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Unread postAuthor: ramses » Tue Sep 13, 2011 8:58 pm

jsefcik wrote:dont have a fridge compressor,
my dad is an electrican, he said it will work just fine,

i believe an electrician i dont know about you guys

if my dad is wrong, im going to buy a small generator instead that puts out 7 amps!!!


I'm not sure what kind of training electricians receive now days, but you may well have a compressor that will run on less than 750W continuous.



Things get a little weird when you consider that the motor must start off the grid. In home wiring, the breakers are generally "slow" in that they allow starting currents 2-3 times their rated "blow" current. With solid state devices like an inverter, that capacity is not there. MOSFETS (used in inverters) really don't like being overcurrented.

Even with a small generator, you have the inertia of the spinning crankshaft and generator to supply some extra power through the windings. Sure it'll bog pretty fast, but by then the compressor's started and the engine can bring the RPM's back up. Essentially, there's a way to store the energy required for start-up over a relatively long period, then dump it through very robust components.
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Unread postAuthor: jsefcik » Tue Sep 13, 2011 9:00 pm

My dad isnt an idiot, he knows his crap, being an electrican for over 25 years i think i can trust him
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Unread postAuthor: MrCrowley » Tue Sep 13, 2011 9:08 pm

Just calm down a bit, no one said your dad was an idiot. You asked what we thought and a few nice people have offered their opinions and concerns.

Perhaps the best thing to do would be to check these concerns with your dad. If he can explain why they are not a problem and the guys here think his explanations are justified, you can buy the compressor without having to worry about any issues. Hell, the members even said themselves you may not have any issues but they suggested a few things you might have a problem with.

This is how we help one another, don't jump at our throats for trying to help.
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Last edited by MrCrowley on Tue Sep 13, 2011 9:12 pm, edited 1 time in total.
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Unread postAuthor: Fnord » Tue Sep 13, 2011 9:10 pm

What we're saying is, it might work or it might not. With a smallish compressor it's quite possible but a full-sized shop compressor is pushing it. You might trip the overload protection if your inverter has any.

Also, my dad was an electrician for about the same length of time and I wouldn't trust him farther than I could throw a polepig so it's a bit of a subjective matter.
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Unread postAuthor: jrrdw » Tue Sep 13, 2011 9:32 pm

WTF is a "polepig"?

Why o why do people wait until the lights go out before they check their resources? There was a weeks notice before each storm rolled up the coast and still waaaa waaaaa waaaa my generator wont start waaaaa waaaaa waaaa.
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Unread postAuthor: POLAND_SPUD » Tue Sep 13, 2011 9:38 pm

polepig

FNORD you bastard!!

it's a racial slur

BAN HIM BAN HIM BAN HIM!!!
:-D
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Unread postAuthor: Zeus » Tue Sep 13, 2011 9:52 pm

Jrrdw, a pole pig is a power pole transformer.

jsefcik, I've worked with electricians, there's a few sorts, domestic who don't do a lot of theory, industrial who have to do a lot, and repair/test & tag, they tend to know the least. I've known some just qualified that know a heap, and quite a few more experienced domestic only electricians who wouldn't come close to passing the grade A exam.

A generator that puts out 7 amps at 110/120VAC is only 770-840 watts, that's not a lot. Most mains powered compressors (in my experience) are at the very least 1100 watts, usually 1500 and up. Watts and VA(volt amps) are basically the same thing.

And car batteries aren't for running appliances off, that's why deep-cycle lead-acid batteries were made. Car batteries' plates are foamed lead, discharge them too deeply and the plates erode a lot. They don't come back from the discharge well. Deep Cycle batteries are a solid lead grid, they can be discharged down to 20% of their capacity, car batteries can only be regularly discharged down to 80% of their capacity.

What I said applies to electricians down here, I don't know the American licencing system. We have a 4 year apprenticeship, with yearly exams, then at the end of the fourth year there's a exam to obtain your grade A license.

I don't even know if it's a licensed trade in America.
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/sarcasm, /hyperbole
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Unread postAuthor: jrrdw » Tue Sep 13, 2011 10:27 pm

I don't even know if it's a licensed trade in America


Varies by state but a certification is in order mostly...
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Unread postAuthor: Zeus » Tue Sep 13, 2011 10:44 pm

Good to hear, I recall reading an excerpt from basically the electricians bible (dated around 1950, I don't recall the name), anyway, it suggests checking 110/120VAC lines by touching them to see if they're live. (I know 110/120VAC is rarely fatal, but still I'd rather use a multimeter).
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Unread postAuthor: Crna Legija » Wed Sep 14, 2011 2:41 am

jsefcik wrote:Guys check out this

instead of using a generator, you can use this, plus its cheaper, and will run a small compressor!!!!! im def buying one, hooks right off of your car battery !!!! only 12v to power it

http://www.harborfreight.com/750-watt-c ... 66817.html

i am mainly getting to hook up to my truck to power my compressor for my gun , therefore i can shoot it in the field where i dont have power

tell me what you think!!!



lol me and jsr linked you to them in your generator thread, 750w might not run your compressor, and this thing will absolutely demolish a stock car battery.

