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Noisy power from typical backup generators
Johnny B Good:
@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. :)
Johnny B Good:
--- Quote from: richard.cs on June 23, 2021, 10:05:25 am ---
--- 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.
--- End quote ---
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.
richard.cs:
--- Quote from: Johnny B Good on June 23, 2021, 02:27:29 pm ---
--- Quote from: richard.cs on June 23, 2021, 10:05:25 am ---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.
--- End quote ---
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.
--- End quote ---
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.
james_s:
--- Quote from: richard.cs on June 23, 2021, 10:05:25 am ---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.
--- End quote ---
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.
Johnny B Good:
@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.
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