7.1 State TWO disadvantages of a relay control system in comparison with PLC control systems - NSC Electrical Technology Power Systems - Question 7 - 2023 - Paper 1

Soft-wired systems allow for easy modifications and updates without the need for extensive wiring changes. This flexibility reduces the time and costs associated with installation and reconfiguration compared to a hardwired system.

The timer in FIGURE 7.3 is designed to prevent the lamp from being switched on before a predetermined time has elapsed after pressing the ON button. This ensures that the lamp operation is controlled and prevents immediate activation.

When the ON button is pressed, the contact closes, energizing coil Y to close relay Y. After a preset time, contact T closes, causing the lamp to turn on. The lamp will remain on until the OFF button is pressed, allowing for control of the lamp's illumination.

A relay is a switch that uses electromagnetic force to operate contacts, and is typically used in low power circuits. A contactor, on the other hand, is designed to switch larger current loads and provides isolation and control for high power applications.

An OFF delay timer keeps the contacts closed for a predetermined time after being de-energized. This allows machinery or systems to continue operating briefly, preventing sudden disconnections.

1. Proximity sensors are used for detecting the presence of objects without physical contact, such as in automation systems.

2. They can be utilized in counting objects on a conveyor belt to monitor production rates.

1. Inductive proximity sensor.

2. Photoelectric proximity sensor.

This requires a visual representation; candidates should draw a ladder logic diagram that captures the sequential operation as described in the given figure.

Each overload is connected in series to ensure that if one motor overloads, it will interrupt the circuit and protect the motor from damage due to excessive current.

The MC1 N/O contact ensures that motor 1 must be energized before motor 2 can be started, providing a sequence of operations that prevents potential overload conditions.

Voltage frequency control using Variable Speed Drives (VSD) involves adjusting the frequency of the electric supply to the motor, allowing for precise control of motor speed and torque, leading to energy savings and improved process efficiency.

1. Regenerative braking methods are used in electric trains to recover energy during braking.

2. They are also utilized in cranes and elevators for energy efficiency during operations.

Regenerative energy is the energy that is recovered from a motor when it slows down, converting mechanical energy back into electrical energy, which can either be used immediately or stored for later use.