Alternator pulleys

Ultradog MN

Well-known Member
Location
Twin Cities
I'm converting a 3 cyl tractor to an alternator.
Pulley on the new 10-SI I bought from Napa was for a 7/16" wide belt. I need a 1/2" belt.
So I went back and bought a 1/2" pully ~ $9.
The 1/2" pully is 3 3/16" dia vs 2 5/8" on the original 7/16".
That's about a 30% difference.
I know an alt will charge at idle but am curious how that 30% will affect charging rate on a slow revving tractor engine.
Back when the machine shop was still operating it would have been an simple task to widen the small pully on a lathe. But alas, no lathe now.
Thanks in advance...

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once she's excited.. I doubt you will have a problem keeping up with charge rates to refill the battery and run a couple 55w headlamps.

Unless you have an ac and some KC lamps and a huge stereo on that tractor.. should be fine at even less than pto rpms..
 
The original pulley on the genny on my 3 cylinder 4000 is 3 1/2". Folks who do the conversion to an alternator with the tach drive have to use the same size pulley as the original generator for the tach to be accurate, and I've never heard of any charging problems from those folks .
 
You would need to know the dia of the engine drive pulley to determine the overall increase / decrease.

My "guess" is you will find the tractor's drive pulley is smaller than a Chevy engine. But even if it is the same, the alt will be turning slower due to the bigger pulley on the alt. My ford with auto alt doesn't charge below 8-900 rpms.
 
I would think the slight difference in speed is made up by the smaller draw and need for recharge of the battery.

You can get a smaller pulley but most I have seen are in the $25 range.
I have even read where someone took a saws all and cut the 7/16 pulley in half and added a flat washer between the two halves making it wider.

I just bought a 7/16" belt of the proper length and let it ride deep in the crank pulley. Been working great for several years that way.
 
It will actually increase the charge rate provided it's got enough adhesion on the smaller pulley to adequately drive the alt at full load. I usually just get the pulley widened...

Rod
 
(quoted from post at 17:30:15 05/21/14) It will actually increase the charge rate provided it's got enough adhesion on the smaller pulley to adequately drive the alt at full load. I usually just get the pulley widened...

Rod

The new pulley is larger, not smaller.
 

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