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%)


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

Electromagnetic Compatibility (EMC)

Electromagnetic compatibility (EMC) is the study of coordinating electromagnetic fields give off equipment, with the withstand (compatibility) of other...

IEC 61439 - The Switchgear Standard

The new standard IEC 61439 replaces the old 60439. Compared to the old standard, the new 61439 is a more clearly defined and takes into account the assembly...

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...

HTML Symbol Entities

HTML supports a variety of entity symbols which can be entered using either numbers or an entity name.  The number or name is preceded by the ‘&’ sign...

Introduction to Lighting

When looking at the design of a lighting scheme it is useful to have an understanding on the nature of light itself and some of the basic theory associated...

Operational Amplifier

The fundamental component of any analogue computer is the operational amplifier, or op amp. An operational amplifier (often called an op-amp,) is a high...

Cable Sizing Tool

Our cable sizing tool is one of the more popular tools on the site.  The tool enables cables to be sized in compliance with BS 7671 (the IEE Wiring Regulations...

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 Efficiency Classification

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

Random Numbers

Using laser optical pulses the random number generator utilizes the time between arrival of random photos to generate the numbers, ensuring true accuracy...

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