May 8, 2019 Single-phase-autotransformers

What is an Autotransformer?

The autotransformer are a kind of electrical transformer where primary and secondary shares same common single winding. In other words, it’s a one winding transformer.

An auto transformer is similar to a two winding transformer but differ in the way the primary and secondary winding are interrelated. In an auto transformer, one single winding is used as primary winding as well as secondary winding. But in two windings transformer two different windings are used for primary and secondary purpose.

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 Autotransformer works as a voltage regulator.

Types of autotransformers

Based on their construction:

  • In one type of transformer, there is continuous winding with the taps brought out at convenient points determined by desired secondary voltage
  • In another type of auto transformer, there are two or more distinct coils which are electrically connected to form a continuous winding.

According to the output voltage:

  • Single phase variable autotransformers: single-phase autotransformers has a single-phase input voltage of 230V and a single-phase output voltage between 0 and 230V variable depending on the position of the selector.
  • Three phase variable autotransformers: three-phase auto transformers has a three-phase input voltage of 400V and a three-phase output voltage between 0 and 400V variable depending on the position selected by the selector.

What is an Autotransformer used for?

  • Used to start induction and synchronous motors to provide up to 50 to 60% of the total voltage to the motor stator during start-up.
  • It is used to give a small boost to the distribution of a cable, compensating for voltage drops.
  • It is also used as a voltage regulator.
  • In rural distribution systems where distances are long, autotransformers can be used with ratios around 1: 1 in order to compensate for the appreciable voltage drops at the ends of the line, taking advantage of the multiplicity of sockets to vary the supply voltage .
  • It is used in the power transmission and distribution system and also in the audio and rail system.

Advantages and disadvantages of using Autotransformers

Advantages of using Autotransformers

  • Less costly: the size of the auto transformer would be approximately 50% of the corresponding size of two winding transformer. Thus auto transformer is smaller in size and cheaper, cause there is a saving in cost of the material.
  • Copper Savings in Auto Transformer. the weight of copper of any winding depends upon its length and cross-sectional area. Auto transformer employs only single winding per phase as against two distinctly separate windings in a conventional transformer.
  • Better regulation: An auto transformer has higher efficiency than two winding transformer. This is because of less ohmic loss and core loss due to reduction of transformer material.
  • Low losses as compared to ordinary two winding transformer of the same rating: Auto transformer has better voltage regulation as voltage drop in resistance and reactance of the single winding is less.

Disadvantages of using Autotransformer

  • The leakage flow between the primary and secondary windings is small and, therefore, the impedance is low. This results in more severe short-circuit currents in fault conditions.
  • Connections on the primary and secondary sides must necessarily be equal, except when using interconnected highlighted connections. This introduces complications due to the change of the primary and secondary phase angle, particularly in the case of the delta / delta connection.
  • Due to the common neutral in a star / star connected automatic transformer, it is not possible to ground the neutral on one side only. Both sides must have their neutrality either on land or in isolation.
  • The secondary winding is not insulated from the primary winding. If an autotransformer is used to supply low voltage from a high voltage and there is a break in the secondary winding, the entire primary voltage comes through the secondary terminal, which is dangerous for the operator and the equipment. Therefore, the automatic transformer should not be used to interconnect high and low voltage systems.

See more single-phase / three-phase autotransformers here

If you are interested in knowing more about autotransformers read here

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