had an interesting conversation with a generator

[larryccf]

engineer for one of the major US mfgrs - this was while on the hunt for a 1800 rpm 5- 8KW gen head

he made some interesting comments i hadn't thought about and figured for the benefit of the forum i'd post em

first off, i'm about as electrically intelligent as a brick - i basically understood what he was relaying (i think) but if i appear to miss-state something below, pls bear the above in mind BEFORE coming out flaming me. I'm just trying to share something i heard that sounded awfully logical and i hadn't thought about

basically his suggestion for best durability factor and best svc, he recommends oversizing gen head by a factor of 3 - regardless of the engine you plan to use to drive the gen, if you determine you need 5KW, buy a 15KW head - price difference isn't that much (and he was right about that) but the reasoning was fairly logical and something i hadn't thought about - if you're only going to use 5KW and size your engine accordingly (a 10-12hp engine), you'll never pull the extra kw out of the gen head, but when a major load hits, like a 5hp inductive motor or your 5 ton central A/C, the extra mass of the higher rated gen head's cast iron & copper windings give the gen head more inertia to meet that momentary surge in current demand, but also serves to better absorb the heat the gen head is generating, ergo the extra durability or service life factor. The engine you couple to the gen will be your limiting factor in how much current the generator will produce, so size the engine according to what you've identified as your needs.

he also advised against using a 12 lead gen head wired for 120/240V as they'd done a study and basically depending on how wired, 1/3 or more of the gen head is never utilized - stay with a 4 lead gen head. I think i understood why when he explained it, but not well enough to try to explain. For what i'll be doing, a dedicated 240V gen head will be best.

there were some other factors or items - i didn't realize battery chargers were "switching" loads (forget the exact term he used) but basically they were "mini-loads" occurring 60 times a minute, comparable to a surge load, in terms of destructive to the gen.

I've measured the current draw i need, that will keep the house comfortable in winter at 3480 watts 120V plus whatever current draw the battery charger takes. Because of his recommendation, i'll either go with 12-15kw gen head and a good inverter with battery charger management (which aren't cheap) or reduce the gen head some and use a double pulley, with one belt turning a 12V 150-200amp altenator and use a clutch on the 12v altenator to disconnect it when batteries are fully charged.

anyway, for what it's worth

 

[brin]

Larry,

I agree with him ..I only need about 12KW to power my house..but I bought a 20KW PTO generator with a 40KW surge capcity powered by my 29HP tractor and it handles the whole house and it is 1800 RPM as you say and not the 3,600 rpm ones that are so common.

 

[RonMar]

That does indeed work if you are not particularly concerned with efficiency. That oversized generator head is capable of delivering it's rateing under rather high ambient temps, so it has a pretty large blower on the shaft to pump air thru the case. This cost's HP and fuel to move this air and serves no purpose on a generator with an average load less than 1/3 of it's rated capacity, as it will never generate that much heat with the available HP and load. Modifying the blower to move less air and taylor the cooling airflow to your load would save fuel.

Another issue is internal energy consumption. I participate in another forum that deals with slow speed engines and home built generator/co-generator plants. A couple of us who have built similar generator sets did some fuel studies and the first killowatt costs the most in terms of fuel consumption. Below is a link to a graph of my generator's fuel consumption. It is a 6HP engine coupled to a 5KW generator head for a maximum sustained electrical power output of 3KW. It takes mine just over 1/8 Gallon Per Hour(.126GPH) to roll everything over at rated voltage, with no load applied. The first KW of load is .192 GPH the second is .266 GPH and the third is .350 GPH. You will note that the 3 KW load does not consume 3 times the fuel of the 1KW load. The reason for this is that the generator consumes a pretty large ammount of energy building the magnetic field in the rotor just to make rated voltage and current and to meet that first KW of electrical load. This energy is created by the generator and causes mechanical load that consumes fuel. Once this is accomplished however, the additional field current/flux to provide for a greater load is not much larger so the generator gains in efficiency and the fuel consumed per KW of electric load goes down untll the point where the field becomes saturated and efficiency peaks. A larger generator head requires more energy to excite and feed small loads than a smaller head loaded to a more efficient point... This could be an issue for extended power outtages when fuel may or may not be easy to come by when all the gas stations have no power to pump gas




I might not go as high as 3 times, but inertia is definitely your friend when it comes to starting larger inductive loads. I would use a generator that is electrically heavy enough to meet the surge demand without toasting it's internal wiring. Remember these startup surge loads are very brief. A heavy flywheel added to the mix will add the inertia to keep the generator up to speed over these startup loads, while still allowing the generator to remain in a more efficient region of it's load capacity(above 50% IMO). The smaller engine will be more efficiently loaded which is also good for efficiency.

