BobG said:

Is there a 'rule of thumb' for figuring out much torque it takes to put

out xx volts and yy amps from a PMA given the gauss of the magnet and

the N turns of the coils? Should spin free open circuit, and really

stall out short circuit, and be somewhere in the middle during

operation. I guess the 'max power' point is when the load drags the

volts down to half the open circuit value? I guess the current here

would be half the short circuit current too? I want to rig the pwm

controller to not load down the pma too much under low power/low

torque/ slow spinning conditions, like from a stirling or micro hydro.

In general volts (and current) is proportional to rpm... someone got

some formulas that are a little more specific?

First of all. the torque to produce a voltage xx is 0. Torque is not related

to voltage per se. Torque is related to current.

If you can find the voltage -easiest done by spinning it and measuring it.

then you will get a relationship E=Kw where E is in volts and w is in

radians/ sec (2*Pi*60 rad/sec =1 rpm)

Then a rough guide is Torque =KI torque in Newton meters and I in amps.

same K as above.

The relationship between E and I depens on the load.

Finding the max power is another problem and it will not be at the half

voltage level. Do you really want to be operating there? the alternator

won't want to be operating there. It is a little different than what is

implied by the maximum power theorem which implies matching load impedance

to the alternator impedance., not the other way around (it also implies <50%

efficiency)