In a single phase system neutral and line wires provide a potential difference over the load that is to be powered. Neutral is commonly earthed, connected to the earth, at some point in your house. Because of this the neutral wire is at the same potential as the ground we are connected to making it typically less dangerous.
The connection between neutral and earth allows any phase-to-earth fault to develop enough current flow to "trip" the circuit overcurrent protection device.
On a three phase system you can have a neutral wire but it is optional. Below are two three phase load configurations, Delta and Y. Neither requires a neutral conductor, though in the Y configuration neutral can be connected to the center, where all the phases meet. As you mentioned on a balanced three phase system no current flows through the neutral wire. In such a system current on the neutral wire can be indicative of a problem.
It may help to visualize the waveform in polar coordinates. The radius is the same all around, indicating each phase magnitude is the same. Each phase is seperated by 120 degrees.
AC is always pusing and pulling current so it can be positive or negative at a particular instant. We can do a calculation to illustrate how a balanced 3 phase system would leave no current on the neutral phase.
- A balanced system will have the same magnitude per phase (equal source and load), we can take that as 1.
- A balanced system will have each phase seperated by 120 degrees.
So the following is true,
I_neutral = I_a + I_b + I_c
where I_x is the instantanious current at one time. Also if each phase is an ideal sine curve then
I = magnitude*Sin(theta)
theta is the phase shift. So,
I_neutral = (1)Sin(0) + (1)Sin(120 deg) + (1)Sin(240 deg)
= 0 + sqrt(3)/2 - sqrt(3)/2
- As an aside, we talk about current and voltage in AC systems by using the RMS (root mean square) voltage or current; this is what a multimeter in the AC setting will read. This is not the peak volatage/current and not the average but the
value of the direct current that would produce the same average power dissipation in a resistive load -source