General > General Technical Chat
Noisy power from typical backup generators
Renate:
--- Quote from: richard.cs on May 28, 2021, 06:41:21 pm ---
--- 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.
--- End quote ---
And you have to consider that the fridge normally runs somewhere around a 50% duty cycle.
So peak power is twice that.
Still, I LOVE my fridge!
Besides, the amazing thing is that it even works.
NH4 + H2O + H2
NiHaoMike:
--- Quote from: SeanB on May 29, 2021, 09:22:05 am ---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.
--- End quote ---
Or what about just drive the existing compressor as the 2 phase motor it really is? I'm aware of a proprietary/research implementation of such (Shannon Liu Quadrature Drive), but it should be possible to build one that's open source.
Or if all you need is to reduce the inrush, a single phase inverter that does V/Hz ramp up would probably do the trick without modifying the fridge at all. Perhaps at least some inverter generators have that feature?
Johnny B Good:
--- Quote from: NiHaoMike on May 29, 2021, 01:29:05 pm ---
--- Quote from: SeanB on May 29, 2021, 09:22:05 am ---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.
--- End quote ---
Or what about just drive the existing compressor as the 2 phase motor it really is? I'm aware of a proprietary/research implementation of such (Shannon Liu Quadrature Drive), but it should be possible to build one that's open source.
Or if all you need is to reduce the inrush, a single phase inverter that does V/Hz ramp up would probably do the trick without modifying the fridge at all. Perhaps at least some inverter generators have that feature?
--- End quote ---
I'm afraid using a separate VFD as you suggest just won't work. The starting capacitor will be the wrong value for the initial low ramp up frequency and won't do anything useful until you get within 10 to 15% of the compressor motor's nominal mains frequency rating.
As far as I know, VFD compressor motor systems driving an otherwise conventional pump use three phase BLDC motors which don't require any capacitor starter to be switched into the circuit, merely a three phase supply that can ramp both supply voltage and frequency together from a very low initial start condition, culminating in nominal voltage and frequency for full output (perhaps as high as 70Hz and 140vac for a compressor designed for 120v 60Hz mains supply).
Such VFD compressors can be controlled with a PID algorithm to run at almost any speed in between to generate the required level of cooling without resorting to the stop start system of conventional fridges and freezers. Basically, other than for unusually low ambient temperature conditions, the compressor runs continuously at a little under the 50%ish duty cycle, typically around an hour on and an hour off. Not only does this provide more stable cabinet temperature management, it also reduces energy consumption, wear and tear and noise levels as well as eliminate the nuisance of very high startup current transients on the supply.
However, I discovered an interesting alternative to this more efficient VFD compressor setup whilst I was researching fridge/freezer models endowed with VFD compressors. This was notably (but not exclusive to) LG's Linear inverter compressor technology as employed in, for example, their GBB92MCBAP fridge/freezer model. This has a very high energy efficiency rating of just 110KWH consumption per annum (an average consumption of just 12.56 watts!).
Unfortunately, its 70/30 split between fridge and freezer isn't ideal especially when it's priced at £1400 here in the UK. If the split had been 60/40, I'd have been tempted even at £1400 or at least seriously considered their slightly lower spec version (still blessed with that all important linear inverter compressor) at just £800. However, despite the wife's declared interest in replacing our current setup with a modern 'American styled' fridge freezer, she's vetoed any further consideration on my part about this particular LG model range.
I've looked at LG and other brand's American styled fridge/freezers that incorporate these new fangled linear inverter compressors only to be surprised by the almost universal fourfold jump in annual energy consumption. TBH, with just shy of a doubling up of capacity, I was expecting an increased annual energy consumption in the region of 50 to 100 percent rather than the rather unconscionable 300% increase I discovered on close inspection of all the American styled fridge/freezers I'd checked out.
Anyway, I might have no choice but to go for one of these obscenely energy hungry fridge freezers (linear inverter compressor notwithstanding) and accept the higher continuous 50 watt average with the only consolation being the absence of the hefty startup current surge associated with the older compressor technology. Even with the higher energy consumption, the running costs will still be lower than our current arrangement of separate chest freezer and under counter-top fridge. Perhaps, if we wait long enough, less energy demanding American styled fridge/freezers might come to market, who knows?
BTW, I did finally get round to testing the inverter genset's behaviour with our ancient fridge (nearly 3 weeks ago now). Surprisingly, it coped just fine with the fridge's startup current surge as the following 'scope screenshots I've attached show. Even when I failed to allow the three minutes pause to clear the back pressure on the compressor when the power is shut off mid duty cycle (second image - I shut it off rather than let it thermally cycle the overload trip to initiate the required delay), the stalled pump motor current was comfortably within the generator's maximum limit.
I remembered this time to select units of Amps in the Y scale settings and used the 10X probe attenuation factor to get traces in the correct units and somewhere within 5% of true. Take note of the sensitivity settings on the Y axis. I changed them to make it easier to assess the peak values (not all of the screenshots included measurement stats).
Also, keep in mind the 50% PF of the compressor motor loading along with the fact that I had no other loads hooked up to the genset during this test and the fact that the 230v rms rated output voltage was likely down to around the 220v mark at the extension socket. The reduced supply voltage (compared to the 240 to 245 vrms typically seen on our UK supply), no doubt helped reduce the startup surge somewhat without dipping below the minimum operating voltage requirement.
Anyway, enjoy the following images. :)
james_s:
--- Quote from: Johnny B Good on June 22, 2021, 10:19:36 pm --- I'm afraid using a separate VFD as you suggest just won't work. The starting capacitor will be the wrong value for the initial low ramp up frequency and won't do anything useful until you get within 10 to 15% of the compressor motor's nominal mains frequency rating.
As far as I know, VFD compressor motor systems driving an otherwise conventional pump use three phase BLDC motors which don't require any capacitor starter to be switched into the circuit, merely a three phase supply that can ramp both supply voltage and frequency together from a very low initial start condition, culminating in nominal voltage and frequency for full output (perhaps as high as 70Hz and 140vac for a compressor designed for 120v 60Hz mains supply).
--- End quote ---
I think his idea was to nix the capacitor all together. Most compressors use a PSC motor so the capacitor is not just a starting capacitor, it's a run capacitor that creates the phase shift necessary to produce a rotating field. I haven't tried, but it should be possible to drive a PSC motor with a 2 phase VFD without any capacitor at all, the phase shift would be produced electronically.
richard.cs:
--- Quote from: james_s on June 23, 2021, 01:31:08 am ---I think his idea was to nix the capacitor all together. Most compressors use a PSC motor so the capacitor is not just a starting capacitor, it's a run capacitor that creates the phase shift necessary to produce a rotating field. I haven't tried, but it should be possible to drive a PSC motor with a 2 phase VFD without any capacitor at all, the phase shift would be produced electronically.
--- End quote ---
I've only had a couple of (older, UK) fridges apart, but they were all capacitorless motors with thermally controlled switching of a resistive start winding.
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