Wiring 3 phase transformers

Kva rating is still exactly that. It doesn't change. You can step the voltage up or down with a transformer and you must not exceed its rated amperage rating

Hello yardbird,

No effect on transformer rating.

Before you try this, check the transformer terminals and connections.

Also check that the currents the transformer windings are capable of, are suitable for your project.

Normally the transformer may be Delta connected Primary (480 Volts) and Star Connected Secondary (240 Volts).

Each Winding is normally brought out as a separate pair, with moveable copper or brass strips to "jumper across the various terminals to achieve Delta/Delta, Delta/Star, Star/Delta, Star/Star.

If your transformer is of reasonable size, there should be a nameplate or connection diagram, inside the connections box/es.

Ensure that your transformer is truly a 3 phase transformer, if you need 3 phase. (It may be 480-0-480 Volt to 240-0-240 Volt = Centre tapped both primary and secondary) = Single Phase.

See how it goes for you, let's know how you progress.....

hi,

its not at all possible because the py. wdg will be designed to withstand 480v and viceversa.

P=VI,if V INCREASES THEN I DECREASES and viceversa.

but u can give the sy voltage and achieve py. voltage.

thank u,

naveen.m

It is possible. You are technically backfeeding the transformer. Instead of the incoming current being on the 480V, or the H1, H2, H3 terminals (normally the primary on this type of transformer) the incoming current is being fed into the 240V terminals X1, X2....

The information provided is incomplete. You have not specified weather the transformer you are using is Delta/Star , D/D S/S or three phase to single phase. If star connected weather the star point is internally done or externally done . You want to use a transformer designed for step down to be used as step up. So post complete details so that correct guidance can be given.

hi,

i too understand that concept but what i mean is that, if we are reversing the voltage it will not effect the KVA rating.the thing is it wii act as stepup transformer .but we can use the 3ph 415 load with the py. current rating only and not with the sy. current rating.

thank you and best regards,

naveen.m

To answer your question... there is no effect on KVA rating of the transformer at all. I do hope you recognize the current voltage relationship of this setup.

A few posters have asked about whether or not the winding configurations are D/S or S/D or S/S or D/D. A 480/240 volt transformer would more then likely be a D/D.

Using such a transformer as a step up one would raise some interesting issues around grounding, as if it is in fact D/D there is no way to ground it other then to corner ground one leg of the 480 three phase primary.

Best advice... be very careful and well understand what you are doing and how to do it.

Rick...

i may mention to take care about the vector group relationship of the reversed 3-phase transformer will be changed

so take care if you want to connect it as parallel supply

No Effect. Because over all KVA rating will be Same.

Do not forget that the short circuit current, in this case, for the low voltage winding will be severaly increased.

Regards

Hi everyone, I think you all put in something interesting.

Idealy I tend to agree with those who claim there's no difference and you can use a transformer both ways (as step up or step down).

However, there's hickup though because some transformers are really designed to be used in just one way as they work more efficiently than if you try to use them the other way.

Most small general purpose TRFs do work equaly well in both ways but not necessarily the larger 3 phase ones including the toroidal types.

I will leave this puzzle up to you to figure it out but will be happy to get back about it later.

This is not a mind game it is quite serious.

Thanks to everyone for their responses. The transformer in question is designated as a step down transformer with a primary of 480 and a secondary of 240 with a rating of 75KVA. The unit was already in service in the reverse mode when I ran across it. The application is showing a current draw of 28 Amps on the secondary side. Sorry that I did not give enough info regarding the wiring configuration(Star,Delta/Wye). I will have to look when I go back into the plant. I tend to think that due to the physical size of the respective windings that one would lose some capacity as well as a greater chance of over current shorting on the secondary winding.

There is a difference in the construction of a step-up and step down transformer. I don't know the specifics. I have ordered both for customers. I have seen a few installations were they are used opposite and they do work, however they get noisy and fail prematurely. I suggest you contact a few of the specialty transformer Manufactures ( in North America Olsun Electric, and MGM transformer come to mind).

Hi yardbird

Ther are some good answers in this thread. Hopefully they will enlighten you

BUT if you didnt know the answer before you asked the question then you shouldnt be messing with electricity. The results of ignorance are aften fatal!

Thanks for your response. According to all of the responses all of which were very informative, this is not a condition ran across by electricial engineers everyday. As for my experience with electricial issues, I have never met but one man who claimed to know it all and he almost got me killed. Remember the old saying; The only dumb question is the one not asked.

Very very true!

Did a little bit of checking on this issue. A plasma cutter unit is being supplied by this arrangement. With a current draw of 29 amps on the 480VAC(now the secondary) side of the transformer, a current draw of 72 Amps is seen on the 240VAC(Primary)of the transformer. Transformer is fed with a 100A circuit on the 240VAC side.Transformer is a 3 phase unit.

How did you ground the 480 volt 3 phase secondary? Please don't tell me it is not grounded! If it is not grounded, did you install a ground fault indication system?

I did not install this system. It was already in service when I was brought in to do some controls design and installation. The said equipment was to be set up for motor voltage of 480 volts. I informed the management staff that the primary coming into the building was 240 volt 3 phase. They in turn said no we have 480 volts. This is how I became involved with this issue. No ground fault system is in place. As for grounding I have not had the time to open up the unit. All I can say is there is a ground wire leaving the unit and does show continuity to other grounds and the conduit from the unit.

Thanks

Yardbird

Regards !

All such trasformers can be used & are made to be used both way.

NOT AT ALL ANY DIFFERENCE IN CONSTRUCTION FOR USE IN ONE WAY OR THE OTHER.

If it is a multi Windings/Taps Transformer, then using only one tap as primary

will give only the VA of that winding /tap.

Single tap Step-up or Step-down are the same thing & only the ratio of windins /voltage will decide what it is Step-up or Step-down.

Simple rule of TRANSFORMATION.

pls clarify that for a star-star transformer of 15MVA we have to include terdiary delta winding or noy.

if yes pls give solution.