Changing the number of phases

In order to provide power from a 3 phase supply to a multi-phase load (e.g. 12 pulse rectifiers where 6 phases are used), it is necessary make a power conversion. This can be achieved by employing a transformer with a multi-phase secondary circuit. 

There are various schemes possible including double star, double delta and so-called fork winding.

A range of transformers that convert from three phase to multi-phase systems are shown below:

Special systems of transformer connection.

Transformer with non-typical construction and special properties can be created to meet specific technical requirements.

Scott’s system – 3/2 phases:

In Scott’s system, shown below, two single-phase transformers Tb and Td are employed to convert from
a 3-phase supply to two single phase circuits. The advantage of this system is that the single phase circuits have the same current magnitude (with π/2 phase angle between them) and the 3-phase supply is loaded symmetrically.



Vv - connection – 3/1 phases:

The Vv system is the next most widely used method for conversion from a 3-phase supply to a single phase load. Here the transformer iron construction is the same, but with primary and secondary windings only on the two outer legs. Connection is as shown below. This system has the advantage of providing high single phase power to the load, but with the disadvantage of asymmetric loading of the 3-phase source. The centre phase has twice the current of the outer two phases.