dr bob

For those playing along at home... The capacitor-start motor on Dean's compressr likely has "start" windings used to get initial rolloff and low speed operation. There's a centifugal "speed switch" that disengages those "start" windings and engages the "Run" windings. Trying to run that motor on a too-small generator will likely result in the following. With no load, the generator engine is running at 3600 RPM and a very small throttle opening. When the compressor starts, that initial "inrush" load used to accelerate the compressor from stopped is seen by the generator as it's speed is reduced. The governor on the generator adds throttle opening to try and get the speed back up to 3600 RPM. The reduced speed on th generator actually reduces the target speed on the compressor motor. With less energy available and a lower target speed, the compressor motor accelerates more slowly, and herefre remains trying to roll up on the "start" windings for a longer time. The "start" windings are drawing more current than the generaor can produce, and at the same time the amount of current in those windings is high. From that point it's a drag race to see if the overcurrent protection (the circuit breaker on the generator) opens before the over-temperature protection on the motor is tripped. The situation is more acute when there is pressure in the tank, since the compressor wil accelerate even more slowly with the higher initial load. It's possible that the compressor may start OK when there is no residual pressure in the tank, but fail to start when the compressor tries to cycle on again after pressure drops below the pressure switch set pressure of 125 PSIG.


Dean, my thumbnail guesstimate is that you'll need a generator with at least 18-20HP to reliably start that compressor when it's loaded with air. I had a 20KW natural-gas-fired standby generator at the last house that was barely adequate for starting one 5HP AC compressor with its fan and blower motor running and less than 2KW additional house load. I had to put a time delay feature on the compressor so all the other high-inrush loads (refrigerators, etc) would be completely started before the AC compressor was even allowed to try to start.

Were it my situation, I'd try to negotiate with the landlord to pay some of the electric bill for the 240V. It wil be cheaper than just the cost of gasoline for the generator. Next would be a smaller 120V compressor, the biggest oil-type unit you can find that will run on your 120V circuit. If your landlord was stingy with power to start with, verify that you have a dedicated 20A circuit you can use, or adjust your compressor purchase accordingly. I'd plumb the smaller compressor to use the bigger tank through an isolation valve, and plan to get coffee when the big tank gets low. Next option is a gasoline-powered compressor. The maintenance on the engine and compressor together makes this less than desirable. But it may be better than buying a new generator big enough to run the existing compressor. Last, if you do end up running it off a generator, get one big enough to do the job running on natural gas if that's available in your shop. You can add conversions to smaller generators for NG operation, or just get one big enough that's designed for NG use. You can use it at home with a transfer switch in weather emergencies, if you have NG available there too. None of these options is cheap though.

Dean_Fuller

Thanks dr bob,

Found my generator...except mine has wheels.

http://www.brandnewengines.com/30430..._DsaAqvE8P8HAQ

But it sounds like this is not going to work.....

The generator / compressor will be used VERY little really. Certainly not every day. NO other draw would be added to the generator while the compressor was plugged in. Not sure if that makes enough difference.

No gas lines available at the shop.

Dean_Fuller

http://www.brandnewengines.com/30430..._DsaAqvE8P8HAQ

What if I had 2 generators like the one listed above??? Is there a way I could wire BOTH gens to run the compressor??? My brother has one just like mine. Again....its not for a lot of continuous use. If that might work how would I wire it? Thanks so far for the advice all.

dr bob

Unfortunately there's no good way to synchronize the two generators so they are in phase with each other as you close the circuit breakers. There are some consumer-class generators like the Honda EX line that are inverter-type, and have the capability of synchronizing and load sharing with a connecting cable. They can get pretty spendy when you think about buying a couple and making the connections and all.


For those who might be curious, trying to synchronize two generators like this is a chore. The way old-school method involved a couple light bulbs that would go completely out when the two lines were approximately in phase. More modern methods use a "sync scope" to display the phase relationship, and have sophisticated protective relays to prevent connections when the phase relationship isn't correct. The price of failure is meltd wiring at minimum, since the circuit breakers on these generators are way too slow for this kind of protection. With the amount of available short-circuit current, there would be significant local gas ionization, expansion, and heat. (read: explosion). You'd get hurt.

Dean_Fuller

Got it. thanks...I see that will not work.

dr bob

Dean--

What power do you actually have? Some places give you one or maybe two 15- or 20-amp circuits for lighting and tools for a space that size. When I rented utility/shop space way back when, I had a 100A service installed for the space (about 1500sqft) and easily managed everything from that. Don't know if that's an option. My local utility was happy to pull the feeders to the service that I purchased and mounted. I put the compressor right next to that, along with a welding outlet, so the wiring would be short, and everything else was 120V for outlets and lighting. Knowing what I know now, I should have put in HVAC since I had gas service too. But I was young and immortal at the time and the SoCal "cold" never really was. AC would have been real nice though. A lot depends on how long you plan to use the space. All those "improvements" stayed behind when I gave up the lease, but I was there over ten years so it was easily worth the investment.

Speedtoys

That doesn't solve the inrush problem.

These generators are only delivering what they need to, they are not pushing out what they CAN at idle.

Two gen's at idle still dont have the inrush required to do what you need.

And as noted, phase is a HUGE problem, you'll start something on fire doing that wrong pretty fast.

You have the wrong generator, you need a 120v unit.

dr bob

I think this is what Jeff meant to say.


I have a 5hp C-H oilless rotary-vane style compressor with a 30 gal tank that is very loud and under-delivers on air for the noise it generates and the power consumption. Meanwhile, a measurement of actual electrical load shows that the "5 horsepower motor" is really a lot closer to 2.5 to maybe 3 horsepower running. There are a few times like painting where the compressor runs almost continuously, other times like car detailing and tire inflating where I have to pressurize the tank, yet use only a small amount of the air. So I'm looking at smaller piston-type compressors for most-of-the-time duty. I've nailed it down to a few 2.5 - 3 horsepower units intended for the construction guys who run nailers/staplers/etc. Gotta decide between Makita and maybe Senco. They are well-rated by Amazon users, all run on 120V, and don't take up a lot of room. I figure I can valve the 30gal tank in and out if I have a medium-size project and power up the bigger one when I need it for painting. Look at your actual duty, and consider that one or two of these smaller 120V compressors would do the duty for you, assuming you have enough 120V available to run them (two separate circuits). Valve the big tank in when you need it for painting/bodywork stuff, just pressurize it with the smaller units. For the rest of the time, one of the smaller ones alone would do the trick for casual needs.