Variable Speed Compressors

Multi-speed compressors are a step in the right direction, but what about outright variable speed ones? Think about it, a two or three speed compressor has to pick the best speed for a given environment and it can only run at those two speeds. When it’s incredibly hot, full speed ahead. When it’s cooler, low speed. In between however, it becomes a game of the lesser evils.


Fixed-Speed Flaws

This fixed-speed issue can be a bigger problem in the right scenarios. There’s only really a handful of conditions where the compressor can hit peak efficiency. You can crawl around the parking lot or go at highway blazing speeds, but you have no support for those middle roads between town. The same is true of the two speed compressor.

The system can be forced to oscillate between stages or even be trapped in a single stage if the conditions are right. When it’s hot out, you’ll never get to use that second, lower speed. When it’s in between, the low speed might not be able to keep up and it’ll have to cycle between speeds, never really saving anything or improving comfort.

It’ll almost always be this battle of the lesser evils: power hungry high speed and be too cool or have to work on wider margins of error, or fighting a losing battle with the low speed constantly running and only delaying the inevitable need for the high speed setting. There needs to be an in-between, or even, a near infinite number of in-betweens.


Variable Speed Drives

It turns out, we have a solution for this. Manufacturing and high end industries have used variable speed drives for decades. These are motors which have drive controllers that allow a near arbitrary output speed to be set. These drives can crawl along or run at ludicrous speeds. They’re used in cars, lathes, and even washing machines. It’s industrial technology becoming usable in small business and consumer applications.

In practice, this allows us to have an air conditioner create exactly the right amount of coolness to reach and maintain a preset temperature. We have a throttle where we can perhaps run the machine between 30% and 100% speed to achieve a desired outcome. Going lower than a certain minimum threshold would likely create too little pressure and compression to matter, but above that, we have free-reign.

These types of systems use the amount of power necessary to get the job done at any given time. This can mean longer cycles, running near continuously at times, but they sip power while doing it. It operates to match the load and demand in real-time.


The Drawbacks

The pitch of these systems is to save on electrical costs, but there are other costs to consider. A variable speed motor requires some more expensive design and parts to actually work. Your standard compressors have just massive relays to turn them on and off. There might be some extra relays to include a hard-start capacitor, but that equipment is downright simple compared to variable speed systems.

In order to control the speed of these motors, there needs to be an extra, dedicated controller and sensors inside the motor itself. By varying the electricity going to the motor and measuring the output speeds and electrical output, we can precisely control speed. This is all done in a special controller, that can handle all the input power of a compressors’ relay, but mostly in solid-state chips.

These extra electronics can drive up repair costs significantly. Instead of paying perhaps up to $100 for a simple, but beefy relay, you could be charged hundreds more for a purpose-built and programmed drive controller. And once you’re using a variable-speed system, there is no going back either, without replacing the entire unit and potentially modifying your thermostat’s wiring to the new unit.

The electrical savings and improved comfort are likely worth it in the long run, but it’s important to remember more performance like this does come with a premium attached until this becomes the industry-wide, standard way to build air conditioning systems.

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