Scott's given an excellent and thorough description there. I'd just add that as you're in the U.K., we don't have delta supplies at low-voltage. Motors and other loads may be wired in 415V delta configuration across the supply as you know, but the actual source in our public supply system is always a 4-wire wye system with solidly grounded (earthed) neutral.
say the red phase and yellow phase are connected to the element, the red phase current would run down the yellow phase when at 0 volts potential (at mid point of cycle)and the other way around when yellow phase is pulling current?
If one phase is at the zero point, the direction of current flow depends upon the polarity of the other phase at that instant.
Let's look at it from a slightly different angle and analyze the potential
with reference to ground/neutral on each phase.
If you plotted the voltage on phase A (red in Britain) and that on the phase B (yellow) on the same axes, you have two sinewaves each of the same amplitude (240V RMS, or about 340V peak) but with a 120-degree phase shift between them. (Any of our friends here with fancy CAD software have an image they could provide?)
I'll assume
conventional current flow from positive to negative in the following, although
electron flow is, of course, negative to positive.
If you place a ruler vertically on the graph and move it slowly allow the horizontal axis, you'll see that at any given moment you can have any of the following state of affairs:
1. One phase positive, the other negative. Current flows from positive phase to negative phase.
2. One phase at zero, the other positive. Current flows from positive phase to zero phase.
3. One phase at zero, the other negative. Current flows from zero phase to negative phase.
4. Both phases positive. Current flows from whichever phase has the higher positive potential to the other phase.
5. Both phases negative. Current flows from whichever phase has the lower negative potential to the other phase.
Note that in all cases, current will flow from whichever phase is the
more positive to whichever is the
more negative.
So to go back to your 415V element connected across two phases, if phase B (yellow) is at the zero crossing, current direction will depend upon the polarity of phase A (red) at that instant:
If A is positive, current flows A to B.
If A is negative, current flows B to A.
If you moved your ruler slowly along the two sinewaves and plotted a third wave which at any point is the instantaneous sum of the two phases A and B, the result would be another pure sinewave of 415V RMS amplitude.
So as far as that element is concerned, it is just receiving single-phase power at 415V.
[This message has been edited by pauluk (edited 07-17-2003).]
