# Whats the justification to determine the phase sequence?

I have been told many times that one way to determinate the phase sequence of a three phase system is to use two resistances and one capacitor (or inductor) in star (wye) connection, then connect two voltmeters across the resistances and watch the one gives more voltage like:

$$\V_{R1}>V_{R2}\$$$$\=ABC\$$

$$\V_{R1}$$\=ACB\$$

simulate this circuit – Schematic created using CircuitLab

But I was never told why is so? So someone could give me a reference? Thanks

• watelectrical.com/… Need to draw some phasors which I haven't done in a really long time. Can't work on it right now, it nobody gives an answer by tomorrow I will strain my brain. The impedance of the cap should be roughly the same as the resistors at the line frequency to get a good voltage separation. Mar 4 '21 at 23:50
• Yeah the values of the components must match the same impedance/reactance in all components. Mar 4 '21 at 23:51
• I don't think CircuitLab can show you the RMS voltage on the resistors. Mar 5 '21 at 2:45

CIVIL: In a C I leads V which leads I in an L.

Assuming your phase sequence is V1, V2, V3, then the current in C1 will lead V3 and end up being closer to the phase of V2. This will reduce the current drawn by phase 2 relative to phase 1 and therefore the voltage will reduce on R2. Obviously reversing the sequence will give the opposite result.

Apparently you can replace R1 and R2 with lamps while choosing C1 to have a similar impedance value as the lamps to get a simple and cheap indicator. I've never seen one in real life.

The phase sequence determines which way an AC motor will rotate. This is very important with pumps and other pieces of mechanical gear.

• This seems to be an answer to a different question. Mar 4 '21 at 23:07
• Yup! You do not want to run some equipment backwards, even for a second or two. Best to know the rotation before flipping the switch. I use my old Tektronix PowerScout for that. Mar 5 '21 at 0:39

The "neutral" must move so that the current vectors sum to zero. When B follows A, this will tend to push the neutral toward C.

I chose a frequency such that 1 degree equals 1 mS so I could easily check my answer in the simulator. The impedance of the cap at this frequency equals the impedance of the resistors.

simulate this circuit – Schematic created using CircuitLab

Here I have normalized the currents to have the same vector length as the voltage. I then estimated the location of the neutral so the current vectors sum to zero. Maybe someone better at geometry than me maybe can give an exact answer.

I can't prove it, but I believe that it is impossible to get zero current through R3.