Re: [CAD_CAM_EDM_DRO] Re: mill servo conversion ?'s

On Thursday 05 August 2004 11:22 pm, Stephen Wille Padnos wrote:

> Roy J. Tellason wrote:
> >On Thursday 05 August 2004 11:57 am, Jon Elson wrote:
> >>For performance. To get full peak torque, you need to deliver 12 A
> >>to the motor. With a 36 V power supply, you can't deliver 12 A to a 4
> >> Ohm motor, even standing still. At 900 RPM, the back EMF of the motor
> >> is approximately 36 V, so you can produce zero torque! This severely
> >> limits performance. I have a client in town who has an 800 Lb Grizzly
> >> style machine with these motors and a 36 V supply, and the performance
> >> is pretty weak. To get full peak torque at rated speed, you need 36 +
> >> 12 * 4= 84 V minimum, appled to the MOTOR itself. You need some margin
> >> for the modulation scheme of the servo amp. too. So, a 120 V supply
> >> sounds good, but if using Gecko 320 drives, 75 V or so is the limit.
> >> With a 5 TPI screw driven directly, you get 180 IPM, which should be
> >> plenty for most users. But, if belt reduction is used, or if the screw
> >> is finer than 5 TPI, then the max rapid rate goes down, even with the
> >> higher voltage.
> >
> >This part of your post I don't quite follow. How are you figuring out
> > what the back EMF of a motor will be? I feel like I'm missing something
> > here...
>
> That's a rating that usually appears on the label of the motor as Ke.
> It will be in Volts per Radian/Sec or Volts/kRPM.

I guess all the (small) motors I have on hand here must be missing that info
or something, as I've never seen that anywhere.