"Paul B. Webster VK2BZC" wrote:
>   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).

I am not quite sure, I think that the difference between using a single
coil or two coils in series attached to the *same* drive VCC, as you
said, results in a half current, half dissipated power, but the torque
*would not be the same*, it should be less.  If this is true, than
multiplying the coil turns by 100, it will go for an infinite low
current and power, keeping the the same torque, what unfortunately I
think is not true. I really wish you would be right :)

I really don't know how to calculate some "gross" torque, but I believe
it has a direct relation to several factors in the motor, mainly applied
power.  There must have a "perfect" relation between torque, maximum
possible magnetic flux, core size (dictated by the winding coil size to
generate the maximum magnetic flux with the maximum current), and heat
dissipation.

Using a bipolar motor in a unipolar fashion (coils in series) almost
doubles the torque if you double also the VCC applied, but care need to
be observed to the dissipated thermal power.  A bipolar motor is
designed to stand one coild of each net energized all the time, not
both, so I think it is not a good idea to feed double VCC to the double
coil (unipolar) fashion.  First because the motor will dissipate double
heat, second that it will be trying to generate a double magnetic flux,
and the core can get saturated and the motor productivity would not be
the same as in bipolar fashion.

Each case is different, how long the motor is used (pwm, rest time and
so on), against real extra torque required from the motor.

I use bipolar motors in "H" bridges, but always try to feed them with
less than double voltage.

--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm