Electric air compressor and generator

[mad welder 4]

On paper it sounds like the generator should start the compressor with ease. It's a 2.5hp electric air compressor and a 7kw 13hp 420cc generator.
I just always assumed it would work.
One day I figured that I should test it after I switched the house over to dimable LEDs and noticed how much compressor startup dims the lights.
The dimmable ones were on sale and the cheapest so that's what I got.
To my surprise the compressor has a momentary power draw of up to 47 amps. The instantaneous power draw that I can not measure is probably much higher.
I thought that 47 amp reading was awfully close to what my generators 10,800w surge rating is. But the compressor only draws 11 after the initial start when empty then shuts off at about 16 to 17 amps, well with in the 28 amp limit I know it can get from it to power my plasma cutter.
So I ran my smaller 25ft 10 gauge extension cord from the generator to the compressor and applied power.
The generator bogged down under the surge current and could not recover, after several seconds of running at 46Hz the thermal overload tripped on the motor.

I would like for these 2 machines to be able to work together. I just need to reduce the starting amps a little.

The cheapest thing I can think of is barrow the go cart clutch that I put on my gasoline powered air compressor to make cold starting a lot easier. Then put it on my electric air compressor. The gas motor and the electric both have 5/8 shaft and both have the same size or very close in size pulleys. See if that reduces starting current.
If not I'm open to suggestions.

[scsmith42]

Electric motors typically have a very high current inrush when starting. Sometimes you can assist a single phase motor with a "hard start kit", which consists of a higher capacity capacitor. These are used frequently in the HVAC industry.

If the compressor will operate acceptably plugged directly into the generator, you should be able to solve your problem by increasing the wire gauge for your extension cord. Electrical wiring is sized in order to transfer a maximum amount of amps with an acceptable voltage drop. The longer the wire, the more resistance and the higher the voltage drop. Your 10 gauge extension cord is inadequate to carry the current demand at the required voltage during startup.

If you size your extension cord based upon the maximum current inrush for the compressor and based upon carrying that load over the entire length of the cord, the compressor should start off of your generator.

I dont have my reference books handy right mow to check the current inrush for your motor and the requisite cable size, but can do so later in you wish.

[mad welder 4]

I thought that might be the case so I tried plugging the 25 foot cord into grid power and it started right up, no delay.
Then I added another 10 foot section to the mix just to see what happens and again it starts right up with no delay.
I can move the compressor next to the generator or plug it in with my stick welder's 6 gauge extension cord but I don't think it will change the result.
But I will try it.

[12V71]

Is your generator an auto idle model and going to idle when the compressor kicks off? If so you need to run it at high idle to start the compressor. That's too much load for the gen to pick it up off idle.

[MinnesotaDave]

I had the same problem when I tried my old 5hp 20 gallon twin cylinder craftsman compressor.
Bogged a 5000/6250 genny so bad that the motor died.

So I bought a used oil-less 30 gallon noisemaker off Craigslist and used it instead.

Always wondered if I could replace the start capacitor with another one that would allow the old one to be used on the generator.

[mad welder 4]

The generator is a troybilt, direct drive fixed speed 3600rpm machine.

Testing some ideas, taking the belt off the compressor and starting the motor with out anything on it seems to be the only way to dramatically reduce starting amps.

Since starting the motor with no load works so well I have been experimenting with some of the spare electric motors I have laying around.
It turns out if the motor is not driving anything you can switch out the start capacitor for a slightly smaller one to save a few starting amps and get the motor up to speed faster.
But I believe this also reduces starting torque so that could be bad for a motor that needs real high starting torque.

I also tried resistor starting a completely unloaded motor and this saves even more start up amps too. Only problem is the motor needs to be totally uncoupled form the load or the motor will just burn up.

So it looks like using some kind of clutch like in my original go cart clutch idea is kind of I the right direction.

Also in my motor collection a have a 1hp 1800rpm 4 pole motor. Compared to my other 2 pole 1hp motors 4 pole draws a lot less on start up. This is just comparing the unloaded 2 pole and 4 pole motors to each other.
So my ideal compressor setup starts to looks like:
A 4 pole motor if I can find one.
Some kind of clutch to unload the motor on start up.
A resistor starter to ease the unloaded motor up to speed before it switches to full power.
And some controls to take care of it all so I don't have to stand there and run it up manually every time.

[MinnesotaDave]

My electrical knowledge is rudimentary so bear with me.

