What is an Open Delta transformer? 3 phase open delta ratings

An open Delta Transformer is a conventional three phase transformer manufactured to give a single phase output. The specialist windings used produce a single phase output which is:

The transformer load is split across the three input phases but in a 2:1:1 ration, so while not exactly even it is still the most even loading technique without active transformation.


Wherever you want to spread a large single phase load over all three phases an open-delta transformer is a cheaper option than a 3/1 UPS.

The `open delta' transformer is still the best way to transform the three phase output of a UPS into a single phase.

When used on a UPS output: In general it is better to try and avoid the transformer all together. However there are some considerations about the load characteristics on the UPS. The phase currents supplied by the UPS will be unbalanced. This unbalance is calculated thus:

assume the load is resistive assume the transformer ratio is 1:1 assume the supply is 400Vac and the load required is 1kVA. The load in the input centre limb must equate to the sum of the other two inputs the voltage across the load must be 400 square root 3 = 692.8V for 1kVA then 1000/692.8 = 1.44A therefore current in common leg is 2.88A for each kVA of 1PF load

In specific applications care must be taken to assume that:

the transformer is only 94% efficient

the voltage may not be 400 ph-ph

the UPS may have difficulty with the unbalanced load

the PF of the load may affect the calculation

A sample calculation is shown:

for a 20kW single phase load a 20kW transformer is required the transformer will require 20*2.88A * 230Vac=13248 watts on the common transformer leg the UPS will need to provide 3 times this if it has three separate inverters and 2 times this if unbalanced loads are tolerated the power is 13248*3=39744 watts - assuming the UPS has a capacity to provide only a 0.8PF the UPS size is 49600VA! the transformer is about 94% efficient meaning a final figure of 52.851kVA is appropriate

A much better solution is to use two units rated at 12kW and run them in parallel

If the application cannot supply a single phase at the required load a cost effective solution is to use a single phase input UPS with a phase/phase input transformer rated 400/230Vac on the input to the UPS.

If you need any more help please ask!