Puppeteer

Author Topic: SIZING A GENERATOR FOR HOME USE  (Read 21909 times)

diesel guy

  • Full Member
  • ***
  • Posts: 164
    • View Profile
SIZING A GENERATOR FOR HOME USE
« on: December 09, 2007, 10:59:04 PM »
This is the reasoning why I choose oversized generator heads with at least 2.5 times and up to 4.5 times the continuous rating of the engine and with additional flywheel mass on the generator. Large surges need lots of headroom, especially when two or more motors start at the same time, refrigerator, freezer, well, ect.

http://gillettegenerators.com/sizing/sizing02.html

Diesel Guy

Doug

  • Hero Member
  • *****
  • Posts: 3386
  • Why don't pictures ever work for me?
    • View Profile
    • Doug's Petteroid Stuff
Re: SIZING A GENERATOR FOR HOME USE
« Reply #1 on: December 09, 2007, 11:20:06 PM »
Too each his own.....

A little oversized is fine but as has been said over and over and over again.
The Bigger the generator the more the no load losses at some point you have to ask yourself is the extra fuel I burn realy worth?

I drag a parachute behind my car because I want to shorten up my braking distance then I guess I can afford the extra fuel....
It's a Good Life, If You Don't Weaken

Stan

  • Guest
Re: SIZING A GENERATOR FOR HOME USE
« Reply #2 on: December 10, 2007, 12:10:36 AM »
I think the real point, which has been stated by myself and others several times on the forum is, Instead of sizing your generator to your engine, size your needs to your generator.

The cheapest and most efficient way to design a "generating system" is to first get rid of loads that are not really necessary.  Many people have air conditiioning up here in S.E. BC, and they only use it 4 or 5 days per year!  Why have an electric stove if you want to go off grid when propane/gas is much nicer to cook on and uses infinitly less electrical power. Get a fridge that uses fewer kwhrs than the fancy 2 door models with heaters in them to keep butter soft for gods sake!!!   yada yada yada

If you are serious about getting off grid, make your electrical cuts, then see what sized "generating system" you need.  You could run an entire house on less than 3kw and still be comfortable.
Stan

diesel guy

  • Full Member
  • ***
  • Posts: 164
    • View Profile
Re: SIZING A GENERATOR FOR HOME USE
« Reply #3 on: December 10, 2007, 12:42:43 AM »
Doug,

I can appreciate what your saying and you can say it over and over again all you want but it is a proven method that saves fuel not waste it when the engine is at an efficient rpm and when a surge hits the stored inertia in both flywheels assist in powering the load and the generator does not fall on its face when the headroom is exceeded.

If there was a generator design with less stored inertia the continuous engine speed would have to increased to produce the needed power from the lack of stored kinetic energy.

Today’s latest series hybrid propulsion systems (I know first hand with four provisional patent applications applied for) that the military and commercial systems “all” have a 1.75 to 2 to one oversized which has proven to offer the best efficiency.

The reason behind the 2.5 to 4.5 to on oversize previously stated is that the engine can operate in a “maximum” fuel economy mode 24/7 if required, then still be able to handle several surges all at once if need be from the stored kinetic energy, which can happen many times during an usual day. This is mechanical storage and a hybrid system has electrochemical storage from the battery bank or capacitors to perform the needed work for assisting the engine. 

This design enables cleaner electrical output to sensitive equipment without an increase of rpm of the engine.

This also keeps a more constant load on the engine (24/7) due to it will be operating in a higher percentage of available power than operating an engine in a high rpm range 24/7 just to meet the peak load requirements from the surges. When an engine operates under minimum load percentage due to a high rpm requirement, it burns “much” more fuel and carbon buildup than an engine properly operating under a higher load percentage at a lower rpm.

Stan,

You bring up a good point, if everyone did that we wouldn’t need to oversize generator nearly as much, closer to what Doug mentioned.

Diesel Guy

Doug

  • Hero Member
  • *****
  • Posts: 3386
  • Why don't pictures ever work for me?
    • View Profile
    • Doug's Petteroid Stuff
Re: SIZING A GENERATOR FOR HOME USE
« Reply #4 on: December 10, 2007, 12:52:22 AM »
I came across a little abrasive there....

If going with a head twice the size you need works for you specific system then you are the best judge of that.

