Transformer tap

[kingpb]

Meaning that you should get the full KVA of the transformer if you connect 504V to the 504V tap, 480V to the 480V tap, or 456V to the 456V tap. But that doesn't tell us what happens if 480V is supplied to the 456V tap.

-Jon

That is incorrect.

Assuming taps are on the HV side, and they are the standard 2 x +2.5% and 2 x -2.5% and nominal voltage is 480V, 3ph - applying 480V to the -5% tap (456V) and no-load on the LV side of the transformer will raise the voltage by 5%. So, assuming 208V on LV side, the actual measured voltage would be approx. 219V. Of course ignoring transformer and cable losses. The fact that you change taps does not diminish the rating e.g. it's still a 30KVA transformer.

What will change on the nameplate is that the rated current will inversely go up or down by the same percent as the voltage change.

 

[elec_eng]

That is incorrect.

Assuming taps are on the HV side, and they are the standard 2 x +2.5% and 2 x -2.5% and nominal voltage is 480V, 3ph - applying 480V to the -5% tap (456V) and no-load on the LV side of the transformer will raise the voltage by 5%.

I can see where this might be a problem like Jon pointed out.

What if the utility voltage swings, say 504V (+5%), which is no surprise, then the LV side of the transfomer will raise the voltage by 10%.

Will the transformer be capable of operating without core failure?

 

[winnie]

Clearly, if you increase the volts per turn in the transformer primary, magnetizing current and core flux _must_ increase. That is the physics of the situation.

At the same time, if you keep the same KVA loading, than as the secondary voltage increases the load current goes down. The reduced load current will compensate for the increased magnetizing current.

If I understand kingpb correctly, then nominal voltage may be applied to _any_ of the transformer primary taps without causing excessive heating or changing the KVA rating (in, of course, a transformer built to the specification that he referenced). There must be a limit to this, since eventually the magnetizing current is so great as to overheat the transformer with absolutely no secondary load at all. But it is entirely plausible that the standard used for building these transformers gives a significant range of acceptable usage volt/turn values.

I do not know how to answer the question: what happens to a transformer if 504V is applied to the 456V tap. Well, I guess I could figure it out with more detailed design info, but for that you have to ask the same entity that can just give the answer directly.

-Jon

 

[macmikeman]

The taps aren't on the transformer by accident. The transformer with taps is designed to work over a voltage range specified by the taps. It is ok to use them for the purpose, thats what they are there for. And, the use of them is pre-engineered and tested for that use so that the transformer does not have a failure, provided the correct voltage range is connected to the primary.

 

[kingpb]

I can see where this might be a problem like Jon pointed out.

What if the utility voltage swings, say 504V (+5%), which is no surprise, then the LV side of the transformer will raise the voltage by 10%.

Will the transformer be capable of operating without core failure?

ANSI C84.1 stipulates that for a 480V nominal system the maximum service voltage can be 504V. The transformer is designed for the maximum service voltage. So, no damage should occur.

However, it is why taps should not be used arbitrarily, because overvoltage on the bus at no-load, could occur. Taps should be investigated to resolve specific problems, like the OP, and used without overall system consideration.

 

[winnie]

The transformer with taps is designed to work over a voltage range specified by the taps. It is ok to use them for the purpose, thats what they are there for.

But in this case, the question is what happens when a _different_ voltage than specified on the nameplate for that tap is applied to a given tap.

Given a 480D to 208/120Y transformer with 3 taps, one nominal tap, one 5% FCAN tap and one 5% FCBN tap. The nameplate will show three different primary voltages (one per tap) and a single secondary voltage.

Is it acceptable to apply the 480V to the FCBN tap in order to raise the output to 218/126V? I know the physics of the situation (magnetizing current goes up, load current for the same KVA goes down), but I don't know what the results will be given the actual construction of the transformer.

kingpb seems to be saying that under the standards used to build transformers, the transformer will behave acceptably if 480V is applied to the 456V tap.

You seem to be saying that the specific taps are designed for specific supply voltages.

These two views are not necessarily in conflict; the 456V tap may be designed for optimal transformer operation at 456V, but still have acceptable operation at 480V.

I'm just trying to clarify/understand.

Thanks
Jon

-Jon

 

[kingpb]

Instead of thinking of it as a specific applied voltage position, maybe think of it as just a change in turns ratio, and try to forget about the actual voltage being applied. That way, whatever voltage you apply to the HV side, the LV side is adjusted up or down by the same percentage.