4.1 Refer to FIGURE 4.1 and answer the questions that follow - NSC Electrical Technology Electronics - 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 Electronics - 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 setup generates three-phase currents flowing in the stator windings, creating a rotating magnetic field within the stator. This magnetic field induces a current in the rotor conductors, resulting in the rotor spinning. The interaction between the magnetic fields of the stator and rotor causes the rotor to rotate, generating torque.
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 is an open circuit, the motor will still rotate, but it will not develop the correct torque. This leads to an imbalance in the magnetic fields, risking overheating or damaging the motor.
Step 4
4.2 State TWO advantages of a three-phase induction motor over a single-phase induction motor.
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Three-phase induction motors require less maintenance as they have fewer components compared to single-phase motors.
For the same size, a three-phase motor can deliver a higher torque than a single-phase motor.
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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It is crucial to ensure that the integrity of the insulation is sound to avoid electrical faults when the motor is energised. This helps prevent short circuits and ensures safe operation.
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 to ensure that the motor is properly supported.
Step 7
4.5.1 Synchronous speed.
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Answer
The synchronous speed can be calculated using the formula:
n_s = rac{60 imes f}{p}
Substituting the given values: n_s = rac{60 imes 50}{3} = 1000 ext{ r/min}
Step 8
4.5.2 Rotor speed.
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The rotor speed can be calculated using the formula:
nr=ns(1−S)
Substituting the values gives: nr=1000(1−0.04)=960extr/min
Step 9
4.6.1 Input kVA of the motor at full load.
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The input kVA of the motor can be determined using the formula:
S = rac{ ext{√3} imes V_L imes I}{1000}
Substituting known values: S = rac{ ext{√3} imes 380 imes 8.5}{1000} = 5.59 kVA
Step 10
4.6.2 Active power output of the motor at full load.
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Active power output can be calculated using:
P = rac{√3 imes V_L imes I imes ext{cos} heta}{1000}
For this case: P = rac{√3 imes 380 imes 8.5 imes 0.8 imes 0.95}{1000} = 4.25 kW
Step 11
4.7 Describe the function of an overload unit in a motor starter.
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The overload unit acts as a safety device that protects the motor from overheating by disconnecting power when excessive current is detected, thus preventing damage.
Step 12
4.8.1 State ONE practical application of the automatic sequence starter.
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One practical application is in conveyor belt systems, where motors are started in sequence to manage load and ensure smooth 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 is faulty, motor 1 (MC1) would not energise when the start button is pressed, leading to a failure in starting that motor.
Step 14
4.8.3 Describe the function of the starter.
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The starter is responsible for controlling the sequence of motor activation. When the start button is pressed, it initiates the process of energising the first motor, and after a predetermined time the second motor is activated, ensuring coordinated operation.
