Refer to FIGURE 6.1 below and answer the questions that follow - NSC Electrical Technology Power Systems - Question 6 - 2021 - Paper 1
Question 6
Refer to FIGURE 6.1 below and answer the questions that follow.
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FIGURE 6.1: EXPLODED VIEW OF A THREE-PHASE MOTOR
6.1.1 Name parts A, B and C.
6.1.2 Explain h... show full transcript
Worked Solution & Example Answer:Refer to FIGURE 6.1 below and answer the questions that follow - NSC Electrical Technology Power Systems - Question 6 - 2021 - Paper 1
Step 1
6.1.1 Name parts A, B and C.
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Answer
A – Terminal Box (Wiring Cover)
B – Motor frame (Enclosure/Housing/Stator)
C – Rotor
Step 2
6.1.2 Explain how a rotating magnetic field is created in the stator windings.
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Answer
A rotating magnetic field is created by connecting the three-phase supplies to the stator windings. Each coil generates an alternating magnetic field at alternating intervals (120°). The coils are uniformly distributed around the stator, producing the impression of a rotating magnetic field that moves around the stator.
Step 3
6.1.3 State TWO applications of squirrel-cage induction motors.
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Cranes
Conveyor belt systems
Step 4
6.1.4 State TWO contributing factors that determine the speed of rotation in a three-phase induction motor.
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Number of pole pairs
Frequency of the supply
Step 5
6.2.1 Is this motor connected in star or in delta?
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This motor is connected in delta.
Step 6
6.2.2 State how the direction of rotation of this motor can be reversed.
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The direction of rotation can be reversed by swapping any TWO of the supply lines.
Step 7
6.2.3 Name TWO important procedures to follow before testing the insulation resistance of the motor in FIGURE 6.2.
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Answer
Disconnect the supply lines from the motor.
Remove the internal connecting plates.
Step 8
6.3.1 Synchronous speed.
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Answer
The synchronous speed can be calculated using the formula:
n_s = rac{f imes 60}{p}
For this motor, where p = 4 and f = 50 Hz:
n_s = rac{50 imes 60}{4} = 750 ext{ rpm}
Step 9
6.3.2 Percentage slip if the rotor speed is 700 r/min.
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Answer
The percentage slip is calculated as:
ext{Slip} = rac{n_s - n_r}{n_s} imes 100
Where:
