Power draw on compressor

[Gene Michael]

I used to have a Porter Cable pancake compressor (it died after 2 years of medium use) that I could hook up to my Honda 2000 generator. Placed everything on a utility trailer and use the rig to do chores around the farm in areas with no electricity. When the PC died, bought a Sears unit with an oil lubricated compressor. It's rated at 10.5 amps; the PC was rated at 10. The generator never had a problem with the PC, but won't drive the Craftsman. The compressor kicks on, but labors at a very low and erratic speed, warning lights flash on the generator and the overload protector trips (this all happens in less than 2 seconds). Does anyone have any ideas what may be causing this? Any and all feedback will be appreciated.

[Joe Jensen]

It seems the new compressor is drawing more amps than the old one. I suspect the old compressor was really drawing less that 10 amps and it was rated for more as a way to make it look more powerful. It's possible that the new compressor draws more than the rating, but that would be a safety hazard and I doubt an manufacturer would do that...joe

[Tom Veatch]

Just kinda thinking out loud, but maybe the generator can't supply the start surge requirement of the new compressor. Does the compressor motor dataplate list the Locked Rotor Amperage (LRA or LRC). That's what the generator will have to supply for a short period of time until the compressor motor comes up to speed. That should only be for a very short time but if the compressor is extra hard starting, maybe it lasts long enough or is high enough to exceed the generator surge rating.

I assume the compressor has a functioning unloader valve to reduce the startup load on the motor. Plugged into line voltage does the compressor give any indication that it's having a hard time starting? I've also seen compressors act like that when the start capacitor is bad - and I've had motors in which the start cap was DOA.

Assuming the surge rating on the generator is sufficient to meet the compressor's requirement, my first two thoughts are bad unloader valve on the compressor or bad start cap on the motor.

[Joe Jensen]

Tom has a very good point. Do you hear a Hiss sound when the compressor stops (this is the unloader valve releasing the pressure in the cylinders)...joe

[Charles Lent]

My son and I just tried to do the same thing yesterday with a 4,000 watt generator and a 2 hp motor driving an 8 cfm compressor. It could start and run the compressor, but only if there was no pressure or a very low pressure in the tank. After a bit of experimenting we discovered that the motor doesn't come up to full speed because of the sudden pressure build-up in the compressor's output line. The unloader empties this pressure but it builds too quickly when the compressor is starting, causing the motor to never reach full speed. It's quite obvious that the generator doesn't have enough reserve to handle the startup requirements of the motor, but seems to be able to easily handle the load once the motor has reached it's full speed.

Since we really needed to make this arrangement work without buying a bigger generator or a smaller compressor we decided to try to make an accumulator out of a 12 inch piece of galvanized 3 inch diameter pipe, an end cap, and a 3 to 3/4 reducer to close the ends. Then connect this assembled accumulator into the line running from the compressor to the tank inlet. We believe that this will provide enough delay in the pressure buildup to give the motor sufficient time to reach full speed before it has to drive the load.

Since we haven't built and tested this idea yet I'll have to get back to you with the results.

Charley

[Pat Germain]

I'll be master of the obvious and ask if you're using an extension cord on the compressor. Many compressors won't run on an extension cord even when plugged into a standard outlet.

[David G Baker]

My compressor works on an extension cord that is 50 feet long, the cord is #10 wire. I have run it on my 25 foot #12 wire cord but if I use it for a long time the cord gets a little warm.