Refer to the losses that occur in transformers and answer the questions that follow. 4.1.1 Name TWO types of losses that occur in transformers. 4.1.2 State TWO factors that may contribute to the excessive heating of transformers. 4.2 Describe how electromotive force (EMF) is induced in the secondary windings of transformers. 4.3 Explain why the following are important for effective and efficient use of transformers: 4.3.1 Cooling methods 4.3.2 Protective devices 4.4 A 10 kVA star-delta transformer has primary and secondary line voltages of 6 kV and 500 V respectively. Given: S = 10 kVA VLS = 500 V VLP = 6 kV pf = 0,97 Calculate the: 4.4.1 Secondary line current 4.4.2 Transformer ratio 4.4.3 Input power if the total loss is 80 W 4.5 Explain, with a reason, whether the secondary line current in QUESTION 4.4.1 is higher than the primary line current.

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Refer to the losses that occur in transformers and answer the questions that follow - NSC Electrical Technology Power Systems - Question 4 - 2019 - Paper 1

Question 4

Refer to the losses that occur in transformers and answer the questions that follow. 4.1.1 Name TWO types of losses that occur in transformers. 4.1.2 State TWO fac... show full transcript

Worked Solution & Example Answer:Refer to the losses that occur in transformers and answer the questions that follow - NSC Electrical Technology Power Systems - Question 4 - 2019 - Paper 1

Step 1

4.1.1 Name TWO types of losses that occur in transformers.

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Answer

Two common types of losses that occur in transformers are:

Copper losses: These occur due to the resistance in the windings and are proportional to the square of the current flowing through them.
Iron losses: Also known as core losses, these losses occur due to the alternating magnetic field in the transformer core, primarily comprising hysteresis and eddy current losses.

Step 2

4.1.2 State TWO factors that may contribute to the excessive heating of transformers.

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Two factors that contribute to excessive heating in transformers include:

Insufficient circulating air for cooling: This can lead to a buildup of heat within the transformer, affecting its efficiency and lifespan.
Internal faults: Any faults within the transformer can lead to excessive current flow, resulting in overheating.

Step 3

4.2 Describe how electromotive force (EMF) is induced in the secondary windings of transformers.

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Electromotive force (EMF) is induced in the secondary windings of transformers through the principle of electromagnetic induction. When alternating current flows through the primary windings, it generates an alternating magnetic field. This magnetic field links with the secondary windings, inducing an EMF according to Faraday's law of electromagnetic induction. The magnitude of the induced EMF is proportional to the rate of change of the magnetic flux linkage.

Step 4

4.3.1 Cooling methods

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Cooling methods are crucial for effective transformer operation as they help dissipate heat generated within the transformer. Proper cooling ensures that the temperature remains within optimal limits, preventing overheating, which can lead to insulation failure and reduced lifespan of the transformer.

Step 5

4.3.2 Protective devices

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Protective devices ensure that the transformer is safeguarded against faults. They help isolate the transformer from the supply during internal faults, which protects the equipment from damage and increases the overall safety and reliability of the electrical system.

Step 6

4.4.1 Secondary line current

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To calculate the secondary line current (I_L), we use the formula: $I_L = \frac{S}{\sqrt{3} \times V_L}$ Substituting the given values: $I_L = \frac{10,000}{\sqrt{3} \times 500} = 11.55\ A$

Step 7

4.4.2 Transformer ratio

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The transformer ratio is determined using the formula: $\text{Transformer Ratio} = \frac{V_{ph}}{V_{sl}}$ Given that: $V_{ph} = \frac{V_{L}}{\sqrt{3}} = \frac{6000}{\sqrt{3}} = 3464\ V$ Therefore, the transformer ratio is: $\text{Transformer Ratio} = \frac{V_{ph}}{V_{LS}} = \frac{3464}{500} \approx 6.92:1$

Step 8

4.4.3 Input power if the total loss is 80 W

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To calculate the input power (P_in), we use the formula: $P_{in} = S \times \cos \theta$ Where:

S = 10 kVA
Cos \theta = 0.97 By substituting: $P_{in} = 10,000 \times 0.97 = 9,700\ W$

Step 9

4.5 Explain, with a reason, whether the secondary line current in QUESTION 4.4.1 is higher than the primary line current.

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The secondary line current (11.55 A) is higher than the primary line current. This is because the secondary voltage (500 V) is much lower than the primary voltage (6000 V), and since the power (S) must remain constant in an ideal transformer, a lower voltage results in a higher current according to the formula: $P = V \times I$

Refer to the losses that occur in transformers and answer the questions that follow - NSC Electrical Technology Power Systems - Question 4 - 2019 - Paper 1

Worked Solution & Example Answer:Refer to the losses that occur in transformers and answer the questions that follow - NSC Electrical Technology Power Systems - Question 4 - 2019 - Paper 1

4.1.1 Name TWO types of losses that occur in transformers.

4.1.2 State TWO factors that may contribute to the excessive heating of transformers.

4.2 Describe how electromotive force (EMF) is induced in the secondary windings of transformers.

4.3.1 Cooling methods

4.3.2 Protective devices

4.4.1 Secondary line current

4.4.2 Transformer ratio

4.4.3 Input power if the total loss is 80 W

4.5 Explain, with a reason, whether the secondary line current in QUESTION 4.4.1 is higher than the primary line current.

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