I made a homemade generator.

mihi(Southeast)October 18, 2006

I recently built a homemade generator. I decided it would be nice to have if we lose power and I want to keep the refrigerator and freezer going.

Here is how I did it:

1) I asked around and got an old lawnmower given to me. It was a Murray with a 4.5 horse Briggs engine. The handle was shot, the wheels were falling off, but I got the engine running good. I cleaned the air filter, which I don't think had ever been done, and changed the oil and sparkplug. It runs fine now.

2) I bought a steel plate for $25 from theepicenter.com. This plat has universal holes drilled for a lawnmower. It also has the mounting holes for the alternator I would need. I bolted the lawnmower engine to the plate. I took the blade off the lawnmower first though. I also used 3 washers between the plate and engine to lift it off the plate a bit. This was needed to insure it could get in "depth" alignment with the alternator, in other words so the shaft didn't stick down to far.

Also, the blade acts as fly-wheel weight, which you lose when you take it off. I've found the pulley and alternator bring enough weight to make it work OK though and start easy enough.

3) I put a pulley on the lawnmower crankshaft. This to run the belt that goes to the alternator. I also got this pulley from epicenter. Theirs is a very good quality one, better than what I saw at Northern Tool.

4) I picked up a GM alternator. You can get the model number needed at theepicenter to from some of the plans they have. I used the alternator where you have a switch in the circuitry so that you can switch on the alternator to charge the battery, or you can switch it off. You can pick-up used alternators at the car junkyard, about $25. Or you can get a newly rebuilt one for about $50 or so. You do need to understand the different types and how they hook-up, which is really very simple. theepicenter website gives a good description.

5) You have to bolt the alternator to the steel plate. I had to go to the hardware store to get the correct bolts and stuff for this. It was easily done though.

6) You have to get a belt to run between the engine pulley and the alternator pulley. I ended up getting one from theepicenter too, as it was about as cheap as anywhere else and I got the right belt. Read their info. to decide on the right belt to use. I put the belt on and tensioned it just by feel.

7) I had gotten some cables that would work to hook the alternator to the battery post of a 12V car battery that I had available to me. Tell the guys at the autopart store what you're doing, they have them. You can also get them from theepicenter too. In this system you have to have a battery because a car alternator needs to be attached to one when it is charging. But that's OK, I like having the battery because I can run small appliances without having to crank-up the engine.

8) Then I ran some wires and a couple of switches, which I got at the local auto parts store, to an inverter that I had ordered. I got a 1200 watt continuous inverter for $99 (and no shipping charge or tax!). You can get small 400 watt inverters for about $25 if you only need to power small stuff.

9) I didn't mentioned that I built a wooden frame for this all to set on. Its a hodgepodge of 2x4's I had laying around. But it does the job of holding the steel plate with engine and alternator, and battery and inverter all together. I have a small wooden post sticking up that has my switches on it, right by where the engine pull-cord is.

10) Also the other day I bought 4 wheels on sale at Northern Tool, along with 2 axels. I'm going to mount the generator on wheels so its easy to move around.

11) It runs great. I start it for a few minutes every few days to keep the battery topped off. Its sitting there waiting for an emergency. I've plugged everything from a small microwave (1150 watts) to a drill in it, it powers them just fine. For the microwave though, the pull is more than the alternator can produce, which is about 750 watts, so it is running off the battery some too when the gun is firing. So charge the battery a while after the microwave has been used.

If you decide to make one, good luck. I think next I'm going to build one using a horizontal shaft engine, I've found a place you can get them for about $150+ for a new one.

If you would like to see a picture let me know.

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wise_guy(NW MN)

You need a battery when using an alternator because the field of an alternator is not self-exciting. You need a power source going to the alternator to get inductive reactance in motion and produce a stator output. A battery also helps level out the voltage output and help sustain short-term high-amp loads if the alternator is not able to.

You could install an old A or B-type generator in the system instead of an alternator if you didn't want to use a battery.

    Bookmark   October 19, 2006 at 12:25PM
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A few of the older engines had a starter/generator. that would work with a storage batt. you would have to put a couple of toggle switchs going the Regulator, to direct what you wanted it to do.

