[londons_explore]

> practically cause brownouts on the electrical circuit

That's because a compressor has an odd property... The motor requires substantially more starting torque than running torque. That's because the cylinder can be stuck on the compression cycle pushing against a cylinder of gas, and has no momentum to get it through. Also, the very first compression typically has far more gas in it (because the 'cold side' is actually warm before it starts).

If nothing were done to compensate, the motor would stall, overheat, and burn out.

Designers of fridge compressors have instead added a very high current starting coil, which will provide massive amounts of torque to get the thing going for a few seconds. Sometimes that might be 20 amps or more for a few seconds. When the motor is spinning fast enough, that coil gets disconnected.

[jcims]

That starting coil can get stalled too, esp if head pressure is still very high. I did HVAC 30 years ago and back then you could reliably thermal-trip residential and commercial systems by simply turning them off then back on within 10-15 seconds. Motor would just hum for a few seconds then 'click. We'd always have to let it equalize for a minute or so before turning back on.

Not sure if that applies to newer systems.

[rainbowzootsuit]

The gas distribution part can be significantly compensated for by "unloading valves" of some kind—this is very common on piston air compressors—but with unloading, a major component of the starting torque is overcoming the angular momentum of the motor and compressor assembly. This is expressed as Locked Rotor Amps (LRA) on a compressor's spec sheet. There are devices that can limit the inrush current to about 25% of the listed LRA.

On most air conditioning compressors today, a scroll plate compressor is used, which doesn't have the unloading problem of the piston type of compressor.

[jcims]

>On most air conditioning compressors today, a scroll plate compressor is used, which doesn't have the unloading problem of the piston type of compressor.

Just replied to the same comment wondering if this has changed. I used to help my dad install residential and commercial HVAC systems and all of the compressors back then were piston type. I would tick him off by getting impatient when we were testing by calling for cool too soon after shutting it down and tripping the thermal beaker. (Come on dad it's hot up here!!! lol)

I imagine the scroll type are probably quieter as well and probably work nicely with inverter-based drives.

[rainbowzootsuit]

Yes, they do tend to be quieter, and as far as I know are the only kind on inverter drive AC compressors. It's also a continuous process without having peaks and valleys in the motor load which probably lends itself to work well with an inverter drive, but I'd have to think about it a little more to say if that's accurate.

I did go back and look that there are sometimes valves used in scroll AC compressors. Even with that the full compression cycle is much more gradual, and continuous happening over ~3 revolutions of the compressor instead of half a revolution for a piston so there's not a slug compressed gas to hold the compressor back in the same way as a piston compressor.

This pages has some diagrams that shows the spiral plates and compression cycle pretty well. https://www.sciencedirect.com/topics/engineering/scroll-comp...

[c_o_n_v_e_x]

I did HVAC work in the late 90's till the late 00's. At that time, only the really expensive, high SEER rating units were variable speed. Cheaper 10-13 SEER systems had scrolls but they were fixed speed.

[jcims]

Awesome, thank you!