An autotransformer is a type of electrical transformer that has a single winding that acts as both the primary and secondary coil, as opposed to the traditional transformer where there are two separate windings for primary and secondary. The main difference lies in the fact that the autotransformer has a part of the winding common to both the primary and secondary circuits.

Key Features of Autotransformers:

  • Single Winding: In an autotransformer, the primary and secondary windings are not entirely separate but share a common portion of the winding. The voltage is stepped up or stepped down by tapping into different points along the same winding.
  • Voltage Relationship: An autotransformer can step up or step down the voltage, but the voltage change depends on the portion of the winding used. For example, if part of the winding is tapped for the secondary, the output voltage will be a fraction of the input voltage.

Basic Structure:

  • Primary Side: The input side that is connected to the power source.
  • Secondary Side: The output side, which is connected to the load.
  • Common Winding: Part of the winding is shared by both primary and secondary.

How It Works:

In an autotransformer, part of the winding is common between the primary and secondary circuits. The voltage transformation occurs based on where the secondary is tapped from within the winding.

For instance:

  • In a step-down autotransformer, if the secondary is tapped at a lower point on the winding, the voltage output will be lower than the primary input voltage, but the current will be higher.
  • In a step-up autotransformer, if the secondary is tapped higher on the winding, the output voltage will be higher than the input voltage.

Voltage and Current Relationships:

  • The voltage ratio between primary and secondary is determined by the ratio of the number of turns in the portion of the winding used for the secondary to the total number of turns in the primary.

  • The current ratio in an autotransformer is different from that in a conventional transformer. Since part of the winding is common, the current in the primary side is generally smaller than the current in the secondary side for a step-down transformer (and vice versa for a step-up transformer).

    • For a step-down autotransformer, the current on the primary side is smaller, but the current on the secondary side is higher, although the total power is conserved.
    • For a step-up autotransformer, the primary current will be higher than the secondary current, but the voltage is increased.

Advantages of Autotransformers:

  1. Efficiency: Autotransformers are more efficient than traditional transformers for the same size and power rating because they use a smaller amount of copper and iron, as they share a common winding.
  2. Smaller Size and Weight: Since they require fewer windings, autotransformers are physically smaller and lighter than conventional transformers with the same power capacity.
  3. Lower Cost: Due to the reduced material requirements, autotransformers are less expensive to manufacture.
  4. Higher Efficiency for Voltage Regulation: Autotransformers can achieve better voltage regulation and operate more efficiently under light load conditions.

Disadvantages:

  1. Less Isolation: Unlike traditional transformers, autotransformers do not provide full electrical isolation between the primary and secondary circuits. This can be a disadvantage when isolation is required for safety reasons.
  2. Higher Fault Currents: Since the primary and secondary windings are electrically connected, faults on the secondary side can result in higher fault currents on the primary side.
  3. Limited Voltage Transformation: The voltage transformation ratio is limited by the number of taps available in the winding, which means autotransformers are less versatile compared to traditional transformers in providing a large range of voltage transformations.

Applications of Autotransformers:

  • Motor Starting: Autotransformers are commonly used in motor starting applications to reduce the inrush current when large motors are started. They allow the motor to start at a lower voltage and then switch to full voltage once it has gained speed.
  • Voltage Regulation: They are used in some applications for regulating voltage in situations where full isolation isn't necessary.
  • Power Transmission: Autotransformers can be used in high-voltage transmission systems to step up or step down voltages with reduced size and weight compared to traditional transformers.

Example of Voltage Transformation:

  • Suppose a 440 V primary supply is connected to an autotransformer, and a secondary voltage of 220 V is tapped from the winding.
    • The voltage ratio will be 220440=0.5\frac{220}{440} = 0.5 or a step-down ratio of 2:1.
    • The primary current would be lower, and the secondary current would be higher than that in a conventional transformer.