# Permanent Magnet Alternator Torque and volts?

B

#### BobG

Jan 1, 1970
0
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?

J

#### John - KD5YI

Jan 1, 1970
0
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?

Horsepower = Volts*Amps/746 = torque*RPM/5252 with torque in lb*ft.

John

D

#### Don Kelly

Jan 1, 1970
0
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
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)

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