Question

Explain with the help of a neat diagram the working of a 3-phase autotransformer. Write any two applications.

Answer 1

To explain the working of a 3-phase autotransformer, let’s start with the basic concept. An autotransformer is a type of transformer that has a single winding, which acts as both the primary and secondary winding. This design allows for a more compact and efficient transformer compared to a conventional transformer.

### Working Principle:

1. **Construction**: A 3-phase autotransformer consists of a common winding divided into two parts: the primary section and the secondary section. The primary side is connected to the input voltage supply, while the secondary side provides the output voltage.

2. **Voltage Transformation**: The input voltage is applied across the entire winding. By tapping into the winding at a specific point, the transformer can deliver a different voltage (either higher or lower) on the secondary side. The relationship between primary and secondary voltages can be represented by the equation:
   \[
   \frac{V_s}{V_p} = \frac{N_s}{N_p}
   \]
   where \(V_s\) is the secondary voltage, \(V_p\) is the primary voltage, \(N_s\) is the number of turns in the secondary, and \(N_p\) is the number of turns in the primary.

3. **Operation**: In a 3-phase system, three sets of windings are arranged to ensure balanced load and equal voltage transformation across all phases. The phase shift between the three phases ensures that the transformer operates efficiently, minimizing losses.

### Diagram:

While I can't draw directly, envision the following:

- Three sets of coils, each representing a phase (A, B, C).
- A winding loop that starts from one end of the coil for the primary input and taps at a point along the winding for the secondary output.
- Arrows indicating the direction of current flow and voltage levels for clarity.

### Applications:

1. **Voltage Regulation**: 3-phase autotransformers are used in power distribution systems to adjust voltage levels efficiently, allowing for long-distance transmission with minimal losses.

2. **Motor Starting**: They are often employed in starting large induction motors, providing a reduced voltage at startup to limit the inrush current and mechanical stress on the motor.

If you have more specific aspects you’d like to delve into, let me know!

Answer 2

A 3-phase autotransformer is a type of transformer that uses a single winding per phase to provide both primary and secondary voltage connections. It differs from a conventional transformer by having a common winding for both input and output, which allows it to be more compact and efficient. Here’s an explanation of how it works, followed by its applications.

### Working of a 3-Phase Autotransformer

#### Diagram

Consider a 3-phase autotransformer with the following key components:
1. **Primary Winding (or Coil)**: The part of the winding that is connected to the input voltage source.
2. **Secondary Winding (or Coil)**: The part of the winding that provides the output voltage.
3. **Tap Point**: The point where the winding is tapped to provide a different voltage level.

The diagram below illustrates a simple 3-phase autotransformer:

```
       Primary Side                Secondary Side
        ________                     ________
        |      |                     |      |
        |      |                     |      |
        |______|                     |______|
       __|__   __|__               __|__   __|__
      |    |   |    |             |    |   |    |
      |    |   |    |             |    |   |    |
      |    |   |    |             |    |   |    |
      |____|   |____|             |____|   |____|
        |______|                     |______|
       |  |  |  |                   |  |  |  |
       |  |  |  |                   |  |  |  |
       |__|__|__|                   |__|__|__|
        \      /                     \      /
         \    /                       \    /
          \  /                         \  /
           \/                           \/
```

In the diagram:
- The primary winding consists of a single continuous winding with a tap point.
- The secondary winding is connected to the part of the winding from the tap point to the end.

#### Working Principle

1. **Input Voltage**: The 3-phase input voltage is applied to the primary winding of the autotransformer.
2. **Induced Voltage**: Due to electromagnetic induction, a voltage is induced in the entire winding, both above and below the tap point.
3. **Voltage Transformation**: The output voltage (secondary voltage) is taken from the portion of the winding beyond the tap point. By adjusting the position of the tap point, the output voltage can be varied.
4. **Common Winding**: The key feature is that the portion of the winding used for both primary and secondary is common, leading to a reduction in the size and weight of the transformer.

### Applications of a 3-Phase Autotransformer

1. **Voltage Regulation**:
   - **Application**: Used in electrical substations and industrial power systems.
   - **Description**: It helps in voltage regulation by stepping up or stepping down the voltage levels according to the load requirements. The autotransformer adjusts the output voltage to maintain a stable voltage level in the system.

2. **Starting of Large Motors**:
   - **Application**: Used in starting large induction motors.
   - **Description**: When large motors start, they draw a high inrush current. A 3-phase autotransformer can be used to reduce the starting voltage, thus limiting the starting current and reducing mechanical stress on the motor and the power system.

### Summary

A 3-phase autotransformer is efficient for applications where voltage adjustment is needed with minimal size and weight. Its design allows for cost-effective voltage regulation and smooth starting of large electrical equipment.