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# Difference Between Autotransformer and Isolation Transformer

Autotransformers and isolation transformers are two different forms of transformers that work on the same principle. Both use an energy conversion between the alternating magnetic field and the alternating current in the coil to convert the voltage. So what's the difference between them? The following article lists six major differences between autotransformers and isolation transformers.

### Autotransformer vs. isolation transformer

Coil structure

The primary coil and the secondary coil of the autotransformer are common coils. The secondary side voltage outputs a voltage generated by the self-inductance of the coil. The magnitude of the output voltage is calculated according to the ratio of the number of turns of the common output part of the coil to the total number of turns of the coil. If it is a three-phase autotransformer with only one coil per phase, there are three coils in total. The primary and secondary coils of the isolation transformer are independent of each other. The voltage output of the secondary side is generated by the mutual inductance between the coils. The magnitude of the output voltage is also calculated according to the turns ratio of the coils between the primary and secondary sides. Therefore, if it is a 3 phase isolation transformer, it has six coils, and the two coils on each phase are combined into a whole.

Coil connection methods

The primary coil and the secondary coil on each phase of the autotransformer are actually one coil, and its connection methods include "Y" and "△" connections. "Y" means to connect the common points of the primary and secondary coils together as a common neutral point, that is, the zero line. The tap drawn from the middle of each phase coil is the output end of the transformer. The other end of the coil is the input end of the transformer. This autotransformer can actually output two different voltages. The transformer with a "Y" connection can output two different voltages, namely three-phase 380V and single-phase 220V.

The voltage between the live wire and the live wire is the line voltage, and the voltage between the live wire and the neutral wire is the phase voltage. The line voltage is 1.732 times the phase voltage. The output voltage is different due to the different connection points. This is a characteristic of an autotransformer and a characteristic of a "Y" connected transformer.

The autotransformer also has a "△" connection, which is also called an extended triangle. But there is often only one connection method in an autotransformer.

Since the primary and secondary coils of the isolation transformer are independent of each other, it allows the connection of the primary coil and the secondary coil to be the same or different. For example, the primary side coil is "△", and the secondary side coil is "Y" or "△", and vice versa. Depending on the phase of the coil, dozens of different connection methods can be extended.

Whether it is an autotransformer or an isolation transformer, the "△" connection method has no neutral wire. If the secondary coil of the isolation transformer is "△", it can only output one voltage for the load, which is not as convenient as the "Y" transformer connection. It is generally used in applications where a resistive or inductive three-phase balanced load does not require a neutral wire.

Operation mode

In the operation of the autotransformer, the input side and the output side have both electrical and magnetic connections, because the output voltage of the common part is the voltage generated by the self-inductance of the coil. It uses the induced current generated by the alternating magnetic field in the coil to realize the induced voltage. On the other hand, since the primary coil and the secondary coil of the autotransformer are themselves a coil, the input and output sides are directly connected together during operation, but the output voltage has changed. Therefore, there are both electrical and magnetic connections in the operation of the autotransformer.

In the three-phase four-wire power grid, if the human body accidentally touches any non-neutral output terminal of the autotransformer, there is a danger of electric shock.

In the operation of the isolation transformer, because the primary coil and the secondary coil are not connected, but independent of each other, the isolation transformer only has a magnetic connection but no electrical connection. In its operation, when the human body accidentally touches a single end of the output of the isolation transformer, there is generally no danger of electric shock. This is because the input and output of the isolation transformer are not isolated from each other and cannot communicate with each other. It actually plays an isolation role. Therefore, isolation transformers are also called safety transformers.

Impedance voltages and short circuit currents

The radial thickness of the autotransformer coil is small, so the impedance voltage is low. The primary and secondary sides of the isolation transformer are independent of each other, the number of layers of the coil is large, the thickness of the radial direction is large, and the impedance voltage is higher than that of the autotransformer.

Due to the difference in impedance voltage, the short-circuit current in the autotransformer is much larger than the short-circuit current in the isolation transformer. When the autotransformer and the isolation transformer of the same capacity are short-circuited on the secondary side, the short-circuit current that the autotransformer coil has to bear is higher than that of the isolation transformer. The transformer is much larger. Therefore, the ability of the autotransformer to resist the short circuits is worse than that of the isolation transformer, and the damage rate in the secondary short circuit fault is high. At the same time, due to the small radial thickness of the autotransformer, the magnetic leakage during operation is smaller than that of the isolation transformer, and the requirement for the iron core is lower than that of the isolation transformer.

Power quality

The primary and secondary sides of the autotransformer are straight-through, the interference in the primary grid will be directly added to the load of the transformer, and the interference generated by the load will also be added to the primary grid to interfere with other load equipment in the same grid, which is not conducive to the grid Optimization of power quality.

The interference in the primary grid of the 800 VA to 300 kVA isolation transformer is partially attenuated or isolated due to the characteristic of the coil itself that hinders the change of the alternating current so that less interference is applied to the secondary load. The interference generated by the secondary load will also be attenuated by the transformer Or isolated part of it without making more and more disturbances in the public grid. It purifies the grid, which is what isolates the disturbance.

Volumes and costs

Autotransformers are smaller and lighter than isolation transformers and cost less to manufacture than isolation transformers of the same capacity. For the two transformers of the same capacity, the core section of the autotransformer is smaller than that of the isolation transformer. In the utilization of the core, the autotransformer is higher than the isolation transformer.
Since the autotransformer has fewer iron core consumables and wire consumables than the isolation transformer, the manufacturing cost of the autotransformer transformer is lower than that of the isolation transformer, the volume and weight are smaller than the isolation transformer, and the transportation is also convenient. The isolation transformer is used on some occasions with high safety levels and power waveform requirements.