Re: [CAD_CAM_EDM_DRO] Adequate torque from VFD driven motors?

scyvt 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?

No, not true. Torque is derived from the "magnetizing induction", or the
excess voltage above that which is needed to induce the magnetic field
in the rotor. As long as the VFD is correctly programmed for the motor,
you can get constant torque down to practically zero speed. Advanced
VFDs, with flux-vector control, as well as AC induction servo drives, can
deliver full rated motor torque AT zero speed, ie. perform as a torque motor.

> 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?

Well, now, the mechanical VS drive does MULTIPLY torque at the lower
speeds, and a VFD just gives you the SAME torque at lower speeds.
Also, self-cooled AC induction motors don't work too well at VERY low
speeds, as the cooling fan can't move any air. As long as you have a
back gear to get torque for threading and other very low speed work, you
may not notice the difference. There IS, indeed, a loss of POWER when
running an AC induction motor at lower speeds, as power is torque X speed.

Jon