I saw this posted on another list.  I am passing it along for general interest, with no warranty for the accuracy of the information contained in it.


From Pifer's Airmotive, Inc. Pontiac MI

DIFFERENCES BETWEEN AIRCRAFT & AUTO ALTERNATORS USING A FORD BELT DRIVEN 12v OR 24v ALTERNATOR FOR A COMPARISON

1. Although alternators are bi-rotational, aircraft engines turn opposite of automotive. This means cooling fans must be canted in the opposite direction. Also, pulley and belt size vary due to coming-in speed.

2. The thru bolts are of a higher tensile strength utilizing an anti-rotation device in the form of a lock tab. The rectifier assembly has a heavy duty diode with higher voltage and amp. capacity. Also, one excites at 90 PIV (Peak Inverse Voltage) and the other at 150 PIV. Radio suppression is designed for 108 frequencies and up which is the VHF and 108 and down which is FM band.

3. The brushes have a higher graphite content and they utilize a tin plate on the brush leads to prevent corrosion.

4. The stator is of the Delta wind rather than the “Y" wind and it does not utilize the stator terminal. The aircraft unit also carries “H" insulation which is capable of 200 degrees centigrade temperatures. It also is rated at 60 amp. instead of 55.

5. The rotor has a shorter shaft and a smaller thread size. Because of the opposite rotation it is wound in the opposite direction. It also uses "H” insulation and Havel varnish.

6. The front and rear housings are the same as automotive.

With this brief description, I hope I have enlightened you on the differences between aircraft and automotive alternators. Using automotive units in an aircraft creates a potential safety hazard as well as a short alternator life and unreliability.