Idea 1: Wouldn't the answer be in using a larger start capacitor, or capacitor bank of some kind, to provide the electricity needed to initially start the motor under load?

Idea 2: An unloader valve that allows the compressor to start without load like a chainsaw engine does? My new saw has one and I thought maybe that concept might work.

Idea 3: The clutch would need some kind of delay, an electrical delay timer perhaps for an electric clutch. Or a centrifugal type clutch and set the springs so it won't engage until the motor is nearly up to speed?

Idea 4: pressure switch only runs the clutch. Turn electric motor on manually and let it run like a gas engine air compressor would operate.

[mla2ofus]

You might try what I did when installed an extra compressor on my welding trailer and powered by the Wisconsin engine. I just connected an air governor like used on 18 wheeler compressors. Plumbed the unloader line to the gov and it works great. With the electric motor you may need to tee a valve into the line from the head to the tank to relieve the pressure until the motor gets up to speed. You'll also need to bypass the pressure switch. The compressor motor will have to run continuous as long as you need air.
Mike

[mad welder 4]

I did try a bigger start capacitor, the motor really did not like that. Going 10% bigger the motor just hummed and barely moved.
But go 10% smaller and the motor starts faster and draws less amps when the belt is removed.

I have power factor corrected my air compressors, the power factor correction capacitor does reduce starting amps. When I added power factor correction a few years ago I only cared about reducing running amps. Turns out they also reduce start amps.

I tried doing an air unload and it still draws 11 amps while it's running. I can't see a difference when I remove the air discharge line and start it, which effectivelygives you an air unload start. The air discharge line on this compressor was pretty long. So my compressor builds pressure slower than the store bought ones with short compressor to check valve lines.
Also running an air unloader would waste a lot of power, 11 amps unloaded is more than 1/3 of its capacity.

I work on industrial automation so time delays and wiring up controls stuff are kind of my thing.

[MinnesotaDave]

Ah, then electric clutch with time delay sounds like your best option and fortunately ends up being an option you're well suited to carry out. That's handy

[mad welder 4]

I tested out running up the motor with the belt slacked and suddenly jerking the slack out of the belt to kind of simulate the sudden engagement of a magnetic clutch. The jerk test only peaked at 20 amps, which is a whole lot less than 47 amps.

So I found a magnetic clutch for a 56C frame motor but don't have a 56 frame motor with a C face.
If I'm going to buy one a 56C face I will get a 4 pole.

[bent]

Your four pole motor will run at 1800 nominal rpm. It is possible your compressor won't get enough splash lube at 1/2 speed.

[mla2ofus]

I'm guessing part of the starting load is the cast iron flywheel. How about a soft start setup? We used them on 300 to 400 horse motors to start rock crushers with a lot of mass to get moving to relieve shock load on the generator.
Mike

[MinnesotaDave]

Wow, decided to look in my bobcat 225nt manual to see what the generator chart says.
Starting watts on air compressors is shockingly high compared to running watts.

[mad welder 4]

Your four pole motor will run at 1800 nominal rpm. It is possible your compressor won't get enough splash lube at 1/2 speed.

Yes I was going to run roughly double the motor pulley size. Plus I don't want to cut the air out put in half from the compressor because it makes a pretty good amount.

[mad welder 4]

I'm guessing part of the starting load is the cast iron flywheel. How about a soft start setup? We used them on 300 to 400 horse motors to start rock crushers with a lot of mass to get moving to relieve shock load on the generator.
Mike

Yes I think I'm going to soft start the motor a 4 pole motor on a resistor completely unloaded.
For example I have a 1hp 4 pole motor plus a 1hp and a 1.5hp 2 pole motor.
When I resistor start the 2 pole motord unloaded I can only get the 2 pole motors down to 15 to 17 amps. When I resistor start the 4 pole motor unloaded it pulls 7 or 8 amps.

I can't automatically soft start a single phase motor hooked to a load.
I kind of can if I use an auto transformer and turn the voltage down from 250v to 220v. But it only reduces starting amps from the original 47 amps to 36 amps.
This would be a good option for manually soft staring occasional use equipment that is going to run for a while.

Another thing I would like to try some day is a single phase powered variable frequency drive. But that won't be till later on.
Usually big motors are always 3 phase and connected to some kind of electrical and/or mechanical soft start.
The modern trend is to leave the motor connected to the load and control it with a variable frequency drive.

[mad welder 4]

Wow, decided to look in my bobcat 225nt manual to see what the generator chart says.
Starting watts on air compressors is shockingly high compared to running watts.

