Using alternator and motor as torque converter??

[rudyauction509]

I have a gokart I built last year, it has a 6.5hp lawn mower engine on a 3 speed riding mower transmission, and a belt slip clutch. I'm sick of replacing the belt weekly and having it jump every couple minutes. I'm thinking of running the engine to a 180 amp alternator and wiring that to 1 or more old truck generators (the kind that are also used to start the truck) that will drive the transmission.

I know the an alternator can do the job, but will the old generators hold up? I may use 2 in parallel to ease the load (and may do that with the alternators as well if needed). I plan to do lots of hill climbing and will have a small radiator fan blowing over the generators to keep them cool and possibly a water cooling jacket or some heatsinks mounted to them.

Also an alternator regulator works by powering the field coils with varying current based on the battery voltage. If I used a pwm controller to drive those coils instead of the regulator I should be able to work them almost like a throttle, while the engine would run on its governor at about 3600rpm and the alternator would be connected by a belt and pulleys with about a 2-1 gear ratio, allowing it to spin around 8000rpm. Does this sound correct? I know I'll have to be careful not to overvolt the generators but I'm not worried about that causing a problem.

The reason I want to do this vs other options is cost. I have all the tools, a good alternator, all the belts and pulleys and a truck generator at a junk yard is only $10. I know there's better ways to finish this project but I just can't afford $100+ in clutches and other parts on something I literally built for free out of scrap metal last year.

I should mention that I have been working with electric motors for over 10 years but have never touched a truck generator before, so the wiring and electrical parts won't be a problem.

 

[rudyauction509]

Just had an idea. What if I skipped the alternator and used a generator to generate the electricity as well? Then I could add a small car battery and be able to kill the engine when cruising flat trails between hills using battery power and start it again by hitting the generator with 12v from the battery. Basically a hybrid, which would be cool for messing around in my back yard without making a bunch of noise as well as allowing me to go farther into the woods on a tank of gas.

 

[itsid]

think about it this way:

A typical two pulley nontoothed belt unit has an efficiency of about 95%
a good alternator has an efficiency of converting mechanical into electrical power of around 80%, the same goes for a good electric motor.

A generator (two phase instead of three) has an efficiency of around 60%.. so yeah.. alternator if you can

Anyways, you will be left with 60.8% of the engines power using an alternator.
(45.6% using a generator)

You work with electric motors for over ten years.. and still think you can overvolt a generator/alternator?
hmm... *shrugs*
The problem you'll be having is heat, not volt!

In the end you will be spending more on fuel than you do now on belts.

So back to square one:

The initial problem will be your gear ratio , that's why you burn through belts on a regular basis.
adjust the ratio to - allow me to make an assumption without any necessary information- about half the top speed you theoretically have now;
and your problems will be gone I guess.

'sid

 

[rudyauction509]

think about it this way:

A typical two pulley nontoothed belt unit has an efficiency of about 95%
a good alternator has an efficiency of converting mechanical into electrical power of around 80%, the same goes for a good electric motor.

A generator (two phase instead of three) has an efficiency of around 60%.. so yeah.. alternator if you can

Anyways, you will be left with 60.8% of the engines power using an alternator.
(45.6% using a generator)

You work with electric motors for over ten years.. and still think you can overvolt a generator/alternator?
hmm... *shrugs*
The problem you'll be having is heat, not volt!

In the end you will be spending more on fuel than you do now on belts.

So back to square one:

The initial problem will be your gear ratio , that's why you burn through belts on a regular basis.
adjust the ratio to - allow me to make an assumption without any necessary information- about half the top speed you theoretically have now;
and your problems will be gone I guess.

'sid

By overvolting I mean like running 36 volt battery pack to it with no current limiting. Motors are limited by current, and current by the load on the motor. So a stalled 12v generator at 24+ volts will fry very quickly but would run fine with a light load. That's what I mean by overvolting. In reality I'll likely be limited by the 6.5hp engine instead of the current the generator can handle safely.

As for gear ratios I have a 3 speed transmission. 1st gear 5 mph 2nd gear 12 mph 3rd gear 25 mph. Any lower and this thing would no longer be fun to ride. I have to slip the clutch at every start and a bit while shifting. I should mention that I had to decide between free and lightweight when building this, I chose free and that left me with only angle iron for the frame, the gokart weighs about 250lbs so slipping just to 2-3mph quickly is enough to have the transmission pulley hot enough to burn my hand.

And as for more fuel, maybe that's worth it to not have to clutch at every stop and put the belt back on every couple blocks (I've adjusted the belt as well as I can and it still falls off when I'm slowing down with the clutch pulled). Plus I'd always have the option of going electric easily in the future if the motor dies for some reason. And one more reason, I've always wanted to do a gas-electric combo on a small vehicle like this. 60% of my motor's power is plenty for the hill climbs I do because I only go about 3-4mph on the steepest hills.

 

[rudyauction509]

Man, truck generators are really hard to find! I visited 5 wrecking yards today and no one had any. I'll try a couple more tomorrow. Otherwise I'm going to have to figure out what else has powerful DC powered motors that can be found cheap.

 

[rudyauction509]

Also I got 2 old alternators today for free. Theyre the kind where the regulator isn't built in, with the field coils tied to a seperate connection. Add a 12v 10a PWM controller and I have a variable voltage and current generator that's perfect for driving the final drive motor.

 

[rudyauction509]

Side note from another project, about how efficient is a toothed V-belt and pulleys? I'd assume a bit better than non toothed. And I'm assuming by toothed you mean the belt with the notches on the inside not one that goes on a toothed pulley like a timing belt. The pulleys are 3 inches around and the belt is 24 inches around (smallest one at local parts shop).

 

[rudyauction509]

OK managed to get an old generator for $15 today. It's not what I thought it was and I had to re-wire the field coils to make it work as a motor. They now run in parallel with the stator which is powered through a brush and commutator and the polarity can be reversed to change direction of rotation. Running on 12v it spins about 1,500 rpm and had enough torque to rip out of my hand. I left it free spinning for 3 hours and it was cool to the touch, pulling about 3/4 of an amp without load, so it is working properly. It's a bit smaller than most generators (wrecking yard guy said from some import car), but I got it half off just to see if it'd work. I may be buying 2 full size versions for the final project. This one will probably end up on a scooter or some other project. I should also mention that this motor runs on AC or DC current so I may be able to skip the rectifier on the alternator to reduce heat and increase power output by a couple percent.

 

[rudyauction509]

Just discovered today that I can change the torque and speed of the motor using only the field coils. The stator coils run 48v constant, the field coils run on 12, 24 or 48 volts, 12 volts gets me lots of speed with low torque, 48 volts gets me low speed and really high torque, 24 volts is about half way in between. Its kind of like a virtual transmission. And I don't have to worry about the magnets losing strength with heat, only the insulation on the coils. But the generator does have a cooling fan which will help when under heavy load.

This is on an electric wheelchair I got for free that had a burned motor. I figure this generator-motor will be perfect when combined with the big wheelbarrow wheels I welded on. I haven't hooked it up to the transmission (that's for tomorrow) but used an old piece of tire to put load on the motor (pressed it to pulley for friction) to check the torque during testing.

 

[rudyauction509]

Ended up getting a nice 5hp golf cart motor, I'll just use that.