Three phase + other

[mildmanneredjanitor]

Just out of curiosity, yes I am aware of the cat!

With three phase, 3 x 230 = 690, so why is three phase 415v? (i think?)

Also, doesn't three phase share a common neutral? Has this and the phase word got something to do with this?

I seem to remember a much earlier thread about 110v site transformers actually being 55+v/55-v (so fluctuating 110v). So does 230v AC fluctuate (at 50 htz) 0 - 230v?

Laymans terms please!

 

[ban-all-sheds]

Just out of curiosity, yes I am aware of the cat!

With three phase, 3 x 230 = 690, so why is three phase 415v? (i think?)

AFAIK that's the RMS value for 3 230V sine waves separated by 120°.

Also, doesn't three phase share a common neutral?

Yup. But sometimes there is no neutral.

Has this and the phase word got something to do with this?

I have no idea what you mean by that.

I seem to remember a much earlier thread about 110v site transformers actually being 55+v/55-v (so fluctuating 110v). So does 230v AC fluctuate (at 50 htz) 0 - 230v?

It fluctuates from -325.27V to +325.27V with respect to neutral.

 

[plugwash]

note angles in this post are in radians there are 2pi radians in a circle

normal domestic electricity supply is a sine wave that is it follows the graph V=Asin(2piFT) where F is freqency T is time and A is the peak voltage and V is the voltage at any point in time

mesuring relative to the neutral wire which should generally have a voltage close to earth

it should be noted that we mesure voltages in power distribution by RMS not peak but this is not important here

diagram from wikipedia. ignore the dotted line that is cosine

now imagine we took three of theese cosine graphs and spaced them by (2/3)pi radians (you will note that after 3 such spacings we will have gone 2pi radians which is a complete cycle) we consider such waveforms of the same type but spaced in time to be out of phase with each other hence the term 3 phase

it can be shown that the waveform found between any two of theese three waveforms is also a sine wave

furthermore it can be shown (by trig identities) that the waveform found between any two of the 3 waveforms has a peak value sqrt(3) times bigger than the peak value of the original sine wave

now since for any sine wave the peak voltage is a constant value of sqrt(2) times the rms voltage we can say that the rms voltage between any two phases is sqrt(3) times the rms votlage between any phase and neutral

so 240*sqrt(3)=415.69219381653055044658712196141
this is normally rounded to 415V

for 230V which is what we apparently have now (fake harmonisation don't you just love it but thats another rant)
230*sqrt(3)=398.37168574084177751131265854635
this is normally rounded to 400V