22-03-2012, 03:08 PM
PLC Based Load Sharing
PLC Based Load Sharing.docx (Size: 360.02 KB / Downloads: 114)
Introduction
Efficient load control of electrical generating plant is of critical importance to the operation of a DP Classed vessel. Although DP vessels have been operating for almost thirty years and Classification Society Rules governing the design and construction of such vessels have been in place, and undergoing continuous revision, since the nineteen eighties, experience has shown that a number of misconceptions remain where load sharing systems are involved.
This paper seeks to identify the most common load control and power management philosophies, their strengths and weaknesses in relation to DP classification and to assist the system designer in determining the most suitable implementation for a given project application.
Although a number of design solutions exist in the more complex implementations the basic topologies and philosophies are largely similar and will be treated generically in the paper. However, in less complex systems solutions may be unique to a particular manufacturer. In such instances the manufacturer will be specifically identified. This identification is not intended to indicate the author’s endorsement or preference for any one product in relation to another.
CPU
Like other computerized devices, there is a Central Processing Unit (CPU) in a PLC. The CPU, which is the brain of a PLC, does the following operations: Updating inputs and outputs. This function allows a PLC to read the status of its input terminals and energize or de- energize its output terminals. Performing logic and arithmetic operations. A CPU conducts all the mathematic and logic operations involved in a PLC. Communicating with memory. The PLC’s programs and data are stored in memory. When a PLC is operating, its CPU may read or change the contents of memory locations.
Memory
Memory is the component that stores information, programs, and data in a PLC. The process of putting new information into a memory location is called writing. The process of retrieving information from a memory location is called reading. The common types of memory used in PLCs are Read Only Memory (ROM) and Random Access Memory (RAM). ROM location can be read, but not written. RO Misused to store programs and data that should not be altered. For example, the PLC’s operating programs are stored in ROM.
Input modules and output modules
A PLC is a control device. It takes information from inputs and makes decisions to energize or de-energize outputs. The decisions are made based on the statuses of inputs and outputs and the ladder logic program that is being executed. The input devices used with a PLC include pushbuttons, limit switches, relay contacts, photo sensors, proximity switches, temperature sensors, and the like. These input devices can be AC (alternating current) or DC (direct current).The input voltages can be high or low. The input signals can be digital or analog. Differing inputs require different input modules. An input module provides an interface between input devices and a PLC’s CPU, which uses only a low DC voltage. The input module’s function is to convert the input signals to DC voltages that are acceptable to the CPU. Standard discrete input modules include 24 V AC, 48 V AC, 120 V AC, 220 V AC, 24 V DC, 48 V DC, 120 V DC, 220V DC, and transistor-transistor logic (TTL) level.