Author Topic: Noisy power from typical backup generators  (Read 9554 times)

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Offline Johnny B Good

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Re: Noisy power from typical backup generators
« Reply #75 on: April 20, 2021, 10:54:34 pm »
 I have a feeling that the 140W figure I'd quoted for our home fridge might have been the immediate reduction a second or so after the 1KW surge since I can't shake off the memory of figures of 72W and an 'average' figure of 36W.

 It looks like I'll get a chance to measure the fridge's startup and running demands in the next day or two. The missus told me today that she's going to see her friend tomorrow or Thursday, probably landing up at their favourite garden centre or some such for lunch.

 That'll give me a chance to set up the generator to test its capacity to handle the fridge compressor startup surge which might well be significantly higher than that almost 1KW peak reading I saw on the MetraWatt's 1000W scale (5A 200V settings). I might even try repeating these tests with the chest freezer if I have enough time. Hopefully, I'll be able to report the results of these tests in another day or two.
John
 

Offline james_s

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Re: Noisy power from typical backup generators
« Reply #76 on: April 20, 2021, 11:31:00 pm »
I have a feeling that the 140W figure I'd quoted for our home fridge might have been the immediate reduction a second or so after the 1KW surge since I can't shake off the memory of figures of 72W and an 'average' figure of 36W.

 It looks like I'll get a chance to measure the fridge's startup and running demands in the next day or two. The missus told me today that she's going to see her friend tomorrow or Thursday, probably landing up at their favourite garden centre or some such for lunch.

 That'll give me a chance to set up the generator to test its capacity to handle the fridge compressor startup surge which might well be significantly higher than that almost 1KW peak reading I saw on the MetraWatt's 1000W scale (5A 200V settings). I might even try repeating these tests with the chest freezer if I have enough time. Hopefully, I'll be able to report the results of these tests in another day or two.

I've measured my fridge before, once it starts up, the consumption increases steadily for a bit as the head pressure builds up, then it levels off at around 140W steady state until it shuts off. This varies ambient temperature, which doesn't really vary much inside my house. US fridges are typically quite a bit larger than UK fridges though.
 
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Offline Johnny B Good

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Re: Noisy power from typical backup generators
« Reply #77 on: May 21, 2021, 03:30:59 am »
I have a feeling that the 140W figure I'd quoted for our home fridge might have been the immediate reduction a second or so after the 1KW surge since I can't shake off the memory of figures of 72W and an 'average' figure of 36W.

 It looks like I'll get a chance to measure the fridge's startup and running demands in the next day or two. The missus told me today that she's going to see her friend tomorrow or Thursday, probably landing up at their favourite garden centre or some such for lunch.

 That'll give me a chance to set up the generator to test its capacity to handle the fridge compressor startup surge which might well be significantly higher than that almost 1KW peak reading I saw on the MetraWatt's 1000W scale (5A 200V settings). I might even try repeating these tests with the chest freezer if I have enough time. Hopefully, I'll be able to report the results of these tests in another day or two.

I've measured my fridge before, once it starts up, the consumption increases steadily for a bit as the head pressure builds up, then it levels off at around 140W steady state until it shuts off. This varies ambient temperature, which doesn't really vary much inside my house. US fridges are typically quite a bit larger than UK fridges though.

 That doesn't surprise me. :)

 I still haven't had a chance to run that fridge compressor start up surge load test on my little 1KW rated inverter genset. My missus didn't get the chance to meet up with her friends that week as I'd hoped would allow me the opportunity to run the test free of domestic discord. In fact, all further opportunities to do so have failed to materialise ever since to this day. :(

 However, shortly after announcing my intent, I realised it would be more instructive (and quieter!) to make up a current probe for the SDS1202X-E I'd used to determine the PM alternator's pole count and let it capture the event using the same one shot trigger technique. That would give me a more accurate measure of the peak current surge than the camera on a mini tripod setup I'd previously used to record a movie of the analogue wattmeter to estimate the strength of this start up surge current.

