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Three-phase emonTx negative power values?

We’ve got a couple of emonTx units running three-phase firmware installed monitoring some three-phase circuits. One or two of the phases often drops below 0W e.g


Any ideas what could be causing this? @Robert.Wall

Are you sure that is not a genuine reverse power flow in those two phases? What exactly is the load that it’s measuring?

I’ve just set an emonTx running here, no load, and certainly that’s showing negative powers on phases 2 & 3 due to noise. It’s showing approx. 100 mA of current on each phase, 6-7 W on phase 1 and -1.5 W on phases 2 & 3.

244.74 0.107 0.109 0.090 0.000 6.36 -0.77 -1.64 0.00 49.941 0.2423 -0.0291 -0.0742 0.0000 16.75 Pulses=0  PLL is locked 
244.68 0.112 0.109 0.090 0.000 6.28 -1.08 -1.69 0.00 49.943 0.2298 -0.0403 -0.0767 0.0000 16.62 Pulses=0  PLL is locked 

(The values are V, I1 - I4, P1 - P4, f, p.f. 1-4, t, pulses.)

But I can’t think of a reason for it to be reading a false negative power of 100 W or so unless there’s something seriously wrong with the c.t. wiring. For example, if you put the 3 c.t’s on the reference phase and there’s a wrong value for phasecal (let’s say the same wrong value for all three c.t’s), then phase one will be roughly correct, but phase 2 will read minus half-power plus or minus an error, and phase 3 will read minus half-power minus or plus the same error, due to the angle being wrong.