Closed Delta Transformer

[Nedhal Al-Faraj]

Hi every body

I want to replace 3 phase transformer used to supply 1-phase load.
the damaged transformer that will be replace is very old and no information can be collected from name plate. However, I brought new one ( 30 KVA, 3phase, 60 HZ , 480/208 V, Closed Delta /Y)

The previous ( damaged) one was supplied by 2 lines (un-greounded conductors) in primary side as appears from connections.

But according to my information & knowledge, the closed delta transformer
( the new one) can't be operated ( functions) without suppling power to each winding (leg) which means it needs 3 lines ... Am I Right ?

Is this mean that the damaged transformer was Open Delta in Primary Side?

Please Help Me

Thanks

 

[winnie]

Two 'hot' wires doesn't tell you if the primary is open delta or not.

1) Two ungrounded conductors probably indicates a _single phase_ supply, with the load connected between the two conductors. The 'load' in this case would be the transformer primary. This 'single phase' supply may be derived as two phases of a three phase system.

2) Two ungrounded conductors plus a third grounded conductor could be part of a true three phase 'corner grounded' delta system. Full phase voltage is present between all three conductors, but one is bonded to ground. If you have corner grounded delta, then you have a full three phase supply, and can simply use your transformer at full ratings.

It is probably possible to use a three phase transformer with a single phase supply and load, if suitably derated. You may not be able to use all of the secondary coils, and you certainly won't be able to use the full KVA of the transformer.

With your transformer, with a 480V single phase input, I believe that you could get 10KVA of 120V single phase output and 10KVA of 60/120V center tapped output. If you can reconnect the secondary coils (most are permanently connected wye) then you might be able to get a full 20KVA of 120/240 center tap. You will not be able to get 208V with a single phase supply.

-Jon

 

[Nedhal Al-Faraj]

Dear Jon

I will use now 3-phase transformer (NOT 1phase) to provide power to 1-phase load.

The Transformer is Closed-Delta in Primary Side and Y in Secondary.

To make 3-phase indut I need to connect three windings of Primary to Hot lines ? ( Am I Right)

Waht will be the KVA I will get? is it only 10 KVA ?

Thyanks

 

[Smart $]

But according to my information & knowledge, the closed delta transformer
( the new one) can't be operated ( functions) without suppling power to each winding (leg) which means it needs 3 lines ... Am I Right ?

Is this mean that the damaged transformer was Open Delta in Primary Side?

Not necessarily true for either question... but we need more info to make that determination. Jon essentially mentioned this but with different wording.

You said only a 1? load was supplied by the to-be-replaced transformer. Could be that the primary already had one (or two) bad winding(s), such as the winding connected to H3 on the right side of the following diagram, and someone made the decision to use it as a 1? transformer, using one primary winding for 1? in, 1? out. Yes, this is a rather uncommon usage of a transformer but it does happen on occasion... perhaps a "why buy a single phase transformer when we have a bad 3? transformer which is still good for 1??" scenario.

What is the single phase voltage of the equipment powered? Is it single or dual voltage? 120, 208, 240, or a combination of such?

Of the primary windings, which terminations were connected to what lines? (If this is a dry-type transformer, don't go simply by the terminal markings... determine by the actual winding connections... they may have been rearranged and/or shorted.) You mentioned two ungrounded lines. Is there a third grounded line such as in the corner-grounded delta system Jon mentioned?

 

[winnie]

Nedhal,

Again, a three phase primary will require _three_ supply conductors. It is possible that only two of these conductors will be 'hot', but all three conductors must be used.

If you only have a single phase primary and a single phase load, then you _cannot_ use the entire capability of the three phase transformer. You _must_ change either the connections of the primary or the secondary coils. If you connect the transformer properly, then you can use it as a single phase transformer

Smart$ provided a change in connections of the three phase primary which uses only two of the primary coils, connected to a single phase supply. This connection will permit single phase 480V to produce 120/240V single phase secondary output. IMHO this is the correct way to use a delta-wye transformer with a single phase supply. The output will be single phase, and the capacity will be 66% of the three phase capacity.

I was imagining reconnecting the secondary and using all three primary coils. My approach would also give 66% capacity, but is much uglier as well as less practicable given normal transformer construction. Use Smart$'s approach.

-Jon