Generator won't power compressor....ideas?

[Boeing]

Fellas,
I guess I made a bad decision...(never done that before) I bought one of the Aurora Generators thinking it would run almost anything I own. It's a diesel putting out 6500 watts. I have an air compressor with a 4 HP motor, currently wired at 115V and draws 16Amps. The generator is rated at 20 amps so I thought I was good. Don't know much about surge or starting load. Anyway, it won't budge it; trips the breaker every time. The motor CAN be wired to 230V and draws 7.3Amps. Haven't tried that yet.
I was wondering if anyone in the TBN braintrust knows if there is a way to make an electric motor easier to start? add an additional capacacitor or larger capacator? Maybe put a larger pulley on the motor making it easier to turn the pump (I know that the pump would put out less air)
Are their more efficient motors on the market? Perhaps less HP?
Any ideas here? I also wanted to run a radial saw which pulls 12A at 115 or 6A at 230V, I think .....haven't tried it yet.
The generator data stated that it would run well pumps etc......is this true or do I have to sell all my power hungry stuff and go lean?
Any ideas would be appreciated.
Thanks, Rob

 

[SPYDERLK]

Yes. Wire it for 230 1st thing! Your gen will do ~28A @ 230 if 6.5KW. Starting a 1 phase motor usually draws about 4X running current. Thats ~60A on 115, but only 30A when wired for 230. It should start it fine if no other loads ... and be able to run it in conjunction with other loads after startup.

Motors always draw huge as they start, but compressors are big time bad because most start under load and the large draw lasts much longer. You will have no trouble with the radial arm [maybe even at 115V] as it starts w/o load. Unless your wellpump is a big one - over 3/4HP you should have no trouble starting it even if running one of the others.
larry

 

[orezok]

Does the compressor have an unloader on it? If not, add one.

 

[Gary Fowler]

Dont run the power thru an extension cord either, power it straight into the genset with the 4 foot or so cord supplied with the compressor. Extensions rob a lot of voltage even in just 50 feet of cord.

 

[Soundguy]

all good advice.

no ext cord. DO setup for 220v.. otherwise a start aid cap MIGHT help.. but way better if you can wire 220 and do away with an ext cord.. unless it is super beefy.. like 8g individual conductors..

 

[Raspy]

6500W at 120V = 54A. 6500W at 240V = 27.5A


Neither one is 4 times the rated amps at the running voltage, but probably plenty to get the compressor started. You just need to get that power to the compressor in a practical way.

Your compressor is listed as 4 HP. But 115V X 16A = 1,840W and that equals 2.47 HP. Many compressors have exaggerated HP figures.

Will your compressor start if you plug it into a 20A 120V household plug with no extension cord?



A 6500W, 120V generator can have a much larger breaker than 20A. A larger breaker to handle the surge amps might be a good method and the generator has the capacity. But the internal wiring in the generator might not handle a larger breaker on that circuit.

If you rewire the compressor for 240V and use a 20A 240V breaker, you'll be doubling the number of watts to the compressor and still be within the generator's watt rating. This would make more sense than a 50A breaker on the 120V side. And it would only require #12 wire. What size breaker is on the 240V generator output?

See if you can find a 12-3 extension cord and 240V plug to match the generator plug. Or hardwire it in. Keep the cord short.

If the generator is permanently mounted near the compressor you could use stranded wire in flex conduit and hardwire it through a switch.

Large capacitor start kits work well on air conditioners. I did it on one one of mine to get my generator to start the unit and it worked. But I have never tried it on a compressor. You can get them at Grainger.

I suggest just altering the motor voltage first and going with a 20A breaker on the 240V output.

During emergency power outages you probably don't need a large compressor. So you could always buy a cheapo small one to use for filling a tire or something. Sears makes a 1hp oil lubed unit with an iron compressor that can be had for $100. on sale. I have had one for years and they are great. Paid $69. for it at some blowout sale and felt like I was stealing. There's no way my 5 HP compressor will ever run from my 2000 watt diesel emergency generator. And that's OK.