Good luck

 

[larryccf]

interesting - hadn't thought about it in terms of efficiency loss but that makes sense - my focus to the engineer was a) durability and b) a 3hp motor i've got on a pump

be curious to visit that generator forum if you don't mind sharing the url (if board rules allow) - what got me started on all this was seeing a honda cogenerator unit and then brain cells lit up

i've got a liquid cooled honda 13hp engine that i'm rebuilding and was thinking about putting it into cogenerator useage (converting to nat gas fuel) and using the "homebuilt" cogen to recharge a bank of batteries

 

[RonMar]

That big pump will be the limiting factor on how small you can go. IMO, less is more. Does it have to be a 3HP pump?

As to the Forum, unfortunately it is not an open forum, but a "by invitation" affair, and mainly centers around slow speed listeroid type diesel engines...

 

[larryccf]

pump can't be eliminated -
i've currently got a 10.5/13.5KW gen that handles it fine
and a 4.8/5.8kw gen (makita w/11hp eng) that handles it but it's at it's limit, from the sound of how slow the engine dips when pump kicks on - it's sucking all the current available out of the gen, and overpowering the engine momentarily - which is why that engineer's logic was so appealing

i might settle on a 9 or 10kw head - i've also considered adding a dbl pulley and driving a 100-150 amp 12V alt for the batteries, and freeing up gen head capacity for other loads but need to find out output at 1800 rpm on 12V alternators

 

[RonMar]

pump can't be eliminated -
i've currently got a 10.5/13.5KW gen that handles it fine
and a 4.8/5.8kw gen (makita w/11hp eng) that handles it but it's at it's limit, from the sound of how slow the engine dips when pump kicks on - it's sucking all the current available out of the gen, and overpowering the engine momentarily - which is why that engineer's logic was so appealing

i might settle on a 9 or 10kw head - i've also considered adding a dbl pulley and driving a 100-150 amp 12V alt for the batteries, and freeing up gen head capacity for other loads but need to find out output at 1800 rpm on 12V alternators

That 3HP pump when up to speed is pulling probably 15A at 240VAC? That would be 3600W and within the makita's ability to operate, but as you describe, difficult at startup with the startup loads overpowering the engine. That same genset with a substantial flywheel would probably do OK with the stored inertia taking up the slack in the system.

I think you would be fine with a 10KW head copled to your 13HP engine as the engine will only be able to sustain about 6.5KW(2HP/KW rule of thumb). You mentioned an 1800 RPM head. What RPM does your engine deliver 13HP? At any rate, either belt drive or direct drive, building it yourself will give you the opportunity to add flywheel inertia which is also good from a frequency stability standpoint.

As for alternator output, that is the beauty of belt drive. You can adjust pully size to get the desired/peak alternator current output RPM for a given/fixed engine RPM. From an efficiency standpoint, a typical automotive alternator is only about 50% efficient. It's claw rotor design was built to be compact to fit under the hood, not efficient. From an efficiency standpoint, You would probably be better off with a AC powered charger taking 85% efficient generator power and making it into 85% efficient DC charge current(about 72% overall efficiency). If you are running a 10 KW head, depending on your loads, I could see the head itself running below 50%, so the added load of the battery charger would help to push it into a more efficient area of operation.

I know if I were contemplating a battery/inverter hybrid system, which has distinct advantages like the light switches working anytime during the night without having to run the generator all night, I would not be looking at anything less than a 24VDC system. My preference would be 48 VDC. The main reason for this is required current and system infastructure. Watts are watts regardless of the source. A 1200W load is 10A at 120V. It is 100A at 12V. This of course isn't taking into account inverter efficiency which is probably going to be in the area of 90%. So that 120V 10A load will take 110A at 12V to supply from battery to inverter. 110A at 12VDC needs a really big cable. It also needs big fuses. I arc weld with 100A and less, so a fuse big enough to supply a load can still lead to some pretty serios hazards and damage without opening the circuit. That same load fed out of a 48V battery bank would only need about 27A(4X the voltage = 1/4 the current)... A lot safer to fuse/protect and with the cost of copper these days, the added cost of a 48V inverter is pretty easilly offset with the payback in smaller wire and parts.
FYI

Good luck with your project.

 

[Stimw]

A good web site that has a lot of generator info is- Antique Engines and Old Iron Tractors on SmokStak
Not sure what you are trying to accomplish with a generator. I have REAL world experience with a gen and power outage. I have a Wacker 5.6kw contractor gen with a 11hp Honda. I had to use it for 12 days after a close call with a hurricane.
The Honda used .5 gal of gas an hour, after 12 days it used about 2oz of oil total running at 3600rpm. I shut it off twice a day to ad gas and check oil.
I had it hooked up to a 240v welder plug in my shop, main breaker turned off. I ran two 120v window AC's , fridge, lights, TV, satilite, microwave, computer, plus other things. What I did not have was hot water and I had to turn off one AC to use water. I have a well and pump.
I can't imagine running my tractor for 288 hours straight, at full throttle ( 540rpm) to have had power!

