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What is a Delta connection?

The delta connection is a standard three phase connection with the ends of each phase winding connected in series to form a closed loop with each phase 120 degrees from the other.

  • What is a Delta connection?

      The delta connection is a standard three phase connection with the ends of each phase winding connected in series to form a closed loop with each phase 120 degrees from the other.

  • What does the term Banked describe?

      Two or three, single-phase transformers can be inter-connected to make a three-phase bank. The primary windings of the single-phase transformers can be connected in Delta or Wye. Likewise the secondary windings can be connected in either a Delta or Wye configuration.

      The equivalent capacity of the bank will be equal to three times the nameplate rating of each single-phase transformer. Usually, this type of installation is more expensive than using a single three-phase transformer. Advantages to banked transformers include:

      • For utility applications, the loss of one transformer in a delta configuration creates an open delta configuration using only two transformers. This will continue to supply power albeit at a reduced total kVA rating.
      • Banked transformers can be used to applications where the size or weight of a three phase transformer is too large. An example would be the ability to move a banked transformer in three lighter and smaller sections in a freight elevator.

  • What does radial-feed mean?
  • What does loop-feed mean?
  • When can you Reverse Connect a transformer

      In general, distribution transformers can be reverse connected without de-rating the nameplates KVA capacity. However, this is rarely considered in modern applications due to NEC code changes. Several precautions need to be taken for reverse connection of some smaller transformers. These would include:
      Dealing with higher current inrush which can cause nuisance tripping.

      HPS transformers under 6kVA three-phase and 3kVA single-phase, there is a “turns ratio compensation” on the low voltage winding. When backfed the turns compensation actually reduces the output voltage. When a three-phase transformer is reverse connected thus resulting in a Wye-Delta configuration, the neutral terminal must be isolated. This modification may violate the warranty and agency listings such as U.L.

      Back-fed transformers increase the installer’s liability since a future user may not realize what is the primary while de-energizing the transformer.

      In general HPS suggest that a proper step up transformer which is designed with the low voltage terminals as the primary terminal be used.

  • Should Buck-Boost transformers be used to develop 3-phase 4 wire Wye circuits from 3-phase 3 wire Delta circuits?

      No – a three-phase “Wye” buck-boost transformer connection should be used only on a 4-wire source of supply. A delta to Wye connection does not provide adequate current capacity to accommodate unbalanced currents fl owing in the neutral wire of the 4-wire circuit.

  • What is an Open Delta transformer?

      An open delta transformer is a three phase transformer that only has two primary and secondary windings, with one side of the delta phase diagram “open”. Open delta transformers are rare and are typically only used for small loads where cost is important. More common is critical loads being wired with three single phase transformers in a banked configuration. Should one of these transformers fail, the three phase circuit can remain active although the two remaining transformers are limited to about 57% of the total load. This allows a circuit to remain powered during a failure of a transformer, albeit at a lower overall load factor.

      Open Delta