When a two-winding transformer is converted into an autotransformer, the KVA rating of the resultant autotransformer:

Explanation:

When a Two-Winding Transformer is Converted into an Autotransformer

Definition: A two-winding transformer is a traditional transformer configuration where the primary and secondary windings are electrically isolated from each other. When this type of transformer is converted into an autotransformer, the primary and secondary windings are interconnected, resulting in a single continuous winding that serves both functions. This connection allows for a portion of the power to be transferred directly (conductively) between the input and output, while the rest is transferred inductively via the magnetic field in the core.

Concept: The kVA rating of a transformer indicates its capacity to handle apparent power. In a two-winding transformer, the entire power transfer is achieved inductively. However, in an autotransformer, part of the power is transferred conductively, which significantly increases its kVA rating for the same physical size and thermal capacity. The increase in the kVA rating depends on the voltage ratio of the transformer.

Formula for kVA Rating of an Autotransformer:

The kVA rating of an autotransformer is given by:

kVA_auto = kVA_two-winding × (1 + k)

Where:

• k = Voltage ratio = (V₂ − V₁) / V₁
• V₁ = Lower voltage side
• V₂ = Higher voltage side

This equation shows that the kVA rating of the autotransformer is greater than that of the original two-winding transformer, depending on the voltage ratio.

Working Principle:

In a two-winding transformer, the primary and secondary windings are completely separate, and the entire load power is transferred magnetically through the core. In an autotransformer, the windings are interconnected, and part of the power is transferred directly through the winding itself (conductive transfer), while the rest is transferred inductively via the magnetic field. This dual transfer mechanism leads to a more efficient utilization of the winding and core material, thereby increasing the apparent power (kVA) rating.

Example:

Consider a two-winding transformer with a kVA rating of 100 kVA, operating with a primary voltage of 200 V and a secondary voltage of 400 V. When converted into an autotransformer:

• Voltage ratio, k = (400 − 200) / 200 = 1
• kVA_auto = 100 × (1 + 1) = 200 kVA

Thus, the kVA rating of the autotransformer is double that of the original two-winding transformer.

Advantages of Autotransformers:

• Higher kVA rating for the same physical size and material cost compared to a two-winding transformer.
• Better efficiency due to reduced losses in the windings.
• Lower cost and weight, as less copper and core material are required.

Limitations of Autotransformers:

• Lack of electrical isolation between the primary and secondary sides, which may pose safety concerns.
• Not suitable for applications requiring galvanic isolation.

Correct Option Analysis:

The correct option is:

Option 4: Increases

When a two-winding transformer is converted into an autotransformer, its kVA rating increases due to the conductive transfer of power in addition to the inductive transfer. The extent of this increase depends on the voltage ratio, as demonstrated in the formula above. This characteristic makes autotransformers highly efficient and cost-effective for certain applications where electrical isolation is not required.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Remains same

This option is incorrect because the kVA rating of an autotransformer is always greater than that of the original two-winding transformer. The conductive transfer of power in an autotransformer contributes to this increase, as explained in the formula.

Option 2: Decreases to half of the original rating

This option is incorrect because the kVA rating of an autotransformer does not decrease. On the contrary, it increases due to the efficient utilization of winding material and the dual power transfer mechanism.

Option 3: Decreases to 3/4th of the original rating

This option is also incorrect. As mentioned earlier, the kVA rating of an autotransformer increases compared to the original two-winding transformer. There is no scenario where the rating decreases to 3/4th of the original value.

Conclusion:

The conversion of a two-winding transformer into an autotransformer results in an increased kVA rating due to the combined conductive and inductive power transfer mechanisms. While autotransformers offer significant advantages in terms of efficiency and material utilization, their lack of electrical isolation may limit their use in certain applications. Understanding the principles of operation and the factors influencing the kVA rating is crucial for selecting the appropriate transformer configuration for a given application.