Posted by jdemaris on October 16, 2009 at 07:35:03 from (67.142.130.47):
In Reply to: 12v generator low rpm posted by JerryK on October 15, 2009 at 20:28:14:
You are wasting your time with a generator if you want high output at 12 volts at low engine speeds.
A generator, in general, wastes more then half the power it makes - as compared to modern generator that uses recifiers - i.e. an "alternator."
An old (non-rectified) generator wastes a huge amount of output powering its own fields, and also only selects part of the power from the armature, in order to make DC current.
An alternator (rectified generator) uses very little field current and uses all the power from the armature (rotor) by running it through recifiers.
A John Deere B using the OEM Delco 6 volt 1101371 generator is rated:
6.5 to 8.5 amps output at 7 volts and 1800 RPM at the generator. It wastes 4.5 amps just to run the field while it makes the charge current. I think the B ran the generator at twice engine speed. So, it could put out 8 amps at 7 volts with the engine at approx. 900 RPM.
A typical Deere 1101859 "high output" 6 volt generator (like used on a 420) puts out:
20 amps output at 7 volts and 3000 RPM at the generator. It wastes 2 amps just to run the field while it makes the charge current.
Ratings are RPM that the generator is turning, not the engine. Since a Deere B runs full bore at around 1200 - 1300 RPM, the generator needs the smaller pulley than the drive pulley on the engine to reach max output of 8 amps.
Alternators on cars and trucks are usally run 2 to 3 faster than the engine turns. So an engine idling at 800 RPM has its alternator spinning at least 1800 RPM. A typical auto gas engine has an alternator running 2.3 times faster than the engine. Some diesels run a bit slower with the alternator running 2 times faster.
A cheap Delco 10SI alternator can put out 25 amps at 14.5 volts at 1600 RPM and 70 amps at 6000 RPM , i.e .the engine revving at 2600 RPM. It needs to spin at least 1100 RPM to make 8 amps.
A Delco 12SI alternator can put out 30 amps at 14.5 volts at 1600 RPM and 100 amps at 6000 RPM. Again, with the engine revving at 2600 RPM.
A Delco 15SI alternator can put out 40 amps at 14.5 volts at 1600 RPM and 85-105 amps at 6000 RPM. So with the engine idling, this can put out over 40 amps.
Some CS130 Delcos can put out 100 amps at engine idle and 220 amps when the engine is revved to 3000 RPM.
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