rmsthree phase
What is the difference between line, phase and RMS voltage?
We get many questions where it is not mentioned whether the question is specifying line, phase or RMS voltage. I will attach some example questions to be more specific:
In all the three questions, how do you know if it means line, phase or RMS voltage?
Any help would be appreciated
You can measure phase-neutral voltage and if the supply and load are balanced then you can infer line voltage by: -
line volts = phase volts\$\times\sqrt3\$
This should be one of the easier measurements yet you say you can calculate "real power, reactive power and apparent with ease". This does make me think that you have used one of these measurements, and the RMS measurement of current and back-calculated phase/line voltage and that's when you are seeing a discrepency.
If this is so then I suspect your current measurement may be at fault either through incorrect use of a current transformer or some ratio being incorrect. Or, it could be the \$\sqrt3\$ thing mentioned above.
If you need any further help with this you should consider detailing how you make the other measurements and why you believe voltage to be incorrectly calculated/measured.
No need for a complicated formula.
If you have balanced three-phase power, where all three phase voltages are equal in magnitude and 120° apart in phase, then:
$$ V_{L-L} = \sqrt{3} \times V_{L-N} $$
To see why, consider the phasor diagram:
Applying some basic trig:
Best Answer
On AC power questions, unless otherwise specified the default measurement is RMS.
On 3ph power questions, unless otherwise specified the default voltage is always line-voltage\$^1\$.
Phase voltage is between any of the 3 power-conductors and a neutral wire (as per star connected loads. Line voltage is between 2 of the 3 power-conductors and, for a balanced load, this will be \$\sqrt3\$ times higher than phase voltage.
RMS is the preferred method of measurement.
\$^1\$ Except in aerospace where phase voltage is always the default.