Power Electrical Autotransformers: Definition, Characteristics and Applications

what is an autotransformer?

An autotransformer, or auto-transformer, is a type of transformer with only one winding that serves both as the primary and secondary winding. Unlike traditional transformers with separate primary and secondary windings, an autotransformer has a common winding shared by both input and output sides. The winding has at least three taps: one common tap and two additional taps for connection to the input and output circuits.

The basic operation of an autotransformer involves varying the point of connection along the winding to adjust the voltage transformation ratio. The voltage across the common winding is the sum of the voltages across the tapped portions, providing a variable voltage output.

 

What is the difference between an autotransformer and an isolation transformer?

There are four main differences between autotransformers and isolation transformers: structure, voltage transformation, voltage regulation and impedance, size, and efficiency.

put up:
Autotransformer: An autotransformer has a single winding that acts as both primary and secondary windings. It usually consists of a common winding with taps at different points to provide various voltage ratios.
Isolation Transformer: The isolation transformer has two independent windings – primary winding and secondary winding. These windings are electrically isolated from each other.

Voltage transformation:
Autotransformer: In an autotransformer, voltage transformation is performed by tapping at different points along the winding. By changing the connection points, different voltage ratios can be obtained. The primary and secondary voltages are interconnected, meaning the secondary voltage comes from the same winding as the primary voltage.
Isolation Transformer: In an isolation transformer, voltage transformation occurs through mutual inductance between the primary and secondary windings. The primary voltage induces a magnetic field, which then induces a voltage in the secondary winding. The primary and secondary voltages are electrically isolated from each other.

Voltage Regulation and Impedance:
Autotransformer: Autotransformers generally provide better voltage regulation due to the direct connection between the primary and secondary windings. However, they have lower impedance than isolation transformers, so they may not provide as much protection against voltage spikes or fluctuations. Lacking any phase shift, an autotransformer cannot help eliminate smaller harmonic distortions.Isolation Transformer: Isolation transformers provide complete electrical isolation between input and output, making them suitable for applications where safety and electrical fault protection are critical. They have a higher impedance which helps reduce voltage spikes and provide isolation from the mains supply. In systems with smaller harmonic distortions, the isolation transformer provides phase shifting (and possibly a grounded neutral conductor in DY systems), which helps eliminate smaller harmonic distortions.

Scale and efficiency:
Autotransformers: Autotransformers are generally smaller and lighter than isolation transformers because they require only one winding. Although autotransformers are not subject to the U.S. Department of Energy’s 2016 energy efficiency requirements, their efficiency is generally improved due to their low impedance and common conductive path.

Isolation transformer: Because of its two independent windings, an isolation transformer is larger and heavier than an autotransformer. They need to meet current U.S. Department of Energy efficiency guidelines and are not as efficient as autotransformers.

 

Key features and characteristics of autotransformers include:

Voltage Transformation: Autotransformers can step up or step down voltage levels depending on the point of connection along the winding. The turns ratio is determined by the ratio of the number of turns between the common tap and the tapped points.

Reduced Size and Weight: Autotransformers are generally more compact and lighter than traditional transformers of equivalent power rating because they share a common winding.

Efficiency: Autotransformers tend to be more efficient than two-winding transformers since they have fewer losses associated with a single winding.

Cost-Effectiveness: Due to reduced material and winding costs, autotransformers are often more cost-effective than their two-winding counterparts.

 

autotransformer Applications:

Voltage Regulation: Autotransformers are commonly used for voltage regulation in power systems.
Motor Starters: Autotransformers are employed in motor starters to reduce inrush current during motor startup.
Adjustable Voltage Systems: Autotransformers are used in applications where adjustable voltage levels are required, such as testing laboratories.
Despite their advantages, autotransformers have some limitations:

Lack of Electrical Isolation: Autotransformers do not provide electrical isolation between the input and output circuits, which c