A steady field voltage is required to force an alternator up to the rquired 60-90 vdc for striking a good arc.
It's better if your engine has a built in charging system. Lots better. Spend up to an extra
$100. It makes the job sooo much easier and is more compact, The field can be excited from the charging
system without an extenal extra generator,
A battery can provide this steady voltage for quick jobs. I prefer an autonomus always capable system
such as useing a permanent magnet DC motor to act a generator. Stick with something designed to run constantly.
An Ametek or any other DC motor capable of providing 4 amps at 12vdc will sufficiently energize the field. Wind
power sites list the voltages and rpms for many permanent magnet motors, You could also guestimate by looking at the
motors specs and then plan on turning it around 20% faster to achieve the voltage it would ordinarily consume then calculate your
pulley size based on this. I used a permanent magnet alternator off a Kubota diesel because it was self energizing and
at the time free. I rectified the output to provide half wave DC (less power to the alternators field)....... vice
fullwave (more power to the alternators field). This built in inefficiency reduces the alternators output and approximately
matches it to the available engine power.