Guy's, There have been testers around to do this sort of test, for years. I note that a number of two probe testers give a Phase Rotation reading on them, in the form of a circular arrow on an LCD screen. Could someone please correct my way of thinking, in so much as,I was under the impression, to effect a decent 3 Phase Rotation test, you had to have a tester that would connect to and test all 3 of the phases. The question I ask, is this, how can a 2 probe tester do the same job as a 3 Clamp tester?
I cannot imagine a wiring configuration with only two phases that would tell a phase rotation for all three. Could you share a product manufacturer and model? This one has three wires connected. http://www.traknet.com/wei/aemc9.html My understanding is many rotation testers are really small induction motors that rotate in a direction relative to the phase rotation measured/applied.
Maybe an electronic meter could see phases AB, determine that they are 120 deg out of phase and assume that the third phase is either before the A or after the B, resulting in clockwise rotation either way ABC or CAB.
[This message has been edited by Ron (edited 08-13-2004).]
Hmmm..... I don't recall ever seeing a two-probe tester, but I don't get involved with 3-phase much.
Let's say you connected to two phases, A and B. There are only two basic possibilities no matter which two of the three phases you picked at random:
#1. A leads B by 120 degrees. Therefore C must lag B by 120 and lead A by 120 deg.
#2. A lags B by 120 degrees. Therefore C must lead B by 120 deg. and lag A by 120 deg.
So as I see it, you could prove rotation in this way.
You would still need three connections to be able to get a reference point for A and B.
In the case of, say a British/NZ 415Y/240-volt system, we know that A and B are 240V to ground. But unless you have a third reference point, you have no way to determine which lags or leads the other with respect to ground.
Any instrument connected solely across A and B with no other point of reference will see only a simple 415V RMS sinusoidal waveform, nothing more.
A suitably sensitive device might be able to use capacitive coupling to the user's hand to obtain a ground reference. That's the only way I can think of.
[This message has been edited by pauluk (edited 08-13-2004).]