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

A soft-wired system is cheaper because modifications can be made easily without the need for rewiring. This reduces labor costs and downtime associated with implementing changes.

The timer in FIGURE 7.3 functions to avoid the lamp being turned on immediately when the system is activated. Instead, it creates a delay, ensuring the lamp only turns on after the pre-set time has elapsed.

When the 'ON' button is pressed, it closes the contact, energizing coil Y, which eventually closes the lamp circuit. After a pre-set time, contact T closes, allowing the lamp to remain on. The lamp will stay on until the 'OFF' button is pressed, which breaks the circuit.

A relay is used for low power circuits and usually has contacts that operate on low voltage. In contrast, a contactor is designed for high power applications and often features contacts that can handle larger loads.

Upon de-energization, an OFF delay timer maintains its contacts closed during the preset time, after which they open, ensuring a delay before the output is cut off.

1. Detection and positioning of objects in automated systems.
2. Measurement of rotating speed in machinery.

1. Inductive proximity sensors.
2. Capacitive proximity sensors.

The ladder logic diagram should maintain the same functionality as in FIGURE 7.6, illustrating the interconnections of the motors, contactors, and control elements as per the provided schematic.

Each overload is connected in series to provide individual protection for each motor, ensuring that if one motor experiences an overload, the contactor opens, de-energizing its circuit.

The MC1 N/O contact ensures that motor 1 must be energized before motor 2 can start, providing a sequential control mechanism for the operation of both motors.

Voltage frequency control involves changing the frequency of the input voltage to control the speed and torque of the motor, allowing for variable speed operation based on load requirements.

1. Regenerative braking in lifts.
2. Regenerative braking in electrical locomotives.

Regenerative energy refers to the energy recovered from a motor when it slows down, converting mechanical energy back into electrical energy, which can be reused immediately or stored for later use.