On 29.05.12 06:57, John Thornton wrote:
> There is no N connection on the transformer so I remain confused as well
> and I'm on my second cup of joe. I didn't measure the 390v to ground but
> I assume one leg is pretty high, but I can wander out and measure it
> easy enough.
It is 40 years since I fooled with 3 phase in the power lab at uni, and
I've kept to digital electronics and software most of the time since, so
the following relies on dusty memories of a mix of theory and practice.
It is not unconditionally necessary to connect a fourth wire to the
neutral of a Y. For example, take the case of a simple 3 phase resistive
load. If the load is balanced, its neutral hovers near the supply's
neutral, i.e. ground. If the load is severely unbalanced, its neutral
can be (many) tens of volts off, as I found when we conducted the
experiment, by leaning on the ventilated metal cabinet of the mobile
loadbank, and was zapped. (Grumping about the inadequacy of the
loadbank's isolation from it neutral did seem to meet a view that there
were plenty more students where we came from ... and a good zap is long
remembered, I will admit.)
If the neutrals are connected, neutral current flows, and the phase
voltages are pulled into equality if the supply is low impedance.
The only critical difference between Y and delta that I can remember is
that for identical transformer or motor windings, the voltage across the
Y is 1.73¹ times what the same windings give/need in delta. (Here that
is 240 and 415 volts.)
Erik
¹ sqrt(3)
--
There are three principal ways to lose money: wine, women, and engineers.
While the first two are more pleasant, the third is by far the more certain.
- Baron Rothschild, ca. 1800
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