6 ms·

I would expect a refrigerator that has EC motors running the compressor(s) and fan(s) to be around 2-2.5x as efficient as one with fixed speed motors, based on

I would expect a refrigerator that has EC motors running the compressor(s) and fan(s) to be around 2-2.5x as efficient as one with fixed speed motors, based on what I know about variable frequency drives and three-phase induction motors. For those, 80% speed uses 50% of the power, 63% uses 25% of the power. For an 1800 rpm motor that is 1440 rpm and 1134 rpm. VFDs work well for most applications with variable torque (fans and pumps), but applications requiring constant torque (saws, grinders, etc) are better served by fixed speed starters.

amluto 1y ago
> variable frequency drives and three-phase induction motors. For those, 80% speed uses 50% of the power, 63% uses 25% of the power. You’re presumably thinking of the “Affinity Laws”, which, according to Wikipedia (and plenty of other sources), “apply to pumps, fans, and hydraulic turbines. In these rotary implements, the affinity laws apply both to centrifugal and axial flows.” This is, IMO, one of the worst kinds of science writing. Wikipedia, and plenty of other sources, make little mention of when the do and don’t apply or, relatedly, why they’re true and why they can’t always be true. They generally apply to situations where a pump is pumping fluid through something like a filter or a long pipe where the pipe is a closed loop or at least the ends are at the same elevation (e.g. a swimming pool pump, except when pumping from a pool into a higher hot tub). So you have no actual work being done by moving fluid, and you can run the pump slower, and thus move less fluid per unit time, thus reducing friction in a manner that the pressure that the pump needs to overcome goes all the way to zero as the flow rate approaches zero. But the affinity laws are not really anything fundamental about pumps, and they certainly do not override conservation of energy. Now consider a refrigerator. The compressor is pumping refrigerant from an (approximately) fixed low pressure to a fixed high pressure. (The fluid goes back from high pressure to low pressure via a capillary tube or expansion valve or similar lossy device -- it gets its pressure increased in the gas phase and decreased in the liquid phase.) There's some friction, but after subtracting friction, the pressure is independent of flow rate, and thus the work done per unit flow is independent of flow rate, and the pump power scales linearly with flow as opposed to super-linearly as the affinity laws suggest. Also, the compressor is a positive-displacement pump, and the affinity laws don't even pretend to apply to these. (A well pump is another common system where the affinity laws will lead to nonsensical results. If you want to size a well pump properly, you need to know the height that you're raising the water, the output pressure you need, and the range of flows that you want. And then you look at the actual measured performance curves of the pumps (and their drives) that you are considering, and you pick something appropriate.) All that being said, variable-speed fridges exist, and they're kind of nice in that they try to run continuously and quietly instead of alternating between full-power (and loud) and all the way off. And they are probably a bit more efficient because there's less friction and because the motors are likely to be more efficient three-phase designs instead of the not-actually-amazing single-phase motors you'll find in older fridges.
- stevesimmons 1y ago
  This is the kind of comment that makes HN special and precious. Thank you.
- bombela 1y ago
  > variable-speed fridges exist, and they're kind of nice in that they try to run continuously and quietly instead of alternating between full-power (and loud) and all the way off. Modern continuous variable speed compressor fridges drive me absolutely crazy. They sound like two ceramic plate rubbing together with some maddening flutter. Some also add incredibly annoying high pitch whines. That seemingly nobody seems to notice but me. In the same vein as coils whine from power supplies and other modern electronic. Old bang bang fridges are loud, on lower frequency, and with a sound that is more consistent and stable. Not varying one second to the next, which I find easier to ignore. I have started looking at how reasonable it is to move the compressor of my expensive and low quality 2025 fridge across the wall into the garage (refrigerant capture and refill, brazing new lines etc).
  - axiolite 1y ago
    > Some also add incredibly annoying high pitch whines. That seemingly nobody seems to notice but me. That drove me crazy for about a week trying to figure out what the noise was coming from... Pinhole water pipe leak? Cat stuck in the flue? Once I realized what it was, I didn't mind it much. It is better than loud old compressors suddenly kicking on and burrr'ing away then stopping.
  - dlcarrier 1y ago
    It's not just modern electronics. I used to be able to know, the minute I walked in, if there were any CRT TVs on in someones house. Flyback coil whine was a constant presence.
    - ewoodrich 1y ago
      Same. Sometimes I'd be sitting in a different room not exactly "hearing" anything except a vague sense of heightened perception and unease, and eventually decide to get up to check the TV room on the other side of the house and sure enough, someone left the TV on. The feeling of utter blissful silence afterwards in comparison was incredibly relaxing though, I do kinda miss that part...
rpcope1 1y ago
VFDs and inverters are nice, but have you ever looked inside of an AC unit with an inverter and asked what it would cost if the electronics has a problem? Simple dumb single speed stuff looks like it costs more to run, until you have a problem, which you will. On simple single speed fridges and AC that don't do anything special maybe you need a motor cap or motor or something every parts house has for little money and easy to swap; the moment you introduce more complexity is the moment you bend over for GE or Mitsubishi or Samsung to gouge you for basically any part.
themafia 1y ago
VFDs also fail about 2x to 2.5x as often as anything else in the system. In small commercial applications they often end up bypassed within a few years of installation of that equipment. They're also unusually good at catching fire due to the design requirements of the part. Properly sized multistage A/C systems are a much better idea.
- quickthrowman 1y ago
  You do see drive failures, but they’re less common than they used to be, almost every drive my guys install is an ABB drive and they’re pretty reliable. You are correct about soft-starters being a lot simpler and requiring less maintenance, as it’s more or less just another contactor inside a regular across-the-line starter with some extra control wiring to handle the extra contactor. Adding an inverter, rectifier, and solid state electronics does make the complexity much higher.
bdavbdav 1y ago
I suspect a cheap fridge is still a fixed speed compressor. Same with AC units - only the nicer ones are “inverter”