 All I can report at this stage is that I'm still formulating the best way to make up a suitable current transformer module that I can plug into the outlet which I can then plug the fridge into to generate a sensor signal to feed to the DSO. Unfortunately, I've had a few other distractions to content with - it isn't just my penchant for procrastination that has delayed things this time.

 Once I've captured a few of these startup events with the DSO, provided the peak amperage readings don't exceed the generator's 5.2A overload limit (1200W for 30 seconds max), I'll repeat the test using the generator. Obviously, if the start up surge demand peaks much higher than the 1200W limit, there won't be much point in testing with the generator (I already know how it reacts to a 1250W overload - instant shut off!). In either event, I'll make a follow up report when I do eventually make up that current sensor module and have some test results to share.
John
 

Offline james_s

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Re: Noisy power from typical backup generators
« Reply #78 on: May 21, 2021, 06:54:58 pm »
Why not just make things simple and cut straight to testing with the generator? If you overload an inverter generator it just shuts down the output, nothing will be damaged. The only test result that really matters is real world behavior.
 
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Offline Johnny B Good

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Re: Noisy power from typical backup generators
« Reply #79 on: May 23, 2021, 12:14:12 am »
 If the opportunity arises to avoid the domestic discord over running the generator before I get the current transformer sensor adaptor made up, I'll do just that. I'm not worried about burning out the genset since I already know the inverter overload protection works quite well.  :)
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Offline Johnny B Good

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Re: Noisy power from typical backup generators
« Reply #80 on: May 27, 2021, 11:37:15 pm »
@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.

John
 

Offline richard.cs

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Re: Noisy power from typical backup generators
« Reply #81 on: May 28, 2021, 06:41:21 pm »
It runs me about 7.5 US gallons of propane a month ~$30 with tax (in an expensive area).
(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.
 
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Offline Johnny B Good

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Re: Noisy power from typical backup generators
« Reply #82 on: May 28, 2021, 08:49:53 pm »
@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
John
 

Offline james_s

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Re: Noisy power from typical backup generators
« Reply #83 on: May 29, 2021, 12:06:20 am »
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.
 

Offline SeanB

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Re: Noisy power from typical backup generators
« Reply #84 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.

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.
 

Offline Renate

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Re: Noisy power from typical backup generators
« Reply #85 on: May 29, 2021, 11:49:31 am »
It runs me about 7.5 US gallons of propane a month ~$30 with tax (in an expensive area).
(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.
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.
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Offline NiHaoMike

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Re: Noisy power from typical backup generators
« Reply #86 on: May 29, 2021, 01:29:05 pm »
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.
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?
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Offline Johnny B Good

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Re: Noisy power from typical backup generators
« Reply #87 on: June 22, 2021, 10:19:36 pm »
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.
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?

 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. :)
« Last Edit: June 22, 2021, 10:25:08 pm by Johnny B Good »
John
 
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Offline james_s

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Re: Noisy power from typical backup generators
« Reply #88 on: June 23, 2021, 01:31:08 am »
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).


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.
 
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Offline richard.cs

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Re: Noisy power from typical backup generators
« Reply #89 on: June 23, 2021, 10:05:25 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.
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.
 

Offline Johnny B Good

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Re: Noisy power from typical backup generators
« Reply #90 on: June 23, 2021, 01:24:39 pm »
@james_s, I was addressing the idea he'd suggested in the last paragraph to eliminate the startup surge current of a conventional fridge by using an inverter genset that had a frequency and voltage ramp up feature (or more realistically, a converter module to perform the 'soft start' from a fixed frequency and voltage supply since there's no sane reason to incorporate such a feature into inverter gensets to begin with).

 His initial suggestion of a 2 phase VFD married to a 2 phase compressor motor would work well, but the common practice is to use a 3 phase VFD married to a 3 phase motor since you get smoother torque delivery using just three motor winding connections, the same number as used in the 2 phase case (3 phases, no neutral versus 2 phases plus a neutral).

 The linear inverter compressor is an interesting alternative to either of those systems in that it generates the pumping action directly, bypassing the mechanical complication of converting rotary motion to a fixed stroke length linear motion of the compressor pistons, allowing the inverter to drive this setup using only a single phase variable voltage and frequency output with even finer control over the compressor's output (and with less friction in the reduced moving parts count).