    Bookmark   October 19, 2006 at 9:31PM
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wise_guy(NW MN)

Giventake made a pretty nice proposition.
A compound wound B-type starter/generator with a vibratory-type voltage regulator system would be very easy to assemble.

That would be a pretty nice set-up due to the electric start feature. Of course you would need to retain your battery for the electric start; but it's a good thing to have for surges of higher current draw loads.

Starter/generators like all generators do have short-comings though. The increased weight, reduced output, and added cost might not shift the balance away from the alternator system that mihi owns.

It would also be nice to add a welding feature to mihi's contraption. I wonder how easy it would be to incorporate the 3-phase output of the alternator that is present before the diode rectification into an adjustable output transformer system. You would still want to rectify the output to the stinger into DC for better weld characteristics and reduction of "sticking" the stick when the arc is first made.

Here's a proposition I just sketched up:

This would produce a nice constant voltage DC arc welder. You could adjust the voltage at the stick by adjusting the rheostat controlling the DC coil in the transformer. A DC current field in an AC transformer will soak the field and control the amount of inductance through the AC fields.

    Bookmark   October 20, 2006 at 1:06AM
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Why not hook up a small photovoltaic panel (solar panel) to your battery and charge it that way? Then you wouldn't have to start the generator up every few days. Solar power would do it for you. At Harbor Freight you can get 1.5 watt battery trickle charger when on sale for $9.99! They work good.

I have a gas engine/alternator similar yours that I use to charge my household 12 volt PV/battery system. I only use it during cloudy spells in late autumn and early winter that we are going into now....

    Bookmark   October 20, 2006 at 11:41AM
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Here's a picture of a small dc generator I made a few years ago. Self-excites every time, no battery. Used an old Cadillac alternator, lawn mower engine. Followed plans I found on the web. Sorry about photo quality - digital cameras were very new and low resolution back then.

Here is a link that might be useful:

    Bookmark   October 22, 2006 at 10:05AM
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It's been about 10 years since I made the generator (see post above) but if memory serves correctly, I believe the key component in the self-exciting ability is in residual magnetism inside the alternator. This ability to self excite was discussed in the plans I followed. If a battery had been required, doubt if I would have built it.

    Bookmark   October 22, 2006 at 11:11AM
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wise_guy(NW MN)


Alternators typically do not posses any residual magnetism. Did you get a special alternator or modify a stock one? There is really no safe way of altering a stock armature safely. If you were to attach a small permanent magnet to the armature (rotating field) it would excite the stator but would not last long.

Alternators can spin upwards of 15,000 RPM and any modification of the armature would be very dangerous.

So, how did you get an alternator to posses a residual field and self-excite?

Generators DO have a residual field. Sometimes the field has to be re-esablished in them. And the field is so weak that the generator has to be spinning close to 1000RPM to self-excite (depending on type of voltage regulator).

    Bookmark   October 22, 2006 at 5:30PM
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Nope, didn't modify it - it's stock. Removed it from junk Cadillac myself. I have used it in past for 12 volt lighting and it always comes up. Occasionally it has been idle for 2-3 years, but no problem. I have read how to re-magnetize them, but never had to. Wish I could remember what my construction plans said about the self-exciting aspect. About all I remember after studying the plan was that I decided it would work without a battery to get it started - and it does. Think the plan specified a GM alternator with internal voltage regulation, which I was careful to get. I searched the web a bit but can't find the plan I used. I printed it out years ago and if I find it, I'll let you know.

    Bookmark   October 22, 2006 at 6:14PM
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wise_guy(NW MN)


Typically the armature of an alternator is made of laminated layers of steel with a very high permeability and very low retentivity. An alternator is many times more efficient than a generator. It is nice to have residual magnetism in a generator for self-excitation. But, typically the same amount of residual magnetism applied in an alternator would adversely affect the amount of control a voltage regulator would have over the amount of induction taking place within the alternator.

Keep my posted! I'm interested.

I am by no means an expert on this. I just know what I have been told and have read.

    Bookmark   October 22, 2006 at 7:43PM
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Generally speaking, If a permanent magnet is used to excite an alternator, it is small enough not to adversly effect the efficiency of the alternator, not to mention the fact that it is rotating in a seperate coils field, not in the main outputs field. (This is usually what you find in a self excited generator you can buy - the kind with a portable gasoline engine/ac output type).