 

[Soundguy]

Your compressor is listed as 4 HP. But 115V X 16A = 1,840W and that equals 2.47 HP. Many compressors have exaggerated HP figures..

I doubt that. it's just people don't read the tags / or understand what they read.... or how the co listed the info / did the conversion.

rms and peak.. etc.

I BET you his comp is listed using PEAK wattage, then the hp conversion was done. looking at the rms conversion and the hp listed.. that would drop the rms hp into the 2.x range.. right near your calculation.

I bet somewhere hidden on that tag or in the manual it states a peak measurement..

For those not familiar with rms, peak and peak to peak.. hit an ac/dc / electronics 101 coursebook...

 

[Qapla]

Some years back I used to work with a generator and several tools that required power. This was before everyone had cordless tools. All our tools ran fine, except the compressor. It would start fine if nothing else was running and the generator hadn't been running long. But, when the generator got hot or we were using the portable table saw and the generator tried to kick on ... blew the breaker every time.

When a compressor starts it is not only starting the motor, it is also trying to compress air, after all, that is what it is supposed to do. On most compressors, the supply tube that runs from the compressor head to the tank is quite short ... just a few inches. That works fine when the power is coming from a steady source, like the line from the house. But, when the generator is powering it and you are "pushing" the setup, the generator will lug down while the compressor tries to overcome the head pressure, thus causing the breaker to overload because of low power and heavy load.

What we did to fix the problem was this ...

We replaced the supply line running from the compressor head with a new line that was about 30" long, coiled to allow for movement and vibration. Never had a problem with the compressor starting after that. By the time the compressor hit head pressure, the motor was out of start mode and into running mode.

 

[Raspy]

We replaced the supply line running from the compressor head with a new line that was about 30" long, coiled to allow for movement and vibration. Never had a problem with the compressor starting after that. By the time the compressor hit head pressure, the motor was out of start mode and into running mode.

That's a very creative fix. Makes sense.

 

[namesray]

i am by no means an expert in this area, but just something to throw out there, check the phase and Hz loads the generator is rated for and what the compressor is. my generator says do not connect "3 phase" loads and do not connect "50Hz" loads. maybe this helps, maybe this was a waste of time. good luck with your issue.

 

[Raspy]

I doubt that. it's just people don't read the tags / or understand what they read.... or how the co listed the info / did the conversion.

rms and peak.. etc.

I BET you his comp is listed using PEAK wattage, then the hp conversion was done. looking at the rms conversion and the hp listed.. that would drop the rms hp into the 2.x range.. right near your calculation.

I bet somewhere hidden on that tag or in the manual it states a peak measurement..

For those not familiar with rms, peak and peak to peak.. hit an ac/dc / electronics 101 coursebook...

Many compressors advertise a horsepower rating, but the motors say SPL instead of the actual running horsepower. It's common to see a "4 HP" compressor that actually has a 2 1/2 HP motor and the CFM confirms the rating. A true 5 HP compressor will say 5 HP on the motor plate. Others will advertise 5 HP, but the motor will say SPL and the running amps will be far below 5HP.

I think the exaggerated claims are listing the locked rotor amps and converting that to some advertising claim. I call it "Sears horsepower". It's commonly done on shop vacuums where they draw 13 amps at 115 volts, but claim to be 5 hp or so. Bogus. It would be equal to the amps it draws as it explodes, not as it runs. It's misleading. Best just to look at the CFM at 90 PSI for comparison o compressors.

 

[Soundguy]

if he has a 110/220 motor.. it's gonna for sure be compatible with 60hz and or single phase if sold for the domestic market.

 

[Soundguy]

I think the exaggerated claims are listing the locked rotor amps and converting that to some advertising claim. I call it "Sears horsepower". It's commonly done on shop vacuums where they draw 13 amps at 115 volts, but claim to be 5 hp or so. Bogus. It would be equal to the amps it draws as it explodes, not as it runs. It's misleading. Best just to look at the CFM at 90 PSI for comparison o compressors.

well.. that's pretty much exactly what I said..

peak draw.. possibly at stall / max torque, converted to hp figure.

and yes.. i'd call that the same type of logic used to rate small gas engine hp.


somewhere in some lab, using 0w1 oil, and straight ether for ful, with no belted equipment, no pumps.. no fans , etc.. carb running out of epa spec in a manner the end user could never see it operate, at an rpm band 200 nanoseconds before it detonates on the test stand. they clock that hp .. and then poof.. you have an engine the size of a basket ball making 25hp..

