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 ------------------------------------------------------------------------------ RSA(R) Conference 2012 Mar 27 - Feb 2 Save $400 by Jan. 27 Register now! http://p.sf.net/sfu/rsa-sfdev2dev2 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
