Re: Thanks - was: Things to check? is now: RUNNING~!

Weyland:

Some will flame me but...

Forget Ohm's law for the steppers. You are using Gecko's to control
the current through the motors. The Geckos don't care what voltage
is required, they just want the current through the stepper's winding
to be at a certain value.

This does not apply to the input voltage of the Gecko's - too much
and you'll blow them. When your stepper motors are turning, they
also act as generators. The voltage they generate is called Back
Electro-Motive Force (or Back EMF) and can be higher than the voltage
of your power supply. This voltage is sent back to the Gecko's and
is why it is strongly recommended that you run the Gecko's with less
than their maximum voltage at your power supply.

Note that since the Geckos use microstepping the 'certain value' I
mentioned above is in reality a series of values that are repeated
for each 'full step' of the stepper motors. The current is ramped up
in discrete steps, then ramped down in discrete steps to provide
ten 'microsteps' per full step of your stepper motor.

The 'iron losses' mentioned below translate to heat. What it means
is that you have diminishing returns as the voltage multiplier goes
up. In your example of a 4.5V motor the 75V power supply is 16.7
times the nominal voltage - well within the boundary.

Tony

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Weyland <weyland@v...> wrote:
> On Thursday 18 September 2003 11:09 am, Tim Goldstein spake:
>
> > > No, no... we're down to 75VDC now, thanks to Mr. Variac.
> > > (...and what a kewl little thing he is, too...)
> >
> > In the context of limits and motor heating 75 VDC is way up
there. What
> > is the voltage rating of the motors? If is about 3 you are way up
on the
> > curve where the majority of your heating will be due to iron
losses
> > (result of high over voltage). Look at the Gecko whitepaper
curves and
> > you will see that at the upper end of the recommended 10 - 25X
> > overvoltage range the iron losses go up very steep.
>
> <huge deafening bell goes off in Weyland's head at this point>
>
> Oh-oh...
> Wait a minute.
> See if I'm doing this correctly, please.
> Looking up Ohm's Law, if I multiply the current
> by the resistance, I should get the normal voltage.
> Izzat right?
>
> Then, if I'm using 3.5A, and the phase
> resistance is 1.29Ohms, I should be using 4.5 VDC.
> Correct?
>
> So, while 75VDC might be at the higher
> end of the scale, it should still be okay, no?
> Knowing this, what would be safe?
>
> 65VDC?
>
> More in another post...
>
> Weyland