What is an Open Delta Transformer 

By on

In three phase systems, the use of transformers with three windings (or legs) per side is common.  These three windings are often connected in delta or star, resulting in common transformer configurations such as delta-delta or delta-star.  An open delta transformer is a special arrangement which uses only two windings.

If your new to transformers, have a look at the Power Transformers - An Introduction note.

Transformer Configuration

The illustration shows how an open delta transformer is connected.  On both the primary and secondary there are only two windings.  Even in this configuration, it is still possible to transform a three phase voltage.

Open Delta Transformer
Open Delta Transformer

Open delta transforms are not the commonly used.  Typically they would be used for small loads where cost is important.  Alternatively, they could be used as an emergency measure, should one winding only of a transformer fail. 

Sometimes you may hear an open delta transformer referred to as a V-connection transformer.

Power Delivered

Sometimes the power delivered by an open delta transformer is compared to that of an equivalent three winding transformer.  Typically figures like having 57.7% of the capacity of an equivalent three winding transformer or 87% of two transformers (same winding size) are quoted. While you can think of the transformer in this manner, it is more fruitful not to consider comparisons but to the necessary calculations on the open delta transformer.

Open Delta Transformer Current

Consider the illustration, showing the output from both a close delta and open delta transformer.  Note, that in the delta connection to line current is √3 times the phase current, whereas in the open delta, they are the same.

The transformer output power (in VA) is for a balanced transformer system for the closed delta connection (using phase current), this give:

  VA=3 V L I ph

And for the open delta connection:

  VA= 3 V L I ph

Taking the ratio of open delta to closed delta power, gives:

  3 V L I ph 3 V L I ph =0.577 (or 57.7%)

Summary

Open delta transformers are three phase devices, with only two windings on each of the primary and secondary sides.  While cheaper than a conventional three winding transformer, the open delta will only deliver 57.7%  of the power of a conventional transformer (not two thirds, 66.7% as may be expected).  There is limited adoption of open delta transformers, although they can be useful in certain situations.



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



Large Hadron Collider

The 27 km, Euro 6 billion  Collider lies on the border between France and Switzerland, took nearly 30 years to complete.  Some of the lofty goals for the...

Restricted Earth Fault Protection

The windings of many medium and small sized transformers are protected by restricted earth fault (REF) systems. The illustration shows the principal of...

Maxwell's Equations - Gauss's Electric Field Law

Gauss's Electrical law defines the relation between charge ("Positive" & "Negative") and electric field.  The law was initially formulated by Carl Friedrich...

Why is electricity so hard to understand?

It's been a busy few months on different projects or busy couple of decades depending on how I look at it. I can say that on the odd (frequent) occasion...

Robots - Interesting Videos

The robot folding towels post below was interesting enough at the time to post a link.  Recently I’ve come across a couple of other interesting videos...

How Electrical Circuits Work

If you have no idea how electrical circuits work, or what people mean then they talk about volts and amps, hopefully I can shed a bit light.  I’m intending...

The dc resistance of conductors

This is the first of two posts on the resistance of conductors. In the next post I will look at the ac resistance, including skin effect and we deal with...

Nikola Tesla

Nikola Tesla was born exactly at midnight on July 10, 1856 in the tiny village of Smiljan, Lika in Croatia. In his late teens, Tesla left the village to...

Periodic Electrical Installation Inspection – How Often?

How often installations are inspected is up to the owner of the installation, provided such durations do not exceed any regulatory maximums in force. ...

Lighting Design - An Introduction

From the earliest times, humans have found ways to create light. Pre-historic peoples used natural materials (moss, grass, etc.) soaked in animal fat and...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note