Brian Whittaker wrote:

> One way to almost double the low speed torque is to drive both sides
> of each coil simultaneously with a PIC and bipolar driver using twice
> the motors rated voltage because the coils are in series. The more
> turns of wire you drive the larger is the generated magnetic force.

  Assuming you are not limited by power dissipation in the coils.

  If you *are* however, this logic is worthy of closer examination.  If
we put both windings in series across the *same* supply voltage, we
halve the current but with twice the turns, develop the same torque.
Each coil with half the current, dissipates one quarter the heat, but
there are now two, so half as much heat is dissipated overall at exactly
the same steady-state voltage.

  The *inductance* is now quadrupled however, but compensated by the
doubled resistance, the "charging" time is only doubled so the usable
motor speed is only halved in the process.

  OK, we can now increase the overall drive voltage by a factor of
sqrt(2).  This increases the torque by that factor and doubles the heat
in the winding so it now equates to the original single winding at the
original voltage.  It also increases the motor speed in proportion, so
we now have sqrt(2) times the original torque and 1/sqrt(2) times the
original speed.

  Interestingly, the power developed by the motor is half the torque
times the square of the angular velocity, which suggests that while this
alteration has increased torque (by sqrt(2)), it has reduced power
output by the same factor.

  Can someone spot a flaw in this reasoning?

(Please note that the two windings per pole share the same space and
thus the allowable power dissipation for driving the pair together is
the same as for only one at a time).
--
  Cheers,
        Paul B.