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

The functions of the central processing unit (CPU) include:

1. It controls the PLC's operations, ensuring the execution of the control logic.
2. It performs all calculations necessary for processing input data and generating output signals.
3. It runs the PLC program, enabling communication between hardware and software components.

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

The term scan time refers to the amount of time the PLC takes to complete one full scan cycle. This includes reading the input states, executing the control program, and updating the output states. A shorter scan time means the PLC can respond more quickly to changes in inputs, enhancing performance in real-time applications.

Software in the context of a PLC refers to the machine language or programming that is installed on a computer or written into the PLC's control program. It defines how the PLC interacts with its input and output hardware, essentially translating the control logic into actions that the PLC can perform.

A sensor is a device that detects a physical condition, such as temperature, pressure, or motion, and changes its electrical characteristics accordingly. This allows the PLC to receive input signals from the sensor, enabling it to monitor and respond to various conditions.

Three types of sensors other than the overload sensor include:

1. Light sensor
2. Level sensor
3. Proximity sensor

The overload sensor is connected to an analogue input on a PLC because it continuously sends an analogue signal that provides varying levels of resistance based on the load. This allows the PLC to monitor the load accurately and make real-time adjustments as needed, rather than operating in a binary on/off mode.

The PLC ladder logic program can include:

• N/O contact for Start FWD
• N/C contact for O/L
• N/C contact for Stop
• Actions for MC1 (Forward Motor) and MC2 (Reverse Motor) as appropriate. This setup will ensure that the motors are controlled according to the designed logic.

Stage A is identified as the DC-to-AC Inverter.

A semiconductor component that could replace the switches used at A is the Insulated Gate Bipolar Transistor (IGBT).

Two advantages of using variable speed drives are:

1. They improve energy usage by allowing precise control of motor speeds according to demand.
2. They reduce motor wear by maintaining optimal operating conditions.

Vector drives utilize a mathematical model of the drive-in software. By measuring the current vectors in relation to the applied voltage, they can maintain a constant field at all frequencies below the line frequency. This allows for precise speed and torque control, enabling enhanced performance of the motor during varying load conditions.