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 are more flexible and can be easily reprogrammed for different applications, reducing the need for physical rewiring.
2. Space Efficiency: PLCs take up less space compared to traditional hardwired systems that require numerous relays.

Before switching on, it is critical to check PLC wiring and connections to ensure safety and functionality. Loose connections can lead to malfunctions, short circuits, or equipment damage, while improper wiring may cause unexpected behavior in the system.

A PLC system is safer because it can be programmed to include safety features such as emergency stops and fault detection routines. This allows for automatic corrective actions, while hardwired systems may not have the ability to respond effectively to fault conditions.

The central processing unit (CPU) in a PLC is the brain of the system. It performs all logic operations, processes input data, and controls output devices. The CPU runs the ladder logic program, executing instructions in a sequential manner.

Soft-wired systems utilize programming rather than physical connections to create control circuits. This reduces the complexity and space required for traditional wiring while allowing for easier modifications and updates.

PLC software is the programming interface that enables users to create logic programs that control the PLC’s operations. It compiles the logical instructions into executable code that can be processed by the CPU.

An analogue signal is a continuous signal that represents physical measurements, varying smoothly over a range. In contrast, a digital signal represents information as discrete values or steps, making it more suitable for binary processing by digital devices.

Markers or flags are used to:

1. Store status data that is relevant to the program state.
2. Indicate specific conditions or events within the programming logic.

A contactor in a PLC program is used to control the power to other devices. It can be programmed to turn on or off based on the operational needs of the system.

A sensor is a device that detects environmental conditions and converts them into an electrical signal that can be read or processed by another device. For example, temperature sensors measure heat levels and provide signals proportional to the temperature sensed.

1. Temperature Sensor: Measures temperature.
2. Light Sensor: Detects light intensity.

1. To detect the presence or absence of an object without physical contact.
2. To measure the distance of an object in relation to the sensor.

The ladder logic diagram should include rungs that perform the functions of the described manual sequence starter using contacts and coils corresponding to the circuit diagram given.

1. ON-delay Timer: Activates an output after a specified time delay when the input is turned on.
2. OFF-delay Timer: Keeps an output activated for a specified time delay after the input is turned off.

The operation involves:

1. When the STOP button is pressed, the motor coil MC is de-energized.
2. Pressing the START button energizes MC, allowing the motor to operate until the STOP button is pressed again.

1. Transistor: Used for switching applications.
2. Relay: Provides isolation and control for high current loads.

FIGURE 6.14 represents a regenerative braking block diagram showing how regenerative braking is utilized in an electric motor control system.

The braking resistor dissipates excess energy during braking to prevent the motor from overheating. It allows the regenerated energy to be safely released, preserving system integrity.

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

1. Reused within the system (e.g., stored in capacitors).
2. Returned to the grid or harnessed for other operational needs.

Block A in the diagram refers to the AC to DC Converter, which converts the incoming AC voltage to DC for further processing.

The main component used in the filter circuit is the capacitor, which smooths the output voltage and minimizes ripple.

The inverter takes the DC voltage output from the rectifier and converts it back to an adjustable AC voltage to control motor speed and torque by varying the frequency and voltage supplied to the motor.

1. Energy Savings: VSDs offer significant energy savings by adjusting the motor speed to match the load requirements.
2. Better Control of Motors: They provide precise control over motor speed and torque, allowing for optimized performance.