Ferret Alternator Question

[Malcolm Towrie]

We have some Canadian Ferrets here that were converted to alternators in the 1970's or '80's. The conversion included disconnecting the original "P" wire. This supplied voltage to the original field winding through the 6W charging bulb on the switchboard so the light went out when the generator was charging.

It replaced the "P" wire connection to the alternator field winding with the "R" wire, which is ignition-switched 24V. This meant the charging light no longer worked so I assume the voltmeter was added to show the charging state.

I recently bench-tested a couple of alternators. Instead of using +24V to excite the field windings (equivalent of the "R" wire), I used a test light connected between battery +24V and the field winding, which is equivalent to the P wire (although I don't know the wattage of the test light bulb). I spun the alternator with a drill. It loaded up and generated 15A charge at only 1200 rpm and the test light dimmed as the voltage across it equalized. I assume it would have gone out if I could have spun the alternator as fast as the engine would at idle.

So I'm curious why the conversion used the "R" wire rather than the "P" wire which seems to work and would have kept the charging light working.

Malcolm