Generating operating current

[fusername]

So this is a strange question. I am re-purposing a VW generator for a project because it is what I have lying around. Does anyone with a generator in their car and an amp meter know the typical output of the generator when driving around? I am wondering what a safe 100% duty cycle load is for this thing (kinda). It will be run in a cool envirnoment so I think my cunning plan should work out

[Piledriver]

If memory serves the 12v gens were rated for 30A.
That was with forced air cooling from the VW fan.

I picked up a Ford F350 ambulance alternator for $50... shipping cost was ~35.
I bought it to weld with, it is rated for 300A continuous @14v.
Monster thing the size of your head, I have a 2 cylinder 18 HP briggs to drive it if I ever get around to that project.

[fusername]

oh man, I forgot they had that little duct in the fan shroud for forced cooling, I thought they were compeletly passive cooled. While they are rated to 30 A, I was trying to figure out what to de-rate them to for continuous (4 hour max realisticly) use.

The reason I want to use a generator is I want to use it as a spindle drive for my milling machine, and I could easily use the field for ghetto speed control with an encoder for feedback. since I have the electronics already for a low power controller, I figured I would be nice and easy. Current motor is 1/4 hp and is plenty, so I think a generator lands in the right power range, as that is probably about 15 amps, who knows what voltage they use for rating these things.

Good call on the forced air tho, I completly forgot about that.

[Marc]

If cooling's the only issue you should be able to rig up a 12VDC fan from a computer case. Parallel-field DC motors don't have much torque, though - would it even do the job?...or were you figuring on a gear-reduction setup of some kind?

[Dale M.]

Think it would be easier to just get a proper DC motor and be done with it.... You will probably never be really happy with something that is sort of a abortion, kludged together..... Sort of believe in my own mind something as good and precision as milling machine deserves correct application of modifications so its reliable and accurate....

Dale

[fusername]

I have a belt drive already on the mill head, so I can use that, has a 3:1-.3:1 range. Should get me good gearing up to 12k rpm, or much slower, hard to say without testing it first. I think I'll try running one up on the bench at see what the speed clocks at. at ~400+ bucks for a proper setup, I am looking around first.

The thing is I could buy a motor, but the generator is physically a great fit. Good size, face mount, shaft on either end to keep the encoder away from the work, oh and I have a few.

[Dale M.]

Don't think generator will produce enough HP even with gear reduction...

Dale

[telforddorr]

You can probably run the generator as a motor at 1/2 of it's max rating. So assuming an efficiency of 80% that's:

((12 volts x 15 amps) / 746 watts/hp) x 80% = 0.193 hp = ~3/16 hp

You can control the speed by varying the field current (less current = higher speed), by either adding series resistance between the field connection and ground or reducing the field voltage directly (remember, the field winding is wired between the two generator terminals, not to the grounded generator housing). Don't reduce it too much, as the generator will draw too much armature current and/or over-speed.

Don't forget to supply some kind of forced air cooing through the generator.