Auto Transformer Overview

Autotransformer (Auto Transformer) is a transformer with only one winding wound on a laminated core. An auto transformer is similar to a two winding transformer but differ in the way the primary and secondary winding are interrelated. A part of the winding is common to both primary and secondary sides. On load condition, a part of the load current is obtained directly from the supply and the remaining part is obtained by transformer action. An Auto transformer works as a voltage regulator.

Auto Transformer Construction

Construction is characterized by a single copper wire coil common to both the primary and secondary circuits, wound around a silicone steel core. While theoretically separate parts of the winding can be used for input and output, in practice the higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap.

Auto Transformers Advantages

  • Increased KVA rating for a given size and mass.
  • Less amount of copper.
  • Due to the smaller size, the cost is less compared to a two – winding transformer.
  • The resistance and reactance are less compared to a two – winding transformer and hence it has superior voltage regulation.
  • Volt-ampere rating is more compared to a two – winding transformer.
  • It is possible to get smooth and continuous variation of voltage.

Auto Transformers Applications

  • Compensating voltage drops by boosting supply voltage in distribution systems.
  • Auto transformers with a number of tapping are used for starting induction and synchronous motors.
  • Auto transformer is used as variac in laboratory or where continuous variable over broad ranges are required.

Power Transmission & Distribution

Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 132 kV to 66 kV for transmission. Another application in industry is to adapt machinery built (for example) for 480 V supplies to operate on a 600 V supply.

They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100 V–130 V and 200 V–250 V). The links between the UK 400 kV and 275 kV 'Super Grid' networks are normally three phase autotransformers with taps at the common neutral end.

On long rural power distribution lines, special autotransformers with automatic tap-changing equipment are inserted as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the autotransformer compensates for the voltage drop along the line.

Audio System

In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage audio distribution systems, and for impedance matching such as between a low-impedance microphone and a high-impedance amplifier input.

Autotransformer Starter

Autotransformers can be used as a method of soft starting induction motors. One of the well-known designs of such starters is Korndörfer starter and X-Ray detectors.

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