A 6/1 and a ST5 seems to fit your criteria and some people say that a ST7.5 is even better for the 6/1.

I believe its better to match a Generator to the load and prime mover to the generator. By all means mass is a good thing.
Hey can anyone here burn me a 13 inch diameter, 2 inch steel plate machines and bushed for a 22 mm shaft?

But a stationary generating set is not a car.
« Last Edit: December 10, 2007, 12:58:30 AM by Doug »
It's a Good Life, If You Don't Weaken

cujet

  • Hero Member
  • *****
  • Posts: 957
  • Lister power rules!
    • View Profile
    • www.cujet.com
Re: SIZING A GENERATOR FOR HOME USE
« Reply #5 on: December 14, 2007, 12:42:23 AM »
This is all great in theory, AND, I agree to an extent.

But, experience with Listers has shown that those great big flywheels do not store all that much energy. They are "going slow" after all. A surge load will drop the frequency, obviously slowing the engine in a fraction of a second. Within 2 or 3 seconds, it is all over with, and the engine speed is down below acceptable limits.

Even with a hand on the governer, applying full rack, accelerating the engine to a higher RPM just before the load is not enough. They bog right down.

Couple that with the fact that the surge overload is often well within the design specs of a good gen head.

I put an ST15 on my 20HP twin. I could have easily used an ST12. And my electrical power quality is very poor until I have the gen head highly loaded.

It may be that if you choose to use a WAY oversized ST, you will always have a crappy sine wave when running modest output.

Chris
People who count on their fingers should maintain a discreet silence

diesel guy

  • Full Member
  • ***
  • Posts: 164
    • View Profile
Re: SIZING A GENERATOR FOR HOME USE
« Reply #6 on: December 14, 2007, 01:54:10 AM »
cujet,
You are correct with your annalist on how the engine will react when placed under a fast load ie: motor starting. Then you are also correct about manually throttling the engine to full load would not enough. There is one thing you missed and it is the most important. The stored kinetic energy "within" the flywheel on the generator "exceeds the stored kinetic energy on the heavy flywheels on the 14/1. This is my pulley from Mike on coppermine:

SuperMax Flywheel/Double PulleyThe pulley shown weighs around 120 lbs and is built with three pieces, making it a modular construction so that custom pulley diameters can be easily upgraded or changed. The flywheel section weighs 70 lbs and is 13" diameter and 2" thick. These pulleys use a standard SK taper bushing. This one is for 600 and 850 rpm, 7 7/8" and 11 3/16" diameter pulleys

To see what is happening in slow motion. A large motor is started, this places 3 to 5 times the running wattage momentary load on the generator head for 2 -3 seconds. The generator head is oversized for two reasons. First, it provide plenty of head room for high surges and it has a much heavier rotor than a smaller generator head, also the bearings are much larger than if I used a properly matched head. This extra strong shaft and bearings allow me to place a flywheel/pulley system weighing over 120 lbs. without over working the bearings and reducing bearing life.

Now if I used a properly sized head 7.5 kw, the rotor ( I haven't weighed them but its close) would be about 60 lbs. The rotor on the 20 kw would be 120 lbs. approx. I have 120 lbs in a small diameter (rotor) spinning 30 times a second and a 120 lbs flywheel/pulley in a 10.5+ inch average mass diameter spinning at 30 times a second. This is just in the generator head alone, the produced surge has to overcome and slow down this stored potential before it even starts to place a load on the belt then to the engine. Then the engines flywheels stored potential assist the generator head stored potential. As far as slowing the engine down, since the surge has to slow down both flywheels, the slow down would be much slower than if the load was suddenly placed on just the engines flywheels, therefore the engines governor can more accurately follow the power requirement needed, without the stored potential in the generator head the engines governor and engine could not meet the demand nearly as well.

This is no fix all to replace an electronic governor with a properly sized generator head, what I'm trying to archive is having a somewhat tighter frequency regulation when multiple motors are engaging at once ie: refrigerator, freezer, well and not have to worry about unacceptable frequency drop without any electronic controls. You are also correct with the poor sine wave under lighter loads. Everything is a give and take in this world and this is the way I do it and as you and Doug point out very accurately it has it drawbacks as well as it's strong points.