 The technology is proprietary and therefore expensive right now but no doubt, as production and competition is ramped up, it will become commoditised as the older fridges and freezers become obsoleted by government mandated climate change directives.

 I hate to be on the bleedin' edge of such innovative technology but it makes more sense to solve the emergency genset's inadequacies to handle the start up surge loadings of devices soon to become ancient history not by investing in a more powerful unit that may never be called into action but by eliminating the start up surge problem at its source by investing the extra cash in something that is guaranteed to be used from day one to provide better quality refrigeration at a greatly reduced energy cost, hence my (new found) interest in a better fridge/freezer rather than a more beefy emergency inverter genset. :)
« Last Edit: June 23, 2021, 02:30:47 pm by Johnny B Good »
John
 

Offline Johnny B Good

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Re: Noisy power from typical backup generators
« Reply #91 on: June 23, 2021, 02:27:29 pm »
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.
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.

 I didn't think they used the R/L phase shifting technique in domestic fridges (or even that it was "A Thing") hence my DDGing to find this description on the following page:-

https://www.theengineeringknowledge.com/starting-methods-of-single-phase-induction-motor/

 It (described as a Split Phase Induction motor on that page) offers the lowest startup torque option second only to the shaded pole single phase induction motor typically use to drive zero torque startup fan loads. I'm surprised it's used at all with a potentially high start up compressor load. The more popular system is the switched start winding capacitor, with or without the smaller run capacitor.
John
 

Offline richard.cs

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Re: Noisy power from typical backup generators
« Reply #92 on: June 23, 2021, 02:43:00 pm »
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.

 I didn't think they used the R/L phase shifting technique in domestic fridges (or even that it was "A Thing") hence my DDGing to find this description on the following page:-

https://www.theengineeringknowledge.com/starting-methods-of-single-phase-induction-motor/

 It (described as a Split Phase Induction motor on that page) offers the lowest startup torque option second only to the shaded pole single phase induction motor typically use to drive zero torque startup fan loads. I'm surprised it's used at all with a potentially high start up compressor load. The more popular system is the switched start winding capacitor, with or without the smaller run capacitor.

I agree capacitor approaches are superior, I'm simply saying that I've not encountered a fridge compressor that uses one (out of a very limited sample size of three  :) ). I guess if time is allowed for the backpressure to bleed down then the actual starting torque requirement is quite modest as many rotations will be required to build pressure back up. Presumably capacitorless approaches are simply cheaper.
 

Offline james_s

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Re: Noisy power from typical backup generators
« Reply #93 on: June 23, 2021, 06:20:55 pm »
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.

Now that you mention it yes, our fridges are like this too. Air conditioners on the other hand typically use a PSC motor in the compressor. I'm not sure why this is, maybe it has something to do with the fact that AC compressors are much larger and more powerful than domestic fridge compressors.

Whatever the case, my little 2kW inverter generator has no trouble starting my fridge, even with the eco-throttle enabled.
 

Offline Johnny B Good

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Re: Noisy power from typical backup generators
« Reply #94 on: June 23, 2021, 08:49:57 pm »
@richard.cs

 Well, quite honestly speaking, our Whirlpool fridge (model # ART417/G) could be using exactly the same cheap split winding starting technique for all I know. :-//

 I've always assumed they all used the capacitor start method (with or without the smaller value run cap). Whichever method is employed, they always use a thermal cut out to guard against excessive back pressure in the event of an interruption mid active cycle (or the result of the refrigerant being displaced during transport).

 I found that allowing three minutes delay before reconnecting it to the genset after a disconnection in order to repeat each startup surge current test was enough to avoid putting the thermal overload device to the test. If you have to transfer to another mains outlet, unless you're certain the compressor wasn't running, it's a good idea to wait another 3 to 5 minutes before powering it up again since we're dealing with a device not intended to deal with such events on a regular basis, built down to a price and designed to "Fail Safe" i.e. fail open circuit.
John
 


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