Last week, I introduced Arduinos, and mentioned that I would be discussing my air compressor controller project using an Arduino. I will try to complete this series in 5-6 posts with this introduction, details about the hardware, and details about the programming.
Disclaimer: I am providing this information for educational and informational purposes only. Although this looks simple, working with 120V and 240V components and manipulating them with projects like this can be very dangerous, potentially resulting in severe injury, death, damaged equipment, fire, and will definitely void any warranty you may have on the item(s). If you wish to perform a project like this, seek assistance from an experienced electrician first.
So why am I doing this project, when you can just pick up a new stock controller (pressure switch) for $50 – $60? The standard pressure switch for my air compressor typically starts the air compressor when the air pressure drops below 90 psi, and stops when it reaches around 135 psi. It becomes somewhat problematic when trying to run air tools in the 110-120 psi range – when it drops below 100 psi, the air tool just doesn’t get the power it needs, requiring constant adjustment. Yes, you can manually set these pressure switches to cycle on sooner, but leaving it like that will drastically increase the start/stop cycles on the compressor, causing it to ware out much quicker.
So, for this project my goal is to create a microcontroller for my compressor that allows me to change the start/stop points of my compressor, safely. The first step in a project like this is to figure out how it works, so you can figure out what you need in order to control it. Normal household compressors are relatively simple – they use a 120 or 240 volt AC induction motor which are controlled by a mechanical pressure switch. The mechanical pressure switch is what I am replacing, so I won’t get into the other aspects of the compressor.
The motor on my compressor is a 240V motor, so it has 2 terminal leads and a ground. The ground is always connected, but the two leads are what the pressure switch controls – it is what my new controller has to do, switch those two terminal leads on/off to start/stop the compressor (it is possible to control it by switching just one terminal lead, but it is just not a good idea as one motor lead will always be ‘hot’).
Now that we know the basics for how to control the start/stop of this compressor, I’ll move on to safety. Most people should already be aware of the hazards of electricity, but this introduces another hazard – over pressurization of the pressure vessel (air compressor tank). I wish it weren’t so, but mechanical and electrical components fail – there is a possibility that the device we use as a switch could fail, leaving the compressor running, over pressurizing the tank, causing it to rupture.
The main key in preventing these bad things from happening, is by getting the properly rated equipment/material. In the image below, you will see my compressor motor is rated for 240V at 15 amps – The switch I use must meet or exceed this rating, failure to do so will definitely result in the switch failing. It is also important to use the proper internal wiring and any other parts/components that will support the voltage and current (amp) that will be flowing through it.
Preventing the over pressurization can be done with a pressure release – a valve held shut by mechanical means, such as a spring, that will open at a certain pressure set point to reduce the pressure. The safety relief must be rated for the proper pressure, and flow – it will do no good if the air compressor can supply more air than the relief can release. Most air compressors only output a small volume of air, so most pressure reliefs you find will suffice, just make sure. You can see in the picture below, that my air compressor is rated to output a volume of 10.3 SCFM at 90 psi (at a higher pressure, like 130 psi, it will output a lower volume). Also, it is important to note that the sticker on my compressor states 135 max psi, I looked up the details which state it is capable of 150 psi, and the tank is also rated for 150 psi. So, I need to get a pressure relief for 150 psi or less, that is capable of relieving 10.3 SCFM or more.
Next week I will continue posting about this project, moving into discussing the hardware I’ve chosen for my controller. Thanks for reading and following my blog! Please leave comments, and check back next week for more!