I have been observing my VM-3P75CT on a single-phase system (my house with a solar system) for some time and starting to wonder about its accuracy in low current situations.
It almost never gives a reading much below 2A (import or export) even when power is well under 200W (on a 240V grid). When I look at PF, it shows stupid values like 0.048 (see pic, an extreme example) - which I don’t believe! And I’m skeptical about the near constant 2A draw overnight. I would expect more variation as loads (eg fridge/freezers) cycle on and off. What makes me most suspicious is the transition period when the solar production roughly balances the load (ie zero current flow to/from the grid), the current will jump between 2A and -2A (see expanded current graph), never anything nearer to zero.
I have tried the usual troubleshooting like checking the CT is properly closed and swapping CTs but no improvement. If I disconnect the CT, it reads zero so that discounts a calibration issue. I can’t turn off all the loads to check at zero current because it will disrupt the household - and also the meter is powered from one circuit.
Is anyone else seeing this sort of behavior at low currents? Any troubleshooting suggestions?
Hi @Dunnp, it’s the same explanation as @wjmb links to. Added below for completeness.
In addition, the “flip-flopping” between -2A and 2A isn’t actually a difference of 4A current flow. The VM-3P75CT reports the RMS value of the current, which by definition is always positive. We made the choice to report the current with the same sign as the power flow. So in this case, at +1W it reports 2A while at -1W it reports -2A, even though the actual RMS current hasn’t changed at all.
Thank you for the explanation Luc and @wjmb. As long as the power (W) reading is accurate and reflects what I’m paying for/getting paid for, the current (A) is fairly irrelevant at low values. For interest, is there any scope to lower the current threshold? Or will noise/harmonics get in the way?
The reason I raised it here (other than me being pedantic) was to rule out the possibility of a faulty meter.
I will chalk this up to another quirk of the VM-3P75CT - along with the power gauge in VictronConnect that only reads from 0 to +100W, making it rather useless.
You say if the power is +1W, show +2A and for -1W, show -2A. What I meant was whether the +/-2A reading can be made smaller and closer to the actual current? Or better still, can it be calculated (W/V) to display the correct current (presuming the watts reading is correct)?
On VC, there is another quirk: the Neutral current seems right in a direct VC display, but in VC-R, it initially displays correctly then changes to one-tenth the value - eg 3.6A will drop to 0.3A. Not a big deal, just a small bug somewhere. A VC bug or a VRM bug?
Hi @Dunnp, the 2A is the actual current that the wires have to carry. A value close to 0A wouldn’t represent a physical value, and is something we won’t implement. I agree that it isn’t really nice to look at though those flip-flopping values though The only real way to lower that value is to improve the power factor of your system.
I’m not able to reproduce the neutral current initially displaying as a different value.
This is a VC-Remote connection (via VRM). It initially shows the correct Neutral Current (5.1A) then after a short (but random) time (sometimes a couple of seconds, sometimes 15 or so), it divides the current by 10. Clearly a wrong value.
It happens on all my devices - iPhone, iPad and PC. And it’s consistent (apart from random time) so should be reproducible. Running v1.10 but was the same with v1.09.
Like I said Luc, it’s no big deal - and rather pointless in a single phase system anyway. But presenting wrong measurements is not ideal. Same goes for the original +/-2A situation I raised - measurements can’t be assumed correct.
This morning the power gauge in VC is now working much better, not limited to 100W. Some nice work in the background?? (Still no negative readings though).
The only real way to lower that value is to improve the power factor of your system.
