QUESTION 5: THREE-PHASE MOTORS AND STARTERS 5.1 State THREE mechanical inspections to be conducted on a three-phase motor after installation, but before commissioning. 5.2 State THREE advantages of a squirrel-cage induction motor. 5.3 Study FIGURE 5.3 below and answer the questions that follow. 5.3.1 Identify the control circuit in FIGURE 5.3. 5.3.2 Explain the function of the following components in FIGURE 5.3: (a) Stop button (b) MC₁ (N₀₁) 5.3.3 Describe the starting sequence of the control circuit. 5.4 A three-phase induction motor with 6 pole pairs per phase is connected to a 380 V/50 Hz supply and has a slip of 0,05 units. Given: p = 6 f = 50 Hz s = 0,05 units V = 380 V Calculate the: 5.4.1 Synchronous speed. 5.4.2 Rotor speed. 5.5 An 18 kW three-phase motor is connected in delta to a 380 V/50 Hz supply. The power factor is 0,8. Given: Vₗ = 380 V f = 50 Hz P = 18 kW Calculate the: 5.5.1 Line current drawn from the supply. 5.5.2 Apparent power of the motor.

NSC Electrical Technology Power Systems Motor Applications: QUESTION 5: THREE-PHASE MOTORS A

QUESTION 5: THREE-PHASE MOTORS AND STARTERS 5.1 State THREE mechanical inspections to be conducted on a three-phase motor after installation, but before commissioning - NSC Electrical Technology Power Systems - Question 5 - 2020 - Paper 1

Question 5

QUESTION 5: THREE-PHASE MOTORS AND STARTERS 5.1 State THREE mechanical inspections to be conducted on a three-phase motor after installation, but before commissioni... show full transcript

Worked Solution & Example Answer:QUESTION 5: THREE-PHASE MOTORS AND STARTERS 5.1 State THREE mechanical inspections to be conducted on a three-phase motor after installation, but before commissioning - NSC Electrical Technology Power Systems - Question 5 - 2020 - Paper 1

Step 1

5.1 State THREE mechanical inspections to be conducted on a three-phase motor after installation, but before commissioning.

96%

114 rated

Answer

Check that the bolts securing the motor to the frame are tight.
Ensure that the end plates are fastened properly.
Inspect the bearings for smooth rotation.

Step 2

5.2 State THREE advantages of a squirrel-cage induction motor.

99%

104 rated

Answer

It is cheaper and more robust compared to other types of motors.
It has slightly higher efficiency and power factor.
These motors are explosion proof, reducing the risk of sparking due to the absence of slip rings and brushes.

Step 3

5.3.1 Identify the control circuit in FIGURE 5.3.

96%

101 rated

Answer

The control circuit in FIGURE 5.3 refers to a typical motor control circuit, which includes start and stop buttons, contactors, and overload protection components.

Step 4

5.3.2 Explain the function of the following components in FIGURE 5.3: (a) Stop button

98%

120 rated

Answer

The function of the stop button is to disconnect the supply from the control circuit and stop both motors.

Step 5

5.3.2 Explain the function of the following components in FIGURE 5.3: (b) MC₁ (N₀₁)

97%

117 rated

Answer

The function of MC₁ (N₀₁) is to allow current to flow in the parallel circuit and engage the starting motor, maintaining contact even after the start button is released.

Step 6

5.3.3 Describe the starting sequence of the control circuit.

97%

121 rated

Answer

When start button 1 is pressed, current flows through the stop button and operating line O₁/L₁.
MC₁ (Motor 1) will energize, and contactors MC₁N₀ and MC₁N₁ will close.
Motor 1 will start running.
When start button 2 is pressed, MC₂ (Motor 2) will energize and close contact MC₂N₁, engaging and switching Motor 2 on.
The two motors will run respectively.

Step 7

5.4.1 Calculate the synchronous speed.

96%

114 rated

Answer

The synchronous speed (nₛ) can be calculated using the formula:

$nₛ = \frac{120 \times f}{p}$

where:

$f = 50$ Hz, and
$p = 6$ (pole pairs).

Calculating:

$nₛ = \frac{120 \times 50}{6} = 500 \text{ rpm}$

Step 8

5.4.2 Calculate the rotor speed.

99%

104 rated

Answer

The rotor speed (nₜ) can be calculated using the formula:

$nₜ = nₛ \times (1 - s)$

where:

$s = 0.05$ (slip).

Calculating:

$nₜ = 500 \times (1 - 0.05) = 475 \text{ rpm}$

Step 9

5.5.1 Calculate the line current drawn from the supply.

96%

101 rated

Answer

The line current (Iₗ) can be calculated using the formula:

$Iₗ = \frac{\sqrt{3} \times Vₗ \times P}{1000 \times \text{Cos } θ}$

Given:

$Vₗ = 380 V$
$P = 18 kW$
$\text{Cos } θ = 0.8$

Calculating:

$Iₗ = \frac{\sqrt{3} \times 380 \times 18}{1000 \times 0.8} = 34.14 A$

Step 10

5.5.2 Calculate the apparent power of the motor.

98%

120 rated

Answer

The apparent power (S) can be calculated using the formula:

$S = \frac{Vₗ \times Iₗ}{1000}$

Using the line current calculated previously:

$S = \sqrt{3} \times 380 \times 34.14 = 22.5 ext{ kVA}$

QUESTION 5: THREE-PHASE MOTORS AND STARTERS 5.1 State THREE mechanical inspections to be conducted on a three-phase motor after installation, but before commissioning - NSC Electrical Technology Power Systems - Question 5 - 2020 - Paper 1

Worked Solution & Example Answer:QUESTION 5: THREE-PHASE MOTORS AND STARTERS 5.1 State THREE mechanical inspections to be conducted on a three-phase motor after installation, but before commissioning - NSC Electrical Technology Power Systems - Question 5 - 2020 - Paper 1

5.1 State THREE mechanical inspections to be conducted on a three-phase motor after installation, but before commissioning.

5.2 State THREE advantages of a squirrel-cage induction motor.

5.3.1 Identify the control circuit in FIGURE 5.3.

5.3.2 Explain the function of the following components in FIGURE 5.3: (a) Stop button

5.3.2 Explain the function of the following components in FIGURE 5.3: (b) MC₁ (N₀₁)

5.3.3 Describe the starting sequence of the control circuit.

5.4.1 Calculate the synchronous speed.

5.4.2 Calculate the rotor speed.

5.5.1 Calculate the line current drawn from the supply.

5.5.2 Calculate the apparent power of the motor.

Join the NSC students using SimpleStudy...