Explanation and vector-groups to applicate 1 phase and 3 phase autotransformers

Autotransformers do not provide galvanic isolation.
They have electroconductive connection between input and output winding. It`s not allowed to take autotransformers for electrically protective separation.

If difference between input voltage and output voltage is small:

  • mains power may be higher
  • transformed power is lower
  • size can be reduced (core, copper winding)

Example shows 90% power saving compared to an isolating transformer

Equation to calculate the requested size

Example: 1 phase autotransformer / Input voltage 440 V / Output voltage 400 V / „Rated power“ needed for application 5 kVA

Identical equation works with 3 phase transformers.





Regarding input/ output “upper“ or „lower voltage“ is independant, voltage related only for 3 phase autotransformers value of phase voltage is important.

Standard circuit design of 3 phase autotransformers is YNa0. Max. load at neutral wire is approx. max. 10% of nominal current (phase current).

If 100% neutral wire load is requested, circuit design ZNa0 is manufactured.

Note: YNa0 design can be used in certain circumstances, if neutral wire at input provides full load and transformer star point is permanently connected to load. Please contact us for further informations!