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

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

Autotransformer:
An autotransformer is a transformer with a single primary and secondary winding. Based on its structure, it can be categorized as either adjustable voltage or fixed voltage. The coupling in an autotransformer is electromagnetic. Conventional transformers transfer energy through electromagnetic coupling between the primary and secondary windings. There is no direct electrical connection between the primary and secondary sides. In an autotransformer, the primary and secondary sides are directly electrically connected, and the low-voltage winding is part of the high-voltage winding. Autotransformers and other protective devices are also used as protective devices for communication lines.

An autotransformer is a type of self-coupling transformer. Its structure is relatively simple and low-cost, and it shares a common input/output and neutral line. The secondary side of the transformer is part of the primary side. The primary and secondary sides share a single winding, like two coils running from a single wire. The two coils use the difference in current to interrupt the magnetic flux lines, converting voltage. It is generally used to boost DC power. The transformer’s output and input are directly connected, resulting in poor safety performance.

Isolation transformer:
An isolation transformer is a transformer that provides electrical isolation between the input and output windings. Isolation transformers are used to prevent accidental contact with live objects. Transformer isolation separates the currents of the primary and secondary windings. Early applications in the power industry in European countries, it is widely used in the electronics industry and general circuit control power supplies for industrial and mining enterprises, machine tools and mechanical equipment, and safety lighting and indicator lights.

The primary function of an isolation transformer is to isolate electrical equipment from the power grid. It is suitable for safety applications or those requiring interference immunity. There is no direct electrical connection between the electrical equipment and the power supply. Seismic isolation structures require a lot of materials and are expensive. In addition to voltage conversion, isolation transformers can also electrically isolate the input and output windings from each other and isolate the input/output from the neutral line to prevent the danger of simultaneous contact with live objects (or metal parts that may be live due to insulation damage) and the ground. Furthermore, isolation transformers have a certain ability to suppress various interferences, provide filtering properties, and offer high safety.

Differences between autotransformers and isolation transformers:
1. Structural differences
Isolation transformers are the most common transformers. Their low-voltage and high-voltage windings are wound on separate coils with no electrical connection. There is no direct electrical connection between the primary and secondary sides, and the secondary winding is not grounded. The primary and secondary sides of an autotransformer are directly electrically connected. Its low-voltage coil is part of the high-voltage coil, meaning it has a single-circuit winding. A tap is located in the middle of the winding, serving as a common point between the high- and low-voltage coils.

2. Poor Safety Factor

Typically, the AC power supply voltage we use has one wire that is grounded, while the other wire has a potential difference of 220V or 380V from ground. Touching this wire can result in an electric shock. An isolation transformer’s secondary high- and low-voltage coils are electrically insulated from each other and ungrounded. There is no potential difference between any two wires and ground. Touching any of these wires prevents electric shock, making it much safer. Therefore, regardless of whether the secondary voltage exceeds the safety threshold, as long as a person does not simultaneously touch the two secondary output points with a voltage difference, no harm will occur. This is the greatest advantage of an isolation transformer. However, it is bulky and expensive.

Because the high- and low-voltage coils of an autotransformer overlap, some coil wire material is saved, resulting in both a compact size and low cost. However, its high- and low-voltage coils are connected. Even though the output voltage on the low-voltage side of a step-down transformer is very low, because its two windings share common ground, the primary high voltage can “sneak” into the secondary winding, which can easily cause harm.

3. Differences in Usage
Three-phase isolation transformers are primarily used for power loads in power systems and industrial and mining enterprises, as well as power sources that require isolation from the grid. They convert mains power into magnetic power. As electrical equipment, they are used for precision measurement and testing of power and grid power, as well as for purification (anti-interference). Isolation transformers are a safe power supply, commonly used for machine maintenance, providing protection, lightning protection, and filtering.

The primary and secondary sides of an autotransformer are directly electrically connected. If the wiring is damaged or leaks, it can easily cause harm to people. They are generally used in applications where safety requirements are not high. They only need to change the voltage to meet the equipment’s requirements. However, these devices cannot be equipped with leakage protection devices, and the neutral line cannot be grounded.