L OADING C O N S I D E R AT I O N S
L OADING C O N S I D E R AT I O N S
The system power supply provides the DC power required by the logic circuits of the CPU and the I/O circuits. The power supply has a maximum amount of current that it can provide at a given voltage level (e.g., 10 amps at 5 volts), depending on the type of power supply. The amount of current that
a given power supply can provide is not always sufficient to satisfy the requirements of a mix of I/O modules. In such a case, undercurrent conditions can cause unpredictable operation of the I/O system.
In most circumstances, an undercurrent situation is unusual, since most power supplies are designed to accommodate a mix of the most commonly used I/O modules. However, an undercurrent condition sometimes arises in applica- tions where an excessive number of special purpose I/O modules are used (e.g., power contact outputs and analog inputs/outputs). These special pur- pose modules usually have higher current requirements than most commonly used digital I/O modules.
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Power supply overloading can be an especially annoying condition, since the problem is not always easily detected. An overload condition is often a function of a combination of outputs that are ON at a given time, which means that overload conditions can appear intermittently. When power supply loading limits have been exceeded and overload occurs, the normal remedy is to either add an auxiliary power supply or to obtain a supply with a larger current capability. To be aware of system loading requirements ahead of time, users can obtain vendor specifications for I/O module current requirements. This information should include per point (single input or output) require- ments and current requirements for both ON and OFF states. If the total current requirement for a particular I/O configuration is greater than the total current supplied by the power supply, then a second power supply will be required. An early consideration of line conditions and power requirements will help to avoid problems during installation and start-up.
Power Supply Loading Example. Undoubtedly, the best solution to a problem is anticipation of the problem. When selecting power supplies, current loading requirements, which can indicate potential loading problems, are often overlooked. For this reason, let’s go over a load estimation example.
Consider an application where a PLC will control 50 discrete inputs and 25 discrete outputs. Each discrete input module can connect up to 16 field devices, while each output module can connect up to 8 field devices. In addition to this discrete configuration, the application requires a special servo motor interface module and five power contact outputs. The system also uses three analog inputs and three analog outputs.
Figure 4-22 illustrates the configuration of this PLC application. The first plug-in module is the power supply, then the processor module, and then the I/O modules.
Slot 00 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
processor
power supply
contact output
digital output
digital output
digital output
digital output
digital input
digital input
digital input
digital input
servo motor
analog input
analog output
Application Note Power supply requires one slot (slot 00).
Processor requires one slot (slot 0). Twelve I/O slots are used, four are spare. Auxiliary power supplies, if required, must be placed in slot 8.
Figure 4-22. Configuration of an example PLC.
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The first step in estimating the load is to determine how many modules are required and then compute the total current requirement of these modules. Table 4-1 lists the module types, current requirements for all inputs and outputs ON at the same time, and the available power supplies for our programmable controller example.
T y p e B B 5 a m p s Table 4-1. Listing of modules and their current requirements.
The total power supply current required by this input/output system is 4655 mA, or 4.655 amps. Adding this current to the 1.2 amps required by the processor results in a total of 5.855 amps, the minimum current the power supply must provide to ensure the proper operation of the system. This total current indicates a worst-case condition, since it assumes that all I/Os are operating in the ON condition (which requires more current than the OFF condition).
For this example, there are several power supply options. These options include using a 6 amp power supply or using a combination of a smaller supply with an auxiliary source. If no expansion is expected, the 6 amp power source will suffice. Conversely, if there is a slight possibility for more I/O requirements, then an auxiliary supply will most likely be needed. The addition of an auxiliary supply can be done either at setup or when required; however, for the controller configuration in Figure 4-22, the auxiliary source must be placed in the eighth slot, resulting in I/O address changes if the auxiliary supply is added after setup. Therefore, the reference addresses in the program will have to be reprogrammed to reflect this change. Also, remember that the larger the power supply, the higher the price in most cases. You must keep all these factors in mind when configuring a PLC system and assigning I/O addresses to field devices.
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
S ECTION Components Processors, the Power Supply, C HAPTER 2 and Systems
and Programming Devices 4