An explanation of open delta transformers

It is common in three phase systems to use transformers that have three windings (or legs) on each of the sides. Common transformer designs include delta-delta and delta-star, which use these three windings to form a delta or star configuration. In an open delta transformer, only two windings are used.

Configuration of transformers

An open delta transformer is depicted in the diagram. Windings on both primary and secondary are identical. Even in this configuration, three-phase voltage can still be converted.

There aren’t a lot of open delta transforms in use. In most cases, they’d be used to transport small amounts of material at a low cost. Alternatively, they could be used in the event that only one winding of a transformer goes down.

V-connection transformers are sometimes referred to as open delta transformers.

Delivered Power

The power delivered by an open delta transformer is sometimes compared to the power delivered by an equivalent three winding transformer. Typical figures include 57.7 percent of the capacity of an equivalent three winding transformer or 87 percent of two transformers with the same winding size. While you can think of the transformer in this way, it is more beneficial to focus on the necessary calculations on the open delta transformer rather than on comparisons.

Consider the following diagram, which depicts the output of both a close delta and an open delta transformer. It is worth noting that the delta connection has a line current that is three times the phase current, whereas the open delta has the same.

For a balanced transformer system with a closed delta connection (using phase current), the transformer output power (in VA) is as follows:

In addition, for the open delta connection:

Taking the open delta to closed delta power ratio, we get:


Three-phase transformers with only two windings on each of the primary and secondary sides are called open delta transformers. Even though an open delta transformer costs less than a standard three-winding transformer, it can only deliver 57.7{8e0181f9b4047a7790fd20bbf2d43faff96926569f2820f3da0bb199a786bafe} of the power of a standard transformer (not two thirds, 66.7 percent as may be expected). Open delta transformers aren’t used very often, even though they can be useful in some situations.

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