Why must an A/C compressor wait a few minutes after shut-off before it can start again?
In: Engineering
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A motor has inrush current, to go from 0 RPM to its rated speed. This can be 3-5x what the motor is actually supposed to draw. If it has to push against a significant load, that current will be even higher and may trip breakers.
Letting the system sit for a little bit will allow the pressures to equalize, reducing the amount of work the motor has to do to start.
HVAC systems work by pumping a fluid around a loop of tubing. In this loop is a pump and a tiny hole called the orifice. The fluid in this tube is normally a gas, but if it is put under enough pressure, it will condense to a liquid. It is important to remember that from a physics perspective, bot liquids and gasses are considered fluids.
The pump in this loop tries to push the fluid through a small hole. As this happens, pressure builds up, the fluid is compressed, and eventually condenses to a liquid. It travels down the tube to the orifice (small hole) where it escapes into a larger tube.
There is more space in the larger tube, so the pressure drops rapidly. This causes the fluid to quickly evaporate from a liquid to a gas state. Because pressure and temperature are linked, the fluid rapidly cools off when this happens. Blowing air over the tube full of cold fluid (gas) is how our A/C makes the air cold.
Now back to the original question: Why does the pump need a break when it shuts off?
When the pump is running, it’s building up pressure on one side. When the pump shuts off, the pressure on that side actually levels off, and is much lower. So if the system is sitting for a while, starting the pump happens when there isn’t much back pressure.
But if you try to start the pump right after it has shut off, there is already quite a bit of pressure on one side. This makes it extra difficult to get started.
Think of it like trying to get a wheel barrow full of dirt rolling on level ground versus starting uphill. It’s much easier to get started on level ground, *then* go uphill.
There are two other factors as well. When you start an electric motor from a full stop, it requires an extra “kick” to get going. This extra kick places a lot of strain on the motor. Ideally, you don’t want to do this when it is already hot. Since the motor gets very hot as part of its normal operation, it’s a good idea to let it cool down again before you try and start it.
So you have:
* A pressure build-up that makes it difficult to start.
* A motor that is already hot and would be placed under additional stress when starting.
By letting the system sit, you let the pressure level off and the motor cool down.
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