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Harbor Freight 800 Watt Generator Modifications
#1
Harbor Freight had their little 800 watt generator on sale for $88 so I went ahead and bought one.
The idea is to use it for a battery charger when needed.  As they are pretty small and this one is quite cheap, they can come in handy.

After doing a lot of research, it seems the story on these generators is Yamaha designed them, and had them made in China, and then the Chinese just copied them and starting selling the clones to resellers.
So this model is available under many different brand names.  

   

It seems there is zero quality control, they slap them together and put them into a shipping box.
The manual says to break them in with 25 hours of runtime before a full 800 watt load.  
Really?  25 hours.   Oh well, so after buying this, I had a couple gallons of old gasoline from a couple jetskis I tore apart, and thought this is a good way to get rid of this old gas.  After running for about 4 hours unloaded, I hooked up some lights and a fan, and ran it another 8 hours till the 1 gallon tank was empty.
Then I put in another half gallon, and it ran for another 6 hours, and then I tried to restart it the next day, and busted the pull cord mechanism.   This doesn't seem to be uncommon.  So with 18 hours of runtime, I disassembled the unit just about completely, and took a look at the parts.   There was already a good amount of carbon buildup on the piston and exhaust port.  I suppose the nature of a two stroke, and my gas was probably overly rich in oil.   So next up, weigh the parts and decide how to modify it.

   
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#2
So here is a breakdown of the weight of this portable generator.  Assembled it weighs in about 35 lbs
Weight is lbs.ounces I apologize it isn't grams

8.00   Generator Stator
5.00   Generator Rotor
6.08   Engine Block/Crank/Piston
3.03   Engine Cylinder
2.14 Gastank
1.10 Muffler
1.05 Generator End/Cover
1.04 Pull Cord/Recoil Cover
 .14 Flywheel
 .13 Cylinder Head w/plug
 .13 Electric Outlet/Panel
 .08 Misc Bolts
 .09 Carb

Roughly you can say half the weight is the Generator ~16lbs and Half is the Engine ~16lbs.
So what can be modified to reduce the weight?  

Two simple ones:
Replace the steel gastank with a plastic tank.
Remove the steel PullCord/Recoil and use portable drill instead.
Make a new muffler out of aluminum.

After this, the level of difficultly and complexity goes up a lot.

The engine cylinder is cast steel and most small engines use aluminum
with a steel cylinder liner. This would be much lighter, but not easy to DIY!

The generator itself is not particularly lightweight for 800 watts.
The way the stator windings are done, there is a lot of wasted copper.
The rotor is also rather heavy with its copper windings.

One could also just replace it altogether with another motor/alternator.
Given only want 14v dc output and not 120 vac, this would make the most sense.
I have a 700 watt 36v BLDC motor that is only 6 lbs, so half the weight.
But it isn't cheap, and not easy to change the motor/gen drive coupling.

I think another way is to replace the rotor with a permanent magnet rotor.
With a permanent magnet, we could then operate the generator as a motor,
and not need a portable drill to start the engine.  This would be a really nice feature.
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#3
The gen head is setup to be a 120 vac output generator at 3600 rpm.
It is a self-excited brushless design.  This design, while simple, is not very efficient, because it isn't generating power through the full 360 degree cycle 60hz waveform, and essentially half the poles are used to charge up the wound rotor that is connected to a capacitor.
This wound rotor is shown below.  It has heavy steel laminations and two coils.
   
So the plan is to modify the generator head to use permanent magnets.
But just replacing the rotor with magnets isn't going to be sufficient.
We want to rewind the stator so it doesn't have as much wasted 'overhang' as a one-pole winding.
All this copper that winds outside the laminations is just extra weight and resistance.
   
Furthermore, we want to change the gen head so it can run as a motor to start the engine,
and a single pole motor isn't going to work.  

So we are going to rewire the stator into a 3-phase design.
Effectively, we are creating a 3-phase brushless permanent magnet motor from the existing single phase generator head, and then use this motor to generator electric power. Given the size of this stator and rotor, we should have no problem getting twice the power out of it 1.6k, perhaps even more.
Here I have removed the windings from the stator.
   

It turns out all that wasted copper was really cheaper aluminum wire, so it didn't really weigh very much!  But the additional resistance of the aluminum wire would make it even less efficient.

Next step is to figure out how to wind this stator to get a 3-phase design.

A good video that explains how a 3-phase brushless motor works is the one below





A good paper that describes the options in how to wind a 3-phase motor is this paper by Hernershot.
3-phase windings

Our generator head has 30 slots, so given we must have the number of poles divisible by 3, we really only have three options, 3 poles, 5 poles, or 10 poles.
We use 10 poles, so we wind around a single slot, which minimizing the wire overhang.
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#4
This program http://www.bavaria-direct.co.za/scheme/calculator/ was written that generates motor windings, and it came up with a couple choices.
I have decided to go with a 5-pole winding with 20 magnets.  

   

This skips the even tooth, and so I have modified the stator fairly extensively to eliminate the skipped teeth and lighten up the stator a few pounds.
After drilling holes and removing the teeth by working and prying, I then had to construct a modified grinder so I could get a grinding wheel inside the stator and grind out the rough edges, and make the odd teeth a little longer, and of course there is more room for more windings around the tooth.
This took many hours, and I felt like I was grinding cylinder heads as I did in my younger days trying to make more horsepower from my race engines.  Smile

According to my bathroom scale, it went for 8 lbs to 4 lbs, but using my 5lb postage scale, it reads over weight,
so not sure exactly how much weight was saved. OK, so I bought another scale that goes to 11lbs, and it shows it now weighs 6lbs.
But one thing is certain, it is a LOT easier to wind it, when the even teeth are removed.

   

I have remove all the windings and the laminations from the rotor, which leaves it a bare hollow shaft,
and weighs very little at this point.  

   
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#5
Here is the new rotor with the neo magnets epoxied on.

   
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