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
It requires regular maintenance due to wear and tear over time.
Troubleshooting takes longer because locating faults can be challenging.
Making modifications involves extensive rewiring, which can be time-consuming and costly.
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 status of all input devices.
Program Execution: The PLC processes the program instructions based on the input states.
Output Scan: The PLC updates the status of the output devices according to the program.
Step 3
Explain the term scan time with reference to the scan cycle of a PLC.
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Scan time refers to the total time taken by a PLC to complete one full cycle of reading inputs, executing programs, and updating outputs. This duration is critical as it affects the responsiveness of the control system, with shorter scan times allowing for more dynamic and responsive control.
Step 4
Draw the ladder logic diagram.
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[Insert ladder logic diagram here based on FIGURE 6.4.]
Step 5
Draw the truth table for the NAND gate.
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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 interprets logic to control output devices by scanning input signals, processing the programmed instructions, and accordingly activating or deactivating outputs based on the logic defined within the program.
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 can generate continuous signals that a computing device cannot process directly. Therefore, they are converted to digital signals to enable accurate processing, monitoring, and controlling using discrete values.
Step 9
Describe how a PLC uses a relay to drive a motor.
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The PLC sends a control signal to the relay, which then energizes the relay coil, closing its contacts and allowing power to flow to the motor, thereby driving it.
Step 10
State the purpose of the timer function.
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The timer function in a PLC is used to perform operations after a specific duration or to create delays within the program, allowing for time-sensitive control.
Step 11
Explain the latching concept with reference to retaining circuits.
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Latching refers to the ability of a control circuit to maintain its output state even after the input signal has been removed. This is typically achieved using retaining circuits or specific relays that keep the output active until a reset condition is met.
Step 12
Identify the control circuit in FIGURE 6.10.
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The control circuit is identified as 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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[Insert ladder logic diagram here executing the same function as FIGURE 6.10.]
Step 14
State the function of the MC1/N1O in ladder logic circuits.
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The function of the MC1/N1O is to latch the control circuit, ensuring that output continues to run 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 prevents MC2 from engaging when MC3 is in operation, ensuring safe switching between star and delta configurations.
Step 16
Name THREE types of motors used with variable speed drives (VSD).
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Induction motors
AC synchronous motors
DC motors
Step 17
Explain voltage frequency control with reference to VSD.
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Voltage frequency control involves adjusting the voltage and frequency sent to the motor to control its speed and torque, allowing for efficient operation under varying load conditions.
Step 18
Explain the purpose of the braking resistor with reference to regenerative braking.
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The purpose of the braking resistor is to dissipate excess energy generated during braking, preventing over-voltage conditions and allowing the energy to be safely absorbed and converted into heat.
Step 19
Explain the function of the VSD when used in motors.
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The VSD controls the speed of an AC motor by varying the frequency of the power supply, thereby providing precise control over motor operations, improving energy efficiency, and reducing wear on mechanical components.
