FIGURE 5.1 below shows the rotor of an induction motor. Answer the questions that follow. 5.1 Name parts A and B. 5.1.2 State ONE important advantage of using this type of a rotor compared to using a motor with brushes and slip rings. 5.1.3 Give ONE reason why the rotor bars are skewed. 5.2 Explain the following terms with reference to motors: 5.2.1 Slip 5.2.2 Commissioning 5.3 State ONE type of mechanical inspection that must be conducted after installation and before commissioning. 5.4 A three-phase delta-connected motor has a total of 12 poles and is connected to a 380 V/50 Hz supply. The input power to the motor is 25 kW with a lagging power factor of 0,95. The total losses on the motor are 800 W. Calculate the: 5.4.1 Pole pairs per phase 5.4.2 Synchronous speed of the motor 5.4.3 Rotor speed with a 3% slip 5.4.4 Efficiency of the motor FIGURE 5.5 below shows the control circuit of a three-phase motor starter. 5.5.1 Identify the control circuit in FIGURE 5.5. 5.5.2 Explain the function of the following components used in the circuit. (a) OLN/C (b) MC2/N/O 5.5.3 Explain why the MC1/N/C contact is connected in series with the MC2 contactor coil. 5.6 The following information is given about a three-phase induction motor with reference to the setting of the overload: Given: VS = 380 V I_max = 100 A Calculate the full-load current of the motor if the maximum starting-line current is seven times the full-load current.

NSC Electrical Technology Power Systems Motor Construction: FIGURE 5.1 below shows the rotor

FIGURE 5.1 below shows the rotor of an induction motor - NSC Electrical Technology Power Systems - Question 5 - 2021 - Paper 1

Question 5

FIGURE 5.1 below shows the rotor of an induction motor. Answer the questions that follow. 5.1 Name parts A and B. 5.1.2 State ONE important advantage of using this... show full transcript

Worked Solution & Example Answer:FIGURE 5.1 below shows the rotor of an induction motor - NSC Electrical Technology Power Systems - Question 5 - 2021 - Paper 1

Step 1

Name parts A and B.

96%

114 rated

Answer

Part A is the 'End ring' and Part B is the 'Bearing'.

Step 2

State ONE important advantage of using this type of a rotor compared to using a motor with brushes and slip rings.

99%

104 rated

Answer

This type of rotor requires less maintenance due to the absence of brushes and slip rings, which eliminates any sparks and reduces arcing.

Step 3

Give ONE reason why the rotor bars are skewed.

96%

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Answer

The rotor bars are skewed to help reduce cogging, which refers to the locking tendency of the rotor.

Step 4

Slip

98%

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Answer

Slip is defined as the difference between the synchronous speed ( $v_s$ ) and the rotor speed ( $v_r$ ) of an induction motor expressed as a percentage.

Step 5

Commissioning

97%

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Answer

Commissioning is the process of connecting the electric motor to the power supply and ensuring all preliminary checks (electrical and mechanical) have been completed before operation.

Step 6

State ONE type of mechanical inspection that must be conducted after installation and before commissioning.

97%

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Answer

Inspect the mounting bolts to check if they are properly tightened.

Step 7

Pole pairs per phase

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Answer

To calculate the number of pole pairs per phase, divide the total number of poles by 3.

Pole pairs per phase = $rac{12}{3} = 4$ .

Step 8

Synchronous speed of the motor

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Answer

The synchronous speed can be calculated using the formula:

$n_s = rac{f imes 60}{p}$

where $f = 50 Hz$ and $p = 12$ poles:

$n_s = rac{50 imes 60}{12} = 250$

Therefore, the synchronous speed of the motor is 1500 RPM.

Step 9

Rotor speed with a 3% slip

96%

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Answer

Using the synchronous speed calculated previously, the rotor speed can be calculated using the formula:

$n_r = n_s - (n_s imes slip)$

Here, the slip is 3%, thus:

$n_r = 1500 - (1500 imes rac{3}{100}) = 1455 ext{ RPM}$

Step 10

Efficiency of the motor

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Answer

Efficiency can be calculated using the following formula:

$ext{Efficiency} ("]) = rac{P_{in} - ext{losses}}{P_{in}} imes 100$

Where:

$P_{in} = 25000 W$
losses = 800 W

Substituting in these values:

$ext{Efficiency} = rac{25000 - 800}{25000} imes 100 = 96.8 ext{%}$

Step 11

Identify the control circuit in FIGURE 5.5.

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Answer

The control circuit is a 'Forward reverse control circuit.'

Step 12

Explain the function of the following components used in the circuit.

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Answer

(a) OLN/C: This component opens when the current surpasses the preset rated current, stopping the current flow to MC1.

(b) MC2/N/O: This is the retain contact for the reverse contactor, ensuring that current continues to flow to MC2 after the start button is released.

Step 13

Explain why the MC1/N/C contact is connected in series with the MC2 contactor coil.

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Answer

MC1/N/C is used as an interlocking contact. When MC1 is activated, MC1/N/C will open, ensuring that MC2 cannot be activated when MC1 is active.

Step 14

Calculate the full-load current of the motor if the maximum starting-line current is seven times the full-load current.

99%

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Answer

Given that: $I_{max} = I_{FL} imes 7$ , where $I_{max} = 100 A$ , we can rearrange this formula to find:

$I_{FL} = rac{I_{max}}{7} = rac{100}{7} o 14.29 A.$

FIGURE 5.1 below shows the rotor of an induction motor - NSC Electrical Technology Power Systems - Question 5 - 2021 - Paper 1

Worked Solution & Example Answer:FIGURE 5.1 below shows the rotor of an induction motor - NSC Electrical Technology Power Systems - Question 5 - 2021 - Paper 1

Name parts A and B.

State ONE important advantage of using this type of a rotor compared to using a motor with brushes and slip rings.

Give ONE reason why the rotor bars are skewed.

Slip

Commissioning

State ONE type of mechanical inspection that must be conducted after installation and before commissioning.

Pole pairs per phase

Synchronous speed of the motor

Rotor speed with a 3% slip

Efficiency of the motor

Identify the control circuit in FIGURE 5.5.

Explain the function of the following components used in the circuit.

Explain why the MC1/N/C contact is connected in series with the MC2 contactor coil.

Calculate the full-load current of the motor if the maximum starting-line current is seven times the full-load current.

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