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

 

[waross]

5KW with a service factor of 1.15. You must supply 5 KW plus the losses in the generator. Any motor above 5.5 KW whether the rating or the rating times a service factor should do. If you will be pushing the envelope in terms of ambient temperature during times of peak loading you may want to go a little larger.

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

 

[dvx]

Thank you Bill,

I was worried about the generator being a strict half or quarter size of the motor. I am not an electrician so this may be a some what preposterous question to you but your answer helps me a great deal.

Thanks again,

Dennis

 

[waross]

Remember that although transformers will often take an overload in stride, (distribution transformers in particular may at times be grossly overloaded) generators are limited by the power of the prime mover.
If you anticipate any overloads on the generator, the motor must be large enough to handle the peak loading or it may slow down and stall.
But basically the law of conservation of energy holds. Energy in equals energy out plus losses.
A note to avoid confusion: AC gen-sets that must start induction motors across the line will generally have the diesel engine and the gen-set sized a minimum of three times the size of the motor plus other loads to handle the starting surges.
You should not have that problem with a DC gen-set but if you anticipate short term peak loading much above the rating of the generator tell us now.
Yours
Bill

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

 

[itsmoked]

Bill: Wouldn't he need more like a 6kW motor? DC generators are only about 80% efficient. That would suggest to me he needs 20% more prime mover than the DC generator's rating.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Good point Keith. lps I hope dvx takes note of my comment "You must supply 5 KW plus the losses in the generator."
and does his due diligence checking the losses of the generator.

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

 

[itsmoked]

Yeahhh: I knew something!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I've been following your posts for years now Keith. You know more than just "something". Grin


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

 

[dvx]

Thank you very much everybody for your input it is very helpfull and appreciated.

I presumed that the motor should not be much larger than the generator. If mechanical losses in the generator are combined to 2-3% from output of the generator 5.5KW x 3 / 100 = 0.165KW. Mechanical losses in the motor 2-3% from total power 6KW x 3 / 100 = 0.180KW total maximum losses in the gen set are no larger than 0.5KW (0.165 + 0.180 = 0.345KW)

That means that a 6KW motor should easily handle a 5.5KW generator.

What puzzelled me is on a trip to a local hardware store I looked at a GenSet specifications of which are as follows:

5.5KW, 110v, 60HZ, 3600RPM, 1phase
Gas motor 14.7HP x 0.746 = 11KW ??!!

I wonder if anybody knows the reason why a manufacturer would make such a design where the motor is 14.7HP = 11KW which clearly doubles the size of the generator??!!

Is the design of the motor so poor that half of the power the motor produces are lost in it's own mechanical losses?

Thanks,

DVX

 

[waross]

I knew a gentleman who instructed a course in small motor repairs for years. He used to say when it comes to the horsepower rating of small motors, the first liar didn't stand a chance.
I doubt that that motor would come close to 11 KW on a dyno. That is the nature of small motor HP ratings. Calculate what you need and buy double.

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

 

[itsmoked]

You always need a much larger internal combustion engine to run a generator than the generator's rating.

Running a generator with a too-small engine causes the engine to run at 100% which forces it to maximum internal temperature, maximum mechanical stress, often beyond its peak torque curve, and certainly beyond its peak efficiency. That just covers the static issues. Then there's the dynamic issue. A generator can take large instantaneous hits that only result in a temporary thermal excursion. An engine hit with a massive but brief overload generally cannot respond at all. It just slows down or something breaks. They slow down rapidly too once they're past the peak of the torque curve. This is why you need much larger engine kW to drive a small generator kW.

