generator power help

[eyeballengineer]

I have a 7 hp.220v 80 gallon air compressor that I need to run off a generator. I tried to run it from a 5500 watt generator and it would run up to pressure but when it came time to cycle back up the motor didn't want to spin and would kick the breaker. So I assumed I needed a bigger generator. Now I have a bobcat 225 welder generator 8500 watt diesel and the compressor does the same thing when it comes time to cycle. The start and run capacitors test good in the compressor motor.

Is this still not enough generator? Could there be another issue? Is there another way around it? Add more or bigger capacitors?

 

[pattenp]

The momentary high amps on start up is exceeding the gens output capacity. A 7HP compressor should be on a 40A circuit and can draw 3 times the running amps on start up. So you could be pulling at least 60A or more on start up. A 8500W Gen is only giving you aboout 35A peak.

 

[rlitman]

Just to check, your unloader is working properly, right?

 

[eyeballengineer]

Just to check, your unloader is working properly, right?

The head pressure release on the compressor head? If so then yes

 

[pattenp]

Starting off with zero pressure in the tank vs. starting with a 100 PSI or more in the tank is enough to increase the motor load to trip the breaker.

 

[theoldwizard1]

Cold motor versus warm motor ?

Buy a 5hp motor and a properly resized pulley and sell the 7hp. If you are running if off of a generator, I'll bet you don't have a need for that much volume anyway.

 

[larry4406]

Any chance compressor has a continuous run feature for the motor?

 

[eyeballengineer]

Defiantly need the volume my 13 gallon 4 horse won't run even 1 da sander.

No continuous feature **** that gives me an idea. Maby a couple 120 blow off valves might keep it from cycling

 

[jkwilson]

If this is a long term need, why not invest in a gas powered compressor?

 

[ishiboo]

If you are close, adding a soft start device might get you there.

 

[eyeballengineer]

If you are close, adding a soft start device might get you there.

? More info please

 

[ishiboo]

? More info please

http://en.wikipedia.org/wiki/Motor_soft_starter

They are commonly used with air conditioners, as lots of people have air conditioners and more limited power options. Many generators like the Hondas will easily run a large BTU air conditioner, but cannot start it without a soft start setup.

It's pretty simple... you slowly ("softly") ramp up the motor instead of going instantly from off to on, since you have plenty of current to run the motor but not enough to start it full blast which can be many times the running current.

A true 7hp is going to be a bit big for most soft start setups you'll find, it may be easier/cheaper to get a VFD to do the task if they have a ramp up rate option?

 

[theoldwizard1]

Defiantly need the volume my 13 gallon 4 horse won't run even 1 da sander.

1. How many DAs are you trying to run simultaneously ?
2. What was the bore & stroke of the old compressor head compared to the new one ?
3. An 80 gallon tank will give you a lot of reserve compared to 13 gallons. Do you run your DA continuously ?

If that soft starter doesn't work and you don't want a smaller motor, your going to need a bigger generator.

 

[volleyball]

Swap the electric motor for a gas one. No generator needed.

 

[Todd.Brock]

Starting off with zero pressure in the tank vs. starting with a 100 PSI or more in the tank is enough to increase the motor load to trip the breaker.

Would this be a factor if the unloader and/or check valve is functioning properly??

 

[48RON54]

Personally, I wuold start by putting a clamp on meter on your cord and see what your amp draw on start up is.... that would be a good starting point to figuring out how to proceed with your problem.... I am by far no expert though.

 

[eyeballengineer]

I'm screwed now anyway the plastic part that holds the contactor in the back of the motor cracked. It was sparking so I took the end cap off to clean the contacts and adjust the tabs and it just crumbled. So I'll be looking for a new motor. I think converting to gas or preferably compression ignition would be more trouble than it worth

 

[pattenp]

Even with the unloader valve working there is a difference of work load for the motor/pump if the tank is empty vs. already having pressure in it when starting up. The unloader only relieves the high head pressure from the end of the fill cycle. On a cycle to maintain pressure the pump is starting off pushing against about 100-PSI vs. 0-PSI on an empty tank.

Would this be a factor if the unloader and/or check valve is functioning properly??

 

[volleyball]

Could also be the difference between a cold motor and one that is warm.

 

[rlitman]

I'm screwed now anyway the plastic part that holds the contactor in the back of the motor cracked. It was sparking so I took the end cap off to clean the contacts and adjust the tabs and it just crumbled. So I'll be looking for a new motor. I think converting to gas or preferably compression ignition would be more trouble than it worth

Is it just a DA sander you need to run?
Check out the Mirka Deros. That's one of the few electric sanders that can truly compare to a DA like a Dynabrade or Hutchens.
It's a WAY expensive sander, but still way cheaper than the compressor you'd need to run a DA.