Also if you are planing on running it off just a battery no alternator to charge it you will be lucky to get 10min of ON time
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Unread postAuthor: Technician1002 » Wed Sep 14, 2011 11:40 pm

jsefcik wrote:Guys check out this

instead of using a generator, you can use this, plus its cheaper, and will run a small compressor!!!!! im def buying one, hooks right off of your car battery !!!! only 12v to power it

http://www.harborfreight.com/750-watt-c ... 66817.html

i am mainly getting to hook up to my truck to power my compressor for my gun , therefore i can shoot it in the field where i dont have power

tell me what you think!!!


From an engineering standpoint, this is the considerations.

1 When power is first applied to an electric motor, it is not spinning at all.
2 When the motor is not spinning at all it will draw a value known as Locked Rotor Amps. A good electrician is aware of this. Many electric motors have this value marked on the name plate.
3 Locked Rotor Amps can be from 2-10 times the run current depending on the motor. A low starting torque motor draws less than a high starting torque motor.
4 Time delay breakers are designed to protect house wiring and heat up the same rate as house wiring.
5 Many inverters have very little overload capacity. Often it is only 20% for a few seconds to start motors.
6 Some inverters have no overload capacity and shut down on any overload.

To properly engineer your system, the LRA value must be below the overload capacity of your inverter.

Can an inverter be used to run a small compressor in the field? Yes.
I use a low starting torque compressor (no start switch and cap) of 1/3 hp on a 1KW inverter. My inverter will not start my 1HP compressor.

7 At the maximum draw the low voltage wire must be low enough resistance to prevent a voltage drop that will trip the low voltage protection on the inverter during the starting of the compressor. Simple battery spring clips are not up to the job. This needs to be a solid wired connection with very short large gauge wire as stated in the inverter manual.


Put the flame war aside and follow rule number 1. My dad is smarter than your dad does not answer the simple math required to solve the problem. I am not a licensed electrician, so my advise may be completely wrong. I do however double check my figures and can explain the reasons for my conclusions.

My installation is in this thread.
http://www.spudfiles.com/forums/how-to-pump-up-a-large-cannon-in-the-field-t18124.html

For anybody that looked at the inverter, it is 750 watts capacity with 1500 watt momentary overload capacity. It will start a compressor under 1500 VA LRA rated. For a 120V compressor that would be a LRA rating of 12.5 Amps LRA as the absolute maximum.

Do you know the LRA current of your small compressor?

Note the black wire off to the left of the inverter. I sized it to prevent voltage sag when starting my compressor.
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Unread postAuthor: MrCrowley » Fri Sep 16, 2011 5:40 am

Back to presidents and politics, I've found an unlikely candidate:
[youtube]http://www.youtube.com/watch?v=WibmcsEGLKo[/youtube]
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Unread postAuthor: Crna Legija » Fri Sep 16, 2011 7:45 am

Technician1002 wrote:
jsefcik wrote:Guys check out this

instead of using a generator, you can use this, plus its cheaper, and will run a small compressor!!!!! im def buying one, hooks right off of your car battery !!!! only 12v to power it

http://www.harborfreight.com/750-watt-c ... 66817.html

i am mainly getting to hook up to my truck to power my compressor for my gun , therefore i can shoot it in the field where i dont have power

tell me what you think!!!


From an engineering standpoint, this is the considerations.

1 When power is first applied to an electric motor, it is not spinning at all.
2 When the motor is not spinning at all it will draw a value known as Locked Rotor Amps. A good electrician is aware of this. Many electric motors have this value marked on the name plate.
3 Locked Rotor Amps can be from 2-10 times the run current depending on the motor. A low starting torque motor draws less than a high starting torque motor.
4 Time delay breakers are designed to protect house wiring and heat up the same rate as house wiring.
5 Many inverters have very little overload capacity. Often it is only 20% for a few seconds to start motors.
6 Some inverters have no overload capacity and shut down on any overload.

To properly engineer your system, the LRA value must be below the overload capacity of your inverter.

Can an inverter be used to run a small compressor in the field? Yes.
I use a low starting torque compressor (no start switch and cap) of 1/3 hp on a 1KW inverter. My inverter will not start my 1HP compressor.

7 At the maximum draw the low voltage wire must be low enough resistance to prevent a voltage drop that will trip the low voltage protection on the inverter during the starting of the compressor. Simple battery spring clips are not up to the job. This needs to be a solid wired connection with very short large gauge wire as stated in the inverter manual.


Put the flame war aside and follow rule number 1. My dad is smarter than your dad does not answer the simple math required to solve the problem. I am not a licensed electrician, so my advise may be completely wrong. I do however double check my figures and can explain the reasons for my conclusions.

My installation is in this thread.
http://www.spudfiles.com/forums/how-to-pump-up-a-large-cannon-in-the-field-t18124.html

For anybody that looked at the inverter, it is 750 watts capacity with 1500 watt momentary overload capacity. It will start a compressor under 1500 VA LRA rated. For a 120V compressor that would be a LRA rating of 12.5 Amps LRA as the absolute maximum.

Do you know the LRA current of your small compressor?

Note the black wire off to the left of the inverter. I sized it to prevent voltage sag when starting my compressor.
Image



Come on that's way to on topic. :lol:
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 16, 2011 9:49 am

Image

Image

spotted on ENDO, DO WANT!
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Unread postAuthor: Fnord » Fri Sep 16, 2011 10:01 am

I hope it says "Do not look directly into the operational end of the device" somewhere...
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