In this topic, you study Three Phase Auto Transformer.
For transformation of polyphase voltages, it is preferable to use auto transformers instead of two winding transformers in case the voltage ratio is not greater than 2. The auto transformers in three phase circuits can be connected in many different ways using a bank of three identical single phase auto transformers or three phase three limbed core type auto transformers. But most commonly used connection is a star connection. Fig. 1 (a) shows a star connected three phase auto transformer and Fig. 1 (b) shows its phasor diagram. This star connected three phase auto transformer behaves like a Yy connected two winding three phase transformer in many respects.
Applications of Three phase Auto transformers
Three phase auto transformers have numerous applications, principle among them are listed below:
(i) Small and medium-sized three phase auto transformers are very economical and are satisfactory for a variety of purposes such as
- For obtaining three phase variable output voltage for testing and laboratory purposes. When used for this purpose, they are in the form of a three phase variable-ratio auto transformer commonly known by its trade name as three phase variac (Fig. 2).
Fig. 2: Schematic diagram of three phase variac
- As auto transformer starters for starting of three phase induction and synchronous motors at reduced voltages.
(ii) Large units are used for interconnecting two electrical systems of different voltages e.g. 220 k V and 400 kV systems.
The advantages and disadvantages of polyphase auto transformers are the same as for single phase auto transformers.
