lml@lancaironline.net Mailing List Archive

In a message dated 9/23/2005 9:27:03 P.M. Central Standard Time, lancair403@earthlink.net writes:

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,

Here is an example of a Skytronic's 50 amp alternator :

The 12 V model requires a shaft speed of 4000 rpm to produce the rated max output at some fixed field voltage.

However, strain on an alternator (including heat, hp demand) is determined by the load on it. Say that X amps is needed during all phases of flight for a continuous load. This load can be produced at many different shaft speeds by controlling the magnetic field thru the field voltage (voltage regulator job). At slower shaft speeds, the field voltage would have to be increased to create the same output load whereas at a higher shaft rpm less field voltage is required as is less horsepower and less heat dissipation. That is, 4000 shaft rpm may produce adequate output but may create a higher HP demand to drive it than when running at 7000 rpm.