Re: [CAD_CAM_EDM_DRO] Re: What is Rms?

turbulatordude wrote:

>I assume then that this is the basic idea behind using a higher
>voltage on steppers.
>
>The higher voltage can deliver power faster, so the motor has more
>'snap' OF course, there are other circuits that regulate the power.
>
>

Not really. The real reason is to counteract both the inductance and
back-EMF of the motors. When a stepper motor is spinning, it also
generates a voltage. The stepper driver needs to deliver a voltage
higher than this generated voltage to force current through the
windings. Also, when stepping, the current in each winding needs to be
reversed every 2 full steps. The inductance resists this change in
current. The more voltage the power supply provides, the faster the
current can be flipped to the opposite direction.

>AND.... THIS is why one does not take that 5 watt stepper, and
>calculate 5 watts at a 20x voltage for the power supply.
>
>

If your 5 W comes from the winding specs at idle, such as 4 A at 1.25 V,
that has nothing to do with the power output possible from such a motor.
As the motor accelerates under load, the power that needs to be fed into
it from the driver increases. Some of this additional power is lost in the
copper and iron of the motor. But a significant amount is delivered as
mechanical power from the motor shaft. When the motor is spinning at
such a rate that it takes 40 V RMS at 4 A per winding to drive it, the
input power (neglecting power factor, which is actually very significant
and shouldn't be neglected) would work out to 40 * 4 * 2 (for both windings)
= 320 Watts! Now, 20 - 30 of those watts could be lost in the motor,
some are not "real" due to power factor, and the rest is delivered to
the load. If you vary the load on the motor, you'd find the phase angle
between the motor's voltage and current (power factor) will vary in
such a way that the phase angle will become smaller under higher load.
The power factor is becoming higher when that happens. So, although
a microstepping drive is applying the same RMS current to the motor,
and the back-EMF is responsible for most of the voltage across the
windings, the power IN is varying in relation to the power OUT. The
only way this can happen is if the phase angle between the voltage and
current changes.

(AC induction motors work the same way, too. That is why the current
reading on an AC motor doesn't change that much when the load changes
from zero to full. Most of the change is in the phase angle.)

Jon