To calculate the alternator shaft speed, which should be no more than 16,000 RPM do the following:

Alternator pulley size (ie 2.5")

Crankshaft Pulley Size (ie 5")

Ratio is 2:1 (5 / 2.5 = 2)

Max engine Speed = 5800 RPM

Multiply Max RPM x Ratio (5800 x 2) = 11,600 shaft RPM Max

Idle RPM = 1000

Multiply Idle RPM x Ratio (1000 x 2) = 2000 shaft RPM @ Idle

An alternator needs 1800-2000 at idle to make power and will make maximum output at 6000 rpm, and will start to fail at 16,000 RPM.

If you have Microsoft Excel, from Windows 95 to current, you can download and open this calculator.  Only fill in the yellow fields!  It will tell you the rest.