What is the speed of the external alternator?

The External Alternator connects to a pulley installed between the propeller and the Prop Flange.

See the Installation Manual Section 24-00-00, Chapter 2.6, Pages 14 &  15.

Quote:  "... The speed of the external generator is 1.24 times the Crankshaft speed or 3 times the propeller speed... "

That is CRANKshaft, NOT CAMshaft.

5500 Engine RPM = 6800 Alternator RPM.

The larger the Pulley you install on the Compressor, the slower it will turn.

There is also the option of coupling to the slower turning Power Takeoff on the rear of the CrankShaft.

I installed a B&C 20 amp alternator on my 912 gearbox vacuum pad/accessory mount.  It required a gear set with installation and I believe it runs about half of engine rpm. 

See This - https://www.cps-parts.com/catalog/rtxpages/912914vacuumpump.php

About half was a pretty good guess.  

Looking at the gear train, (21/51 * 29/22 = 0.543) the accessory gear runs at 0.543 of the Engine.

5500 Engine RPM = 2987 Accessory RPM.

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You will be in uncharted territory here.

I do not know how much power you can take off of that gear.

An A/C compressor will be way more load than the Vacuum pump it was intended for, but probably not a whole lot different from a 20 Amp Alternator.

Just something that needs to be considered in the design.

Are you aware of the electric 24-VDC A/C compressor system that the Toyota Prius uses? It includes 3 small are light weight about the size of a 8oz fruit/vegetable can and are plumbed in parallel so you can turn them on and off individually as needed for more or less cooling instead of running a big compressor all the time. Available in most junk yards.

Garrett Wysocki wrote:

Are you aware of the electric 24-VDC A/C compressor system that the Toyota Prius uses? It includes 3 small are light weight about the size of a 8oz fruit/vegetable can and are plumbed in parallel so you can turn them on and off individually as needed for more or less cooling instead of running a big compressor all the time. Available in most junk yards.

How many amps does it need?

But right now I have only the stock generator which is only 250 watts and is already over stressed.