Re: Power Supply for 4-axis CNC stepper driver

Jon Elson is correct. Were he not, you would be confronted with a
motor that got colder with increasing load. :-)

1) An unloaded motor draws power from the power supply.

2) All the power drawn must dissipate as heat in the motor (There is
no load to deliver power to).

3) Applying a load to the motor means the load is absorbing power.

4) If input power does not increase with load, then the power
absorbed by the load must be subtracted from the power the motor is
dissipating as heat.

5) If (4) is true, the motor must get colder with increasing load.
Doesn't happen in real life.

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> John Dammeyer wrote:
>
> >Hi Jon,
> >
> >I think you are confusing the electrical performance of DC Servo
Motors with
> >Stepper Motors. A stepper motor doesn't draw any more power when
> >accelerating than when sitting still. In fact they are almost the
opposite.
> >
> I'm sorry, but that is not true. It is impossible for a drive
system to
> produce meaningful power without drawing more power from the power
> supply. For instance, a 1 A @ 1 V stepper only draws 1 Watt per
winding
> when standing still. Your assertion above would mean it can only
> deliver 2 W
> of mechanical power, which is miniscule.
>
> >Servos draw virtually no power while sitting and steppers are
sitting with
> >full power (depending on step position) running through the
windings.
> >That's why some controllers have a current setback where after x
seconds of
> >no step pulses they reduce current by up to 75%.
> >
> >
> You are confusing power dissipated in the motor windings with the
total
> power in the system. They are not the same.
>
> >
> >
> The back-emf generated by the motors windings means you need to
apply more
> voltage when running than when standing still. That greater
voltage
> times the
> winding current leads to a greater power draw. The I^2 x R
dissipation in
> the motor windings is the same (roughly), but the difference is
power is now
> being delivered by the motor's shaft. (There is also more iron
loss when
> running.)
>
> If you don't believe me, put an ammeter in series with the power to
the
> stepper drive of
> your choice, and accelerate the motor. (This effect will only be
seen with
> chopper-type drives, of course.)
>
> Or, read Mariss' "white paper" on the Geckodrive web site.
>
> Jon