generators
hello maxhelixangle & others,
two important formulae for rotating dc machines :
1) E(voltage) = k(constant) x Magnetic flux( SI unit: weber) x omega( rotational speed in radians/second). this means that a rotating coil in a magnetic field produces a voltage potential(power source), which then can be connected to an electric circuit. in aviation this is a dc generator, connected to so called "bus bars"(+) or metal strips, where different users( pumps, light bulbs etc. ...) are hooked on. as the negative(-) or earth connection, the aircraft airframe is used, saving 50% of the wiring needed.
2) M(moment in meter newton) = k(constant) x Magnetic flux x Ia(amperage or amps). this means that, if ampères are send through a wire, which is in a magnetic field, a force or a moment(for rotational devices) is created. to determine the direction of the force mr. Fleming rule is used with your right hand: palm of right hand on top of magnetic flux, thumb in direction of electric current(amps), then other straight fingers indicate force direction. there is also another rule called the rule of the three fingers to find same direction.
but now back to our generators.
concerning the choice between a shunt or a series wired dc generator, in aviation, only shunt dc generators are used for following reasons :
1) for the voltage regulator( which influences the magnetic flux), a series voltage regulator would be very difficult & costly to manufacture, mechanically as well as electrically. a shunt voltage regulator, on the otherhand, would be very easy & cheap to produce.
2) concerning reverse currents(short circuits). for a series wired dc generator, the magnetic flux would be reduced dramatically & the moment being constant, the amperage would increase catastrophically( see second formula above), resulting in a burned out generator, as where, for a shunt dc generator, a reverse current would just cause the generator to swap function & to run as a dc motor, which is of course much better than to burst in flames.
hope this helps, but i need a cup of coffee now,
kind regards,
bm