Thanks for your input
Diesel Guy


diesel guy

  • Full Member
  • ***
  • Posts: 164
    • View Profile
Re: SIZING A GENERATOR FOR HOME USE
« Reply #7 on: December 14, 2007, 02:39:12 AM »
Jens,

The chirping is directly proportional to the capability to store potential energy vs. mechanical output from the engine. I don't know what engine you have, but my 14/1 has extra heavy 23.5" flywheels and I only 8.5+ horsepower continuous @600 RPM. Therefore, the acceleration of the power stroke is less abrupt than a more powerful 12/2, 16/2 ect with lighter and/or sometimes smaller diameter flywheels.

The stored potential in my engine absorbs and releases the energy more evenly, due to min. HP output vs. the heavy flywheels, so chirping the belt is not my issue. The extra heavy flywheel/pulley is a good match for my system and maybe wouldn't be if it was a 28/2. That is a very good point you bring up for other members that have high HP engines with minimum stored potential capability (light flywheels).

Diesel Guy


BruceM

  • Hero Member
  • *****
  • Posts: 3054
    • View Profile
Re: SIZING A GENERATOR FOR HOME USE
« Reply #8 on: December 14, 2007, 03:18:54 AM »
I noticed the same (serpentine on flywheel) belt slipping as Jens on my lightweight wheel'd Metro 6/1 driving and ST-3. once I added a voltage regulator. At first I though it was something wrong with my regulator, as it didn't happen on the harmonic driven stock setup.  The voltage regulator really slams it to the rotor when the voltage drops, so the tendency for belt slip is greater.  Increasing belt tension did solve it.

Regarding balancing of flywheel and generator mass for the 6/1's I think we'd find that Lister got it just about perfect on the SOM.  Geno did some tests and the SOM flywheels on his 'oid made a huge difference in his ST output 5.5Hz "lister-flicker". Better than 90% reduction measured with my filter circuit and an O'scope.  The SOM generator head was quite a big fella and also had a built in DC generator/starter motor, which provided rotational mass. The SOM combination of DC starter/generator was a pretty sweet setup, you have to admit, plus the auto start/shutdown capability which I also have added to my Metro with a Brit Picaxe controller.  Back to the Future?!

I opted to take Stan's approach for my off grid place-  less is more.   My 6/1 also powers an air compressor that feeds a 500 gallon storage tank at 150psi, which powers my wood shop and all air hand tools (even my Skillsaw has been converterd).  That 500 gallon tank sure makes a nice "flywheel".  My whole setup was designed so that I could get by on just 1000watts, though being able to do two 1/2hp motors at once (pump water to the big gravity tank while running the washer) works on my 6/1 with ST-3 and is a nice convenience.  So my backup genny is a $350. Champion- (Honda OHV knockoff) which will still keep the place going in a pinch.

Now I'm working on a +/- 60VDC battery bank charger/regulator. Shooting for about 1000 watts of charging capacity which is doable on a bank of that voltage, even on flooded batteries. With AGMs I could do my full ST-3 output, but I'm still thinking of sticking with 1000 watts of load so that I could still do laundry or ???. Less is more!

BruceM














diesel guy

  • Full Member
  • ***
  • Posts: 164
    • View Profile
Re: SIZING A GENERATOR FOR HOME USE
« Reply #9 on: December 14, 2007, 03:24:12 AM »
Jens,

Yes it does the power pulse are about the same, but there is one more thing I forgot to mention, I run slightly retarded timing as a previously mentioned in other posts in the past.  I run 16 degrees BTDC, I can get away with this because of the DI head. This makes the engine very smooth with this setting. When the timing is at 20 - 21 degrees and the piston is almost at TDC, then the fuel is injected. This slows the crank speed down momentarily as the stored potential in the flywheels has to overcome the thermal expansion.

As the piston past TDC it is rapidly accelerated from the extremely high cylinder pressures. This causes most of the chirp mentioned, because the generator head is steady and the engines flywheels slow slightly provides a small slack in the belt, then when accelerated from burning the fuel, there is no resistance from the generator for a slip second and the engine accelerates quick, then the slack is taken up and you have the mentioned chirp. As I mentioned before, lazy timing makes for a smooth and reliable engine.

