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

1. Flexibility in Programming: PLCs can be reprogrammed easily for different applications without needing to alter the physical wiring.
2. Space Efficiency: A PLC requires less physical space than a traditional relay system, which needs multiple bulky components.

Ensuring that the PLC wiring and connections are correct is vital to avoid faults that can lead to equipment damage or operational failures. Incorrect wiring can cause short circuits or malfunctioning of the system, potentially risking safety and resulting in downtime.

A PLC system incorporates safety protocols that can safely isolate or shut down sections of the system in the event of a fault. In contrast, a hardwired system may not have easy means of stopping the process, potentially leading to greater risks and hazardous situations.

The Central Processing Unit (CPU) of a PLC is the unit responsible for processing logic operations and managing data. It executes the control program and oversees all functions of the PLC.

Soft-wired systems in PLCs refer to the ability to create logical connections through software instead of relying on physical wires. This reduces complexity and increases ease of modification.

PLC software is used to program the PLC to perform specific tasks. It defines the logic and control sequences that the PLC executes based on input signals.

An analogue signal is continuous and varies smoothly over time, representing physical phenomena like temperature or pressure. In contrast, a digital signal represents data in discrete steps or levels, typically as binary values (0s and 1s).

Markers or flags in PLC programming are used to store temporary data or signals that indicate certain conditions or statuses within a program.

A contactor in PLC programming is used to control electrical circuits by allowing or disallowing current flow based on the program logic.

For the given NAND gate symbols, create a ladder logic diagram featuring:

1. Rung 1: Inputs A and B in parallel, controlled by the contact, leading to the output through a normally closed (NC) relay.

A sensor is a device that detects an environmental condition and converts it into an electrical signal. This signal can be used to inform subsequent actions in a control system.

1. Temperature Sensor: Detects temperature levels.
2. Overload Sensor: Measures load to prevent damage.

1. Object Detection: Used for non-contact detection of objects.
2. Speed Measurement: Measures the rotational speed of rotating components.

The ladder logic program for the manual sequence starter circuit would include rungs that depict the activation of coils MC1 and MC2 based on the position of the STOP and START buttons, in accordance to the control logic described.

1. ON-delay Timer: Delays the activation of an output for a specified time after the input is activated.
2. OFF-delay Timer: Keeps the output activated for a specified time after the input is deactivated.

When the START button is pressed, coil MC is energized, closing the contact MC and turning on the lamp. If the STOP button is pressed, it will disengage the relay, stopping the operation.

1. Transistor: Used to switch the high current load.
2. Relay: Acts as a switch to control high-current loads safely.

FIGURE 6.14 is a regenerative braking block diagram showing a motor connected to braking resistor, indicating a system that recovers energy during braking.

The braking resistor dissipates regenerative energy produced during braking, preventing overvoltage and protecting the system from overload.

Regenerated energy can be converted into another form of energy, such as stored energy in a battery or used to power other components in the system, enhancing efficiency.

Block A is the AC to DC Converter, which transforms the three-phase AC supply into a usable DC voltage for the following components.

The main component used in the filter circuit is typically a capacitor, which helps smooth out the voltage waveform.

The inverter converts the DC voltage back into AC voltage at a different frequency, controlling the speed of the motor according to the load requirements.

1. Energy Savings: VSDs can adjust motor speed to match the load, reducing power consumption.
2. Better Control of Motors: VSDs provide precise control of motor speed and torque for various applications.