Even though it is not designed to be self exciting, just the residual magnetism that exists in the iron core rotor of an automotive alternator can excite itself, especially if it has been in operation recently. The longer it is idle, the weaker the residual magnetism will become, thereby at some point, it won't be able to excite itself.

The rotor is designed not to retain magnetism for two primary reasons. First is so that eddy currents won't build up and work against the electormagnets, and secondly, so that the current can be turned on/off as needed to allow the voltage to be regulated.

The residual magnetism is enough to sometimes excite the alternator even if it was in open circuit operation - meaning nothing connected to the output or if it was connected to a completely dead battery, but it is not enough to cause any voltage regulation problems.

This is all irrelevent however because the automotive alternator is excited the moment current is introduced to the field coil (in this case - the rotor). The voltage regulator monitors the circuits voltage on the battery's side and when the voltage drops below a certain preset level(usually around 13 volts), it energises the rotor(field coil). The rotor then becomes a strong electromagnet. Since it is rotating in the stator coils field, it induces current in the stator coil. This is the output. The voltage regulation is a continous series of turning current on(exciting) and cutting current off to the field. It is then rectified to direct current which maintains the battery voltage and powers any auxillary circuits.

What this means is that the automotive electical power system is designed as a system. Alternator/voltage regulator/battery. A portable genset has a permanent magnet built in to excite itself because a battery is not part of the system. It is supposed to be started and almost immediately generating power(usually 120 or 120/240 volts ac).

In the case of a homemade generator utilizing a small gasoline engine and an automotive alternator, you will not have any problems as long as you understand what I have explained above. Build your system with a battery as part of the system. It is best(easier at least) to use a Gm style alternator with an internal regulator.

If you ever run into a situation where the battery is completly dead, it may not come back to life until you excite the field coil. You could accomplish this by either replacing the battery with one that has at least a little charge left in it or by using jumper cables connecting the dead battery to a hot one. You could even use a vehicle to jump start it. Of course if there is enough residual magnetism in the core, it may excite itself and come back online. Alternatively, as mentioned already in other posts, you could use the older style automotive generators which are permanent magnet generators that will excite themselves everytime regardless of battery's state of charge.

Hope this answered more questions than it created!

Here is a link that might be useful: home generator

    Bookmark   April 25, 2008 at 7:32PM
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My dad built a portable power thing with a delco altenator. It was powered by an electric motor and once started he had no way to turn it off. It self powered itself. That was his version of a perpetual motion machine. Just needed 110AC to start it. Kept burning out his rectifier lightbulbs though.

    Bookmark   April 29, 2008 at 7:25PM
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masiman(z7 VA)

I am sure others will also comment on the perpetual motion machine and how they do not exist, especially with inefficient electric motors.

    Bookmark   April 29, 2008 at 9:20PM
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The homemade generator basic principle is by utilizing the "attraction and repelling" forces of magnets, which is one of the most abundant, it's a very simple yet powerful energy source to generate electricity.


Here is a link that might be useful: Homemade Generator

    Bookmark   May 6, 2010 at 2:55AM
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The homemade generator basic principle is by utilizing the "attraction and repelling" forces of magnets, which is one of the most abundant, it's a very simple yet powerful energy source to generate electricity.


Here is a link that might be useful: Homemade Generator

    Bookmark   May 6, 2010 at 4:44AM
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Many years back I built a little portable generator for camping trips
using a 1hp qualcast engine (vintage sidevalve English motor) and a
Chrysler alternator, externally regulated. It would never self excite after
about a day or two and needed a battery to kick it into life. I tried gluing
a tiny magnet onto the rectifier plate nearest to the armature and reassembled it, the magnet sat about 2mm from the armature. The modification did the trick, no regulation problems at all. I started it recently after about ten years and away it went, instantly. It is probably best to orientate north south poles to suit the armature, which I didn't bother doing, but it has just enough flux
there to bring it to life. The magnets are best sourced from junked hard drives. And watch for heat in the rectifier, I used JB Weld epoxy which stands high temp well, even though the little 75cc motor is out of breath at 25 amps, so not a lot of rectifier stress there.

    Bookmark   December 22, 2010 at 10:12AM
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