THREE-PHASE AUTOTRANSFORMER

Autotransformers are an economical and compact means of connecting
electrical equipment to a power supply of a different voltage.
Part of the winding is common to both primary and secondary circuits so
there is no isolation between the two.
This may not be acceptable on some power systems that do not have a
grounded neutral on the secondary side of the main power transformer.
Typical applications include motor loads of industrial machinery, electric
heating, air conditioners, etc.