 

[Boeing]

You guys are amazing...and thank you.
I will wire to 230V. I bought a male 4 prong plug to go into the generator and 15' of 10/3 wound wire.
And....you're right about the motor too. 4hp PEAK, 2hp running. The compressor is a campbell hausefield vt6196 from about 1993. It puts out 5.8 scfm at 90 psi. Max pressure 120. I like the coiled air line idea too.....
Thanks again.

 

[Soundguy]

yep.. I figured it was a peak hp listing. IE.. the max current it could ever be made to draw.. not something you as the user will ever really see..

pretty common listing nomenclature / method nowadays.. bigger numbers.. looks better...

 

[daybreak1998]

Howdy,
Compressor with a 4hp motor? And that thing has a standard NEMA 5-20 (20amp/125v grounded plug) like your standard but with the right blade sideways? I have a compressor which is a 3hp unit that is also 120/240 capable, which is wired 115, with standard plug, and it starts fine here. If that compressor actually has a 4hp motor, then thats gonna be to big for your standard electrical.

Get a L14-30 twist lock plug which then goes to a NEMA 5-20 (its only about 2 feet long) That way you are going off the 120/240 30amp socket on the generator. L14-30P to dual 5-20R

Even that might not work. Re-wiring the compressor for 240 would be the better way. You would then also need a 240 plug at your shop, or where ever it sits. Are you trying to use it out in the fields?

NOTE; motors really like to start easier with 60Hz cycle. Make sure the generator is really right on the mark.

 

[chim]

Sum Ting Wong

1. Who makes a 4HP motor? Gimme a list of manufacturers [grin]

2. "4HP Peak" has nothing to do with rms/average/peak, etc. It's a sales pitch calc only.

3. Other than watching cute squiggly lines on a scope, the only measurement that means squat is rms (aka effective). RMS is what your clamp-on ammeter and voltage meter display.

4. Full load current for a 115V, 1ﾘ motor is 16 amps:

Electrical Motor - Full Load Current

 

[xzcVAbc5]

You guys are amazing...and thank you.
I will wire to 230V. I bought a male 4 prong plug to go into the generator and 15' of 10/3 wound wire.
And....you're right about the motor too. 4hp PEAK, 2hp running. The compressor is a campbell hausefield vt6196 from about 1993. It puts out 5.8 scfm at 90 psi. Max pressure 120. I like the coiled air line idea too.....
Thanks again.

Your generator will thank you. When you have a generator capable of spitting out 120V or 240V at the same time and you plug in a 120V you are only using half of your generator. When you wire 240V you're using the whole voltage and are able to draw all of the power from the generator from one device.

 

[SPYDERLK]

6500W at 120V = 54A. 6500W at 240V = 27.5A


Neither one is 4 times the rated amps at the running voltage, but probably plenty to get the compressor started. You just need to get that power to the compressor in a practical way.

True, but 240V gens dont work that way. Full rated power is only available from the 240. 6500/240 = 27.5 is the amps available. Only half power will be available from each 120V leg of the spit since the gen will be fused to protect its windings from current over 27.5A. That compressor will never start on the 120V output of that 240V gen.
larry

 

[Soundguy]

take his 4hp peak #.. convert to watts.. then convert to rms, assuming that number is a peak watt rating.. now use the hp conversion and go back to hp... the numbers bang out almost perfectly.

so yes is does matter.. completely.

you just have to understand electronics and economics...

. "4HP Peak" has nothing to do with rms/average/peak, etc. It's a sales pitch calc only.

3. Other than watching cute squiggly lines on a scope, the only measurement that means squat is rms (aka effective). RMS is what your clamp-on ammeter and voltage meter display.

4. Full load current for a 115V, 1ﾘ motor is 16 amps:

Electrical Motor - Full Load Current