According to RoyJ :
A rotary built with used parts is certainly cheaper. If you don't need
to vary the speed, I'd go with the rotary. If speed control is useful
(lathe, mill) it becomes a toss up between function (VFD is NICE!) and
cost (VFD is usually consderably more expensive.

A grinder is one speed, go with the rotary.

But -- you want to take care to properly balance the legs of the
rotary converter. The grinder will *run* anyway, but the torque ripples
show up in the surface when grinding with single phase, or with
unbalanced three phase.

The VFD would assure proper balance anyway.

It is good that the design of your surface grinder kept you from
swapping in a single-phase motor. You would have regretted it (after
you finally started to ask questions about surface finish problem. :-)

To my mind, the choice point between a VFD and a rotary
converter is whether you have multiple machines needing three phase, or
expect to have that. If so -- then a larger rotary converter could
power the whole shop. For any machines which could benefit from
variable speed (which can include larger drill presses as well as lathes
and mills), individual VFDs on a per-machine basis will allow tuning the
speed without having to worry about the effect on other machines which
might be running at the same time -- and will allow you to mount the
speed controls were they will be easy to access from the machine which
they control.

And besides that -- you really don't want to switch the power at
some point between the VFD and the motor -- that can lead to rather
early failure of the output transistors, unless the VFD is massively
oversized for the motor, as in the case of the 7-1/2 HP VFD which
happens to be running my Nichols mill with only a 1 HP motor. :-)

Enjoy,
DoN.
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