I’m just curious because The U and V are very similar and easy to mistake which can be very catastrophic.
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In what context? I’ve usually seen them labeled as either “ØA, ØB, ØC” (most common), “Ø1, Ø2, Ø3” (occasionally) or “ØX, ØY, ØZ” (rarely).
It’s common mathematical nomenclature.
In scalar algebra, you tend to use x, y, z for variables. “x” is just an obvious symbol for “blank” or “unknown” and y, z are standard for 2D and 3D spatial representations, respectively. (For no reason except being the next two letters in the alphabet.) This type of nomenclature was popularized by Descartes around 1637.
In vector algebra, you tend to use “v” for variables. “v” just means “a vector.” If you need to do 3D (or three phase) vector algebra, you’d typically use u, v, w, as they are alphabetically close to “v”. (“x” is avoided since it implies a scalar, per above.) This type of nomenclature was popularized by Cauchy around 1853.
Indices (iterations, matrix row/column position, etc.) tend to use i, j, k. “i” just means “an iteration/index” and j, k are just the next two letters. I’m not sure who popularized this – Fourier was using it by 1829 but I suspect it was used much earlier.
You’d use u / v / w whenever express three things as vectors, such as three phases of a power system. But it has nothing to do with power itself – it’s just standard nomenclature for vectors. You could express the positions and lengths of the hour/minute/second hand of a clock as u / v / w if you wanted.
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