Wow that's a lot.
Hopefully I can get the motor starting down to less than 3,600 with resistor starting the motor unloaded and then stay under 5kw when the clutch engauges.
These are high estimates based on resistor test starting my 1hp 4 pole motor while unloaded and shock engaging the belt on a 2.5hp 3,600rpm motor running at full speed to a stopped compressor.

[tracymobilecnc]

i've gone thru a bit with air compressors and plasma off the same generator.

bottom line for me has been any electrically powered air compressor that met my needs would not be suitable to run while plasma cutting. the starting amps can be handled by grid power but a generator cant deal with the excessive starting current. i've never messed with the delayed clutches and that sounds interesting. and promissing.

my intial fix was to run the air compressor continuously. the compressor was a 2hp splash oil wheelbarrow type Roll-Air. i had it set to bleed air before hitting the shut off. that way i could start it then bring everythign else online and run. but it was a hassle.

the final solution has been gas powered air source for me.

[mad welder 4]

Yes I would like to be able to run the plasma and air compressors off my 17.5kw.
I just think if the compressor kicks on, draws a ton of amps it will cause the plasma to fault for undervoltage.

[MinnesotaDave]

Yes I would like to be able to run the plasma and air compressors off my 17.5kw.
I just think if the compressor kicks on, draws a ton of amps it will cause the plasma to fault for undervoltage.

Cheapest solution in my opinion is to do what I did. Buy an oil-less compressor.

They are loud and annoying, but have low startup amperage and running amperage compared to a normal compressor.

I got one for $100 that needed a $25 seal kit and it runs off my 5000/6250 generator really easily.

It's a 150 psi unit so I turned the pressure switch down to 140 psi to get more life out of the compressor.

With the size of your generator, the plasma and an oil-less noisemaker should be able to run together.

[mad welder 4]

I sold my noise maker electric air compressor last year on account of it making a lot of noise and not a whole lot of air.

[MinnesotaDave]

I sold my noise maker electric air compressor last year on account of it making a lot of noise and not a whole lot of air.

Mine must be the "politician series" - it does both

[Country Metals]

From everything I have read. Air Compressors have a startup draw 4x running amps. It's one of the highest draw "standard" machines.

Every place I have read, they use a vfd to soft start it. Changing the motor/pulley to a different size doesn't help and may make the problem worse. I remember reading on another forum that the air compressor at a machine shop drew so many amps it would slow the spindles on some machines and make problems. After many different things they opted for a vfd slow start.

Of course that was a large air compressor.

Sent from my 2PS64 using Tapatalk

[Ironman916]

You might consider an unloader valve to relieve the head pressure. I know I had to do that on my compressor to get it to start.

[mad welder 4]

You can't soft start an air compressor motor that is still belted to the compressor with resistors or inductors, it will just make starting amps even higher.
The only motor power supply alteration find that would reduce starting amps on a single phase motor connected to a load was an auto transformer. If I reduced the normal 250ish volts to around 220v I could reduce starting amps from 47 down to 38 amps by starting the motor at 220v then quickly turnin up the auto transformer to normal voltage as the motor came up to speed. But that would require manually restarting the compressor every time with the auto transformer. I am looking for full soft start automation.

The easiest thing you can do to reduce starting amps on an air compressor is switch to a 4 pole motor and bigger pulley.
For example my smaller 1hp air compressor had a 2 pole motor, it was hitting between 26 and 30 amps on start up.
I figured I would try swapping it in that 1hp 4 pole 115/230v motor i have wired up for high voltage with a 2x size pulley.
I took the 6 inch pulley off my alternator welder so that I didn't have to buy one to replace the 3 inch pulley that was on the 1hp 2 pole motor.
To my surprise it almost cut the starting amps in half, now it's only hitting 15 amps on start up. I was not expecting starting amps to go under 20 just from a motor swap.
BTW I'm not putting that 2 pole motor back on, it can stay.
Plus the 4 pole motor is quieter.

Only problem is I can't find a real 2.5hp or 3hp 4 pole motor to replace the 2.5hp 2pole motor on that air compressor that draws 47 amps on start up.
All the "2.5hp" or "3hp" 56frame 4 pole motors I am finding appear to be mislabeled or over rated 1.5hp motors just based on their overall weight and full load amp rating.
I would settle for a real deal 2hp 4 pole 56 frame motor to replace the 2.5hp 2 pole motor on that compressor that draws 47 starting amps.