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Series ATT/200 Three-phase

Three-phase Auto-transformers

  • Three-phase auto transformers with ratio 230/400V built according to IEC 61558-2-13
  • Terminal block with neutral
  • Insulation material group II
  • Ambient temperature max. 40 °C
  • Degree of protection IP 00
  • Class H varnish dipped and oven dried
  • Different minimum and maximum voltage ratios, dimensions and prices are available upon request


Single and three-phase auto transformers mainly feature a single winding, which works either as a primary or a secondary; thus input/output galvanic insulation does not exist. The more the ratio between minimum and maximum voltages nears the value 1, the more the auto transformers are inexpensive. Besides the economical aspect, compared to equivalently powered transformers, there are also high efficiency and low working losses to be considered.
To determine the physics dimension of the auto transformer, the core power must be considered:

Pe = Pt x (1 – Vm/VM) (PO = rated output power / Vm = min. voltage / VM = max. voltage)

This value represents the power which is actually transformed, while the other is supplied directly from the mains supply.
The most frequent use is that which requests a voltage adaptation when a galvanic
insulation from mains is not required. A typical example is an apparatus built to work on 380V and the mains supply is 415V.
With all single-phase auto transformers, OV connection is repeated twice to facilitate the wiring and avoid having to insert two wires in the same terminal.
The three-phase auto transformers have a neutral connection in the terminal block, which is advisable to connect to the mains supply neutral (when
present), thus avoiding over voltage in respect to earth on the line at a lower voltage.
The most frequently used and traditional is the star-connection, which is also the simplest to make; although triangle, zigzag and other connections are
It is possible to build three-phase auto transformers with special connections, which create a neutral, if this is not already provided by the mains supply, but
needed to connect single-phase units; in this case, please contact our technicians to define the exact parameters