- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
It isn't that the taps "don't work", it's that they become more DIFFICULT to get correct.
Lets say you have a transformer with 2 x +2.5%, 4 x -2.5% (2FCAN, 4FCBN). The taps are on the HIGH voltage side. There are no taps on the LOW voltage side. So if you back feed it, and the LV side is not what you want it to be, the taps that you have on the other side are now OPPOSITE in terms of "above and below". So for example lets say the transformer is 208V Y to 480V D. But your 208V LV is actually 218V and you want 480V. Your LV side is 5% high, so to get the HV side DOWN to 480V, you would use the 2nd 2.5% tap ABOVE normal on the HV side. That could work, but if your LV side is 221V, you don't have another ABOVE normal tap on the HV side, because it was a 2FCAN design.
Lets say you have a transformer with 2 x +2.5%, 4 x -2.5% (2FCAN, 4FCBN). The taps are on the HIGH voltage side. There are no taps on the LOW voltage side. So if you back feed it, and the LV side is not what you want it to be, the taps that you have on the other side are now OPPOSITE in terms of "above and below". So for example lets say the transformer is 208V Y to 480V D. But your 208V LV is actually 218V and you want 480V. Your LV side is 5% high, so to get the HV side DOWN to 480V, you would use the 2nd 2.5% tap ABOVE normal on the HV side. That could work, but if your LV side is 221V, you don't have another ABOVE normal tap on the HV side, because it was a 2FCAN design.