All output components must operate correctly for a circuit to operate properly. A major advantage to using a PLC to control a circuit or system is that all PLC inputs and outputs can be monitored. The ability to monitor circuit input devices and output components aids in troubleshooting a problem. See Figure


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