I picked up a couple of compressors, one is electric the other gas. The electric starts up fine from my temporary power source a hundred feet away with a 100 ft 15 amp extension cord but as soon as it needs to automatically re start itself cause the air pressure drops it will not start, it just sits there making an unhappy noise until it blows a fuse or damages the outlet it’s plugged into.
What might be the problem?
It operates fine when plugged directly into the same outlet.
Hi,
There are a couple of problems with your set up. Here are some numbers to help illustrait why.
The compressor motor is rated 1.5 HP. In electrical terms this is 1.5 hp x 746W/hp or 1119W (notice how the "HP"s cancel).
To accomidate the starting torque and initial head pressure you have to triple this, 3 x 1119W = 3357W. ( Its a long story where the 3 comes from but it is a minimum value )
I assume the outlet is 120V but if not just adjust this equation. Cord Current = 3357W/120V = 29.97A
This is why it pops the breaker which is likely 15-20A rated.
It takes approximately 29.57A to start the motor/compressor every time it kicks on so technically the cord should be #1/0 AWG otherwise there will be losses. I won't get into the details about what makes up the losses but it is effectively the resistance in the wire x2 (out and back) plus the connections on both ends.
Where your cord is undersized the losses are greater so the voltage droop at the compresssor is much lower than 120V. This is why it won't restart but worse as it tries to restart the currect goes ever higher than the ~29.57A because the voltage is low. Because the cord and connectors are now operating above their rating so, both are getting hot and this is why the connectors are melting and possibly the wire probably inside the casing. It gets worse. The more you run or try to run the compressor the hoter the cables get only compounds the situation. As the losses increase it becomes even more difficult to retart. Overall the situation can become hazarduous because it can start fires or potentially shock someone if it a wire melts through to the surface.
Going back to the beginning and the name plate rating on the compressor motor. We calculated that the compressor needs 1119W of power to operate so, when normally running the motor consumes 1119W/120V = 9.325A. The cable size to support 9.325A at 100 ft is #4 AWG. If the losses are low enough and the compressor can start quickly the period over which it draws 29.57A is significantly reduced. In other words the period when the voltage droops is shorter and not so deep giving the compressor a better chance to start. I didn't mention it earlier this when things heat up they include the motor which takes time to cool back down. With the cord you have now the motor will fail in a short time. The wires just fry.
So, @ 100' #4 cord will get you by but the motor's life may be shortend to some extent, its hard to say how much because there are other factors involved as well.
I get it though, 100' #1/0 cords are astronomically expensive, #4s not much better, even #10s but physics dictates the reality of it and its up to us to make reasonable tradeoffs that won't kill us or the equipment. Compressors are somewhat unique because they are cyclic and one of the most difficult to start repetedily.
Maybe you could move the work a lot closer to the outlet with no cord. As you mentioned it works great there. The manufacturer selected the attached cord length to match the motor's rating and they are not meant to be extended unless you select the extension well. Generally speaking. A good general rule of thumb is to x1.5 the initial cord size AWG for each cord length you add to account length and losses some of which I haven't mentioned. In any case this estimate shows that the cord gets big fast with quick math.
So, I hope this helps.
To just simply recap all of my ramblings. Small cords are always fed by big cords and how big is determined by the length of the run at a given current.
I hope the simplified math helps everyone understand what is going on and why. This is a huge problem in more ways than you can imagine and in places you might not expect. It's not always understood well by some professionals so don't feel bad for not getting it.
