| I am trying to get 208V from a 208V transformer. I was under the | assumption that a 75KVA 600 - 120/208V transformer would deliver 75Kva | at 208V. With 600.00V input they are delivering more like 40 - 50 Kva | at 200V. | | I have 2 of the same transformer that are doing this with similar | loads. I took the cover off them and took some current readings. One | has approx 150A on the highest leg and the other transformer has 180A. | Both transformers are running unbalanced because there is a large | single phase load on each 3 phase panel. There is a lot of heat being | generated too. | | The loads are basically ballasts for high output UV lamps for curing | inks on screen printing press's, with a few small motors for fans | and conveyors etc. I do not know if these ballasts are electronic or | not. I intend to find out tomorrow.
This could be the problem. If the load is very non-linear, it can be causing a substantial pulse load on the transformer if it is also a major part of that load.
A normally resistive load is ideal. But when a load draws all of its current only in a short pulse during each cycle, then it is causing more of a demand on the transformer and wiring than what it would be if averaged out.
A simple explanation works like this. Suppose you have 3 banks of heaters that pull 90 amps of power each. You switch one bank on for
1 second at a time, and cycle through each so that only one bank is on at one time. You get the same amount of heat as if you had just one bank on steady. But now, suppose you turn all 3 banks on at the same time for 1 second, then everything off for 2 seconds and repeat. You get the same heat again, but this time you are drawing
270 amps 1/3 of the time. Consider that impedance loss affects the power in proportion to the _square_
of the current. At 3 times the current you have 9 times the loss. Average that 9 times loss over the 3 seconds in each cycle, and the average loss is 3 times as much.
When you have electronic loads that pull current in short pulses on each AC cycle, it's still the same kind of problem. The math is more complicated, but it puts more stress on the wiring and the transformer windings, and results in more loss, more heat, and potentially even damage or fire.
This problem is even worse with 3 phase power when using loads that connect between any hot phase and neutral. That puts current pulses on the neutral that do not balance out. You can end up with three times the current on the neutral ... and that's just average ... the pulsed current problem can make it worse if the pulses are narrow enough.
Since your loads are connected phase to phase, the neutral overload shouldn't be much of an issue. But you can still cause a 15% extra overload just due to the pulses not matching between phases (they don't average together like sine waves would). But you still could have the I^2*R problem with deep pulses.
If you can get an oscilloscope reading of the current waveform on each of the phases and the neutral, that might be more informative.