It should be noted that it is impossible to create a perfect current balance on a 3 phase supply when feeding a single phase load. Consequently there are several different methods of producing a single phase output each of which may be suitable for different circumstances.
Single Phase Transformer
By far the simplest method of obtaining a single phase output from a 3 phase supply is to place a single phase transformer across two phases of a 3 phase supply. The effect of this system is to pull the full rated current in two of the supply lines and zero current in the third line. This is probably acceptable up to a rating of about 5kVA on a 400V system (13Amps / 13Amps / 0Amps). Above this level an alternative method may be required to achieve a better balance of currents flowing in each of the three phases.
Open Delta Transformers
Above 5kVA the most common type of transformer used to supply a single phase system from a 3 phase supply is the Open Delta Transformer. This is a transformer built on the two outer legs of a three phase transformer core. The primary coils are connected as per a standard 3 phase delta input but with the “B” coil missing and the secondary coils are connected as a standard 3 phase star output, again with the “B” coil missing.
The primary then draws the same current in two of the lines and the third line carries twice this current. Compared to the 5kVA single phase system mentioned above the currents will be 7.5Amps / 15Amps / 7.5Amps.
The kVA rating of the transformer is equivalent to the kVA rating of the load.
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Scott T Transformers
This is another popular method of generating a single phase supply from a three phase supply this time using two single phase transformers, The “Main Transformer” and the “Teaser Transformer” The main transformer is connected to 2 of the 3 supply lines of the 3 phase system. The teaser transformer primary is connected between a centre tap on the primary of the main transformer and the third supply line. Typical currents in the three lines for the 5kVA load at 400V are 7.5Amps / 10.6Amps / 14.5Amps.
Scott T transformers can be used to supply two separate single phase loads or the output of the transformers can be joined to supply one single phase load.
The main transformer rating is equivalent to 90% of the load rating and the teaser transformer about 70% giving a total of 160% of the load rating.
Le-Blanc Transformers
The final 3 phase to single phase converter transformer is the Le-Blanc transformer. The kKVA 400V will have line currents of 3.9Amps / 10.6Amps / 14.5Amps.
The transformer rating is equivalent to 120% of the load rating.

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