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.
Troubleshooting is more complex and time-consuming compared to a programmable system.
Changes to the system require re-wiring, which is labor-intensive.
Step 2
Name the THREE steps that a PLC has to undergo to complete one programmed scan cycle.
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Answer
Input scan: The PLC reads the status of all input devices.
Program execution: The PLC processes the program logic based on the input status.
Output scan: The PLC updates the output devices based on the program logic.
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 the PLC to complete one full scan cycle, which includes reading inputs, executing the program, and updating outputs. It is crucial for timely processing and can impact the responsiveness of the control system.
Step 4
Draw the ladder logic diagram.
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The ladder logic diagram for the NAND gate would consist of two rungs which represent inputs A and B, with the output F being represented as a normally closed contact.
Step 5
Draw the truth table for the NAND gate.
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Answer
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 achieves its function by interpreting logic inputs from various devices, processing these inputs according to pre-programmed instructions, and controlling outputs to perform tasks effectively.
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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An analogue input may be converted to a digital input to enable the PLC to process it. Digital conversion allows for precise reading and easier integration within the digital logic of the PLC.
Step 9
Describe how a PLC uses a relay to drive a motor.
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A PLC sends a signal to a relay, which energizes its coil. This closure allows current to flow to the motor, thus activating it. The relay acts as an intermediary, allowing the PLC to control high voltage systems safely.
Step 10
State the purpose of the timer function.
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The timer function in a PLC is used to delay actions for a predetermined period, enabling timed operations such as start sequences or safety events.
Step 11
Explain the latching concept with reference to retaining circuits.
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The latching concept involves maintaining an output state after a triggering event has occurred, even after the input signal is removed. This is achieved in retaining circuits, enabling the last state to be preserved.
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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The ladder logic diagram would show contacts for Stop, Start buttons, timers, and parallel branches for the MC control with NC/NO configurations.
Step 14
State the function of the MC1/N1 as used in ladder logic circuits.
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The MC1/N1 function latches and keeps the program running 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 ensure that MC2 does not engage while MC3 is in the delta, preventing overload conditions.
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 varies the output frequency of the drive to control motor speed; as frequency increases, voltage is adjusted to maintain performance.
Step 18
Explain the purpose of the braking resistor with reference to regenerative braking.
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The braking resistor absorbs excess energy from the motor when slowing down, converting kinetic energy back to heat and preventing damage from over-voltage conditions.
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 supplied voltage, allowing for precise speed management and energy efficiency.
