[HomeBrew] Auto alternator for a home brew 120 VAC generator

[Flowertime01 at wmconnect.com]

Folks,

Mother earth mag did this topic back in the 70's.

If you spin the alternator at 3600 RPM you get 60 hertz out of the 
alternators 3 phases.

Your 5 hp motor likes to run under a 1000 RPM. So a pulley on the motor 4 
times larger than the pulley on the generator lets you have 3600 RPM 60 hertz 
when the motor runs at 3600 / 4 = 900 RPM.

The generator put out 12 volts. 100 Amp alternators can be had off old chevys 
at the junk yard. Most are 60- 70 amps but look for a truck generator rated 
at 100 amps.

The three phases only produce 33 amps each at 12 volts so that only 396 watts 
per phase. Call it 400 and 13.8 volts

You cannot add phases. You need to use each one separate. Then you need
three transformers to convert 12 volts 33 amps to 120 volts 3.3 amps.

You an operate a lot of thing off this power. The problem is finding a group 
of 120 to 12 volt transformers you can wire in parallel to get the conversion 
done.

Another route is to let the generator run DC and run an inverter to get the 
120 volts at 9 amps. Still 1200 watts. The problem here is about 500.00 for 
that size of converter with good sine output.

If you are going off grid. stay on 12 volt DC. use 12 volt RV lamps. Find a 
12 volt radio and 12 volt small screen TV. run the lap top flat screen computer 
off 12 volts Auto adapter.

Lots of boat and RV folks have been off the grid for years.

Propane works wonders. Gas refrigerators do work. Expensive ones have a pilot 
light and turn the burner on and off from the thermostat in the refrigerator.

The real problems are washer, dryer and big screen TV or a refrigerator.

The power load of the motor gets you.

Rewind an alternator to 1 phase is a no go because of the number of poles in 
current designs. Check the net some have been done but you will not get the 
power.

Best bet is hunt up an old AC DC electric motor of 1 horse power.
You can spin that with a gas engine. you get 60 hertz 120 volt at one horse 
watt power. That much current will run the 1/2 and 1/3 horse motor in the 
washer dryer and refrigerator. Not all at once. The trick is to find a series AC DC 
motor. All the new stuff is AC squirrel cage type and will not excite.

Hunt the web for some stand alone alternators. These are several hundred 
dollars.

If you just want to do some back yard science stuff, you can just use the 12 
volt DC and do a lot of things with it. Shop the RV web pages for the number 
of 12 volts DC appliances and power tools available.


Using your own regulator you can feed 120 volts DC to the field and get 120 
volts out of the generator. Use some large caps rated at 200 volts. Use a diode 
to convert the AC from the generator to DC and charge the caps to 120 volts. 

You start on 12 volts from a battery. Once the generator excites the output 
over 12 volts a charges the caps you open a relay to disconnect the battery. 
The voltage continues to build on the caps and will charge the caps to their 
break down voltage or if the regulator is limiting then to the output voltage.

You can only get 1200 watts out of the generator this way and it is still 3 
phase.

Field voltage  and armature voltage goes up and current goes down. You cannot 
get more power out of the generator by increasing the working voltage. The 
wire insulation will work to about 600 volts. 

At some point power loss in the alternator creates enough heat that you burn 
the insulation off the wires and things short out. This is the factor that 
limits the power output.

It is the three phases and limited current per phase that kills the Auto 
alternator as a practical device for 120 volt power source. The thing has an 
inherent design in the number of winding poles that do not let it be rewound to a 
single phase generator.

Roger.