6.1 Name the TWO types of rotor construction used in induction motors - NSC Electrical Technology Power Systems - Question 6 - 2022 - Paper 1
Question 6
6.1 Name the TWO types of rotor construction used in induction motors.
6.2 Refer to the speed of induction motors and answer the questions that follow.
6.2.1 Expla... show full transcript
Worked Solution & Example Answer:6.1 Name the TWO types of rotor construction used in induction motors - NSC Electrical Technology Power Systems - Question 6 - 2022 - Paper 1
Step 1
6.1 Name the TWO types of rotor construction used in induction motors.
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Answer
The two types of rotor construction used in induction motors are:
Wound rotor
Cage rotor
Step 2
6.2.1 Explain the difference between rated speed and full-load speed.
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Rated speed is the maximum speed at which the motor effectively works under no load or minimal load. Conversely, full-load speed is the speed when the motor is under maximum load. Full-load speed is usually lower than the rated speed due to the load's impact on the motor's performance.
Step 3
6.2.2 Calculate the synchronous speed of a three-phase motor with four pole pairs per phase that are connected to a 400 V/50 Hz supply.
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The synchronous speed can be calculated using the formula:
n_s = rac{60 imes f}{p}
where:
f is the frequency (50 Hz),
p is the number of poles (which is 2 times the number of pole pairs).
Thus, p=2imes4=8.
Now substituting the values:
n_s = rac{60 imes 50}{8} = 375 ext{ r/min}.
Step 4
6.2.3 Calculate the percentage slip if the rotor turns at 725 r/min (rpm).
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The percentage slip can be calculated using the formula:
ext{Slip} = rac{n_s - n_r}{n_s} imes 100
where:
ns is the synchronous speed (750 r/min from previous calculation),
nr is the rotor speed (725 r/min).
Substituting these into the formula:
ext{Slip} = rac{750 - 725}{750} imes 100 = 3.33\%.
Step 5
6.3 Name TWO mechanical inspections that could be carried out on the rotor and bearings of a motor.
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Two mechanical inspections that could be carried out on the rotor and bearings are:
Checking if there is any play on the axis.
Checking if the bearings work smoothly when turned by hand.
Step 6
6.4.1 Input power.
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Answer
The input power can be calculated using the formula:
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Answer
The output power can be determined using:
P_{ ext{output}} = P_{ ext{input}} imes rac{90}{100}
Substituting the input power:
Pextoutput=3092.43imes0.9=2783.19extW.
Step 9
6.5 Redraw the terminal box in the ANSWER BOOK and indicate how the motor can be connected in delta.
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To connect the motor in delta, the terminals should be connected as follows:
Connect W1 to U2, U1 to V2, and V1 to W2.
Ensure that each terminal is wired correctly to allow the three-phase supply to flow effectively.
Step 10
6.6.1 Identify component A.
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Component A is identified as the overload relay, which is a Normally Closed (N/C) contact.
Step 11
6.6.2 Explain the purpose of MC1/N.O2.
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MC1/N.O2 is a hold-out contact ensuring that the timer relay and the star contactor MC2 do not energise until MC1 is energised.
Step 12
6.6.3 Explain the importance of interlocking in this circuit.
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Interlocking ensures that MC2 and MC3 do not energise simultaneously, preventing potential faults in the system.
Step 13
6.6.4 Describe the operation of the circuit after the timer has timed through.
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Once the timer has timed out, the Normally Closed (N/C) contact will open, de-energising MC2. This action will close MC2/N.C and T/N.O, ensuring that MC3 remains de-energised, thus completing the circuit's operational sequence.
