SIZING A GENERATOR FOR HOME USE

[diesel guy]

Doug,

I like redundancy, three engines are better than one and sound better too.

I was think of running a large DC compound traction motor to help assist the engine during peak requirements, but that’s another story.

I also wanted a 20/1 as well, but the 20/1 has more displacement than the 14/1 and I read that at 500 RPM the mechanical governor had something like a 5 or 6 cycle swing from no load to full load, maybe that could be corrected but I don‘t know enough about them to make a conclusion. To provide a tighter frequency regulation the engine speed might need to be increased (maybe 600 RPM?). This would elevate the available power much higher than I need, and increase fuel consumption, which in a 24/7 engine would be not efficient. It could also wet stack from a lighter than required load. If it was attached to a large DC permanent magnet motor/generator with battery bank and inverters, then the speed variation wouldn’t be an issue and it would make a great power plant operating at 350 - 400 RPM.

I think a 14/1 (5.12 bore x 5.75 stroke) or a 12/1 (5.12 bore x 5.5 stroke) is a perfect balance of power and fuel efficiency for my needs at least. Also, on another post, Tom stated that he liked the available parts for his 6/1 which would last years for now and I couldn’t agree with him more. Many of the upgrades used on the 6/1 can be used on the 14/1 as well, they share my components. I also choose the operating speed of my 14/1 to be set at 600 RPM just like a Lister 5/1. I could of went to 650 RPM for more power but opt to the lower speed due to a slightly longer stroke than the 6/1. This way would keep the piston speed slightly lower than a 6/1 @ 650 RPM.

Diesel Guy

[Doug]

redundancy was never the plan.....

What I have is a pair of B&S flat heads from old lawn tractors, combine that with a large induction motor and capacitor bank and I now can supliment the Petteroid power for peak demand. Maybe I could add as much as 2000 - 4000 watts before the system became unstable I don't know.

It wouldn't require any fancy syncronization but I would need to add some series resisitors for bringing a second unit on line to cussion the shock and I'd need to monitor things very closely since voltage and frequency regualtion would be hard to control with two goveners working together or more like against each other.

I could bring a second gas engine on line in about 5-10 seconds this way and thats slow if all I was interested in was trying to increase my surge capacity.

I noticed something interesting at work the other night. An old power factor corrected fan ( over corrected actualy ) I pulled a switch on running ( something I don't normaly do but I didn't feel like walking out to the start stop station on a lazy afternoon ). From the switch room I can guess I had about 3 seconds on induction generation off the unit, just enough to keep the controls live and hold the main contactor in. I happend to have an aprentice with me and restated the fan to show him what was going on ( good teaching tool about the dangers of stored energy and the ability of motors to generate ) How this relates back is I Think with enough mass behind it and a engine to drive it just above syncronous speed an asycronous generator running at minimum might be able to contribute not only a little extra starting current for heavy loads but some stability for lighter loads. 

[buickanddeere]

  As for ideal speed? We should consider piston speed which varies with stroke length and rpm.  Also flywheels store energy with a square of the rpm. So dialing up the rpms from 450 to 600rpm would add  1.78X more stored kinetic energy.

[oliver90owner]

B&D,
I lost the thread a bit there.  Where did the 450rpm come from?  2.1 times is what I make it for stored energy increase (along with about an extra 30% (?) power available with a slow speed diesel) and Diesel Guy was talking of only 50 rpm diffference (c.20% store energy increase).
Regards, RAB

[diesel guy]

Doug,

I was just kidding about the redundant engines I knew what you were getting at. It is interesting but I wouldn’t do it that way. I don’t like to rely on a sensitive electrical systems when I need a power system, but it would be fun to work with and goes to show there is more than one way to achieve the same goal.

