6.1 State THREE disadvantages of a relay control system in comparison to PLC control systems - NSC Electrical Technology Power Systems - Question 6 - 2019 - Paper 1

The CPU is the brain of the PLC, responsible for executing instructions based on the inputs it receives. It processes the program logic and generates the outputs to control the machinery.

The modem enables communication between the PLC and external devices over various transmission mediums. It converts digital signals to analog and vice versa, allowing for data transfer over telephone lines or other communication channels.

Installing a fuse or circuit-breaker protects the PLC from excessive currents that could cause damage or failure. This safety measure is essential for ensuring long-term reliable operation of the control system.

Verifying wirings and connections prior to power-up is crucial to avoid short-circuits or equipment damage. This ensures that all connections meet specifications and operate safely.

An ON-delay timer contact activates after a preset time once energized, meaning it will not close until that time has elapsed. Conversely, an OFF-delay timer contact maintains its active state until the preset time has passed after being de-energized.

An ON-delay timer is typically used in lighting control systems, where a delay in switching on is required to prevent power surges or to allow other system cycles to stabilize.

In the circuit, pressing the start button energizes coil Y which closes the latching circuit, keeping the lamp illuminated. The ON-delay timer (T) starts timing and will ensure the lamp stays ON until the stop button is pressed.

FIGURE 6.6A represents an analogue input because it shows a continuous signal that varies over a range of values.

FIGURE 6.6B represents a digital input, as it displays discrete values, switching between defined high and low states.

The circuit represents an Exclusive OR gate.

In cell phones, light sensors are used to automatically adjust screen brightness based on environmental light levels.

In motor car radiators, temperature sensors monitor the coolant temperature and ensure engine efficiency and prevent overheating.

The PLC ladder logic diagram would diagrammatically represent the conditions under which the forward and reverse start mechanism would function, employing normally open or closed contacts to activate motors MC1 and MC2 based on operational status.

A diode bridge rectifier converts AC supply voltage into DC voltage, allowing the control system to operate using a stable and suitable voltage.

The filtering circuit smooths the ripple in the DC voltage output from the rectifier to provide a stable supply, which is crucial for the performance and longevity of any downstream electronic controls.

The inverter is responsible for converting the filtered DC voltage back into an AC voltage at a variable frequency, thereby controlling the speed of the motor in the system.

1. In water pumping systems, to adjust water flow rates.
2. In heating systems, to optimize heating based on demand.
3. In variable air volume air conditioning systems to manage airflow efficiently.

1. In lifts, to recapture energy during descent.
2. In electrical locomotives, to convert kinetic energy back to electrical energy when braking.

Regenerative energy refers to the energy that is recovered and reused, typically during braking processes, where the system converts kinetic energy into electrical energy which can be used immediately or stored for later use.