@james_s
I finally lashed up a current transformer to test the fridge (and the chest freezer) start up surge current and running power consumption.
I'd made the CT up using a 1.5 inch OD ferrite toroid from my collection of salvage (so of an unknown quality) winding 100 turns with a 13 ohm shunt resistor. It worked well enough to reveal the 5mA multiplier resistor draw of my analogue wattmeter (50K on the 100v range) and the 2.5A of a 600W(240v) travel kettle. However, testing with a 3KW kettle showed a rather distorted waveform which I assume is a sign of core saturation. Testing with a 950W toaster load showed a reasonable facsimile of a sine wave suggesting I could trust the fridge surge results provided the peak amperage didn't exceed 5 amps.
Close examination of the captured waveforms indicate a peak of 9 times the initial running current a couple of seconds after the initial peak. That's a peak of 2.8A versus the 312mA calculated from the wattmeter's initial 75W reading post the 1KW peak transient it had registered (obviously, a touch of ballistic overshoot in spite of the separate magnetic damper built into the meter movement). The running current drops down to 65W after 5 minutes, eventually settling at 62W some twenty minutes or so later.
It seems that our under counter fridge won't be troubling the inverter genset unless it's already driving in excess of a 350W load. That's based on a rounding up of the calculated 675W startup peak to 700W and the fact that the inverter will handle up to a 20% overload for a maximum time out period of 30 seconds.
However, after reviewing the 'scope traces, the peak to running current ratio looks more like 10 or 11 to 1 and, what's worse, it looks like the fridge loading has a PF around the 50% mark which will probably exceed the unmentioned inverter genset's PF rating which I suspect only has a value of unity.
I can see the first ten cycles at about 7A rms before it drops to 5A rms for the next 800ms before finally dropping to the initial 700mA rms running current at what I presume to be a PF of 50% going by the watt meter readings. That 5A loading is right on the generator's overload margin, assuming the brief fifth of a second's worth of 7A doesn't immediately trip it out, so it's rather questionable as to whether it will accept the startup transient overloads or not. IOW, I'd still need to run an actual generator test to determine this.
The chest freezer OTOH, takes twice as much energy once started (eventually dropping to 128W after half an hour). I didn't bother monitoring the current surges with my 'scope and home brewed CT, electing instead to compare its watt meter readings to those of the fridge. The switch on surge gave a 1.5KW peak reading (50% more than the fridge's surge reading) before settling down to its initial 180W demand. Obviously, this is well beyond the generator's peak output rating so I won't be testing this with the generator.
Although I might be able to restart the chest freezer's compressor using the 2KVA/1.5KW rated SmartUPS2000, the complexity of such a power management scheme means I'll just rely on the 24 to 48 hour power cut ride out feature common to most chest freezers when you keep their lids firmly closed.
I'm not anticipating more than 4 hour blackouts due to winter demands exceeding the UK's grid capacity, forcing rolling blackouts to cover the shameful lack of generating capacity margin that the UK government has allowed to develop over the past two decades as a result of all the greenwash politics and unfounded fears over nuclear powered generation so the chest freezer issue can be put on hold for now.
The fridge, otoh, should ideally be addressed and right now, the only way I can see to deal with this with my present backup power solution is to power the fridge from the UPS and fit a load shedding breaker to isolate the genset when the VA hits the 1150VA mark to avoid the need to reset an overloaded genset which can normally only be done by stopping and restarting it.
The load shedding breaker will be configured to reconnect after a 10 to 15 second delay which should be enough time to allow the UPS to deal with the startup surge, allowing the genset to resume normal service and let the UPS switch from battery back to genset power.
Without such a load shedding breaker, I'd otherwise have to nip outside to manually reset the generator by halting the engine to clear the overload lock out state followed by a restart on the pull starter cord. I have in the past managed to reset it by hitting the kill switch to slow the engine right down to almost stopped before unkilling it to allow it to pick back up but this is tricky to get just right as about half of these attempts resulted in a complete stop leaving me to restart it anyway.
The big deal here is that I may only have 5 to 10 minutes of battery reserve in which to reset the generator to avoid losing power to all my IT kit. I suppose I could invest in a couple more of those 7AH SLA alarm batteries to recommission my APC SmartUPS700 to provide a UPS dedicated to the IT kit but I'm rather loath to allow it to squander another 20 watts just to keep it standing by for a mere 700VA/450W's worth of backup power protection. However, that might prove to be the cheapest (if a rather messy) solution for the time being for a scenario that might never actually arise in my lifetime.
What I do have to hand to mitigate any mains outage events has only cost me a hundred quid for the generator and the various UPSes had been bought years before simply to keep the IT kit protected. Over the past four decades, I've spent far more on all those rather over-priced SLAs the UPS manufacturers keep referring to as mere "consumables"
If we ever do decide to treat ourselves to a modern VFD compressor fridge/freezer, the start up surge loading issues will be solved and spending over a grand on a decent 3KW rated inverter genset may no longer be quite so desirable as investing in another of those Parkside inverter gensets as a spare to the existing one (assuming Lidl ever get around to restocking them again).
Anyway, that's the situation I'm in for the time being, essentially looking for the best way to provide emergency backup power within the limitations of a 1KW inverter genset. I'll eventually get a chance to run the genset/fridge startup load test, with or without domestic strife, and I'll post my results as soon as I've collect the data. In the meantime, you can cast your eye over the attached screen shots. They're the first 10 out of a total of 46. The volts per amp value is approximately 100mV. The values were double what I'd been expecting on account of what I've now assumed to be a 50% PF load. I'll do another test with a digital energy monitor that can show the VA and PF figures (the Metrawatt analogue meter can only show true power) and report those results back here in the next day or two.