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914World.com _ 914World Garage _ Generators and power requirements

Posted by: Rand May 13 2010, 08:19 AM

Want to run power tools and a light-duty welder - just a 110-powered flux-core. I know there's math to calculate power requirements, but there are also variables about how long you can run it continuously and all that. So I thought I'd see if those with experience would chime in.

I don't need heavy duty. But I don't want to be worried about when I have to stop and let things cool down.

Just for argument's sake, let's use Harbor Freight as an example... Say you have this 90-amp welder:
http://www.harborfreight.com/90-amp-flux-wire-welder-98871.html

Out of these generators, which could and could not handle it?
http://www.harborfreight.com/engines-generators/gas-engine-generators.html

Posted by: jmill May 13 2010, 08:23 AM

120 volts x 15 amps = 1800 watts. ~~Those 7000 watt generators will be more than enough.~~

Oops didn't scroll down enough to see there were lower wattage units. Any generator that can put out a constant 15 amps at 120 volts would be fine. The smaller 2400 unit would work as long as your not trying to run lighting or a grinder at the same time. With generators it's better to get more than you need.

Posted by: Rand May 13 2010, 08:32 AM

If it's that simple then the 2200 watt generator should be sufficient, right?

Posted by: PRS914-6 May 13 2010, 08:42 AM

Rand, it doesn't take a large generator to run those sexual aid products...get a battery operated one!

On the serious side....I'm not an electrician but what I have learned from first hand experience (that would be failures) is that when motor operated devices are used, it takes a larger generator to start them because of the amp\voltage spike at startup. Once they are running, then it doesn't take much to keep them running, it's just that startup load that hurts you. It seems the larger the motor the more critical to go oversize to compensate. If you put an amp meter on a motor when it started it would momentarily peg the meter and that's what the generator has to overcome. Plug a grinder into a small welder and see the delay when you pull the trigger. Plug the same welder in the wall and see how fast it takes off.

One time I tried to run my well motor and borrowed a large generator. It wouldn't start the motor but would allow continuous running once started.

A welder uses a big draw when you start a weld. It has to fire off the spool feed motor, open solenoids and start an arc all at the same time. Go oversize for sure.

Look at "peak" power vs. continuous power. Some advertise "peak"
I chose the largest anticipated draw and double it. More with large motors.
Hopefully others with more info will chime in.....

YMMV...see ya at South Shore!

Posted by: Rand May 13 2010, 08:52 AM

So the grinder is going to be a tougher client than the welder? I'll just give the wheel a spin before turning it on. Haha.

I'm not going to run more than one thing at a time.

Clearly the $700 unit will handle it. But if the $250 unit is up to the task, that's quite a price difference. Maybe I should shop Sears for something in between.

Posted by: underthetire May 13 2010, 09:19 AM

I can tell you first hand that the 90 amp HF flux welder will kill a 15 amp breaker if used on high.

If your running power tools, resistive load is calculated a little different. It has a longer start up cycle then lets say a tv. I would stay in the 3500-4500 watt range myself.

watts / volts =amps.

Posted by: PRS914-6 May 13 2010, 09:26 AM

QUOTE(Rand @ May 13 2010, 07:52 AM)

So the grinder is going to be a tougher client than the welder?

No but it gives a little demonstration of startup load and how that will affect the generator

Rand, try Craigs List, I see generators there all the time. Some are pretty good deals.....This really is an issue where "size matters"

Posted by: Spoke May 13 2010, 10:37 AM

QUOTE(Rand @ May 13 2010, 10:52 AM)

So the grinder is going to be a tougher client than the welder? I'll just give the wheel a spin before turning it on. Haha.

Power requirements for rotating equipment usually document steady state operation. Start up power to get a motor spinning is a LOT larger.

I powered a tile cutting wet saw with a generator when my house was built and I had to spin the blade just before turning on the saw otherwise the generator would bog down and the motor would not turn at all. Once running, the saw and generator worked fine.

Posted by: DonTraver May 13 2010, 10:43 AM

Rule #1 with generators.

Calculate continuous load, multiply by 5, to get start up load, buy generator that matches or exceeds start up load.

Small motors normally have start up capacitors to assist starting. Larger ones, not so much.

Last generator plant I built and operated, 14.5mw at 4160 volts, 1680 amps. 6 3612 NG Caterpillar Generator Sets. With 6 Gensets online, when they started one of the 1500hp motors, it would dim the lights. Starting current was over 2000amps at 4160volts. For the extra power we ran in Parallel with the utility.

If in doubt for stand alone operation, give yourself an extra 10% power.

Good Luck, Don

Posted by: underthetire May 13 2010, 10:45 AM

QUOTE(DonTraver @ May 13 2010, 09:43 AM)

Rule #1 with generators.

Caluculate continuous load, multiply by 5, to get start up load, buy generator that matches or exceeds start up load.

Small motors normally have start up capacitors to assist starting. Larger ones, not so much.

Last generator plant I built and operated, 14.5mw at 4160 volts, 1680 amps. 6 3612 NG Caterpillar Generator Sets. With 6 Gensets online, when they started one of the 1500hp motors, it would dim the lights. Starting current was over 2000amps at 4160volts. For the extra power we ran in Parallel with the utility.

If in doubt for stand alone operation, give yourself an extra 10% power.

Good Luck, Don

Thats just scary power. The arc flash on that would instantly turn you in to a black powder on the ground !