Refer to the hardware of a PLC and answer the questions that follow - NSC Electrical Technology Power Systems - Question 7 - 2021 - Paper 1

The central processing unit (CPU) performs several key functions:

1. It controls the PLC, ensuring that all operations are carried out sequentially and efficiently.
2. It processes all calculations required for the operation of the control program, interpreting input signals to determine output actions.
3. It facilitates the communication between input and output modules, managing data transfer and processing.

The component used to switch the load connected to a PLC is called a Relay. Relays allow the PLC to control high-power devices safely.

The term scan time refers to the time taken by a PLC to complete one full cycle of operations, which includes reading inputs, executing the control program logic, and updating outputs. The scan time can affect the responsiveness and performance of the PLC, and it varies depending on the complexity of the program and the number of input/output devices.

Software in relation to a PLC refers to the machine language or programming language that is installed on the PLC. It defines the operational instructions and logic that the PLC follows to interact with its input and output hardware, effectively determining how the PLC responds to various conditions in its environment.

A sensor is a device that detects a physical condition or change in its environment, such as temperature, pressure, or light, and converts that information into a signal that can be read and interpreted by the PLC.

Three types of sensors other than the overload sensor include:

1. Level Sensor - Detects the level of a substance in a container (liquid, solid).
2. Proximity Sensor - Detects the presence of an object without physical contact.
3. Temperature Sensor - Measures the temperature of an environment or substance.

The overload sensor in FIGURE 7.4.3 is connected to an analogue input on a PLC because it continuously monitors the load, providing a variable signal that indicates changes in the physical condition. This allows the PLC to interpret varying levels of load and take appropriate actions based on real-time data, rather than just on a binary (on/off) basis.

The PLC ladder logic program for the forward-reverse starter can be structured as follows:

|------[ ]------+---( )----|
|    O/L       |    MC1 FWD |
|              |             |
|---[ ]------[ ]-----|
|   STOP     START FWD |
|                 |             |
|--------------[ ]------+---( )----|
|   START REV    |    MC2 REV |

Stage A in the variable speed drive is identified as the AC-to-DC converter. This stage converts the three-phase AC supply into DC power for further processing.

A semiconductor component that could replace the switches used at stage A is the Insulated Gate Bipolar Transistor (IGBT). IGBTs are commonly used for switching applications in power electronics.

Two advantages of using variable speed drives include:

1. Improved energy usage: By adjusting motor speed according to load requirements, variable speed drives help reduce energy consumption.
2. Reduced motor wear: Operating a motor at variable speeds can lead to less mechanical stress and longer lifespan.

Vector drives operate by calculating the current vectors relative to the voltage applied to the motor, allowing precise control of speed and torque. This method ensures that the motor maintains a constant speed and torque across a range of frequencies, enhancing overall performance.

In regenerative braking, when a load's momentum drives the motor, the motor acts as a generator that converts mechanical energy back into electrical energy. This energy can be fed back into the power supply system or utilized to power other components, effectively slowing down the motor while recovering energy in the process.

The component used to dissipate the excessive energy during the braking process is known as a Braking Resistor. It is responsible for converting the regenerated energy into heat, thereby protecting the system from over-voltage conditions.