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Why do Axperts run hot in Hybrid (SUB) mode, but cool in SBU?


Ben Harper
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I have something curious on my Axpert VMIII 3KW (2019):

image.thumb.png.034b4864829527d423b406186661a777.png

The temperature remains constant around 40 degrees when in SUB mode.

When I switch to SBU mode (Solar is powering the loads by this time), then it drops off pretty close to ambient temperature.

The load is under 500W here, for the duration of this graph.

I'm curious if anybody can explain why this is the case.

Edited by Ben Harper
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5 hours ago, Ben Harper said:

I'm curious if anybody can explain why this is the case.

My wild guess is that as the PV starts generating power, the fans (or at least one of them) runs faster, cooling the heatsinks. Voltronic attempt to have the fans run more often in the daytime (when PV is generating), so it's quieter at night.

At night, the constant 40°C will be the equilibrium of the waste heat with the ability of the heatsinks to dissipate heat with little or no fan cooling. The reported temperature is the maximum of several temperature measurements; the heatsink running the loads (which will probably be varying even at night, as the fridge(s) and/or freezer(s) switch on and off) might be cycling up and down in temperature, but always less than some other heatsink or the transformer, which has a fairly constant (mostly self consumption) load. As I type this, it's not very convincing to myself, but I don't know the VM models well.

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Thanks.. I guess what I'd really love to know is precisely how the "hybrid" circuit works.

I clearly recall seeing in my telemetry a while ago that the load on the inverter was a sustained 4KW for many minutes (it's a 3KW model), and the inverter's overload protection clearly didn't kick in. I have no reason to believe that the overload circuitry is broken, because I often get the "beep beep" for a second or two as the washing machine element kicks in, but before my automated switchover has sent the washing machine back to grid power.

So what I'm wondering is if the design of the hybrid circuity somehow allows more than the rated power to be safely utilized.

This is not something I intend to make a habit of!

But it did make me curious.

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On 2021/02/23 at 10:49 PM, Ben Harper said:

I guess what I'd really love to know is precisely how the "hybrid" circuit works.

My understanding is that the models that have SBU output source priority, when blending AC-in with battery/PV power, simply place the inverter output in parallel with the AC-in (so AC-in is connected to AC-out), and they carefully push power from the inverter into this "AC bus". If the inverter is rated at 3 kW, then the inverter could happily supply 4 kW to a load, with 1 kW if that coming from AC-in, and the inverter running at 3 kW. Of course, the firmware has to synchronise the inverter with AC-in first (they do that all the time anyway), and adjust its output voltage so that the VARs flow is correct (usually voltage determines reactive power, and phase determines real power).

Of course, that means that the inverter could (and will be, for a short time) pushing power back into the AC mains. They obviously don't have the regulatory approvals to do that with all the rigmarole that is required, but they seem to get away with it because it's only a brief burst of power until the control circuit adjusts to the altered load. That's another reason I prefer to stick to the 145 V max SCC models; none of them attempts to do this.

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On 2021/02/26 at 4:44 PM, Ben Harper said:

What are the downsides of this design?

For South Africa, the big one seems to be the puffs of power pushed back into the utility, which may trip certain types of power meters.

Sensitivity to grid transients, yes.

If the grid is at a very low but barely accepted voltage, in the SUB mode your loads will also be at that low voltage.

But it is quite good to be able to blend just enough AC-in to supplement PV power.

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