An autotransformer is a type of transformer that uses a single winding wound around a laminated core. The single winding acts as the primary and secondary winding of the transformer. In a conventional transformer, the primary and secondary windings are electrically isolated but magnetically linked. In contrast, an autotransformer features windings that are both magnetically and electrically connected. Here, a portion of a single continuous winding performs a dual role, functioning as part of both the primary and secondary windings.

Construction and Working

An autotransformer consists of a single winding that acts as both the primary and secondary winding. This winding includes tap points that allow the output voltage to be adjusted by selecting different points along the winding. Unlike a conventional transformer, where the primary and secondary windings are electrically isolated, the windings in an autotransformer are electrically connected.

Conventional Transformer                                                 Autotransformer 

A portion of the winding is shared between the primary and secondary sides, known as the common section. The remaining part of the winding, which is not shared between the primary and secondary, is called the series section. When voltage is applied to the primary terminals, it induces a voltage along the entire length of the winding according to Faraday's law of electromagnetic induction. By tapping into different points on the winding, various output voltages can be obtained, enabling the autotransformer to function as either a step-up or step-down transformer.

When a load is connected to the secondary terminals, the current flows through both the primary and secondary sections of the winding. The voltage drop in the primary section is IZ₁, where I is the load current and Z₁ is the primary impedance. Similarly, the voltage drop in the secondary section is IZ₂, where Z₂ is the secondary impedance. The output voltage seen by the load is the induced voltage in the secondary section minus the voltage drop across the secondary impedance.

The voltage ratio between the primary and secondary sides is determined by the turns ratio between the primary and secondary sections of the winding.

The voltage ratio is given by:

Where V₁ is the primary voltage, V₂ is the secondary voltage, N₁ is the primary turns, N₂ is the secondary turns and 'a' is the turns ratio. For step-up operation, the secondary voltage V₂ is greater than the primary voltage V₁. For step-down operation, the secondary voltage V₂ is less than the primary voltage V_1.

Advantages of an Autotransformer

• Copper Savings: Autotransformers use less conductor material compared to two-winding transformers.
• Efficiency: They offer higher efficiency due to reduced copper and iron losses.
• Compact Design: Their single-winding design makes them more compact and cost-effective.
• Variable Voltage: They can produce variable voltages by adjusting the tap points.

Disadvantages of an Autotransformer

• Direct Connection: The primary and secondary sides are directly connected, which can cause problems if there is an open circuit in the common section of the winding.
• Reduced Isolation: Autotransformers lack electrical isolation, which is undesirable in certain applications.

Applications of an Autotransformer

Autotransformers are used to connect transmission or distribution lines operating at different voltage levels. They serve as booster transformers in electric traction systems to compensate for line voltage drops. In laboratories, autotransformers provide variable-voltage supplies. They are also employed to reduce the voltage for starting induction motors, minimizing inrush current and mechanical stress on the motor.

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