Autotransformers

An autotransformer is a transformer with only one winding, that is, the output and input share a set of coils. The definition of coupling in auto coupling is electromagnetic coupling. When an autotransformer is used as a step-down transformer, a part of the turns are drawn from winding as the secondary winding. When an autotransformer is used as a step-up transformer, the applied voltage is only applied to a part of the turns of winding.
According to structure, the autotransformer can be divided into the variable type and constant type. The common power phase numbers are 3-phase and single-phase. In step-down starters, the transformation ratio of an autotransformer is fixed, while the transformation ratio of a contact voltage regulator is variable. Compared with general transformers of the same capacity, autotransformers have the advantages of simple structure, fewer materials, small size, and low price.

How Does an Autotransformer Work?

The working principle of autotransformer and construction is similar to that of conventional two-winding transformers. However, it differs in the way in which the primary and the secondary are interrelated. In a two-winding transformer, primary and secondary are only magnetically linked by a common core but are completely insulated from each other. But in the case of autotransformers windings are connected electrically as well as magnetically.

The circuit diagram of an autotransformer is shown on the right. When the single-phase AC supply is connected between A and D terminals and output is taken from C and E terminals, this autotransformer will operate as a step-down transformer.

What are the Features of 3 Phase Autotransformer?

• The three phase auto transformer is used in applications where primary and secondary isolation is not required. It has the advantages of small size, low consumables and high efficiency.
• Fundamentally autotransformer is designed to reduce cost by eliminating a coil and has the function of a normal transformer, which means it has the function of raising or lowering circuit voltage, isolating and lowering impedance.
• In addition, autotransformers can be made into AC regulators to flexibly change the output voltage for laboratory use.
• The autotransformer power supply method is very suitable for the power supply of large-capacity load, and the interference to the communication line is less, thus it is widely used by passenger lines as well as heavy-duty freight railroads.

Step Up & Step Down Autotransformer Advantages

• Less consumption of materials and low cost. As the amount of silicon steel sheet and copper wire used in the autotransformer is related to the rated induced potential and rated current of the winding, the material consumption is also reduced, and the cost is also low.
• Less loss and high benefit. Due to the reduction in the amount of copper wire and silicon steel sheet, at the same current density and magnetic flux density, the copper loss and iron loss of the autotransformer are lower than those of the double-winding transformer, so the benefit is higher.
• Easy to transport and install. It is lighter in weight, smaller in size and smaller in footprint than a dual-winding transformer of the same capacity.

Autotransformer vs. 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 lists six major differences between autotransformers and isolation transformers.

• Coil structure
• Coil connection methods
• Operation mode
• Impedance voltages and short circuit currents
• Power quality
• Volumes and costs