| General > General Technical Chat |
| Noisy power from typical backup generators |
| << < (17/19) > >> |
| Johnny B Good:
@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" :wtf: 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. |
| richard.cs:
--- Quote from: Renate on April 20, 2021, 02:44:03 am ---It runs me about 7.5 US gallons of propane a month ~$30 with tax (in an expensive area). --- End quote --- (25.3 MJ/L) * (7.5 US gallons per month) = 273 Watts. Absorption fridges have their upsides, but efficiency is not one of them. I suppose it does compare favourably with a compressor fridge run from a propane fuelled generator. |
| Johnny B Good:
@richard.cs If you're powering your absorption fridge from the same propane tank as the generator, it'll take less fuel than the genset burns to produce that 273W worth. If the genset is an inverter type with 'eco-mode', this will help to keep it running quieter more often and the fridge itself will be quieter than a compressor type, quite apart from eliminating the high startup current surge of a compressor type, easing genset requirements somewhat. In this case, it's a win win. :) My missus expressed interest in replacing our fridge and chest freezer with a modern fridge/freezer combination whilst I was 'annoying her' by testing the fridge with one of those plug in energy monitors that also displays volts, amps, watts and KWH as well as indicating the power factor figure. The power factor figures were all I was really interested in although the wattage and amperage figures compared closely with those I'd obtained in yesterday's test with my home made current probe and the Metrawatt. The dynamic performance of these digital energy monitors doesn't compare to the ballistic response of the Metrwatt. However the peak wattage surge was circa 800W remaining there for several seconds because it had been running before I swapped the energy monitor into the circuit without allowing the back pressure to drop. I unplugged the fridge for 10 minutes before repeating the test which revealed an 800 or so watt surge for the first second or so before it dropped back to just over 80W with a PF of 52% and the amperage reading a tad over 0.55. After leaving it to run for another 20 minutes, the wattage had dropped to 65W, the amperage just about the same at 0.55A but the PF had dropped slightly to 49%, presumably due to less resistive loss in the motor windings as a result of the compressor load stabilising at a lower back pressure level reducing the torque requirement from the motor allowing it to run a tad cooler. It was a gratifying result since the amperage figures between the scope captures from my home made current transformer and the readings I saw today had tallied pretty well with one another. :D Regarding my missus's desire to upgrade to a fridge/freezer, for once I agreed with her on this issue, but only for completely different reasons. Hers being the typical one of style over function (all the eggs in one basket) with mine being only a means of tracking down a unit using a VFD compressor for more consistent temperature control and, more importantly, elimination of the classic compressor start up surge loading that demands double the rating of generator output for the sake of less than one percent of its run time. There's no chance of buying an under counter-top fridge with a VFD compressor in the UK (possibly anywhere in this size) and even with over-priced fridge/freezer units, you have to search carefully to track down the pitifully small number (maybe just two or three models out of the dozen or so available in the UK) so fridge/freezer it has to be. If I have to spend something like a grand on one, I'd rather pay a premium for a VFD compressor type with an extended warranty to mitigate the rather high investment cost involved. It might seem an OTT approach to creating a viable emergency power solution. Spending the money on an expensive fridge/freezer just to save a cheap 100 quid inverter genset from having to handle the hefty chest freezer and fridge startup surge currents rather than spend similar money on a cheapish 3KW rated inverter genset instead. However, when you consider the actual problem you're trying to solve, it makes a lot more sense to invest in something that'll be used all the time rather than something that may never need to be run in an actual emergency. The 1KW/1.2KW peak generator rating is rather marginal but since almost all of the house lighting consists of LED lamps, it turns out to be just sufficient to keep all the lights on and the vital IT kit plus a VFD compressor fridge/freezer running, along with the central heating pump and the lounge TV - pretty much all you need to become a shining beacon of light in a sea of power blackout induced urban darkness. >:D |
| james_s:
Using a generator to power a mains voltage refrigerator in an RV would be a total pain in the butt, even if you came out ahead on fuel (which I doubt) there would be the noise and maintenance issue. The big advantages absorption fridges have is they are silent, virtually maintenance free and very reliable. There is another option that I suspect will become more and more common, refrigerators using a 12V compressor. My dad had one of these on his sailboat, and it was a fairly typical looking small hermetic compressor except for a small control box attached to it and internally it has a BLDC motor instead of the typical induction motor. IIRC it would draw around 30W while it was running, low enough that the 100W solar panel he had could keep it going when it wasn't on shore power. It was smaller than a typical RV fridge but even a larger one would probably be feasible in most RVs. Solar panels are cheap enough these days it's hard to think of a reason not to have at least 100W or so on the roof of just about any RV. |
| SeanB:
Yes those 12VAC 2 phase fridge compressors are available off the shelf, and are a common way to convert the typical bar fridge to boat fridge, though they are rather less than stellar in the cooling department, as they cannot really get down to low temperatures due to the compression being lower. They are pretty expensive though, at least double the cost of the same size refrigerator compressor, plus then you need the inverter to drive them. As to inverter mains power fridges, beware of the fine print in the warranty, they cover the compressor alone for 10 years, nothing else, and typically a failing main inverter board will burn out the compressor, and thus while the compressor will be covered by the warranty, you will need to replace the inverter board as well, which is not covered, and which, as a spare part, is nearly 80% of the price of a replacement unit. The 10 year warranty is a scam, the compressor manufacturers are very capable, after decades of manufacturing, and the modern compressor is almost never going to fail from the compressor failing mechanically, it almost always fails due to a gas leak, allowing moisture in and diluting the oil, or the piping or casing rusting away. I have had 5 year old AC units where the only thing holding it together was the piping, and the wires were structural, holding the compressor from dropping out the base, but it was still cooling. Replaced many compressors after 4 years, because the compressor case rusted through on the mounting ears, or on the suction line seal, which were covered by the warranty. Rest of the unit was rust in close formation, but still had enough integrity to work. As to a DIY current transformer, cheapest to find an old 35 to 50VA non potted toroidal transformer, and strip off the windings, and use it as a core. Did that, and used a spare 5A CT moving iron meter on it to keep a tab on mains draw. Otherwise 60A to 5A CT current transformers are plentiful enough, and easy to wire up as well, only 2 terminals you shunt with the current sense resistor, and a single loop of the supply wire through the hole as primary. They work well under 60A, just select a resistor to give you around 1VAC at the desired peak current, taking into account the 1:12 turns ratio. Currently using one in reverse as a 10:1 transformer to get an extra digit of resolution off a spare 60A consumer meter, where the existing 60A shunt is now the secondary, with 10 turns through the core driving it. |
| Navigation |
| Message Index |
| Next page |
| Previous page |