Electric genset 3

[dvx]

How big should an electric AC motor be to run a 5KW DC generator; idle, 50% load, 100% load for a 1000 hours without problems?

Thanks in advance, your help will be appreciated.

DVX

 

[itsmoked]

Outstanding! Thanks for the info. Will do. It came out of a rail car when propane was recently outlawed in rail service.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Keith, how about running it unloaded for an hour or so and checking the propane consumption?
I was working away from home and n my wife and daughters were fueling the set when we were off the grid. As they remember, it burned about 10 to 15 liters a day. That was after I covered the radiator. At minus 20 C, minus 25 C, and minus 30 C. It was burning a lot more when it was running cold. Covering the radiator so the engine ran hotter made a big improvement in fuel consumption.
I had a gasoline generator for the first couple of weeks and the fuel consumption was insane. 25 liters of gasoline would last 16 to 18 hours. Power is a three cylinder Kubota.

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

 

[waross]

I was nowhere near 24 gallons a day with either set.
A good rule of thumb (derived from hours of checking specs for customers who wanted the best fuel economy) is 13 KWHr per gallon of diesel fuel. This holds true for most loaded diesel sets running 1800 RPM above about 25 KW. Some sets get a little better and some claim better but test on heavy fuel. When you normalize the specific gravity of the fuel, back to 13 KWHr per gallon. For unloaded or lightly loaded consumption, it may be anything. Check the specs.

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

 

[itsmoked]

I can't run it right now but when I do I'll let you know.

I have a 5.5kW one-lunger Generac POS. That baby sucks down the fuel even lightly loaded. It sucks fuel so fast it's a logistical nightmare.

The Onan is a 1800RPM unit which should do a lot better.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Side story off on your engine replacement tangent:
I did a project a few years ago to replace a direct drive 100HP diesel engine with an electric motor and VFD on a WWII era portable rock crusher. The nature of the machine was that depending on the rock being crushed, the speed had to be adjusted to avoid making dust or recycling too much and wasting fuel. So they had a sheave system with 8 belts that had to be changed. California has clamped down on diesel driven machinery of late because of air quality, so they were electrifying it and the VFD was a potential bonus in order to avoid the sheave changing. Not being an engine guy, I did the torque calcs based on the stated HP and speed of the engine so I could match the peak torque with the AC motor, knowing that the VFD would not increase torque so I would be stuck with whatever I designed in. I came up with a 150HP 4 pole motor.

Turned it on and the machine started up, but kept slowing down. My heart sunk thinking I had royally screwed up my math somewhere (knowing it was entirely possible). After a long boring troubleshooting story, it turned out we SMOKED the belts! The VFD driven AC motor was putting out WAY too much torque compared to the diesel.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

So, are you saying Jeff, that a 100HP motor probably would've done the job?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dvx]

Gentelman, I wouldn't have guessed that I would get so much feedback to my questions. I am more informed than ever and your time spent is very much appreciated.

 

[itsmoked]

From all of us - you're most welcome.

Do come back and tell us how things go if/when you do it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

So, are you saying Jeff, that a 100HP motor probably would've done the job?

When all was said and done and I got the torque limit programmed to where it wouldn't squeal the belts but didn't stall under the machine loading, I came to the conclusion that it could have been a 75HP motor. Oops...

But I was not able to test it under worst possible conditions, i.e. hardest rock, highest feed rate etc., because the location had a limited supply and the local rocks were not very difficult to crush. So we agreed that it was a good "safety margine" for the future when they move the machine to another location. But what I didn't exactly tell them is that if they ever need more out of that machine, they will have to change the power transmission system because no way those belts are going to hold under a heavier load.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376