Re: Adequate torque from VFD driven motors?

Steve,

I had the same questions when I implemented CSS on my Hardinge
retrofit. My first stab at the situation was a 5hp 1750 rpm motor
driven by a VFD using a belted drive to the spindle with
approximately 1 to 1 ratio. Everything was fine except when I was
turning large diameter soft aluminum. Almost anything 4" diameter
and larger gave me problems. At those diameters it didn't have the
torque to keep the rpm's up to break chips.

After switching that motor for a 5hp 1150 rpm model my torque
problems went away. And Baldor told me I could safely spin this
motor to 5000 rpm.

Doug

--- In CAD_CAM_EDM_DRO@y..., "scyvt" <scy@v...> wrote:
> Hello list
>
> When the speed of a spindle drive motor is reduced by step or
variable
> diameter pulleys, the torque is multiplied by the same ratio that
the
> speed is reduced. In other words, lower speed - higher torque. This
is
> consistent with what is needed by machine tools.
> But when spindle speed is reduced by reducing motor speed using a
VFD,
> torque is reduced in the same proportion as speed, isn't it? In
other
> words, lower speed - lower torque.
> But VFD's have been recommended on this list, and most new
commercial
> CNC machines seem to control spindle speed this way. So how can
this
> be?
> I am retrofitting a lathe that had an elaborate mechanical variable
> speed drive, and it would a lot easier to go the VFD route,
especially
> so CSS could be implemented. But I just don't see how there could
not
> be a lack of power at lower speeds. What am I missing?
>
> TIA,
>
> Steve Clay-Young