Autoformer Design Considerations

Plitron autotransformers are custom designed to user applications.

  • Voltage step-up or step-down without isolation
  • 30 VA to 30 kVA (per phase)
  • Single or three phase
  • ½ size of toroidal isolation transformer

An autotransformer (autoformer for short) operates on the same magnetic principles as a transformer, however offers no isolation between input and output windings. An autotransformer has only a single tapped winding, and primary and secondary windings share turns and mutual currents.

When a situation exists where turns are shared by primary and secondary windings, then the currents are offset. Thus overall size can be reduced, and efficiency and performance are improved.

Autoformers Can Supply Cost Effective Power Solutions –  Autoformers can offer good solutions in a situation where there is a requirement to step up/down voltages and is not required as the primary means of isolations. As example, a 500VA step down autoformer may have the size and cost of a 220 VA isolation transformer.

Autoformers Are Well Adapted to Audio Installations – In general, the Hi B electrical steels used for toroids are a great choice for audio applications because the long linear portion of the B-H curve ensures low distortion and linear performance at any power level. However, the benefits increase when turns can be shared between windings, because common turns result in a smaller product. In the world of audio output transformers, a smaller transformer can mean tighter coupling between input and output windings, resulting in a lessening of leakage inductance. There is a direct correlation between leakage inductance and high-end frequency response, thus autoformers are inherently ideal to deliver low distortion and extended high frequency performance to the load.


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