Open Star – Open Delta Transformer Connection Diagram Guide

Open Star - Open Delta Transformer connection Diagram

Learn Open Star to Open Delta transformer connection for three-phase systems, enabling reduced-cost supply using two transformers with correct phasing.

Open Star Delta — Fig-1: Star-Delta Wiring

open star open delta transformer wiring

Open Star- Open Delta Connection- This circuit is a combination of one transformer (star-connected) and one (delta-connected) to provide three-phase loads using two transformers only. This design is employed where a full three-phase supply is not required, so that a cheaper cost is possible and the transformer is saved. The main star winding is attached to the AC supply, and the secondary delta winding is used to provide the load. Proper phase order, neutral, and earth connections are essential to prevent unbalanced voltage and motor problems. The open delta side supplies about 86.6 percent of the normal delta capacity, which is enough to serve light industrial demand or in the case of an emergency supply. Some of the tests include checking the line-to-line, line-to-neutral voltages, checking the phase sequence, and ensuring the safe operation of the motors and loads. This arrangement finds extensive application in industrial distribution, temporary installations, and rural electrification.

Work & Installation (Input → Output Summary)

Connectprimary winding of one transformer in starto AC supply.
Connectprimary winding of second transformer in delta.
Connect secondary delta winding to supply three-phase load.
Ensure properphase sequence, neutral, and earth connections.
Check line-to-line and line-to-neutral voltages on the load side.
Connect motors or equipment according to the available three-phase supply.

Testing & Final Adjustments

Verify star and delta transformer connections.
Measure secondary line-to-line and line-to-neutral voltages.
Confirm correct phase sequence using a phase sequence meter.
Inspect neutral and earth continuity.
Test with light load first to check voltage balance.
Ensure load does not exceed 86.6% of normal capacity.
Monitor for heating or voltage imbalance.
Perform multiple cycles to ensure reliability.
Label transformer terminals for clarity.
Document all readings, connections, and phase sequences for future maintenance.

Frequently Asked Questions - Open Star - Open Delta Transformer connection Diagram:

What is an Open Star – Open Delta transformer connection?

A configuration using one star transformer and one delta transformer to supply three-phase load with only two transformers.

When is it used?

For partial loads, emergency supply, or cost-saving in industrial distribution.

How much capacity does it provide?

Approximately 86.6% of full delta transformer capacity.

Is phase sequence important?

Yes, to ensure correct motor operation and balanced load.

Is earthing required?

Yes, for safety and proper operation.

Can it supply motors?

Yes, if motor load is within the available capacity.

How to check voltages?

Measure line-to-line and line-to-neutral voltages on the secondary side.

Why use this instead of full 3-phase?

To save one transformer and reduce cost when full capacity is not required.

Can it be used for industrial loads?

Yes, especially for light or emergency industrial loads.

How to maintain load balance?

Ensure correct connections and do not exceed the rated capacity.

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