tranformer installed reversed

[Toros]

Hi I have heard that if I install a xfrmer (480v prim. to 120-240v, 3-ph. 4w second.) in a reverse fashion , I would get 480v (no nutral) at the secondary. note that , incoming volt. available at the primary is 120/240v-3-ph., 4w , delta is this possible???

 

[iwire]

Yes you can do this, but note that the 2014 now requires the transformer to be marked indicating this is OK.

You will not get a neutral and you will have to either ground one phase resulting in corner grounded 480 delta or you may be able to go with ungrounded delta 480 with the addition of ground fault indicators.

 

[templdl]

And remember that the taps are on the 480v delta side which will prove to be a bit confusing as you have to think backwards. In addition since you will now have a delta secondary you will have to consider ungrounded or grounded.

 

[Smart $]

...2014 now requires the transformer to be marked indicating this is OK.

...

Marked as OK?

(B) Source Marking. A transformer shall be permitted to
be supplied at the marked secondary voltage, provided that
the installation is in accordance with the manufacturer?s
instructions.

 

[iwire]

Marked as OK?

Yes, that is shorthand for oakley doakley,

 

[Smart $]

Yes, that is shorthand for oakley doakley,

IC...

 

[iwire]

IC...

 

[ron]

Keep in mind that due to the primary and secondary winding positioning relative to each other and the core, you will get higher inrush than usual.

 

[GoldDigger]

Also keep in mind that the HV to LV voltage ratio under full load will not necessarily be exactly the inverse of the LV to HV voltage ratio under full load when the transformer is used "backwards".

The voltage ratio under full load will always have a lower output voltage than the mathematical turns ratio of the transformer would predict. When you wire the transformer backwards, any correction made to the turns ratio to allow for the voltage drop (either by design or by tap choice) will actually work against you rather than for you.

 

[Toros]

xfrmer installed reversed

xfrmer installed reversed

Also keep in mind that the HV to LV voltage ratio under full load will not necessarily be exactly the inverse of the LV to HV voltage ratio under full load when the transformer is used "backwards".

The voltage ratio under full load will always have a lower output voltage than the mathematical turns ratio of the transformer would predict. When you wire the transformer backwards, any correction made to the turns ratio to allow for the voltage drop (either by design or by tap choice) will actually work against you rather than for you.

So, if this installation method is correct, true, than I can propose the clients who has been worried or wishing about having 480v for his equimpent that work on 480v and also, note that, the available power is 120/240v, 3-phase, 4w delta only Thank you, for your toughts

 

[templdl]

Keep in mind that due to the primary and secondary winding positioning relative to each other and the core, you will get higher inrush than usual.

Of course there will be higher inrush because inrush is proportional.
Lower voltage =higher current.

 

[winnie]

Of course there will be higher inrush because inrush is proportional.
Lower voltage =higher current.

As you note, the lower voltage coil will operate at higher magnetizing current for the core to be operated at the same ampere-turns.

In addition to this factor, a transformer designed as a step down, but operated as a step up will have higher inrush current that a transformer of the same nominal rating and same over-all build quality, but designed from the get-go as a step up.

My understanding is that the primary coil is generally placed outside of the secondary coil, so that there is more air path leakage flux seen by the primary coil, and this air path leakage flux reduces the inrush current. When you operate a transformer in 'reverse', then what should be the secondary coil is being used as the primary, and you don't have the benefit of the leakage flux reducing inrush.

-Jon

 

[templdl]

As you note, the lower voltage coil will operate at higher magnetizing current for the core to be operated at the same ampere-turns.

In addition to this factor, a transformer designed as a step down, but operated as a step up will have higher inrush current that a transformer of the same nominal rating and same over-all build quality, but designed from the get-go as a step up.

My understanding is that the primary coil is generally placed outside of the secondary coil, so that there is more air path leakage flux seen by the primary coil, and this air path leakage flux reduces the inrush current. When you operate a transformer in 'reverse', then what should be the secondary coil is being used as the primary, and you don't have the benefit of the leakage flux reducing inrush.

-Jon

I agree Jon. But the inrush/magnetizing current is a generality. If you go with a simple proportion that's a starting point. You may considered a greater inrush should there be an issue with nuisance tripping of the PRI. OCPD when the transformer is first energized.

 

[macmikeman]

A couple of times I have done this and used a second transformer at the far end to reverse it again back to the lower voltage and derive a neutral once again . For small loads that were a long long way from a source.

(Prawn farms circulation pumps)

 

[curt swartz]

None of my suppliers stock transformers with 480 primaries and 240 delta secondaries. They would have to be ordered. If you are going to have to order a transformer why not get one that has a 240 delta primary and 480 wye secondary?

I have reverse connected transformers in the past to obtain 480 volts but my primary voltage was always 208. The only reasons I have done this was I could get the transformer from my suppliers stock and there was a rush to get the 480 equipment connected.

 

[jahilliard]

yep...have done the same thing for the same reason...had to get an approval letter form an engineer but they have worked fine for years.

 

[Toros]

Hi I have heard that if I install a xfrmer (480v prim. to 120-240v, 3-ph. 4w second.) in a reverse fashion , I would get 480v (no nutral) at the secondary. note that , incoming volt. available at the primary is 120/240v-3-ph., 4w , delta is this possible???

What if; I had 208V, 3ph, Y as primary to 480V, 3ph. ,secondary Also O.K?????

 

[GoldDigger]

What if; I had 208V, 3ph, Y as primary to 480V, 3ph. ,secondary Also O.K?????

In general, if the primary is a wye, you should not connect the winding neutral point on the primary side to the supply neutral.

 

[templdl]

A couple of times I have done this and used a second transformer at the far end to reverse it again back to the lower voltage and derive a neutral once again . For small loads that were a long long way from a source.

(Prawn farms circulation pumps)

I've provided a pair of transformers for airport applications where power had to be transmitter over a long distance.

 

[curt swartz]

What if; I had 208V, 3ph, Y as primary to 480V, 3ph. ,secondary Also O.K?????

If you are suggesting connecting a 480 primary - 208 volt secondary transformer in reverse to a 240 volt Delta system the answer is NO.