Alternator rpm = engine rpm X (engine pulley diameter/alternator pulley diameter).

My Lycoming has the 7.5" (small) pulley and my alternator has a 2.75" pulley, which I believe to be typical. This results in alternator speeds in the 7,000 rpm range during cruise, which I believe to be excessive. A 4" diameter pulley is available for my alternator, which would reduce it's speed to the 4,000 rpm range and, I believe, produce full-rated output.

I spoke with Mark Landoll who told me that alternators typically require a minimum of 1,000 rpm to produce a stable output, but he didn't know what rpm is usually required to produce full-rated output. I believe it to be in the 4,000 rpm range. He did agree that, while alternator bearings seem to hold up well to high speeds, lower speeds would produce less stress and vibration.

Is anyone on the LML knowledgeable of the alternator speed typically required to produce full rated output?

 

Bill Rumburg

N403WR (Sonic bOOm)