I have a two bearing 10kW AC generator. The instructions state a 20hp minimum is needed to drive it even though the math sez only 13hp. That's for all the reasons just stated.

dvx; I don't agree with your numbers. The conversion efficiency of your DC generator is is only about 80 to 85% why are you stating only 2~3%? I believe you need at least 6kW or you will be sorely disappointed. If you are never going to demand 5kW from your generator then you might get away with a little less. You first stated 5kW but now, I think, you just stated 5.5kW. That would mean a little larger yet.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith. When you get up past about 25 or 30 KW in diesel sets the engine sizing is pretty close.
You are correct about losing speed on overloads. Motor starting is a good example. The under Frequency Roll Off feature of the Automatic Voltage Regulator allows a small (about 3 Hz) drop and then drops the voltage in proportion to the speed loss. This keeps a good Volts per Hertz ratio on the motors and greatly aids recovery.
The OPs mention of 5.5 KW was not his set but an example he saw.
Many smaller sets use capacitor excitation. This is very common on small gasoline driven sets. There is no AVR as such and so no UFRO. Add to that the optimistic HP ratings of small gas engines and you need an engine rated quite a bit more than the generator rating.
A few comments on diesel sets above 25 or 30 KW. The fuel delivery is often set during assembly of the set to limit the output of the engine to the rating of the generator end. A standby set will be rated at the maximum. A prime set will be de-rated 10%.
A single phase set in this size range will often be a reconnected three phase set and the fuel setting will not be changed. So a set rated at 60 KVA / 48 KW will be rated at 40 KVA on single phase. No change to the engine so this set will be capable of 40 KVA / 40 KW on single phase.
We had a situation in our little island utility where the load would come up to the KW rating of the 600 KW generator and we would have to start a second set. We had the idea that if we corrected the power factor we could push out more KVAs and carry a little more load before starting a second set. We tried overloading a 600 KW set briefly as a proof of concept. No joy. At a little above 600 KW (not enough to be useful) we started loosing speed.
But, for small sets with gasoline engines and capacitor excitation rather than an AVR you are right on the money.
Oh, and I have had several diesel sets (50 KW to 100 KW range) that wouldn't put out full power because the fuel stop adjustment had inadvertently been changed.

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

 

[itsmoked]

Thanks for that info Bill. I should've clarified gasoline. I was just thinking "gas" as I wrote that.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Yes both gas and the excitation system used on the small sets. My personal backup is a nice little 6 KW driven by a three cylinder diesel, trailer mounted with a four KW, thirty foot crank up light tower. As well as power failure back up we used it for area lighting for the outdoor grad party at my friends farm.
Back on topic, it has capacitor excitation and the engine is about 50% over sized.

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

 

[itsmoked]

Mine's a 3 cylinder air cooled propane 15kW. I just wish I had one of those micro-trailers to mount it on.. That light is nice on yours!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The 6 KW is quiet and has supplied the house with no trips during an extended power outage. I have found that the biggest expense of running on a gen set is keeping the set running when you are not using any power or very little power. To that end I think that 6 KW is the sweet spot for my home.
How have you found the fuel consumption on the propane set? I haven't worked on a propane set.
Here's a picture of an identical unit.

http://winnrentals.com/images/Light Tower.JPG

That's a three section telescoping tower that cranks up about 30 feet. It will light up several acres.
Last winter my friend called; his power was out. I was hooked up and on the road in about five minutes. The power was restored before we got hooked up, but it sure is nice to be able to respond like that.

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

 

[itsmoked]

Since I've gotten this booger I haven't had a power failure!! So, I don't really know what it does with low loading. Hopefully not too bad.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[wayne440]

I have owned and worked on several sets in that size range. You will be close if you plan on "about" 1gph unloaded and 3 gph at full load. 3 cylinder air cooled? Who made it?

 

[itsmoked]

Wrong generator.. The three cylinder. Sorry for the confusion Wayne. Mine is actually a 4 cylinder Onan air cooled. It has a large centrifugal blower mounted on the front that blows thru ducting up and out around the four cylinder heads.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[wayne440]

That is an Onan JC; the consumption figures I quoted are good for it. They will outlast just about anything you can buy these days. Obtain a spare starter and voltage requlator for it when you have the opportunity, those are the most common failures.