Building a 48V generator from a car alternator

[spinningmagnets]

This video uses a $200 "Predator" gasoline engine similar to a lawn mower engine. The car alternator is a common 100A unit for $100.

It may help to understand the difference between 1950's car generators and what is now called an alternator. The old style generators made 14V at cruise RPM's as you drove down the road, but...when the car was idling, the voltage was lower, and the headlights would "dim". If you idled too long with the headlights on, it would drain down the battery and it might be difficult to start later.

There was a desire for a generator of some type that would put out the full 14V at idle RPM's. What ended up working is using electromagnets in the rotor/armature. The system is designed to put out 14V at idle, and as the RPM's increase, the voltage would normally increase too, but...a small voltage-regulator scales the power to the rotor downwards, so the electromagnets get weaker. In this way, the alternator makes 14V at all RPM's.

By removing the voltage-regulator and connecting the rotor-windings power directly to the systems 14V, I have seen a car alternator produce 120V DC by adjusting the RPM's. By using a belt and pulleys between the engine and alternator, the diameter of the pulleys can be adjusted to produce 48V at the optimum engine RPM.

The AlternatorGenerator, 48V Battery Charging

 

[eee291]

Cheap, but also inefficient.
I had planned a qs138 3kw with a Kelly hooked up to a 2 cylinder Yanmar.
Had all the parts ready to assemble but someone made me an offer I couldn't refuse.

 

[gobi]

Ron, I watched David's video.
He did a good job explaining how the alternator works and what he did to mod it.

I had acquired an alternator from a knelling bus (guy who worked on the knelling controls).

Q: I should be able to get a 24v solid state voltage regulator and switch out the current one and raise the voltage to 28.x volts output from my alternator?

FYI: I bought this 10 years back for a APU I was thinking of, for my diesel overland vehicle (never happened)

 

[spinningmagnets]

Thanks. With the cost of a "real" 120V AC generator going for $1000, I thought there might be some interest in this.

Of course, when there is a disaster with widespread power outages, the "real" generators are sold out immediately, when there are plenty of car alternators and also "lawn mower engines" on the shelf.

 

[CONSIDERABLE SHOUTING]

Cheap, but also inefficient.
I had planned a qs138 3kw with a Kelly hooked up to a 2 cylinder Yanmar.
Had all the parts ready to assemble but someone made me an offer I couldn't refuse.

I wanna say it's about 60-70% efficient at best? Either way like Spinningmagnets says, it also shows if you're expedient and handy how cheap you can make a homemade generator if needed. I've been looking for spent lawnmowers for free gas engines for a while now, and when alternators go bad it's typically the voltage regulators before bearings- A Chevy truck alternator and a lawnmower engine can all be found for free.

 

[spinningmagnets]

"...I've been looking for spent lawnmowers for free gas engines for a while now..."

If you keep your eyes open, you can find used lawn mowers for free. Many people buy the absolute cheapest gasoline lawn mower to use, maybe $150. Then after about four or five years, it starts running rough. A lawnmower mechanic might charge $50 for a go-over and tune up. At this point, many people are either moving to a new place, or even of they are not moving...they just buy a new mower.

So what was wrong with the five year old mower? The air filter was clogged, and the guy never blew it out or replaced it. This makes it run rich, and the spark plug starts building up gunk on the tip. The blade was dull, so it cut badly. Fuel filter is "half clogged", but still runs with some stumbling at the higher RPM's.

If you blow out the air filter and fuel filter, clean and gap the plug, and sharpen the blade with an angle grinder...it suddenly runs well and cuts nice. I would change the oil and put in a new air filter and new fuel filter, but just to get it running nice...it almost doesn't cost a dime.

 

[JackFlorey]

By using a belt and pulleys between the engine and alternator, the diameter of the pulleys can be adjusted to produce 48V at the optimum engine RPM.

A few notes:

1) You generally have to swap out the diodes as well. Make sure they are good to at least 60 volts.
2) Ensure the regulator has the dynamic range to handle the current needed to produce ~55 volts at your desired RPM. (Obviously it can't require 12,000 RPM if you are using a cheap mower engine or something)
3) As others have mentioned this is pretty hideously inefficient. Almost any good (i.e. efficient) motor will work better for this. Turbine starter motors are a good option for this, and are pretty cheap when sold as timed-out parts.

 

[spinningmagnets]

"1) You generally have to swap out the diodes as well. Make sure they are good to at least 60 volts"

"Turbine starter motors are a good option for this, and are pretty cheap when sold as timed-out parts."

Very useful info, thanks!

 

[CONSIDERABLE SHOUTING]

"...I've been looking for spent lawnmowers for free gas engines for a while now..."

If you keep your eyes open, you can find used lawn mowers for free. Many people buy the absolute cheapest gasoline lawn mower to use, maybe $150. Then after about four or five years, it starts running rough. A lawnmower mechanic might charge $50 for a go-over and tune up. At this point, many people are either moving to a new place, or even of they are not moving...they just buy a new mower.

So what was wrong with the five year old mower? The air filter was clogged, and the guy never blew it out or replaced it. This makes it run rich, and the spark plug starts building up gunk on the tip. The blade was dull, so it cut badly. Fuel filter is "half clogged", but still runs with some stumbling at the higher RPM's.

If you blow out the air filter and fuel filter, clean and gap the plug, and sharpen the blade with an angle grinder...it suddenly runs well and cuts nice. I would change the oil and put in a new air filter and new fuel filter, but just to get it running nice...it almost doesn't cost a dime.

Sorry to respond 3 weeks later, but this is literally how I got my YARD MAN mower 2 years ago; I was on a jog and the family had put it out for free.
It was overfilled on oil by over a quart over (on an engine that uses possibly a half of that), filter was completely clogged, carb was junked up, linkage was partially "repaired" with a coathanger, and the mower blade was on backwards.

Seriously, Turbine starters? I didn't know those existed but thank you, I've really wanted to make my own windmill. Have any links or build threads?

 

[CONSIDERABLE SHOUTING]

3) As others have mentioned this is pretty hideously inefficient. Almost any good (i.e. efficient) motor will work better for this. Turbine starter motors are a good option for this, and are pretty cheap when sold as timed-out parts.

Hey Jack, could you tell me more about this in particular, like build threads or the like? I'm super interested in making a turbine but I'd prefer to use a good motor for it from the get-go.

 

[Papa]

Along similar lines, but different purpose... I scored a discarded, but complete, 240cc (8HP) Honda engine from a lawn care business. Re-ringed it and added an electric starter. I then found a 24v 100a Leece-Neville truck alternator at a local recycler/scrap yard for $15. No belts - direct drive with a Lovejoy coupler. My intent is a small, but portable DC welder.

 

[spinningmagnets]

Can't believe I didn't think about this sooner. Use a direct drive ebike hubmotor as a 48V generator. Here's a 4-minute video of that