On Fri, 13 Jan 2012 16:59:48 -0500
Ed Nisley <ed.08.nis...@pobox.com> wrote:
> On Fri, 2012-01-13 at 10:09 -0700, Cathrine Hribar wrote:
>> if the steppers are wired in series, like I wired mine,
>> they would require twice as much current
>
> Having waded through this mess not too long ago, here's what I (think I)
> know...
>
> Putting the two halves of a single pole's winding in series doubles the
> number of turns, doubles the winding resistance, and increases the
> inductance by a factor of four.
>
> Doubling the turns doubles the magnetic flux density in the pole, which
> is easier to see with the old-school unit of Ampere-turn instead of the
> fancy-pants metric Gauss or Tesla. Because torque is proportional to
> magnetic flux, you should get twice the torque for the same current.
>
> Unfortunately, the armature will probably saturate because you're now
> running it at twice its design flux, which will kill the torque and
> perhaps the motor, too. That's not a desirable outcome, so,
> paradoxically, a motor rated at 2.8 A per winding should run at 1.4 A
> with two windings in series.
>
> The resistive power losses would double at the same current, but will go
> down by a factor of 2 at half that current. If the motor has enough
> magnetic headroom, you can reduce the current by 1/sqrt(2) to dissipate
> the same amount of power: 2.8 A * 0.707 = 2 A.
>
> The increased inductance increases the overall L/R rise time by a factor
> of 4, assuming the external circuit is supplying substantial resistance
> (as in antique L/5R DC drives with hulking power resistors). With modern
> current-limiting chopper drivers, however, the rise time depends mostly
> on the winding's internal resistance, which increases by a factor of 2,
> so the net L/R increases by a factor of only 4/2 = 2.
>
> So, with the series-wired windings connected to the same supply voltage,
> the current rise time doubles. If you were pushing the motor's upper
> speed limit, the torque will fall off because the current reaches the
> limit set by the driver much later in each microstep. In the worst case,
> it no longer reaches the limit at all.
>
> It's enough to make your (well, my) head spin...
>
> --
> Ed
> http://softsolder.com
Hi Ed:
What u wrote makes a lot of sense... While I don't have the equpment here at
my shop to check the torque that I used at work, I did some expearmenting
with the amp adj. control. When I increased from 1.8 to 2 amps the torque
felt liked it doubled while holding the hand wheel.
>
>
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