6.1 State THREE disadvantages of hard wiring - NSC Electrical Technology Power Systems - Question 6 - 2018 - Paper 1
Question 6
6.1 State THREE disadvantages of hard wiring.
6.2 Name the THREE steps that a PLC has to undergo to complete one programmed scan cycle.
6.3 Explain the term scan t... show full transcript
Worked Solution & Example Answer:6.1 State THREE disadvantages of hard wiring - NSC Electrical Technology Power Systems - Question 6 - 2018 - Paper 1
Step 1
State THREE disadvantages of hard wiring.
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Answer
Requires regular maintenance due to wear and tear.
Takes longer to rewire if faults occur, leading to increased downtime.
Limited flexibility in modifying the system as changes require physical rewiring.
Step 2
Name the THREE steps that a PLC has to undergo to complete one programmed scan cycle.
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Input Scan: The PLC reads the state of all input devices.
Program Execution: The PLC processes the input information according to the user-defined program logic.
Output Scan: The PLC updates the output devices based on the results from the program execution.
Step 3
Explain the term scan time with reference to the scan cycle of a PLC.
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Scan time is the total time it takes for a PLC to complete one full scan cycle. This includes the time spent reading inputs, executing the program, and updating outputs. Reducing scan time is critical for improving system response and performance.
Step 4
Draw the ladder logic diagram.
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Refer to Figure 6.4 for the appropriate logic diagram representation for the NAND gate.
Step 5
Draw the truth table for the NAND gate.
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The truth table for a NAND gate is:
A
B
F
0
0
1
0
1
1
1
0
1
1
1
0
Step 6
Describe how a PLC achieves its function.
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A PLC translates user inputs into outputs by continuously scanning the input/output states, processing this information through its program, and then controlling various outputs accordingly.
Step 7
Give THREE examples of analogue input devices.
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Light sensor.
Temperature sensor.
Pressure sensor.
Step 8
Explain why an analogue input may be converted to a digital input.
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Analogue inputs are continuous signals that can vary infinitely; they may be converted to digital inputs for processing by the PLC. This conversion allows the PLC to interpret signals effectively, ensuring reliable monitoring and control.
Step 9
Describe how a PLC uses a relay to drive a motor.
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The PLC sends a control signal to energize a relay which closes its contacts. This action connects the power supply to the motor, allowing it to operate as per the desired logic.
Step 10
State the purpose of the timer function.
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The timer function in a PLC is used to delay actions or to activate/deactivate devices for set time periods, which enhances control over the operational parameters.
Step 11
Explain the latching concept with reference to retaining circuits.
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Latching refers to maintaining an output state after the triggering input has ceased. This is achieved through retaining circuits that keep the last activated state until reset by a specific condition.
Step 12
Identify the control circuit in FIGURE 6.10.
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The control circuit in FIGURE 6.10 is an automatic star-delta starter.
Step 13
Draw a ladder logic diagram that executes the same function as the one in FIGURE 6.10.
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Refer to the appropriate ladder logic diagram representation according to the functions outlined in FIGURE 6.10.
Step 14
State the function of the MC1/N0 in ladder logic circuits.
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The function of the MC1/N0 is to latch the circuit, ensuring that the program continues running even after the start button is released.
Step 15
State why the N/C contact of MC3 is connected in series with the star contactor.
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The N/C contact of MC3 is connected in series with the star contactor to prevent MC2 from engaging when MC3 is active, ensuring safe operation during the transition phases.
Step 16
Name THREE types of motors used with variable speed drives (VSD).
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AC synchronous motors.
AC asynchronous motors.
Induction motors.
Step 17
Explain voltage frequency control with reference to VSD.
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Voltage frequency control varies the output frequency and voltage of the motor, optimizing the speed and torque characteristics according to application needs.
Step 18
Explain the purpose of the braking resistor with reference to regenerative braking.
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The braking resistor's purpose is to absorb energy during deceleration, preventing overvoltage and protecting the drive system while converting kinetic energy back into usable electrical energy.
Step 19
Explain the function of the VSD when used in motors.
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The function of the VSD is to regulate motor speed and torque by adjusting the frequency and voltage of the supply, enabling efficient performance across various operating conditions.