You can’t retard too much or it will produce carbon buildup and reduce fuel efficiency. All your internal components will benefit from this setting and increase the operational life of the engine and produce smoother electrical output. The heavy flywheels on both the generator and engine can accelerate more uniformly with lazy timing and belt tension don’t have to be over whelming to provide the proper traction needed. Thanks for your input.

Diesel Guy


diesel guy

  • Full Member
  • ***
  • Posts: 164
    • View Profile
Re: SIZING A GENERATOR FOR HOME USE
« Reply #10 on: December 14, 2007, 03:59:16 AM »
BruceM,

If I could buy a new Start-O-Matic now I would. I think they were the pinical of Lister power.

Diesel Guy

diesel guy

  • Full Member
  • ***
  • Posts: 164
    • View Profile
Re: SIZING A GENERATOR FOR HOME USE
« Reply #11 on: December 14, 2007, 04:43:01 AM »
Jens,

I think you should experiment with your timing first, it's very easy to do. Look up how to do it. One thing I would add to the procedure, scratch a line and mark your TDC on the flywheel then scratch lines and mark in the 18 - 19 - 20 - 21 degrees BTDC on the flywheel. You have IDI heads so this is where you want to be.

Now you can quickly change from one setting to another to match your needs, burn rate of the fuel used and load settings. Hotater is running just over 19 degrees and he has excellent operational life in his engine.

If you are running a more constant load, ie: DC charging you can get away with more retarded timing without carbon buildup and longer engine life than when you power an AC load. I would try 19 to 19.5 degrees and see how that works. Altitude, fuel types and load will determine which is best for you. You can do it, once you do you can't stop playing with the timing to fine tune your engine. Timing is everything.

Diesel Guy


diesel guy

  • Full Member
  • ***
  • Posts: 164
    • View Profile
Re: SIZING A GENERATOR FOR HOME USE
« Reply #12 on: December 14, 2007, 04:57:40 AM »

rcavictim

  • Certified Generator Head and Grand Master Sparky
  • Hero Member
  • *****
  • Posts: 1827
    • View Profile
Re: SIZING A GENERATOR FOR HOME USE
« Reply #13 on: December 14, 2007, 05:53:42 AM »
is there a reasonably accurate way of getting TDC that doesn't involve pulling the head ?

Jens

The valves are close to the piston at TDC.  Why not locate the piston to approximate TDC on the top of the compression stroke (both valves closed) and remove the exhaust valve spring.  Let the valve drop a fraction of an inch to set on the top of the piston.  Place a magnetic dial indicator onto the top of the valve stem and read it`s position to give you TDC and + and - points of TDC.  I see no need to pull the head.
-DIY 1.5L NA VW diesel genset - 9 kW 3-phase. Co-gen, dual  fuel
- 1966, Petter PJ-1, 5 kW air cooled diesel standby lighting plant
-DIY JD175A, minimum fuel research genset.
-Changfa 1115
-6 HP Launtop air cooled diesel
-Want Lister 6/1
-Large DIY VAWT nearing completion

oliver90owner

  • Hero Member
  • *****
  • Posts: 861
    • View Profile
Re: SIZING A GENERATOR FOR HOME USE
« Reply #14 on: December 14, 2007, 07:10:17 AM »
Jens, you wrote : Dang .... I guess I have to break down and finally pull the heads. I have been trying to avoid that up til now.

Think slightly 'out of the box'.  You are not setting it exactly.  You are experimenting with different settings.  You are going to settle on the setting that suits your conditions.  You don't need to know the precise timing you are setting, only the changes you make,  from what you have now.

So, why do you need to know TDC so accurately?
Put a mark on the fly wheel where you are at NOW and vary the timing until you optimise it.  No head pulling required.

OR put some thing in the injector hole and turn the piston until it rests against the item  (thich copper wire would do), then do the same in the opposite direction of rotation.  TDC half way between. Simple, without pulling the head.

It would probably be better to optmise not knowing the exact setting, because you would not be biassed by previous 'knowledge' of where it 'should be'.  Then actually measure the setting at some future date when you can do it accurately without going to a lot of trouble (possibly only to find it was OK where it was at, in the first place!).

KISS is my principle every time.  From reading a lot of your posts, I think you try to make it as complicated as you can.  These engines are simple.  Treat them as that.

Regards, RAB