Electric Motor Rated Vs. Start-Up Current? 1

[smp375]

I'm trying to pair a portable generator to a 3-5hp air compressor. The compressors I'm looking at all all rated 19-22 amps and the generator will mainly be powering the compressor, so it only needs to have enough power for this purpose. I understand electric motors require a significant current draw to start up, but of course this isn't listed in the specs. Should I just assume I need a generator with enough surge wattage to supply twice the rated amperage of the compressor I intend to power, or is there a way I can more accurately calculate my requirement?

 

[jraef]

Generator mfrs typically will supply you with that information. It's not quoite as simple as just looking at surge wattage, but that is a primary issue. A surge rating is typically for 2-3 seconds, but if your compressor takes longer than that to get to full speed, that can be an issue.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[smp375]

That point poses an entirely new aspect to the problem. Not only do I not know how much surge wattage I need, I won't know if I can maintain that surge wattage long enough to keep the compressor running. I think this means I should be sourcing the compressor with the lowest rated current and I probably should make sure the surge wattage of the generator equates to at least twice that number.

 

[dpc]

Don't equate current with watts. A starting motor draws very high current, but at a very low power factor, hence a relatively low power level.

If the compressor manufacturer can provide the motor NEMA starting code, you can estimate the maximum locked rotor amps and provide this info to the generator supplier.

Engine-generator suppliers deal with this issue daily and will be willing to help you.

 

[waross]

I use three times motor current to size generators. If I know all the parameters I may go 2 1/2 times over.

I understand electric motors require a significant current draw to start up, but of course this isn't listed in the specs.

Three phase motors generally have a NEMA code on the nameplate that allows the initiated to look up the ratio of starting current to rated running current in the appropriate references. Not usual for single phase motors.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Head unloading is very important in this situation also.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[StrykerTECH]

Locked rotor current (starting current) for a compressor motor is typically 6 times the full load current (running current).

StrykerTECH Engineering Staff

http://www.stryker-tech.com/

 

[smp375]

Well, based on what I'm reading here, and what I was told by a tech guy at Campbell Hausfeld, this just isn't going to work for me. The compressors I'm looking at don't have three phase motors, they're just single phase and the information on them is limited. However, even when you consider doubling the rated amperage to 36A, we're talking about a generator with a surge capacity greater than 8640 watts, and then I can't even answer the question of will this generator be able to deliver that current long enough to keep the electric motor from being damaged...so now I'm probably looking at 10kw.

I was hoping to avoid using a gas powered compressor because of several factors, mainly the problem of situating the gas engine and compressor pump on my step van, but it would appear this will be what I must do. I'm just going to have to build an under-body rack on which to mount the engine and compressor and plumb it to a remote tank. Of course, this will save me a small fortune on a generator, so it's not so bad.