On Fri, 27 Feb 2004 17:05:02 -0600, Eric Christensen wrote
on Re: [EE ] Stepper motor voltage question:

>I don't understand how this would help.  If I understand what you're saying, you
>have this circuit:
>
>             19R        Step Coil
> 24V--------\/\/\/---+----UUUU----------- GND
>                     |
>                     V1
>
>
>With this setup, V1 will be 5 volts when 1A is flowing through the circuit.
>This means that as far as the stepper is concerned, this is identical to the
>original 5V, 1A setup.  But now you are wasting 20 watts in a resistor.

>What did I miss?

Easy to explain (please see ASCII art below): when you feed
a 5 volts inductive winding from 5 volts, starting from
zero, the law of current in the winding versus time is in
first (rough) approximation linear, and it will take a while
before the current reaches say 90% of its asymptotic maximum
value, this delay depends of the poorly documented winding
inductance (a variable for .

     _      ___         Current in winding
    / |    |   |        1st pulse at 5 Volts
   /  |    |   |        2nd pulse at 24 volts
  /   |____|   |___ ... more surface <-> more energy

  ____     ____
  |   |    |   |        Pulses from control logic
  |   |    |   |
__|   |____|   |___....


Of course, the actual laws are not linear, but look like
this: http://www.cs.uiowa.edu/~jones/step/physicsf/10.gif
(from "Jones on stepping motors" at
http://www.cs.uiowa.edu/~jones/step/ , a recommended read).

If the pulse is short, which is the case when you want to
operate a motor at maximum speed ***while keeping some
residual torque***, then the energy you get is the area of
the trapezium below this ramp, or even the residual triangle
if the pulse stops just when maximum current is reached.

If you energize the winding from a much higher voltage
source (e.g. 24V vs 5V), then the magnetization will be much
faster: as a rough approximation the magnetization time will
be null, and instead of having only the area of the triangle
you will have the area of the rectangle (at max speed of
course), or about twice as much energy per pulse and average
power at a given velocity.

At the same time you ***are*** limiting the current at a
safe value at any time, which implies that you are not
stressing the motor. A big advantage of such a technique is
that there is no need for voltage commutations, etc.

Continuously feeding a 5 volts motor with 24 volts is a bit
on the rough side, IMO.

* Xrobert.soubie@free.frX (veuillez supprimer les "X")
* http://www.astrosurf.com/soubie
* Au royaume des aveugles, les borgnes sont mal vus... - P.Dac

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