 

[larryccf]

That 3HP pump when up to speed is pulling probably 15A at 240VAC? That would be 3600W and within the makita's ability to operate, but as you describe, difficult at startup with the startup loads overpowering the engine. That same genset with a substantial flywheel would probably do OK with the stored inertia taking up the slack in the system.

I think you would be fine with a 10KW head copled to your 13HP engine as the engine will only be able to sustain about 6.5KW(2HP/KW rule of thumb). You mentioned an 1800 RPM head. What RPM does your engine deliver 13HP? At any rate, either belt drive or direct drive, building it yourself will give you the opportunity to add flywheel inertia which is also good from a frequency stability standpoint.

As for alternator output, that is the beauty of belt drive. You can adjust pully size to get the desired/peak alternator current output RPM for a given/fixed engine RPM. From an efficiency standpoint, a typical automotive alternator is only about 50% efficient. It's claw rotor design was built to be compact to fit under the hood, not efficient. From an efficiency standpoint, You would probably be better off with a AC powered charger taking 85% efficient generator power and making it into 85% efficient DC charge current(about 72% overall efficiency). If you are running a 10 KW head, depending on your loads, I could see the head itself running below 50%, so the added load of the battery charger would help to push it into a more efficient area of operation.

I know if I were contemplating a battery/inverter hybrid system, which has distinct advantages like the light switches working anytime during the night without having to run the generator all night, I would not be looking at anything less than a 24VDC system. My preference would be 48 VDC. The main reason for this is required current and system infastructure. Watts are watts regardless of the source. A 1200W load is 10A at 120V. It is 100A at 12V. This of course isn't taking into account inverter efficiency which is probably going to be in the area of 90%. So that 120V 10A load will take 110A at 12V to supply from battery to inverter. 110A at 12VDC needs a really big cable. It also needs big fuses. I arc weld with 100A and less, so a fuse big enough to supply a load can still lead to some pretty serios hazards and damage without opening the circuit. That same load fed out of a 48V battery bank would only need about 27A(4X the voltage = 1/4 the current)... A lot safer to fuse/protect and with the cost of copper these days, the added cost of a 48V inverter is pretty easilly offset with the payback in smaller wire and parts.
FYI

Good luck with your project.

lot of good info there, appreciate

i knew about the advantage of 48V vs 12V
didn't know automotive alternators were that inefficient -
and hp at 1800 is, per honda 8.2 hp, but that same engine has 3 different cams for it available - that engine was also used in japan in the 80s for use in a 2 seat car similiar to the Fiat 500 - cammed for the car use it produced a whopping 27hp - i'm going to play with hp after i finish the rebuild - i've got a 14 hp cam in sight (14 hp was the second variant). If i can't get power where i need, i'll go up on rpm and adjust gen rpm via pulley ratio (don't know why i forgot that for the altenator)

tks for the input

i'm going to finish with the engine rebuild first and look for a 1800 rpm gen over the next few months

 

[larryccf]

A good web site that has a lot of generator info is- Antique Engines and Old Iron Tractors on SmokStak
Not sure what you are trying to accomplish with a generator. I have REAL world experience with a gen and power outage. I have a Wacker 5.6kw contractor gen with a 11hp Honda. I had to use it for 12 days after a close call with a hurricane.
The Honda used .5 gal of gas an hour, after 12 days it used about 2oz of oil total running at 3600rpm. I shut it off twice a day to ad gas and check oil.
I had it hooked up to a 240v welder plug in my shop, main breaker turned off. I ran two 120v window AC's , fridge, lights, TV, satilite, microwave, computer, plus other things. What I did not have was hot water and I had to turn off one AC to use water. I have a well and pump.
I can't imagine running my tractor for 288 hours straight, at full throttle ( 540rpm) to have had power!

appreciate the link -

i've got plenty of experience using a couple of generators - similiar to yours - had hurricane come thru or rather the edge of it, about 1/4 of the city was without power for quite awhile - my 13.5kw gen ran for 9.5 days that one occurrence, and kept the house including central A/C running - have a total of 577 hours on it now, with 70+ on a smaller makita gen

but fuel burn on the 13.5kw gen was horrendous 17-22 gals per day (20hp honda) - that was what prompted the makita 5800w purchase (plus fact that it was a steal), for winter use when i don't need A/C running

i've got my loads etc figured out- the 13HP honda engine i'm rebuilding is liquid cooled, 2 cyl, overcam, real sweet little engine - the one on my tractor is now over 20 yrs and still showing good compression and running as smooth as a honda accord and equally important, it is QUIET - at 3600 rpm with a full load on it, you can have a normal conversation standing 5 feet away, with it's stock muffler

goal is to mate a gen to it, convert engine to nat gas, and then play with building my own cogenerator - go to Honda Power Equipment - Honda Home Energy: Micro Combined Heat and Power
then google for ECR International Inc to see the videos of it - it's a nice concept