3.1 Name and explain TWO types of losses that occur in transformers. 3.2 The delta-connected primary winding of a three-phase transformer is supplied with 11 kV. The secondary winding is star-connected and supplies 400 V to a balanced star-connected load of 10 kW with a power factor of 0,8. Calculate the following at full load: Given: $V_p = 11 \, \text{kV}$ $V_s = 400 \, \text{V}$ $P = 10 \, \text{kW}$ $\cos \theta = 0,8$ 3.2.1 The total kVA of the load 3.2.2 The secondary line current 3.2.3 The secondary phase current 3.3 Describe why the secondary winding of a transformer must be connected in star if the transformer is to supply both a domestic and an industrial load. 3.4 Name THREE causes of heat build-up in transformers. 3.5 Explain what will happen to the primary current of an ideal transformer if the load is doubled.

NSC Electrical Technology Power Systems Transformer Applications: 3.1 Name and explain TWO types o

3.1 Name and explain TWO types of losses that occur in transformers - NSC Electrical Technology Power Systems - Question 3 - 2017 - Paper 1

Question 3

3.1 Name and explain TWO types of losses that occur in transformers. 3.2 The delta-connected primary winding of a three-phase transformer is supplied with 11 kV. Th... show full transcript

Worked Solution & Example Answer:3.1 Name and explain TWO types of losses that occur in transformers - NSC Electrical Technology Power Systems - Question 3 - 2017 - Paper 1

Step 1

3.1 Name and explain TWO types of losses that occur in transformers.

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Answer

Copper Losses: These losses occur due to the resistance of the copper conductors in the winding. When current flows through these windings, energy is lost in the form of heat, which reduces the efficiency of the transformer.
Dielectric Losses: These are losses that happen when the insulation of the windings is damaged, causing leakage currents. It mainly occurs due to the electrical field within the insulation, leading to energy dissipation.

Step 2

3.2.1 The total kVA of the load

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To find the total kVA of the load, we use the formula:

$S = \frac{P}{\cos \theta}$

Substituting the values:

$S = \frac{10 \, \text{kW}}{0.8} = 12.5 \, \text{kVA}$

Step 3

3.2.2 The secondary line current

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The secondary line current can be calculated using:

$I_s = \frac{P}{\sqrt{3} V_s \cos \theta}$

Plugging in the values:

$I_s = \frac{10 \, \text{kW}}{\sqrt{3} \times 400 \, \text{V} \times 0.8} = 18.04 \, \text{A}$

Step 4

3.2.3 The secondary phase current

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For a star-connected transformer, the phase current is equal to the line current:

$I_{ph} = I_s = 18.04 \, \text{A}$

Step 5

3.3 Describe why the secondary winding of a transformer must be connected in star if the transformer is to supply both a domestic and an industrial load.

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Connecting the secondary winding in star allows the transformer to provide multiple voltage levels and ensures balanced load conditions. This arrangement is essential for supplying both single-phase (domestic) and three-phase (industrial) loads effectively. It facilitates a neutral point for grounding and enhances safety.

Step 6

3.4 Name THREE causes of heat build-up in transformers.

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Overloading: When the transformer is subjected to loads beyond its rated capacity, it generates excessive heat.
Poor Cooling: Inadequate cooling mechanisms can lead to heat accumulation.
Poor Connections: Loose or damaged connections can cause additional resistance, leading to increased heat generation.

Step 7

3.5 Explain what will happen to the primary current of an ideal transformer if the load is doubled.

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In an ideal transformer, the primary current will also double as it is directly proportional to the load. If the load resistance decreases, leading to an increased load current, the primary side will draw more current to maintain the transformation ratio.

3.1 Name and explain TWO types of losses that occur in transformers - NSC Electrical Technology Power Systems - Question 3 - 2017 - Paper 1

Worked Solution & Example Answer:3.1 Name and explain TWO types of losses that occur in transformers - NSC Electrical Technology Power Systems - Question 3 - 2017 - Paper 1

3.1 Name and explain TWO types of losses that occur in transformers.

3.2.1 The total kVA of the load

3.2.2 The secondary line current

3.2.3 The secondary phase current

3.3 Describe why the secondary winding of a transformer must be connected in star if the transformer is to supply both a domestic and an industrial load.

3.4 Name THREE causes of heat build-up in transformers.

3.5 Explain what will happen to the primary current of an ideal transformer if the load is doubled.

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