The lights dim when my air compressor starts. Not only does this happen in my workshop, but my wife says it happens throughout the house, too. What’s with that?
So goes a question that a reader posted recently. Why do the lights dim when my air compressor starts?
The Simple Answer
If the lights are now dimming when your air compressor tries to start, it means that the compressor motor is pulling more amps than normal.
So much so, that the compressor motor is actually draining the electrical power supply line, and in so doing is starving other electrical devices that are connected to the same power circuit, in this case, the lights.
Actually, all appliances that are connected to the same circuit are being under-powered if the lights dim. Not good for other appliances that have electric motors, for sure.
The More Complex Answer
Let’s look at the typical reasons a compressor motor is pulling more power than usual when it tries to start
- pump problem
- unloader not working
- start capacitor failing
If there is a mechanical problem with the pump that problem may be adding greater load to the motor, meaning that the compressor motor has to work harder to start.In so doing, the motor pulls more amps, and that drains the household circuit.
Compressor pump problems may manifest themselves by unusual noise in the pump while the compressor is running. A mechanical problem with the compressor pump is one reason, but perhaps not highest in the probability scale. For that, we need to look to the unloader.
Unloader Not Working
If your compressor is equipped with an externally mounted pressure switch, that switch likely has an unloader valve incorporated into it, much like the one in the image below.
Check this great switch here; Pressure switch for air compressor made by Furnas / Hubbell 69JF7LY 95-125 single port w/ unloader & on/off lever
On the left of this Hubbell pressure switch above you can see a brass device. That is the air line connector for the unloader valve on this particular pressure switch.
The unloader valve is there to allow any air pressure that may be trapped over the compressor piston when the compressor stops to evacuate. Pressure trapped over the piston adds load to the motor on start up.
If the unloader valve on the air compressor is not working, when the compressor stops on high pressure cut out, there may be air trapped, and then, when the compressor tries to start again, the added load of the trapped air causes the motor to pull more amps, and the lights dim.
If this is happening, the motor may not start at all. The result may be a tripped breaker or blown fuse.
Start Capacitor Failing
If your air compressor motor comes equipped with a start capacitor to help the motor start, and that capacitor is failing, then the motor may dim the lights when it tries to start.
Buy this capacitor here
Other pages on this site discuss how to check the capacitors, if your compressor motor has them.
That would be on of the first checks to make, if you too are one of the compressor owners that complains, the lights dim when my air compressor starts.