With reference to PLCs: 6.1.1 Name the system used for automated machine control for industrial production before PLCs were developed - NSC Electrical Technology Power Systems - Question 6 - 2019 - Paper 1

1. Maintaining a single ground for the PLC to prevent electrical interference.
2. Ensuring that a main cut-off switch is fitted for emergencies.
3. Using the correct size of wire, ensuring it is at least 2 mm² to handle the current load safely.

Software refers to the machine language or instructions that are programmed into a PLC. It allows the PLC to perform specific tasks such as monitoring inputs from sensors and controlling outputs based on predefined instructions, effectively enabling automation in industrial processes.

Anti-surge protection is critical to safeguard PLCs from voltage spikes, which can be caused by lightning strikes or other electrical disturbances. These surges can damage the PLC and compromise its functionality, leading to costly downtime and potential safety hazards.

A sensor is a device that detects environmental conditions and can change its output based on the variables it measures, such as temperature, pressure, or fluid levels.

1. Proximity sensor
2. Temperature sensor
3. Light sensor

A level sensor is commonly used in water treatment plants to monitor water levels in tanks and control pumps to maintain desired water levels.

An analogue device continuously measures and represents physical variables, providing a range of values rather than discrete states. These devices are essential for applications requiring precise continuous monitoring.

The symbol represented by the truth table is an OR gate.

   A ---|>o--- F
   B ---|>o---

The ladder logic diagram for the truth table, representing an OR gate, can be illustrated as follows:

| I1  |----|   |---( F )
| I2  |----|   |

This diagram shows that if either input I1 or I2 is activated, output F will be energized.

The logic function represented by FIGURE 6.5 can be expressed as:

$F = A + B$

This indicates that the output F is true if either input A or input B is true.

The logic program utilizes an ON-delay timer contact.

When the push button (I1) is pressed, the timer will be energized immediately. The normally open contact of the timer will close, which in turn causes the output (Q) to go high. The output will remain high for a duration of 10 seconds, after which it will revert to low unless the push button remains pressed.

The ladder logic diagram for the control circuit in FIGURE 6.7 can be represented as:

| Start |----| N.C. |----( MC1 )
| Stop  |----| N.O. |----( MC2 )

The first stage of the VFD circuit is designed to convert AC voltage supply into DC voltage through rectification using diode pairs. This process is essential for enabling further stages of voltage control.

In the third stage of the VFD, each pair of switches controls either the positive or negative half cycles of the output voltage. By adjusting the frequency and duration of these cycles, speed control is achieved, allowing the motor to run at various speeds.

If the switches remain ‘ON’ for a longer period, the output waveform will be smoother, and the frequency of the supply will decrease, leading to a slower speed of operation for the motor. This happens because extended conduction time alters the effective voltage seen by the motor.

1. Improved energy efficiency, leading to reduced operational costs.
2. Enhanced process control, allowing for precise adjustments to speed based on demand.
3. Lower mechanical wear on motors, extending their lifespan and reducing maintenance needs.

One application of a VFD is in HVAC systems to control the speed of fans and compressors for efficient temperature regulation.

1. Voltage-to-frequency (V/F) control
2. Direct torque control (DTC)
3. Pulse width modulation (PWM)

1. Cranes
2. Electric locomotives or trains
3. Battery-powered electrical vehicles

Regenerative braking is achieved by using the motor as a generator, converting kinetic energy back into electrical energy during deceleration. This process reintroduces energy into the system or stores it for future use.

Region A is typically referred to as the breakdown speed.

At start-up, the induction motor experiences a torque increase to 200%, allowing it to overcome initial inertia. As speed increases, the torque required gradually falls, stabilizing at a point where the speed can be adjusted based on the load and operational requirements.