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Open Delta Service

Open Delta Service

Open Delta Service

At a remote site the utility company only has two primary phases available and has provided an open-delta 120/240v service to a pumping station containing a 15hp pump and misc. lighting and controls. Any help on the following questions would be great.

1- The % voltage imbalance is +4% leading to high currents.
Is a buck-boost transformer the best solution? How do I size and connect it for the best results.

2- Running under the above condition of voltage imbalance the motor is very, very noisy. Is this the result of the voltage imbalance?


RE: Open Delta Service

The voltage imbalance will cause current imbalance.  Current imbalance can (and will) cause additional noise.  The much larger problem is heat.  Trying to correct with a buck-boost xfmr will be like trying to catch a falling ball of Jello.  You will never be able to get things right since the voltage imbalance will shift as loads change on the transformers.  This problem will be compounded by the fact you have a 120/240, three phase system which surely has a differing load on xfmr for your 120/240 single phase loads.  You just can't balance open delta systems well.  I've seen motor mfr's who will void warranties on motors if used on open delta systems for just these reasons.  The $$$ you would spend on anything like a buck-boost approach would be better spent on paying the utility to put in the 3rd xfmr and just close the delta!!

RE: Open Delta Service

There are AC-DC-AC converters available on the market. Check with major manufacturers to provide info for your application, e.g. ABB, General Electric, Siemens, Robicon, Alstom, AEG, Allen-Bradley, etc. They have some capability to customize the input rectifier or active front end to fit your open delta connection with the voltage/current unbalance present. Check ANSI Std C50.41-1982 "American National Standard for Polyphase Induction Motors for Power Generating Stations," page 9 (6) "Conditions under which the alternating-current supply voltage is unbalanced by more than 1% (See Section MG1-20.55 of ANSI/NEMA MG1-1978)."

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