amluto 1y ago

> variable frequency drives and three-phase induction motors. For those, 80% speed uses 50% of the power, 63% uses 25% of the power. You’re presumably thinking of the “Affinity Laws”, which, according to Wikipedia (and plenty of other sources), “apply to pumps, fans, and hydraulic turbines. In these rotary implements, the affinity laws apply both to centrifugal and axial flows.” This is, IMO, one of the worst kinds of science writing. Wikipedia, and plenty of other sources, make little mention of when the do and don’t apply or, relatedly, why they’re true and why they can’t always be true. They generally apply to situations where a pump is pumping fluid through something like a filter or a long pipe where the pipe is a closed loop or at least the ends are at the same elevation (e.g. a swimming pool pump, except when pumping from a pool into a higher hot tub). So you have no actual work being done by moving fluid, and you can run the pump slower, and thus move less fluid per unit time, thus reducing friction in a manner that the pressure that the pump needs to overcome goes all the way to zero as the flow rate approaches zero. But the affinity laws are not really anything fundamental about pumps, and they certainly do not override conservation of energy. Now consider a refrigerator. The compressor is pumping refrigerant from an (approximately) fixed low pressure to a fixed high pressure. (The fluid goes back from high pressure to low pressure via a capillary tube or expansion valve or similar lossy device -- it gets its pressure increased in the gas phase and decreased in the liquid phase.) There's some friction, but after subtracting friction, the pressure is independent of flow rate, and thus the work done per unit flow is independent of flow rate, and the pump power scales linearly with flow as opposed to super-linearly as the affinity laws suggest. Also, the compressor is a positive-displacement pump, and the affinity laws don't even pretend to apply to these. (A well pump is another common system where the affinity laws will lead to nonsensical results. If you want to size a well pump properly, you need to know the height that you're raising the water, the output pressure you need, and the range of flows that you want. And then you look at the actual measured performance curves of the pumps (and their drives) that you are considering, and you pick something appropriate.) All that being said, variable-speed fridges exist, and they're kind of nice in that they try to run continuously and quietly instead of alternating between full-power (and loud) and all the way off. And they are probably a bit more efficient because there's less friction and because the motors are likely to be more efficient three-phase designs instead of the not-actually-amazing single-phase motors you'll find in older fridges.

stevesimmons 1y ago
This is the kind of comment that makes HN special and precious. Thank you.
bombela 1y ago
> variable-speed fridges exist, and they're kind of nice in that they try to run continuously and quietly instead of alternating between full-power (and loud) and all the way off. Modern continuous variable speed compressor fridges drive me absolutely crazy. They sound like two ceramic plate rubbing together with some maddening flutter. Some also add incredibly annoying high pitch whines. That seemingly nobody seems to notice but me. In the same vein as coils whine from power supplies and other modern electronic. Old bang bang fridges are loud, on lower frequency, and with a sound that is more consistent and stable. Not varying one second to the next, which I find easier to ignore. I have started looking at how reasonable it is to move the compressor of my expensive and low quality 2025 fridge across the wall into the garage (refrigerant capture and refill, brazing new lines etc).
- axiolite 1y ago
  > Some also add incredibly annoying high pitch whines. That seemingly nobody seems to notice but me. That drove me crazy for about a week trying to figure out what the noise was coming from... Pinhole water pipe leak? Cat stuck in the flue? Once I realized what it was, I didn't mind it much. It is better than loud old compressors suddenly kicking on and burrr'ing away then stopping.
- dlcarrier 1y ago
  It's not just modern electronics. I used to be able to know, the minute I walked in, if there were any CRT TVs on in someones house. Flyback coil whine was a constant presence.
  - ewoodrich 1y ago
    Same. Sometimes I'd be sitting in a different room not exactly "hearing" anything except a vague sense of heightened perception and unease, and eventually decide to get up to check the TV room on the other side of the house and sure enough, someone left the TV on. The feeling of utter blissful silence afterwards in comparison was incredibly relaxing though, I do kinda miss that part...

rpcope1 1y ago

VFDs and inverters are nice, but have you ever looked inside of an AC unit with an inverter and asked what it would cost if the electronics has a problem? Simple dumb single speed stuff looks like it costs more to run, until you have a problem, which you will. On simple single speed fridges and AC that don't do anything special maybe you need a motor cap or motor or something every parts house has for little money and easy to swap; the moment you introduce more complexity is the moment you bend over for GE or Mitsubishi or Samsung to gouge you for basically any part.

themafia 1y ago

VFDs also fail about 2x to 2.5x as often as anything else in the system. In small commercial applications they often end up bypassed within a few years of installation of that equipment. They're also unusually good at catching fire due to the design requirements of the part. Properly sized multistage A/C systems are a much better idea.

quickthrowman 1y ago
You do see drive failures, but they’re less common than they used to be, almost every drive my guys install is an ABB drive and they’re pretty reliable. You are correct about soft-starters being a lot simpler and requiring less maintenance, as it’s more or less just another contactor inside a regular across-the-line starter with some extra control wiring to handle the extra contactor. Adding an inverter, rectifier, and solid state electronics does make the complexity much higher.

bdavbdav 1y ago

I suspect a cheap fridge is still a fixed speed compressor. Same with AC units - only the nicer ones are “inverter”