Double-wound transformers and isolation transformers:
Transformers, by definition, have at least two windings, normally (but not necessarily) of different voltages (input and output).
For each phase of the double wound transformer, the primary winding is isolated from and secondary winding.
Autotransformers: In the autotransformer, a common, tapped winding serves as primary and secondary, and therefore no isolation between primary and secondary is provided.
Advantages of double-wound or isolation transformers: The double wound transformer is the normal configuration, and suitable for virtually all applications.
Some advantages are:
- Different connections are possible on the primary and secondary windings of a three phase transformer (e.g. Delta/Star)
- One terminal of the secondary winding may be earthed
- Protection against line transients is provided to sensitive load equipment
- Suitable for extra-low voltage (elv)
Advantages of autotransformers: Autotransformers have the advantage of combining the input and output windings which results in:
- Reduced size, mass and cost (dramatically in some cases)
- Since the input and output terminals are connected internally, the secondary neutral need not (and must not) be earthed
- Greater efficiency (and therefore lower running cost) and lower impedance than equivalent double-wound transformers
Autotransformers: are ideal for small voltage reductions where isolation is not required.
Great advantage is achieved with voltage ratios such as 415/380V, but useful advantage is achieved down to 415/200V.
Double wound transformers: are ideal for any application where isolation is required, such as drive isolation transformers and where a delta winding or large voltage ratio is required.
They may be single-phase, three phase, or many other configurations.
Either way, Powersafe transformers and autotransformers will provide good reliability and efficiency and due to their quiet and cool operation and hassle free installation.