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Rectifier Transformers

A combination of thyristors Rectifier and Transformer is said to be rectifier transformer. For higher voltage and lower current requirement, bridge type connection of thyristors are used in rectifier transformer. For lower voltage and higher current requirement, interphase connection of thyristors are used in rectifier transformer. Rectifier transformer can produce 6 pluse or 12 pulse as per circuit design.

Rectifier Transformer with Interphase connection

Interphase connection gives a symmetrical load to the 3Φ system. The current transfer from one phase to other phase occurs as soon as the potential of the succeeding phase rises above that of the preceding phase. Hence, the phase with the higher positive or negative potential carries the load current and determines the direct voltages. Here, two 3Φ connections are frequently paralleled through an interphase transformer in order to double the load current and to obtain six-pulse performance. This requires 180o phase shifted transformer secondaries. i.e) one secondary is connected in star and the other secondary is connected in inverse star shown below. Each secondary winding conducts during 1/6 of a cycle. Reflected to the primary side, the secondary currents in the two windings on each limb are opposite and 180o displaced and they can perfectly be balanced by symmetrical primary current, regardless of the primary connection. Since the primary side carries the current pulses of both the secondary stars, the primary winding are effectively used but the secondary rating is high as each secondary winding conducts only for 1/6 of a cycle

Rectifier Transformer with bridge connection

In bridge connection, the transformer secondary windings are idle most of the time, resulting in a high transformer rating compared with the converted power. These winding can be put to use more efficiently, if they feed bridge connected rectifier circuit. This is accomplished by combining two 3Φ commutating groups as shown below. A load cnnected between points 1 and 2 is supplied with the total of two voltages when viewed as separate groups. Since the two commutating groups operate with opposite polarity, their ripple voltages are displaced and the total voltage shows 6 pulse ripple.

Uses of Rectifier Transformer

Rectifier transformers are used for industrial processes which require a significant direct current (dc) supply.

  • In traction.
  • In electrolysis.
  • In smelting operation.
  • In large variable speed drive trains.

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