Generator size to start motor

[Soundguy]

Just because a 2X larger generator usually limps the motor up to speed , after several seconds of wide open governors, dim lights from low voltage and 10 seconds up to speed instead of 2 seconds, doesn't mean you should .
Generators are cheap now compared to the 1999 Y2K prices. Buy what is needed instead of barely enough.
I'll look at the code book but iirc that 20 FLA motor requires a 30 amp breaker and at least 12 gauge wire .

I think you are making a blanket assumption. A 2x rated genny doesn't automatically mean a motor is going to limp up to speed and cause dim lights etc.

Size the genny to correctly HANDLE the starting load of the motor you want to use and be done with it.

Not every motor load is the same, not all motors are created equal.. We have a 5500/6500 genny at work that won't start a 2hp submersable pump, but will start and run a 2hp belted motor that is running some industrial equipment without even a thought.

 

[buickanddeere]

Low flow , just wondering how he is managing to get a 2HP pump rolling with a 5000watt generator. Just the 3/4HP pump bogs my Honda 5000 with low voltage and low frequency for approx 3 seconds even when the governor has the throttle blade wide open.

 

[Soundguy]

I think alot of it has to do with the motor and stored kinetic energy, and HP available.

Ideally, a generator head should be matched to an engine that can drive the generator head at it's full overload rating, with -0- hit on the engine... But that's not the case. Builders today put an engine on a genny head that is just barely able to make rated power, and really takes an rpm hit on the overload rating.

Some years back I was over at a buddies farm doing some cleanup mowing on some fields he let go. He was trying to sell the place and needed to do some fence work and mow down the weeds. He had a small ford tractor that was basically just a big lawnmower. He had an 8KW pto generator.

I was mowing with my old ford 5000 and afterwards, we drove around to do some fencing. he had a pancake compressor and an air nailer and a stick welder all strapped to a palate on a carryall that his pto generator was hooked to. The pancake compressor would kick on as needed as we nailed. circular saw didn't seem to do much either. Come to a point and we need to do some gate welding, and that tractor just would not carry that generator while I was striking an arc. We pull my 5000 over and hook it up, and I could strike and weld to my hearts content. engine sound didn't even change striking an arc.

What was the difference? about 50 horse power...

The kinetic rotation mass of the larger tractors engine and pto drivetrain dwarfed the smaller tractors setup. When a big electrical load was applied to the genny, it applied a large kinetic load to the tractor. The tractor / engine didn't have enough mass to push thru, and the governor had to kick in and build the rpms back up. vs having so much flywheel power built up in the form of big rotating parts, that the kinetic load from the genny was absorbed before the governor had to do very much.

Now.. using a 70 hp tractor on a 8kw generator is total overkill and not needed... but.. it illustrates the point well. an old tired 20 hp tractor had a hard time with 8kw.. and old tired 70 hp tractor that weighed 4x as muchg didn't.

I've seen some small generators these days that are basically using weed eater engines. what kinetic reserve do these engines the size of a grapefruit have? ( none )

 

[CobyRupert]

Can this genny start this motor?

It's a complex question.
- Are there other loads being run on genny, if so starting order can be important. (i.e. you may have to start motor before others)
- Characteristics of genny's coil come into play. How easily saturated?
- Starting characteristics of electric motor come into play (Is inrush 3 x full load amps (FLA), 6 times, 8 times, 13 times?). High efficiency motor? Torque speed characteristics?
-Load characteristics come into play. Is load variable torque load like a fan or a pump, where it take almost zero torque to start, or does it require a greater starting torque from the get go like a crusher?
-Genny's engine & motor's combined flywheel effect?
-Genny engine's governor response characteristics (fast/slow?) come into play, as combined with:
-Genny's engine's speed -torque characteristics (powerband) come into play. Many factors, including fuel type (e.g. gas, diesel, or natural gas motor) will alter how a (say) 20 hp motor responds to a sharp increase in load.

 

[HarryVanderpool]

Low flow , just wondering how he is managing to get a 2HP pump rolling with a 5000watt generator. Just the 3/4HP pump bogs my Honda 5000 with low voltage and low frequency for approx 3 seconds even when the governor has the throttle blade wide open.

First of all, my generator is not a Honda.
It is a bare bones Generac with a 10hp Tecumseh.
Got it on sale at Home Depot in 1994 for under $500
It not only starts and runs the pump, but appears to do so easily.
When the well kicks on there is an about 1/2 second grunt and then it runs much the same. When the pump turns off there is a tiny change in tone.
No big deal.

 

[HarryVanderpool]

One thing I often think about in these discussions is having only as much generator as one needs.
We run our generator for 3 hours in the morning and 4 in the evening.
It takes care of all of our needs.
Still, you are looking at about 4 gallons or more each day.
The longest we have ever been out of power was 4 days.
What if it was 2 or 3 weeks? How much fuel does one have on hand?
Then add to that, people want their generator to run 24 hours daily!!????
YIKES!!!

Along with owning a generator, We have a procedure.
First, we burn wood in the winter so heat is not an issue.
When there is an outage and it appears to be serious, we place 2, 4 gallon buckets of water in each bathroom for flushing toilets.
So when the generator is off, our system holds pressure for all of our uses until the next run.
We shower when the generator is running.

Also, there is always talk about generators that provide clean power for electronics.
I can guarantee you that mine is not one of them!!
Yet, we always watch TV in the evening and have never had a problem.
Our computer is on a UPS so it doesn't matter how dirty the juice is.

So, I'm not just blabbing away. I have 23 years of experience with this set-up and have no complaints.

 

[bigdeano]

My 10kw generator would start and run my 3hp submersible pump where I used to live. It won't even budge the 5hp motor on my air compressor even though there's no load at startup.