Ever wondered what a ‘autotransformer’ means? Do not worry—you’re not alone. Autotransformers are electrical transformers seldom known outside of engineering. However, these handy gadgets regulate voltage for massive power networks and industrial equipment. We’ll explain autotransformers, how they function, and their main uses in this guide. You’ll grasp this technology that powers our world by the end.

An autotransformer?

Exactly what is an autotransformer? An autotransformer is a single-winding transformer. Autotransformers have one continuous winding on both sides, unlike ordinary transformers.

How Do Autotransformers Work?

An autotransformer taps into different locations of a single winding to produce different voltages. The “primary winding” is between the input and output terminals, while the “secondary winding” is the whole winding. Tap different spots on the winding to get different voltages in and out.

If you put 120V into an autotransformer and tap the output midway during the winding, you receive 60V. Tap 3/4 through the winding for 90V. Tapping the output steps the voltage up or down. This allows autotransformers to change output voltage.

Advantages of Autotransformers

Key autotransformer benefits include:

Since autotransformers only need one winding, they are cheaper to make.

Autotransformers are 95–99% efficient since there are no magnetic losses from two windings.

By altering the tap point, an autotransformer can smoothly vary its output voltage. Applications requiring changeable voltage benefit from this.

Autotransformers have one winding, therefore they take up less space than transformers.

Most transformers produce more harmonics than autotransformers.

• Energization inrush current—Autotransformers have lower inrush currents than transformers.

In conclusion, an autotransformer is a single-winding transformer that can produce several voltages. They are cheaper, more efficient, and smaller than transformers. For many variable voltage applications, an autotransformer is ideal.

How Do Autotransformers Work?

An autotransformer sends and receives electricity using one winding. A single winding serves both the primary and secondary. As a step-up and step-down transformer, the winding is tapped at various locations to provide varied voltage outputs.

The autotransformer supplies electrical current to the entire winding and taps into it at the desired voltage. It has lower leakage reactance and load voltage drop than a two-winding transformer, but it lacks isolation.

Increase Operations

Step-up transformers apply input voltage across a part of the coil. Using the complete winding’s output increases voltage. For instance, 120V supplied to the first half of a 240V winding produces 240V across the winding.

Step-down operation

The input of a step-down transformer is applied to the complete winding, while the output is taken from a part. The first half of a 240V winding can output 120V when 240V is applied.

The fundamental benefit of autotransformers is their size, weight, and cost. They need less wire and core than two-winding transformers because they have one winding. The space and cost benefits are significant, especially for high voltage applications. Their biggest drawback is that they lack isolation like two-winding transformers. However, cost benefits offset this disadvantage in many applications.

Industry uses autotransformers for motor starters, welding equipment, and power distribution voltage regulation. They are cost-effective when voltage transformation and galvanic isolation are not needed. Understanding these simple yet important devices helps solve electrical problems and develop efficient systems.

Pros and Cons of Autotransformers

Autotransformers have pros and cons.


Autotransformers cost less to make and maintain than transformers. They need less wire and core because they have one winding. They also become smaller and lighter. Repairs and construction are easier with a single winding.


Autotransformers transmit electricity efficiently because there are no leakage losses between primary and secondary windings. Direct energy transfer within one winding. They cut waste and operational costs with 97-99% efficiency.


Autotransformers vary output voltages. Tap into different winding components to smoothly change secondary voltage. This is useful for applications that often modify or control output.


However, autotransformers have drawbacks:

Safety: The primary and secondary circuits are connected, thus they are not isolated. This is dangerous if the system has defects or shorts. Ground and overload protection must be implemented.
Harmonics: The single winding may cause harmonic distortion in other electronics. Harmonic reduction often requires additional filters.
Variable output is a benefit, but autotransformers are limited by tap number to a specific voltage ratio range. Significantly different ratios require various taps.
During starting, the single winding causes larger inrush currents, which might stress the system. Inrush current must be limited.

Autotransformers have various uses, as long as you recognize their restrictions. They provide efficient and cost-effective voltage transformation when correctly selected, installed, and maintained.

Applications of Autotransformers

Due to their versatility and efficiency, autotransformers have many uses.

Regulation of Power System Voltage

Power systems employ autotransformers to adjust voltage. They can optimise power supply voltage for equipment. Autotransformers can enhance voltage for a motor or buck voltage for delicate electronics. Autotransformers are more efficient than transformers for voltage regulation since they use the same winding for input and output.

Power-factor correction

AC power systems can rectify power factor with autotransformers. Power factor measures equipment’s voltage and current efficiency. So the Power is squandered with a low power factor, therefore rectification optimizes the system. Power factor is closer to 1 when autotransformers regulate voltage and current.

Phase Change

Three-phase power systems use autotransformers to switch between delta and wye connections. The can convert 3-phase delta to wye or vice versa. This helps when equipment needs a different phase configuration. A 3-phase wye system can be converted to split-phase power for most homes using an autotransformer.

Voltage Balancing

Autotransformers help balance 3-phase power system voltages. Small voltage fluctuations between phases can damage equipment over time. Autotransformers equalize phase voltages for safe and optimal operation.

In conclusion, autotransformers efficiently regulate voltage, correct power factor, alter phase configurations, and balance voltages between phases, making them useful in power systems. Their AC power optimization and equipment protection benefits are great.

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