4.1 Refer to FIGURE 4.1 and answer the questions that follow - NSC Electrical Technology Power Systems - Question 4 - 2016 - Paper 1
Question 4
4.1 Refer to FIGURE 4.1 and answer the questions that follow.
4.1.1 State whether there is an electrical connection between the stator and rotor.
4.1.2 Describe th... show full transcript
Worked Solution & Example Answer:4.1 Refer to FIGURE 4.1 and answer the questions that follow - NSC Electrical Technology Power Systems - Question 4 - 2016 - Paper 1
Step 1
4.1.1 State whether there is an electrical connection between the stator and rotor.
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Answer
There is no electrical connection between the stator and rotor. The connection is magnetic and not electrical by nature.
Step 2
4.1.2 Describe the operation of the motor.
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Answer
A three-phase voltage supply is connected across the stator windings. This creates three-phase currents flowing in the stator windings, establishing a rotating magnetic field. This rotating field induces a current in the rotor conductors, resulting in interaction between the magnetic fields. The induced rotor current generates a magnetic field, causing the rotor to rotate as the two magnetic fields interact.
Step 3
4.1.3 Describe what would happen to the motor if one phase of the stator winding were an open circuit.
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If one phase of the stator winding were open, the motor would still rotate but would not develop the correct torque required to operate efficiently. The performance would be significantly reduced.
Step 4
4.2 State TWO advantages of a three-phase induction motor over a single-phase induction motor.
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They require less maintenance due to having fewer moving parts compared to single-phase motors.
For the same size, they deliver a higher torque compared to single-phase motors.
Step 5
4.3 Describe why it is important to check the insulation resistance between the stator windings before energising a motor.
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Checking the insulation resistance ensures that the integrity of the insulation is sound. This is crucial to avoid electrical faults when the motor is energised.
Step 6
4.4 State ONE mechanical test that must be carried out on a motor before it is energised.
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Check for secure mountings and a smooth rotation of the motor shaft.
Step 7
4.5.1 Calculate the synchronous speed.
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Answer
The synchronous speed can be calculated using the formula:
n_s = rac{60 imes f}{p}
Thus:
n_s = rac{60 imes 50}{18} = 1000 ext{ r/min}
Step 8
4.5.2 Calculate the rotor speed.
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The rotor speed is given by:
nr=ns(1−extslip)
Inserting the values gives:
nr=1000(1−0.04)=960extr/min
Step 9
4.6.1 Calculate the input kVA of the motor at full load.
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The input kVA can be calculated using:
S = rac{ ext{(Voltage)} imes ext{(Current)}}{ ext{√3}}
Thus:
S = rac{380 imes 8.5}{ ext{√3}} = 5.59 ext{ kVA}
Step 10
4.6.2 Calculate the active power output of the motor at full load.
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The active power output can be calculated using:
P=SimesextPowerFactorimesextEfficiency
Thus:
P = rac{380 imes 8.5 imes 0.8 imes 0.95}{ ext{√3}} = 4.25 ext{ kW}
Step 11
4.7 Describe the function of an overload unit in a motor starter.
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The overload unit relay protects the motor and operator under fault conditions. When activated, it removes power from the main contactor, ensuring safety.
Step 12
4.8.1 State ONE practical application of the automatic sequence starter.
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A common application is in conveyor belt systems where sequential starting of motors is required for efficient operation.
Step 13
4.8.2 Describe what would happen to motor 1 (MC1) if the contact labelled N/O MC1 HOLD IN were faulty and did not close.
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If the contact was faulty, the motor 1 would not be energised when the start button is pressed, making this motor fail to start.
Step 14
4.8.3 Describe the function of the starter.
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The starter ensures that the motors are started in a controlled sequence. For example, after starting the first drive motor, it will start the second drive motor after a predetermined time.
