Transformer Taps

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steve66

steve66

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Standard dry type 300KVA 480V delta to 208V wye transformer.

Shop drawings say "Taps 6 - 2.5% 2+4- Taps"

I would assume these are secondary taps, and that means there are six taps at -2.5%? So I could set the secondary at -2.5% nominal, or at -5% .... clear down to -15% nominal voltage (177Y/102 volts)?

And there are two taps at +4%? That would allow +4% or +8% voltage on the secondary?

Do those both sound correct?

I'm also wondering why anyone would even need to drop the output voltage by 15%? It would be better to have more positive + taps to help make up for voltage drop.
 
electrofelon

electrofelon

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...Also if I can clear this up. I have always been uncertain: What is a - tap vs a + tap? Seems like a dumb question but bear with me: does a negative tap mean "for a primary voltage that is -x% of nominal this will give rated voltage at secondary" Or is it "-x% tap will reduce secondary voltage x% for rated primary voltage" ?
 
steve66

steve66

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electrofelon said:
It seems kinda ambiguous, but if I had a gun to my head and had to choose, Id say after the 6 is a dash, not a negative sign, and its 6 total, 2.5% each step, 2+, 4-
Click to expand...

Yes, not that you say that I would agree - 6 total at 2.5% each, with 2 up and 4 down. That makes more sense

Now, does the 4- mean it lowers the secondary voltage, or its for a lower primary voltage, so it raises the secondary V?

In other words, do I have 4 taps to raise the Voltage or just 2?
 
charlie b

charlie b

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I have always understood the taps to be on the primary side. Thus, a tap of +2.5% would mean more windings on the primary, with no change to the number of secondary windings. The result would be a drop in the secondary voltage of 2.5%. It would help if the manufacturer would be clearer on this question.

My interpretation matches others that have been given. But to be explicit, the tap options are:
  • +5% (result is a lower secondary voltage by 5%)
  • +2.5% (result is a lower secondary voltage by 2.5%)
  • 0% (no change in secondary voltage)
  • -2.5% (result is a higher secondary voltage by 2.5%)
  • -5% (result is a higher secondary voltage by 5%)
  • -7.5% (result is a higher secondary voltage by 7.5%)
  • -10% (result is a higher secondary voltage by 10%)
 
steve66

steve66

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charlie b said:
I have always understood the taps to be on the primary side. Thus, a tap of +2.5% would mean more windings on the primary, with no change to the number of secondary windings. The result would be a drop in the secondary voltage of 2.5%. It would help if the manufacturer would be clearer on this question.

My interpretation matches others that have been given. But to be explicit, the tap options are:
  • +5% (result is a lower secondary voltage by 5%)
  • +2.5% (result is a lower secondary voltage by 2.5%)
  • 0% (no change in secondary voltage)
  • -2.5% (result is a higher secondary voltage by 2.5%)
  • -5% (result is a higher secondary voltage by 5%)
  • -7.5% (result is a higher secondary voltage by 7.5%)
  • -10% (result is a higher secondary voltage by 10%)
Click to expand...

Sounds right. I tried googling transformer taps earlier and came up empty. But a search for "changing taps on a transformer, was more productive, and I found this;

https://www.hammondpowersolutions.com/general-news/how-taps-work/

It seems to agree with you and David. +2.5% means more windings on the primary, and a lower secondary voltage.
 
T

topgone

Senior Member
steve66 said:
Standard dry type 300KVA 480V delta to 208V wye transformer.

Shop drawings say "Taps 6 - 2.5% 2+4- Taps"

I would assume these are secondary taps, and that means there are six taps at -2.5%? So I could set the secondary at -2.5% nominal, or at -5% .... clear down to -15% nominal voltage (177Y/102 volts)?

And there are two taps at +4%? That would allow +4% or +8% voltage on the secondary?

Do those both sound correct?

I'm also wondering why anyone would even need to drop the output voltage by 15%? It would be better to have more positive + taps to help make up for voltage drop.
Click to expand...

I guess tap 4 is your nominal voltage tap (480/208V). Your unit's taps 5, 6 as your + taps while tap nos. 3, 2, and 1 is used as the unit's -taps! All taps are in the primary. There will be more -taps to boost the secondary voltage.
 
Besoeker3

Besoeker3

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Location
UK
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Retired Electrical Engineer
steve66 said:
Standard dry type 300KVA 480V delta to 208V wye transformer.

Shop drawings say "Taps 6 - 2.5% 2+4- Taps"

I would assume these are secondary taps, and that means there are six taps at -2.5%? So I could set the secondary at -2.5% nominal, or at -5% .... clear down to -15% nominal voltage (177Y/102 volts)?

And there are two taps at +4%? That would allow +4% or +8% voltage on the secondary?

Do those both sound correct?

I'm also wondering why anyone would even need to drop the output voltage by 15%? It would be better to have more positive + taps to help make up for voltage drop.
Click to expand...
Apart from special applications I've only come across taps on the HV side, usually 11kV in our case.
 
david luchini

david luchini

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topgone said:
I guess tap 4 is your nominal voltage tap (480/208V). Your unit's taps 5, 6 as your + taps while tap nos. 3, 2, and 1 is used as the unit's -taps! All taps are in the primary. There will be more -taps to boost the secondary voltage.
Click to expand...

Tap "5" would be the nominal voltage.

1,2,3 & 4 would be the "-" taps and 6 & 7 would be the "+" taps

There are 4 below nominal and 2 above nominal.
 
steve66

steve66

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Location
Illinois
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Engineer
topgone said:
I guess tap 4 is your nominal voltage tap (480/208V). Your unit's taps 5, 6 as your + taps while tap nos. 3, 2, and 1 is used as the unit's -taps! All taps are in the primary. There will be more -taps to boost the secondary voltage.
Click to expand...

I'm not sure, but I think the transformer nameplate will explain that in detail. Here is the nameplate from a different xformer that has 7 taps;

IMG_0030.jpg

Looking at a couple of different nameplates, the #1 tap seems to usually be the most windings on the primary side (lowest output voltage).


Besoeker3 said:
Apart from special applications I've only come across taps on the HV side, usually 11kV in our case.
Click to expand...

Yes, from the few nameplates I've looked at, it looks like taps are usually (or maybe always) on the primary side.
 
electrofelon

electrofelon

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Location
Cherry Valley NY, Seattle, WA
steve66 said:
I'm not sure, but I think the transformer nameplate will explain that in detail. Here is the nameplate from a different xformer that has 7 taps;

View attachment 23322

Looking at a couple of different nameplates, the #1 tap seems to usually be the most windings on the primary side (lowest output voltage).
Click to expand...

Looks to me like #1 is highest secondary voltage, lower overall turns ratio, yes?
 
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