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

Three functions of the CPU in a PLC include:

1. Control Execution: The CPU interprets the user program and controls the operation of the PLC by executing instructions.
2. Process Data: It processes input signals from sensors and makes decisions based on the programmed logic.
3. Communication: The CPU manages communication between different modules, ensuring that data is transmitted correctly between inputs, outputs, and the user interface.

The component used to switch the load connected to a PLC is typically a Relay.

Scan time refers to the duration the PLC takes to complete one full scan cycle. During this time, the PLC processes all inputs, executes the control program, and updates the outputs. The scan cycle usually consists of three main phases: reading input states, executing the program, and updating output states. A shorter scan time can lead to more responsive control systems.

Software in the context of a PLC refers to the programmed instructions that dictate how the PLC operates. This machine language allows the PLC to interact with its input devices and perform the necessary control tasks. The software can be modified or updated to change the behavior of the PLC without altering the hardware.

A sensor is a device that detects a physical condition or change in the environment and converts that information into an electrical signal. Sensors are essential for providing feedback to the PLC regarding environmental conditions.

Three types of sensors used in PLC systems, excluding overload sensors, are:

1. Light Sensor: Detects the presence or absence of light.
2. Level Sensor: Measures the level of liquids or solids in a container.
3. Proximity Sensor: Detects the presence of an object without physical contact.

The overload sensor is connected to an analogue input on a PLC because it provides a continuous range of values that represents the load. This allows the PLC to monitor and respond to varying load conditions more precisely than a digital input, which would indicate only an ON or OFF state. By using an analogue input, the PLC can implement sophisticated control strategies, such as adjusting motor speed based on load conditions.

The PLC ladder logic program for a forward-reverse starter typically includes contacts and coils representing the Start and Stop commands, as well as forward and reverse motor controls. The program would reflect the interlocking of the forward and reverse operations to prevent simultaneous activation.

Stage A in the variable speed drive represents the DC-to-AC Inverter, which converts DC power into AC power suitable for driving the motor.

An Insulated Gate Bipolar Transistor (IGBT) can replace the switches used at stage A in the variable speed drive.

Two advantages of using variable speed drives include:

1. Improved Energy Usage: They allow for precise control of motor speed, leading to energy savings during partial load conditions.
2. Reduced Motor Wear: By eliminating frequent starts and stops, variable speed drives reduce mechanical stress on motors, thereby extending their lifespan.

Vector drives utilize a mathematical model of the drive’s dynamics in software to control motor speed. By measuring the current vectors in relation to the applied voltage, vector drives maintain a constant field at all frequencies, improving performance across a wide range of operating conditions.

In regenerative braking, when a load drives the motor, it generates electrical energy. This energy is fed back into the system, converting mechanical energy into electrical energy. This process slows down the motor while reducing power consumption and maintaining system efficiency, instead of dissipating energy as heat.

The component used to dissipate the excessive energy during the braking process is a Braking Resistor.