QUESTION 6: THREE-PHASE MOTORS AND STARTERS
6.1 Name TWO continuity tests to be performed on a three-phase motor - NSC Electrical Technology Power Systems - Question 6 - 2022 - Paper 1
Question 6
QUESTION 6: THREE-PHASE MOTORS AND STARTERS
6.1 Name TWO continuity tests to be performed on a three-phase motor.
6.2 Explain the term cogging with reference to in... show full transcript
Worked Solution & Example Answer:QUESTION 6: THREE-PHASE MOTORS AND STARTERS
6.1 Name TWO continuity tests to be performed on a three-phase motor - NSC Electrical Technology Power Systems - Question 6 - 2022 - Paper 1
Step 1
6.1 Name TWO continuity tests to be performed on a three-phase motor.
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Answer
Continuity test between the ends of each coil.
Continuity test between the frame of the motor and earth.
Step 2
6.2 Explain the term cogging with reference to induction motors.
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Answer
Cogging is the tendency of the rotor rod to remain stuck under a stator tooth due to the direct magnetic attraction between the two.
Step 3
6.3 State TWO advantages of cage-type induction motors over wound rotor-type motors with slip rings and brushes.
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Less maintenance due to the absence of slip rings and brushes.
They are explosion-proof because of the absence of brushes and slip rings that cause sparking.
Step 4
6.4 Name TWO applications of squirrel-cage induction motors where constant speed and torque is essential.
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Answer
Cranes
Conveyor belts
Step 5
6.5 Label points A, B and C on the characteristic curve in FIGURE 6.5 below.
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Answer
A - Pull-up torque
B - Breakdown torque
C - Full load torque
Step 6
6.6 Differentiate between synchronous speed and rotor speed.
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Synchronous speed is the speed at which the magnetic field in the stator rotates.
Rotor speed is the speed at which the rotor rotates in an attempt to reach the synchronous speed.
Step 7
6.7.1 Synchronous speed of the motor.
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Answer
To calculate the synchronous speed, we use the formula:
n_s = rac{60 imes f}{p}
Substituting the values:
n_s = rac{60 imes 50}{3} = 1000 ext{ rpm}
Step 8
6.7.2 Percentage slip.
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Answer
To calculate percentage slip, we use the formula:
ext{Slip} = rac{n_s - n_r}{n_s} imes 100
Substituting the values:
ext{Slip} = rac{1000 - 950}{1000} imes 100 = 5 ext{ \\%}
Step 9
6.8.1 Name TWO types of losses other than copper losses that influence efficiency of the motor.
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Answer
Iron losses
Mechanical losses
Step 10
6.8.2 Calculate the efficiency of the motor.
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Answer
To find the output power, we use:
Pout=Pin−extlosses
Substituting the values:
Pout=5000−600=4400extW=4.4extkW
Step 12
6.9.1 Identify component T.
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Answer
Component T is a Timer.
Step 13
6.9.2 Explain the purpose of having TWO overload units in the circuit.
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Answer
Each overload unit monitors the current drawn by each motor independently.
Step 14
6.9.3 Identify the error in the circuit.
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The error is in the MC1/N/O2 auxiliary contact (Hold contact) connected in parallel with the start button is omitted.
Step 15
6.9.4 Explain how this error affects the operation of the circuit.
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The moment the start button is released, MC1 will de-energize, opening MC1/N/O2, disconnecting the parallel circuit to the control circuit, thus MC2 will not be energized.
