We often think of compressors as single-state devices. They’re on and they’re off. This holds true for most window units, portable indoor units, and even a number of central cooling systems. This simplifies things and lets us save power. Compressor on, hit target temperature, shut down. It’s better than running the compressor 24/7 and achieves the desired result.
The problem with this is that we have to constantly cycle. Imagine if your car had 1 speed. Instead of a gas pedal, you just had a button. Push the button, your engine revs itself through the roof, you do a burn out from your driveway, let off the button to avoid the kid running into the street, and you come to a dead stop in seconds. That’s how a single-stage compressor works and it’s a little wasteful.
With this back and forth, on and off approach, you can never actually hit a precise temperature. You just hit a temperature range. When the thermostat is set to say 70 degrees, the air conditioner kicks on and runs until the room is approximately 65 degrees. Later, when the room has warmed to 75 degrees, it turns back on again.
If we were to run the compressor at 71 degrees, it would be too soon, we’d be constantly starting and stopping. The air conditioner only spits out one temperature of air: really cold. If you run at 71 degrees, by the time you’re out of the start up on the compressor, it’s going to be 69 degrees,you’ve over shot. Even worse, to close that two degree gap, you were using the starter-capacitor, which creates a ton of torque and sucks down a lot of watts. It’s not efficient. This “one size fits all” throttling is only viable if we wait until we’re well outside our target temperature so the compressor gets a chance to run at its most efficient speed.
Adding Another Gear
Motors however, are not limited to one speed. Just look around your house and you’ll find multi-stage electric motors everywhere. The windshield wipers on your car, the motor in your blender and mixer in the kitchen, and even the motor on that nice inground pool out back are all often multi-speed motors. It’s not a hard thing to do, we just need to add some electronics and tweak the design a bit.
This gives us a multi-stage compressor. By adding a stage, we’re able to have a low and high “gear.” When you come in from a hot day and turn the system on, it kicks straight to high gear and turns the building into a nice representation of the north pole. After the target temperature is reached, we can kick back to a low speed or shutdown altogether.
When it comes time to run again, we’ll start in low speed and inch our way back to target temperature from a narrower target range. With this lower speed, we can produce warmer, but still cool air and more gradually adjust the temperature. The system can be more precise. Rather than unleashing a blizzard, it’s feasible to cycle within a degree or two of target temperature.
Ordinarily, this might be a problem as we’re now running potentially longer, more frequent cycles, but there is a difference: we built this compressor to do just that. The low-speed uses less power than high speed, can be started into more quickly, and can run on this cycling without difficulty. Rather than run all this power for start up and full power, we run less power for a lower speed we more easily attain. We cool just as much as we need to and ultimately improve efficiency.
In our car analogy, you now have two buttons. A nice, parking-lot cruising speed and “hold on for dear life on the interstate.” Not quite perfect yet, but better than toggling between on and off.