6.1 Draw a fully labelled diagram of a PLC scan cycle - NSC Electrical Technology Power Systems - Question 6 - 2021 - Paper 1

1. Flexibility: PLC systems can be easily reprogrammed to accommodate changes in the control tasks without needing physical rewiring.
2. Space Efficiency: A PLC system typically requires less space than a hardwired system which uses a multitude of relays and connections, allowing for more compact designs.

It is crucial to check PLC wiring and connections before power-up to ensure:

1. Safety: To prevent electrical hazards that could result from short circuits or exposed wires.
2. Functionality: To ensure that all connections are intact and correct, avoiding malfunctions in the control system.

PLC systems are safer because:

1. Built-in Safety Features: They come with fault detection and diagnostic capabilities that can identify issues before they lead to a system failure.
2. Remote Monitoring: PLCs can allow for safe monitoring and control from a distance, reducing the risk to operators when faults occur.

The Central Processing Unit (CPU) of a PLC is the brain of the system. It performs all the logical operations needed to process inputs and generate outputs. It executes the programmed instructions to ensure the system operates as intended, managing timing, calculations, and logic operations.

Soft-wired systems utilize digital logic instead of physical wiring. They allow complex interconnections between devices to be represented in software, thus reducing the physical space needed for wiring and allowing for easier modifications or updates to the control logic.

PLC software is the program that is written to control the PLC operations. It includes the logic diagrams and sequences necessary for program execution, and it allows for simulation, testing, and debugging before deployment in real-time control environments.

An analogue signal is a continuous signal that varies over time and can represent a range of values (like temperature or pressure), whereas a digital signal represents discrete values and operates using binary states (0 and 1), making it suitable for digital processing.

Markers (or Flags) are used in PLC programming to store and indicate the status of certain operations or processes. They can be set to true or false based on conditions in the program, providing crucial information about the state of the program.

The contactor in a PLC program is used to control a relay which switches on or off circuits. It can be programmed to operate based on various conditions, enabling the control of higher power devices safely.

A sensor is a device that detects an environmental condition (such as temperature, pressure, or proximity) and converts it into an electrical signal that can be processed for monitoring or control by other devices.

1. Temperature Sensor: Used to measure the temperature of a system.
2. Light Sensor: Used to detect the presence of light or changes in brightness.

1. Object Detection: To determine the presence of an object without physical contact.
2. Speed Measurement: To monitor the speed of a moving object by detecting it at multiple points.

The ladder logic diagram will incorporate rungs for each operation in the manual sequence starter control shown in FIGURE 6.10. The diagram will utilize normally open and closed contacts to conditionally energize coils based on the position of the START and STOP controls.

1. ON-delay Timer: Initiates the output after a specified delay once the input condition is met.
2. OFF-delay Timer: Keeps the output activated for a set time after the input condition is no longer true.

The circuit operates as follows: When the STOP button is not pressed, pressing the START button energizes coil MC1, which closes contact MC1/N.O., latching itself on. Coil MC2 is then energized, activating lamp 1. The lamp will remain ON until the STOP button is pressed again.

1. Transistor: Used as a switching device to control the load.
2. Relay: Utilized to control high-power devices, isolating the PLC from high voltage.

FIGURE 6.14 shows the regenerative braking block diagram used in variable speed drive control systems.

The braking resistor dissipates the excess energy generated during deceleration by converting it into heat, thus preventing the DC bus voltage from exceeding safe limits and protecting the drive and motor from overload.

Regenerated energy can be converted into another form of energy, such as:

1. Electrical Energy Storage: It can be fed back into the supply system or stored in capacitors for later use.
2. Mechanical Energy Control: It can also be used to assist in motor control activities, improving energy efficiency.

Block A in the VSD diagram represents the rectifier, which converts AC voltage into DC voltage.

The main component used in the filter circuit is the capacitor, which smooths out the DC voltage generated by the rectifier.

The inverter converts DC voltage back into AC voltage at a desired frequency, allowing for the control of motor speed by varying the output frequency and voltage supplied to the motor.

1. Energy Savings: VSDs allow motors to operate at optimal speeds, reducing energy consumption.
2. Better Control of Motor Speeds: VSDs can provide accurate speed control for various application needs, improving overall system performance.