Thread: Alternator Generating RPM

From the dim recess of my memory, a car alternator was generally geared to run at 150 to 200% of engine RPM.

If this is correct, then you should get useful amperage at 2,000 alternator RPM, and all of it's claimed amps at 6,000 RPM which is, as far as I recall, the standard 'Rating RPM'

Originally Posted by garrycol

but its driving pulley is on the small size.

Thanks

Garry

If the second alternator's pully is on the small side, then it will turn even faster, therefore it should be no problems.

Driving pulley is where the power comes from, driven pulley is where the power goes.
Aaron

Bottom line... how many amps do you need, and at what engine RPM.?

And, to muddy the waters a bit, some alternators specify an 'idle' output, ie, how many amps (probably my loose definition of 'useful'...) you get when the engine is at idle or 'tootling around town' speed.

Work back from engine pulley / alternator pulley size, figure out the ratio, then go and scrounge a suitably sized one if yours is too far out.

Simple.

PS, No, I have no idea if the output is linear or logarithmic or in-between.

Originally Posted by superquag

Work back from engine pulley / alternator pulley size, figure out the ratio, then go and scrounge a suitably sized one if yours is too far out.

Simple.

Thanks - I agree but I already have the bits - I just need to know at what a typical alternator needs to rev at to develop full power.

Cheers

Garry

I would be interested to see a graph of maximum output versus revs for Bosch alternators. Haven't seen much of or taken notice of how similar late model ones are to older types. Also, for comparison, performance graphs for the same types with wires from the three windings in the stator connected to the diodes in delta rather than standard star fashion.


Around 1978 I bought a 55 amp alternator that had this modification and extra bits to use it as a DC welder. While doing so, full battery voltage is fed into the field, ie not through regulator and the current is only going to the welding handpiece. Note the maximum output of these nominally rated 55 amp Bosch alternators is more like 100 amps with delta connection but probably at much higher revs than the max for star. Of course using high output, especially for sustained periods causes problems including shorter bearing life and shorter belt life from slippage unless twin V belts used. Heavier serpentine ones are probably OK. Also, need to remember that alternators get hot with heavy generating loads. I had two of the six diodes burnt and split from not watching this once when with jumper leads charged three flat big batteries connected in parallel on a tractor. I modified two more 55 amp Bosch alternators to delta connection myself. Also recently picked up another one from scrap metal at local tip /recycling centre. It had been a little motor driven welder. and some of the bits including leads were with it. Anyway, have cleaned it up and largely worked out how to install it on Defender without touching standard alternator.