# 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 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.

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.

More interesting Notes:

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

Cable Trumps

Bored at work and would rather be playing trump card game with you son. The next best thing (or not) maybe the online cable trump card game from AEI Cables...

What happened to the cable notes?

If you are wondering what happened to our cable notes, the short answer is that we have moved them to myCableEngineering.com.  The "Knowledge Base" at...

Low Voltage Circuit Breakers

Circuit breakers are switching devices whose primary function is to isolate parts of an electrical distribution system in the even of abnormal conditions...

Fault Calculations - Introduction

Fault calculations are one of the most common types of calculation carried out during the design and analysis of electrical systems. These calculations...

Motor Starting - Introduction

Motor starting and its associated problems are well-known to many people who have worked on large industrial processes. However, these things are, of course...

Railway Electrification Voltages

This post is quick introduction and overview to different railway electrification voltages used in answer to a question sent in via email. While there...

9 power supply issues solved by using a UPS

Installation of a UPS can help in reducing problems due to issues with the power supply.  A lot of people relate this to nine key issues.  Depending on...

IEC Reference Designations

The IEC publishes a series of documents and rules governing the preparation of documents, drawings and the referencing of equipment.   Depending on country...

Electrical Engineering

Electrical engineering is a field that covers a wide variety of sub-fields, including electricity and electronics. It is a field that goes back to the...

Motor Efficiency Classification

Electric motors are one of the most widely used items of electrical equipment. Improving motor efficiency benefits include, reduced power demand, lower...

## 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