Re: [CAD_CAM_EDM_DRO] Re: Power Supply for 4-axis CNC stepper driver

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