Oliver90owner,

The reason behind the 600 RPM set speed is not only for fuel economy and minimum engine wear, but I felt that below that speed, the frequency would vary to unacceptable levels. A reduction in stored kinetic energy is the only drawback from this low RPM. This is part of the reason for the heavy generator flywheel, to help supplement in the loss potential from the reduced engine RPM. Unfortunately you can’t have everything in life, it’s just give and take and everyone has to find there own balance between the two.

Diesel Guy

[jzeeff]

I wonder how hard it would be to take a bunch of old disk brake rotors and make a good 1800 rpm flywheel.

[carlb23]

I wonder how hard it would be to take a bunch of old disk brake rotors and make a good 1800 rpm flywheel.

They are not that heavy so you would need a bunch of them but I like the concept.  I think you would have to mount them on a jack shaft and have the jack shaft turn the turbine.

carl

[Oiler]

Hi there

It is possible to use less than 4000 kw hrs a year.
We do that in a 5 pers household.

Oiler

[Stan]

Anyone can run on even less than that.  The real question is, can you do it and still keep your wife?

I'm not being flip when I say that.  What I really mean is that the amount of power you use is not a function of "hardware" its a personal or "software" issue.
Stan

[bitsnpieces1]

I wonder how hard it would be to take a bunch of old disk brake rotors and make a good 1800 rpm flywheel.

They are not that heavy so you would need a bunch of them but I like the concept.  I think you would have to mount them on a jack shaft and have the jack shaft turn the turbine.

carl

  There's a picture and thread on here about using (truck) engine flywheels on a jack shaft to provide a flywheel with a good bit of mass and capable of high speeds, 3000+.
See Rtqii  pics in  Coppermine gallery.

[buickanddeere]

  Oilver90owner

 I just picked 450 and 600 out of the air as they are in the operating range for Listers. And for 450 and 600rpm not seeming to be a whole lot of difference at 1st glance, the stored energy is a factor or 1.78 times. A sizable difference and can make or break a starting load's success.

[oliver90owner]

B&D,

Sorry, keyed in 650 as standard rpm.  Was late for work by then.  I am hoping the flywheels on the CD (epay) are reasonably heavy duty and will be a bit better than spoked CS flywheels.  Maybe I can hang some more mass on the shafts with the extra bearing size. 

I reckon an outboard bearing on a generator is probably the best place to hang some extra high speed energy store - it will not be affected by any altered engine drive ratios for a start.  More likely to get rid of flicker than increase starting load anyhow, is my thinking. 

With Lister SoMs, there was never any design thought for flywheel mass other than flicker problems.  Keeping it constant at normal loads was all they were worried about.  Your comments about actual real surges and current loadings seem to about sum it up.  I reckon it is more an available product at a seemingly more 'value for money' price that attracts most people to fitting over-sized heads. Efficiency is never (or should not be) the most important design requirement for the military.

Diesel Guy is probably one of the minority who have gone for the extra bearing size as a consideration, and is knowingly prepared to put up with the lower generation efficiency.  Most probably only use the advertised values of generator efficiency which are always massaged to give the most favourable impression to those who do not look through (as in between the lines) the advertising hype.

The energy efficient types, who use their energy to drive a heat pump, to multiply the effective heat energy transferred for use, know that the COP (coefficient  for calculation of energy recovered per unit of energy input) is notoriously optimistic for all scenarios except for the one set of conditions mentioned in their advertising hype.  I suspect the same, although perhaps to a lesser extent, occurs with generator heads.

Regards, RAB

[buickanddeere]

  I agree the best place for additional flywheel kinetic energy storage is on that 1800rpm alternator shaft.
  Also in agreement it would be best not to load a 10KW "rated" alternator 24/7 at more than 7.5KW That full 10KW rating is a stretch at best. Winding temps will be high and insulation durability will be reduced at an exponential rate. Particularly for those alternators operated at temps above 70-80F ambient.
  Diesel guy has raised the important issue of tweaking and tuning the engine to suit the unique application of load(s), temps, elevation, fuel, rpms, flywheel effect, % of max  HP etc.