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Re: Phase Power & Fuses
Reinhard:
Here we go...and for those who aren't interested in finding out more about
three phase power distribution - hit delete now;-)
Assuming the same phase to phase voltage, and ignoring any neutral for the
momemt, you can draw the same total power from a 20A three phase circuit,
regardless of whether the source is wired Y or delta. However, this is not
usually the case here, because the line-to-line voltage for a Y is 208 and
for a delta 240. So a 'Y' gives slightly less power for the same current,
but only because of the lower voltage.
The voltage is lower in a Y configuration because three 120V transformers
are used and (due to the 120 deg phase displacement) the line-to-line
voltage is the sq root of 3 x 120, not 2 x 120. This lets you take 120V
from any phase to the neutral and is frequently used in commercial
buildings where lots of 120V loads (lighting, small appliances, etc.) are
anticipated. You can also use 208V single or 3 phase for heavier loads like
air conditioners, motors, etc. Most, but not all, 240V equipment will
operate on 208V.
The delta configuration is most often used where more three phase loads are
anticipated, like in factories, machine shops, and the like. This is where
things can get confusing, because there are several ways to conect the
neutral in a delta circuit. In some cases, there is no neutral as such,
only the three phases and one phase is often grounded for safety reasons.
In a four wire delta, the neutral is connected to the mid-point of one of
the transformers. Since the transformers are 240V, this gives you two 120V
circuits, line to neutral along with 240V single or 3 phase. However, the
third (or 'wild') phase is at 208V with respect to the neutral and is not
normally used for single phase loads.
An interesting variant of the delta circuit is the 'open-delta' which uses
only 2 transformers to supply a three phase load. The circuit is the same
as a normal delta with one transformer missing. It will only supply 57% of
the power of a full delta, but the power companies frequently use it to
save money; adding a third transformer if and when the actual load
increases.
What this all boils down to is that with a Y circuit you can draw at most
half of the available power from the circuit as low voltage (120V) loads
(for example 3 x 20A x 120V), all of the available power as 208V loads (3 x
20A x 208V), or some combination of the two. With a delta circuit (assuming
there is a neutral) you can draw the full available power at 240V (3 x 20A
x 240V) but only one third of the availabe power at 120V (2 x 20A x 120).
Please excuse my use of US voltages, but I think better in familiar terms
and everything should scale to european standards. If this doesn't answer
your question, perhaps you should mail me off the list, since the others
may be starting to find our non-coil related discussion here tiresome.
More power to ya...
- Mike
> Now comes my question:
> How about the delta connection? Would this mean you could only draw 20A?.
> Or how about (being the same question, really) from phase to phase (i.e
> 380V).This would mean less output on a delta connection.
> I always figured you have to take the phase difference into account. I
never
> could figure this out in a satisfing way.