Hi Phil,
I agree with Dan, in that if you are using a 24V truck to charge a 24V batt bank via a petrol/diesel motor then it would be best to leave the standard exciter coil rotor alone.
The reg changes the strength of the field coils via the brushes depending on how much load is being drawn and how flat the batteries are.
Permanent magnet setups are more for direct drive wind & hydro turbines which need every bit of efficiency they can get, and with an early cut in voltage, you could not use a PM 24 truck alt to charge a 24V batt bank as the cut in speed would be too high, but using a 12V bank the 24V alt reaches it`s cut in voltage earlier.
You also need a dump load to be activated when the batt is full, allowing the turbine to allways be loaded.
IMPORTANT: Using petrol/diesel engines with standard car/truck reg`s are made for charging small 50-80AH batteries,
and only put their full power out when there is a given load.
When the batteries get to about 13.5V with a 12v setup the 40Amp alt will only put in 2-5Amps as any more would damage the small car batterie but not a 500AH bank.
This is why I recommend disconnecting the two wires from the brushes to the reg and connecting them via a large variable resistor, then to the batt bank, which can be set at 70% max output, giving you 30Amps until the batteries reach say 14.4Volts.
This will cut down the run time of the engine and make good use of available horsepower.
When connecting a standard 40A bosch alt to a 3hp engine i got 25amps for one minute and dropped down to 3amps as the batteries reached 13.4V, batts were still
150 Ah down. When converting the brushes, and giving the field coil a continous 10volts, the amps will stay around 30Amps continous until i turn the engine off.
Matt Au.